mid-ir laser-based diagnostics for hydrocarbon fuel … · at the beginning of the engine cycle....

iii

MID-IR LASER-BASED DIAGNOSTICS FOR HYDROCARBON FUEL VAPOR

SENSING AND DECOMPOSITION SPECIES MEASUREMENTS

A DISSERTATION

SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING

AND THE COMMITTEE ON GRADUATE STUDIES

OF STANFORD UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Sung Hyun Pyun

July 2012

http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/ym323bk2464

© 2012 by Sung Hyun Pyun. All Rights Reserved.

Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

ii



http://purl.stanford.edu/ym323bk2464

I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.

Ronald Hanson, Primary Adviser


Craig Bowman


David Davidson

Approved for the Stanford University Committee on Graduate Studies.

Patricia J. Gumport, Vice Provost Graduate Education

This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file inUniversity Archives.

iii

iv

Abstract

The development and optimization of energy conversion systems, such as internal

combustion (IC) engines, gas turbines, and pulse detonation engines, relies on two

important factors. The first is the accurate measurement of critical engineering variables

at the beginning of the engine cycle. Specifically for IC engines, time-resolved in-

cylinder fuel delivery and gas temperature information is needed as an important

optimization parameter for advanced IC engine design and development. The second

factor is the accurate predictive modeling of chemical reaction progress during high-

temperature fuel decomposition. Energy conversion systems can be optimally controlled

based on the understanding and modeling of the chemical processes that control their

performance.

The first goal of this dissertation is to describe the design and implementation of a

mid-infrared absorption sensor for crank-angle-resolved in-cylinder measurements of

gasoline concentration and gas temperature for spark-ignition internal-combustion (IC)

engines. Mid-IR laser light was tuned to transitions in the strong absorption bands

associated with C-H stretching vibration near 3.4 µm, and time-resolved fuel

concentration and gas temperature were determined simultaneously from the absorption

at two different wavelengths. Validation experiments were conducted for a single-

component hydrocarbon fuel (2,2,4-trimethyl-pentane, commonly known as iso-octane)

and a gasoline blend in a heated static cell (300≤T≤600 K) and behind planar shock

waves (600<T<1100 K).

v

A novel, mid-IR, scanned-wavelength laser absorption diagnostic was also

developed for time-resolved, interference-free, absorption measurement of methane

concentration. A differential absorption (peak minus valley) scheme was used that takes

advantage of the structural differences of the absorption spectrum of methane and other

hydrocarbons. A peak and valley wavelength pair was selected to maximize the

differential cross-section (σpeak minus valley) of methane for maximum signal-to-noise ratio,

and to minimize that of the interfering absorbers. Methane cross-sections at the peak and

valley wavelengths were measured over a range of temperatures, 1000 to 2000 K, and

pressures, 1.3 to 5.4 atm. Cross-sections of the interfering absorbers were assumed

constant over the small wavelength interval between the methane peak and valley

features. The differential absorption scheme efficiently rejected the absorption

interference and successfully recovered the vapor-phase methane concentration.

The second goal of this dissertation is to present and analyze fuel decomposition

species concentration time-histories that were measured during the high temperature

pyrolysis of several fuels including major gasoline n-alkane components as well as

dimethyl ether (DME), a bio-fuel.

CH4 and C2H4 concentration time-histories were measured behind reflected shock

waves during the pyrolysis of two n-alkanes: n-butane and n-heptane. Experiments were

conducted at temperatures of 1200-1600 K and at pressures near 1.5 atm, with fuel

concentrations of 1% in Ar. CH4 was measured using the methane diagnostic described

above. C2H4 was measured using a fixed-wavelength absorption scheme at 10.532 µm

with a CO2 laser. The measured CH4 and C2H4 time-histories in n-butane pyrolysis were

vi

compared to simulations based on the comprehensive n-alkane mechanisms and the

chemical model was improved based on the measurements.

High-temperature dimethyl ether (DME) pyrolysis was studied behind reflected

shock waves by measuring time-histories of CO, CH4 and C2H4 in mixtures of 0.5%, 1%,

and 2% DME in argon respectively. Experiments were conducted at temperatures of

1300-1600K and pressures near 1.5 atm. A direct absorption strategy with a fixed

wavelength (2193.359 cm-1) using a quantum cascade laser (QCL) was used to measure

CO concentration time-histories. C2H4 was measured at 10.532 µm and 10.675 µm with

a CO2 laser using a two species-two wavelength scheme to reject fuel absorption. The

measured CH4, C2H4 and CO time-histories during DME pyrolysis were compared to

simulations based on detailed chemical mechanisms, leading to improvements in these

mechanisms.

These measurement strategies can be used to quantify the needed fuel and

temperature loading in IC engines, and the kinetics database obtained provides

quantitative species time-histories that can be used to test, validate and refine the fuel

decomposition sub-mechanism of gasoline surrogate mechanisms.

vii

Acknowledgements

I remember the moment I first talked to my advisor, Ron Hanson five years ago. I told

him that I was interested in the engine application of laser diagnostics because I wanted

to build my career in the automobile industry. He told me he has an interesting project

with Nissan motor company. One year later, in October 2008, I flew to NTCNA (Nissan

Technical Center North America) in Michigan to test and validate a laser diagnostic

sensor for the measurements of gasoline concentration and temperature in real internal

combustion (IC) engines. With his support, I developed a novel Wavelength Modulation

Spectroscopy (WMS) - 1f method and applied this theory to correct and calibrate the

absorption measurement of the sensor. The sensor successfully worked with the theory I

developed and I felt an immense joy of solving tough real-world engineering problems by

applying creatively developed theory. I would like to thank my advisor for giving me not

only a chance to feel the joy of being an engineer but also helpful advice both on

academic and family life. I have also wanted to learn his passion and relentless

motivation for his work and life.

I would like to express my gratitude to Dave Davidson and Jay Jeffries for

helping me solve problems that I faced so that I could advance further. I have been very

fortunate to have their help in my work.

I sincerely enjoyed working with Jason Porter during my first research project.

He is not only a great co-worker, but also an amazing mentor for me. I would also like to

give many thanks to Hanson students, especially Aamir Farooq, Jon Yoo, Wei Ren, Brian

Lam and all my other friends, who have helped me over the years.

viii

I thank my parents, Seokjin Pyun and Enuik Choi and my sister, Jiyoung for their

unconditional love and support for me. I also thank my parents-in-law, Daeho Lim and

Wonok Lee for their warm support and encouragement for me.

Most importantly, I want to thank my wife, Sung Hee Lim for her devotion and

patience. She has been a perfect wife, mother and career woman. My 1 year old son,

Ewan has cheered me up with his unbelievable smile. They always remind me of what is

truly important in my life. I know that I could have not been where I am today without

them.

ix

Contents

Abstract iv

Acknowledgments vii

List of Figures xii

List of Tables xviii

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Experimental objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Two-Color Absorption Sensor for Fuel and Temperature Sensing 7

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Design of temperature and fuel concentration sensor . . . . . . . . . . . . . 9

2.2.1 Fundamental spectroscopy and wavelength selection . . . . . . . . . 9

2.2.2 Two-color mid-IR absorption sensor . . . . . . . . . . . . . . . . . . 13

2.2.3 WMS-1f model for species with broad spectral features . . . . . . . . 14

2.3 Experiments to test sensor performance . . . . . . . . . . . . . . . . . . . . 19

2.3.1 Details of gas handling and experimental apparatus . . . . . . . . . . 20

2.3.2 Measurements of iso-octane absorption for WMS-1f model validation . . 21

2.3.3 Measurements of gasoline vapor mole fraction and temperature . . . . 25

3 Interference-free Mid-IR Laser Absorption Detection of Methane 28

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2 Development of interference-free absorption methane detection scheme . . . 30

3.2.1 FTIR measurements of absorption spectra of hydrocarbon species . . . 30

3.2.2 Absorption theory and the differential absorption scheme . . . . . . . 34

3.2.3 Wavelength selection criteria . . . . . . . . . . . . . . . . . . . . . . 35

x

3.3 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.4.1 Cross-section measurements . . . . . . . . . . . . . . . . . . . . . . 41

3.4.1.1 Methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.4.1.2 Interfering species . . . . . . . . . . . . . . . . . . . . . . . . 44

3.4.2 Demonstration of methane time-history measurements . . . . . . . . 46

4 Decomposition Species Measurements during n-Alkane Pyrolysis 50

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.2 Laser absorption diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.2.1 Methane detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.2.2 Ethylene detection during n-alkane pyrolysis (one wavelength). . . . . 52


4.3.1 Species concentration time-histories and gasdynamic models . . . . . 53

4.3.2 Sensitivity analysis and updated chemical mechanisms . . . . . . . . 59

4.3.2.1 n-butane pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . 59

4.3.2.2 n-heptane pyrolysis . . . . . . . . . . . . . . . . . . . . . . . 62

4.3.3. Measured methane and ethylene time-histories . . . . . . . . . . . . 66

4.3.3.1 n-butane pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . 66

4.3.3.2 n-heptane pyrolysis . . . . . . . . . . . . . . . . . . . . . . . 70

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5 Decomposition Species Measurements during DME Pyrolysis 76

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.2 Laser absorption diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.2.1 Methane detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.2.2 Ethylene detection during DME pyrolysis (two wavelengths). . . . . . 79

5.2.3 Carbon monoxide detection . . . . . . . . . . . . . . . . . . . . . . . 82


5.3.1 Species concentration time-histories and gasdynamic models . . . . . 83

5.3.2 The DME decomposition reaction rate constant . . . . . . . . . . . . 87

xi

5.3.3 Measured species time-histories and comparison with modeling . . . . 90

5.3.3.1 Carbon monoxide time-history measurements . . . . . . . . . 90

5.3.3.2 Methane time-history measurements . . . . . . . . . . . . . . 93

5.3.3.3 Ethylene time-history measurements . . . . . . . . . . . . . . 96

5.4 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

6 Summary and Future Work 100

6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

6.1.1 Two-color absorption sensor for fuel and temperature sensing . . . . . 100

6.1.2 Interference-free mid-IR laser absorption detection of methane . . . . 102

6.1.3 Decomposition species measurements in n-alkane pyrolysis . . . . . . 102

6.1.4 Decomposition species measurements in DME pyrolysis . . . . . . . . 103

6.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Bibliography 106

xii

List of Figures

2.1 Absorption cross section for iso-octane in the region of the C-H stretching vibrational

transitions for temperatures of 230K and 775K; wavelengths selected for temperature and

concentration noted. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2 Sensor architecture showing the DFG scheme for simultaneous production of two mid-IR

laser frequencies centered at ν1 and ν2 with intensity modulation at f1 and f2; EDFA denotes

the erbium-doped fiber amplifier used to increase the intensity of the telecom laser. . . 14

2.3 Iso-octane absorption cross section in the region near 1ν = 2951 cm-1 (panel a) and 2ν =

2970.5 cm-1 (panel b); note that the variation of the cross section with laser frequency is

nearly linear over the range ±a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.4 Experimental arrangement for absorption measurements of iso-octane and gasoline versus

temperature with two-color, intensity-modulated sensor in either a heated cell or across a

shock-heated flow; the transmitted light is focused onto a single detector where two lock-

ins demultiplex the signals at the modulation frequencies f1 and f2. . . . . . . . . . . . 20

2.5 Measurements of σeffective(T) for iso-octane (panel a for 1ν = 2951 cm-1; panel b for 2ν =

2970.5 cm-1) in a heated cell (diamonds) and shock-heated gases (squares); the absorption

cross section )(Tσ at the center frequency of the modulated laser was taken from FTIR

measurements and the effective cross section including wavelength modulation ±a was

calculated from Eqn. 2.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.6 Two-color absorption measurements of iso-octane temperature. (Note shock tube data are

single point at 10 kHz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.7 Two-color absorption measurements of iso-octane mole fraction as a function of

temperature. (Note shock tube data are single point at 10 kHz). . . . . . . . . . . . . 25

2.8 Two-color absorption measurements of gasoline temperature. . . . . . . . . . . . . . 27

2.9 Two-color absorption measurements of gasoline mole fraction as a function of

temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1 Simulation of major species time-histories during n-heptane pyrolysis behind a reflected

shock wave using the Sirjean et al./JetSurF 1.0 (2009) mechanism. Initial conditions: 1%

C7H16 in argon, 1636K, 1.33 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2 FTIR measurement of the absorption cross-section of methane and larger hydrocarbon

species. Heated FTIR cell conditions: 0.4-6% species in argon, 774K, 1 atm. . . . . . 32

xiii

3.3 Detailed view of absorption cross-section measurement near wavelength pair A. Heated

FTIR cell conditions: 0.4~6% species in argon, 774K, 1 atm. . . . . . . . . . . . . . 32

3.4 HITRAN modeling of methane cross-section near the wavelength of pair A. Simulation

conditions: 405K, 1 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.5 HITRAN modeling of methane cross-section near the wavelength of pair B. Simulation

conditions: 405K, 1 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.6 Experimental schematic of laser system used to generate tunable mid-IR radiation near 3.4

µm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.7 Experimental arrangement for laser absorption measurements in a shock tube. . . . . 40

3.8 Measured cross-section of methane at peak and valley wavelengths for wavelength pair A

and its differential cross-section (σ peak minus valley) for pressure of 1.3 to 1.8 atm. . . . . 42

3.9 Measured pressure dependence of the differential cross-section (σpeak minus valley) for methane

for wavelength pair A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.10 Measured cross-section of methane at peak and valley wavelengths for wavelength pair B

and its differential cross-section (σpeak minus valley) for pressure of 1.3 to 1.8 atm. . . . . 43

3.11 Measured pressure dependence of the differential cross-section (σpeak minus valley) for methane

for wavelength pair B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.12 Measured absorption cross-sections of interfering hydrocarbon species for the peak and

valley wavelengths for wavelength pair A and its differential cross-section (σpeak minus valley).

σpeak and σvalley are almost identical in all cases at both temperature. . . . . . . . . . . . 44

3.13 A comparison of measured differential cross-sections (σpeak minus valley) at wavelength pair A

for methane and other interfering species at 1200K. . . . . . . . . . . . . . . . . . . 45

3.14 Measured absorption cross-sections of interfering hydrocarbon species for the peak and

valley wavelengths for wavelength pair B and its differential cross-section (σpeak minus valley).

σpeak and σvalley are almost identical in all cases at both temperature. . . . . . . . . . . . 45

3.15 A comparison of measured differential cross-sections (σpeak minus valley) at wavelength pair B

for methane and other interfering species at 1200K. . . . . . . . . . . . . . . . . . . 46

3.16 Total absorbance time-history of n-heptane pyrolysis at the peak and valley wavelengths

for wavelength pair B and the differential absorbance. Initial reflected shock conditions:

1597 K, 1.44 atm, 1% n-heptane in argon. . . . . . . . . . . . . . . . . . . . . . . . 47

3.17 Simulated and experimental total valley wavelength absorbances (less contributions from

methane) time-histories during n-heptane pyrolysis. Initial conditions: 1597K, 1.44 atm,

1% n-heptane in argon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

xiv

4.1 Simulated pressure profiles with the mechanisms of Wang et al. [63] and Marinov et al.

[60] with the constant volume and the constant pressure gasdynamic models. Initial

conditions: 1% n-butane in argon, 1565 K, 1.45 atm. . . . . . . . . . . . . . . . . . . 57

4.2 Simulated temperature profiles with the mechanisms of Wang et al. [63] and Marinov et al.


conditions: 1% n-butane in argon, 1565 K, 1.45 atm. . . . . . . . . . . . . . . . . . . 57

4.3 Simulated pressure profiles with the mechanisms of Wang et al. [63] and Curran et al. [62]

with the constant volume and the constant pressure gasdynamic models. Initial conditions:

1% n-heptane in argon, 1597 K, 1.44 atm. . . . . . . . . . . . . . . . . . . . . . . . 58

4.4 Simulated temperature profiles with the mechanisms of Wang et al. [63] and Curran et al.


conditions: 1% n-heptane in argon, 1597 K, 1.44 atm. . . . . . . . . . . . . . . . . . 58

4.5 CH4 sensitivity for n-butane pyrolysis using the Wang et al. [63] mechanism. Only the

seven reactions with the largest sensitivities are shown; of these, only the four reactions in

highlighted boxes, Rxn. 4-7, were varied in this study. Initial conditions: 1% n-butane in

argon, 1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.6 CH4 sensitivity for n-butane pyrolysis using the Wang et al. [63] mechanism. Only the four

reactions with the largest sensitivities are shown. Initial conditions: 1% n-butane in argon,

1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.7 CH4 sensitivity for n-heptane pyrolysis using the Wang et al. [63] mechanism. Only the

seven reactions with the largest sensitivities are shown; of these, only the five reactions in

highlighted boxes, Rxn. 8-12, were varied in this study. Initial conditions: 1% n-heptane in

argon, 1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.8 C2H4 sensitivity for n-heptane pyrolysis using the Wang et al. [63] mechanism. Only the

four reactions with the largest sensitivities are shown. Initial conditions: 1% n-heptane in

argon, 1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

4.9 Modeled branching ratio of n-heptane decomposition from Babushok et al. [74]. . . . 65

4.10 C2H4 sensitivity for n-heptane pyrolysis using the Wang et al. [63] mechanism. Only the

four reactions with the largest sensitivities are shown. Initial conditions: 1% n-heptane in

argon, 1600 K, 1.44 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.11 Comparison of measured CH4 time-histories at two low temperatures with the mechanisms

by Wang et al. [63], Marinov et al. [60] and the modified Wang et al [63] for n-butane.

Initial conditions: 1% n-butane in argon, 1254 K, 1.64 atm and 1375 K, 1.58 atm. . . 68

xv

4.12 Comparison of measured CH4 time-histories at two high temperatures with the mechanisms



4.13 Comparison of measured C2H4 time-histories at two low temperatures with the mechanisms



4.14 Comparison of measured C2H4 time-histories at two high temperatures with the

mechanisms by Wang et al. [63], Marinov et al. [60] and the modified Wang et al [63] for

n-butane. Initial conditions: 1% n-butane in argon, 1482 K, 1.50 atm and 1565 K, 1.45 atm.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70


by Wang et al. [63], Curran et al. [62] and the modified Wang et al [63] for n-heptane.

Initial conditions: 1% n-heptane in argon, 1190 K, 1.69 atm and 1350 K, 1.50 atm. . . 71










5.1 Simulations of species time-histories during DME pyrolysis behind a reflected shock wave

using the Curran et al. [82] mechanism. Initial conditions: 2% DME in argon, 1450 K, 1.5

atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.2 Simulations of the CO time-history during DME pyrolysis behind a reflected shock wave

using the Curran et al. [82] and the Zhao et al. [83] mechanisms. Initial conditions: 0.5%

DME in argon, 1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.3 Simulations of the CH4 time-history during DME pyrolysis behind a reflected shock wave

using the Curran et al. [82] and the Zhao et al. [83] mechanisms. Initial conditions: 1%

DME in argon, 1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.4 Simulations of the C2H4 time-history during DME pyrolysis behind a reflected shock wave

using the Curran et al. [82] and the Zhao et al. [83] mechanisms. Initial conditions: 2%

DME in argon, 1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

xvi

5.5 Measured cross-sections of DME and ethylene at 10.532 µm and 10.675 µm. . . . . . 82

5.6 Simulated pressure profiles with the mechanisms of Curran et al. [82] and Zhao et al. [83]

with the constant volume and the constant pressure gasdynamic models . Initial conditions:

0.5% DME in argon, 1542 K, 1.47 atm. . . . . . . . . . . . . . . . . . . . . . . . . 85

5.7 Simulated temperature profiles with the mechanisms of Curran et al. [82] and Zhao et al.


conditions: 0.5% DME in argon, 1542 K, 1.47 atm. . . . . . . . . . . . . . . . . . . . 85

5.8 Simulated CO time-histories with the mechanisms by Curran et al. [82] and Zhao et al. [83]

with the constant volume and the constant pressure gasdynamic models. Initial conditions:

0.5% DME in argon, 1542 K, 1.47 atm. . . . . . . . . . . . . . . . . . . . . . . . . . 86

5.9 CO sensitivity for DME pyrolysis using the Curran et al. [82] mechanism. Only the three

reactions with the largest sensitivities are shown. Initial conditions: 0.5% DME in argon,

1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.10 CH4 ROP for DME pyrolysis using the Curran et al. [82] mechanism. Initial conditions:

1% DME in argon, 1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.11 CH4 sensitivity for DME pyrolysis using the Curran et al. [82] mechanism. Only the three

reactions with the largest sensitivities are shown. Initial conditions: 1% DME in argon,

1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5.12 C2H4 sensitivity for DME pyrolysis using the Curran et al. [82] mechanism. Only the three

reactions with the largest sensitivities are shown. Initial conditions: 2% DME in argon,

1450 K, 1.5 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5.13 Comparison of the current measured DME decomposition rate constant (k1) and RRKM

model from Cook et al. [87], the fit to lower temperature shock tube data and the upper

limit of the data fit with 35% uncertainty. . . . . . . . . . . . . . . . . . . . . . . . . 90

5.14 Comparison of measured CO time-history with the modified and unmodified DME

decomposition mechanisms from Curran et al. [82] and Zhao et al. [83]. Initial conditions:

0.5% DME in argon, 1352 K, 1.60 atm. . . . . . . . . . . . . . . . . . . . . . . . . . 91



0.5% DME in argon, 1445 K, 1.51 atm. . . . . . . . . . . . . . . . . . . . . . . . . . 92



0.5% DME in argon, 1542 K, 1.47 atm. . . . . . . . . . . . . . . . . . . . . . . . . . 92

xvii

5.17 Comparison of measured CH4 time-history with the modified and unmodified DME


1% DME in argon, 1291 K, 1.55 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . 94



1% DME in argon, 1359 K, 1.52 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . 94



1% DME in argon, 1494 K, 1.48 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . 95

5.20 Comparison of measured C2H4 time-history with the modified and unmodified DME


2% DME in argon, 1354 K, 1.53 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . 97



2% DME in argon, 1429 K, 1.49 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . 97



2% DME in argon, 1509 K, 1.45 atm. . . . . . . . . . . . . . . . . . . . . . . . . . . 98

xviii

List of Tables

3.1 Two wavelength pair candidates for differential absorption (peak minus valley) scheme for

methane detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.1 Measured mole fraction of CH4 and C2H4 in the pyrolysis of 1% n-butane in argon at 1ms

derived using different chemical mechanisms and gasdynamic models. . . . . . . . . 59

4.2 Measured mole fraction of CH4 and C2H4 in the pyrolysis of 1% n-heptane in argon at 1ms

derived using different chemical mechanisms and gasdynamic models. . . . . . . . . 59

4.3 All reactions that were modified in the Wang et al. [63] mechanism to form the modified

mechanism of Wang et al. [63] for n-butane. . . . . . . . . . . . . . . . . . . . . . . 61

4.4 All reactions that were modified in the Wang et al. [63] mechanism to form the modified

mechanism of Wang et al. [63] for n-heptane . . . . . . . . . . . . . . . . . . . . . . 64

5.1 Measured mole fraction of CO, CH4 and C2H4 in the pyrolysis of 0.5%, 1% and 2% DME

in argon at 1ms from different chemical mechanisms and gasdynamic models. . . . . 86

1

Chapter 1

Introduction

1.1 Motivation

Energy conversion systems, especially internal combustion (IC) engines, can be analyzed,

developed, and optimized if information on two important areas is available. The

acquisition of this information is the objective of this dissertation.

The first area is the accurate measurement of critical engineering parameters in the

early part of the engine cycle, before fuel decomposition. This information allows active

control of the engine systems. Time-resolved fuel delivery is an important optimization

parameter for modern IC engines, and crank-angle-resolved in-cylinder measurements of

gas temperature and fuel vapor concentration provide a crucial tool for advanced IC-

engine design and development. For example, modern combustion strategies for IC

2

engines, such as homogeneous charge compression ignition (HCCI), rely on accurate

knowledge and control of the in-cylinder fuel concentration (air-fuel ratio) and

temperature to optimize the performance and realize successful operation.

The second area is the accurate predictive modeling of chemical processes that

occur during high-temperature fuel decomposition. The chemical reaction model should

predict accurate time-histories of concentration of important intermediate species and

final products in the pathway of fuel decomposition. For example, bio-fuel combustion

stability in next-generation engines is governed by the chemical kinetics of the mixture of

fuel and fresh charge. The optimal control of this type of energy systems can be realized

when we have complete understanding and a well-developed model of the chemical

processes that govern their performance.

Laser absorption has the potential for fast, time-resolved in situ measurements, and

the development of laser-based gasoline sensing has been an active area of research.

Infrared laser absorption diagnostics have been widely used for non-intrusively

measuring key engineering parameters, such as species concentration, temperature,

pressure and flow velocity.

The first objective of this dissertation is to develop a mid-infrared absorption sensor

for crank-angle-resolved in-cylinder measurements of gasoline concentration and gas

temperature for spark-ignition internal-combustion (IC) engines. The concept of two-

color sensing of gas temperature and fuel concentration was developed earlier for n-

heptane [16] and n-dodecane [17]. However, the simultaneous measurements of gas

temperature and concentration were not possible due to the time-multiplexed approach

used. In this study, simultaneous measurements of gas temperature and fuel

3

concentration were achieved by frequency demultiplexing of the two mid-IR laser

wavelengths that were intensity modulated at different frequencies.

Laser diagnostics are needed for the second objective of this dissertation,

developing a database of fuel pyrolysis species time-histories. Laser absorption

diagnostics have been developed for measuring important fuel combustion products, such

as H2O, CO, CO2, NO, NO2, etc. However, laser diagnostics for selectively measuring

major small hydrocarbon species such as alkanes and alkenes during fuel decomposition

have not been fully developed. Current work in our laboratory addressed the

measurements of alkenes (such as ethylene, but has not yet been extended to larger

alkenes such as propene and butene). In this dissertation, we developed a laser

absorption diagnostics for methane measurements.

Methane is one of the primary alkanes produced from the oxidation and pyrolysis

of larger hydrocarbons such as n-heptane. A time-resolved methane diagnostic that could

be implemented in studies of hydrocarbon decomposition would provide a useful and

unique database for the refinement and validation of chemical mechanisms for fuel

decomposition including n-alkanes and bio-fuels. Laser diagnostics for methane

detection have been developed earlier using many different forms of lasers at wide

wavelength ranges from 1.65 µm to 8.1 µm [27-28, 35-46]. However, they were not

available for the selective measurement of methane in the absorption interference from

other hydrocarbon species. In this study, a novel, mid-IR scanned-wavelength laser

absorption diagnostic for time-resolved, interference-free, absorption measurement of

methane concentration is presented. This diagnostic is particularly suited to methane

concentration measurements in the presence of other n-alkane decomposition species

4

using a differential absorption scheme that takes advantage of the spectral structure

difference between methane and other hydrocarbon products.

For the second objective of this dissertation, developing a database of fuel

pyrolysis species time-histories, n-alkanes (n-butane and n-heptane) and dimethyl ether

were chosen as the target fuels.

Normal alkanes have been widely used as fuels and are major components of many

commercial transportation fuels. In particular, over 90% of the normal alkanes in

commercial gasoline fuels are represented by just the small normal alkanes with carbon

numbers (CN) of four to seven: n-butane (C4), n-pentane (C5), n-hexane (C6) and n-

heptane (C7). n-Butane, is also found as a component in LPG (liquefied petroleum gas)

and gasoline and n-heptane is widely used as a diesel fuel surrogate to study diesel

combustion chemistry because of the similarity of its octane number to that of

conventional diesel. The modeling of practical problems in diesel engines, such as

knocking and auto-ignition can be simplified using n-heptane as the fuel surrogate [56].

The concentration time-histories of two major decomposition species, CH4 and C2H4

were measured behind reflected shock waves during the pyrolysis of n-butane and n-

heptane to improve the chemical reaction mechanism for n-alkanes based on the species

concentration measurements.

Dimethyl ether (DME) has been proposed as an alternative fuel and fuel additive in

diesel engines due to its high cetane number (55) and low emissions of pollutants, such as

soot and CO. DME is considered to be an especially promising bio-fuel because it has

very simple structure that leads to low emissions of particulate matter in combustion, and

because it can be directly manufactured from biomass feedstocks. For better

5

understanding and improvement of DME chemical mechanisms, DME pyrolysis was

studied behind reflected shock waves by measuring time-histories of three major

decomposition species: CO, CH4 and C2H4.

1.2 Experimental Objectives

This dissertation has multiple experimental objectives to achieve the various

experimental tasks toward the optimization of internal combustion (IC engines). The first

objective is the development and validation of a mid-infrared absorption sensor for crank-

angle-resolved in-cylinder measurements of gasoline concentration and gas temperature

before fuel decomposition in engines. The second objective is to develop a novel, mid-IR

scanned-wavelength laser absorption diagnostic for time-resolved, interference-free,

absorption measurement of methane concentration. Intermediate product species

concentration time-history measurement is critical for understanding the chemical

reactions of fuel decomposition and refining fuel decomposition mechanisms. Methane

is one of the important fuel decomposition species. Thus for the third objective, a

database of CH4 and C2H4 concentration time-histories during the pyrolyis of n-alkane

gasoline components: n-butane and n-heptane was acquired. A similar database was

acquired for dimethyl ether (DME), a popular bio-fuel.

1.3 Organization

This dissertation contains four different, but coherent research topics that are essential for

developing, optimizing and analyzing energy conversion systems. Chapter 2 describes

6

the development and validation of a mid-infrared absorption sensor for crank-angle-

resolved in-cylinder measurements of gasoline concentration and gas temperature for

spark-ignition internal-combustion (IC) engines. In chapter 3, a novel, mid-IR scanned-

wavelength laser absorption diagnostic is described that was developed for time-resolved,

interference-free, absorption measurement of methane concentration. In chapter 4, CH4

and C2H4 concentration time-histories were measured behind reflected shock waves in

the pyrolysis of two n-alkanes: n-butane and n-heptane. In chapter 5, high temperature

dimethyl ether (DME) pyrolysis was studied behind reflected shock waves by measuring

time-histories of CO, CH4 and C2H4.

7

Chapter 2

Two-Color Absorption Sensor for Fuel

and Temperature Sensing

2.1 Introduction

Time-resolved fuel delivery is an important optimization parameter for modern

internal combustion (IC) engines, and crank-angle-resolved in-cylinder measurements of

gas temperature and fuel concentration could provide a useful tool for advanced IC-

engine development. Laser absorption has the potential for fast, time-resolved in situ

measurements, and the development of laser-based gasoline sensing has been an active

area of research. For example, the near-IR (~1.6 µm) was used to monitor the overtone

transitions of the C-H stretching vibration in gasoline [1] and this work was extended to

8

absorption tomography using robust telecommunications diode lasers in an attempt to

capture the in-cylinder gasoline distribution [2-5]. Unfortunately, the absorption cross

section of gasoline in the near-IR is weak, and these cross-cylinder measurements were

unable to provide the signal-to-noise needed for crank-angle-resolved, cycle-to-cycle

fluctuations. By contrast, gasoline has strong mid-IR absorption near 3.4 µm in the

region of transitions of the fundamental C-H stretching vibrational bands. This gasoline

absorption overlaps with the 3.39 µm wavelength of an infrared HeNe laser, which has

led to many successful measurements of gasoline and gasoline surrogates using this

fixed-wavelength light source [6-14].

HeNe-laser absorption sensing of gasoline in IC-engines has two primary

challenges. First, quantitative gasoline concentration requires knowledge of the crank-

angle-resolved temperature because of the temperature dependence of the absorption

cross section; although the 3.39 µm wavelength of the HeNe laser overlaps with

hydrocarbon absorption with a cross section that is relatively constant with temperature,

precise measurements must account for the temperature dependence and this dependence

can vary with fuel blend [15]. Second, for many applications, the optical depth at 3.39

µm is too large to infer precise gasoline concentration, even when the optical path is

restricted to a short path near the spark plug. The sensor in this dissertation overcomes

these two difficulties for the detection of gas-phase fuel for multi-port injection IC

engines. First, the two-color design enables simultaneous measurement of gas

temperature, which provides the data needed for quantitative measurement of

concentration in an environment with time-varying temperature. Second, the two laser

wavelengths in this design can be adjusted to avoid optically thick conditions when the

9

product of fuel concentration and the available line-of-sight path length is too large for

precise measurements.

The basic concept for two-color near-simultaneous sensing of gas temperature and

hydrocarbon fuel concentration was demonstrated earlier using a time-multiplexed

approach for n-heptane [16] and n-dodecane [17]. Here, this sensing concept was

extended to simultaneous measurements using frequency demultiplexing of the two mid-

infrared laser wavelengths. A robust sensor design suitable for use in practical IC-engine

testing and development was constructed and tested. The two laser wavelengths were

intensity modulated at different frequencies, and the transmitted signals of the two

wavelengths were separated by frequency demultiplexing. The intensity modulation

imposed a synchronous wavelength modulation and the influence of this modulation is

discussed in detail below. The sensor was tested in the controlled environments of a

heated static cell (300 ≤ T ≤ 600 K) and shock-heated gases (600 ≤ T ≤ 1100 K), and

results for a single-component hydrocarbon fuel (2,2,4-trimethyl-pentane commonly

known as iso-octane) and a fuel blend (gasoline) are described. We found this sensor to

be well-suited for measurements in production engines using the short-path optical access

provided by a special research spark plug [18-20].

2.2 Design of temperature and fuel concentration sensor

2.2.1 Fundamental spectroscopy and wavelength selection

The general principles and wavelength-selection criteria for simultaneous hydrocarbon

concentration and temperature measurements using two-color absorption of C-H

stretching transitions for species without rotational-resolved spectra have been discussed

10

previously in the context of n-dodecane detection [17]. Here we discuss the basic

concepts in the selection of the sensor wavelengths for iso-octane concentration and

temperature measurements, repeating only those details needed to describe the sensor

design. In the absence of losses from scattering by fuel aerosols and absorption of liquid

fuel, the transmission of monochromatic light at wavelength λ through a uniform sample

of absorbing gases is described by Beer’s law:

),,(),,(ln0

pTLnpTI

Ii λαλσ ==

− (2.1)

where I/I0 is the fractional transmission, L the path length, ni the number density of

absorbing species, σ(λ,T,p) the absorption cross section, which depends on the

wavelength, temperature, and pressure, and α(λ,T,p) is the absorbance. In this paper the

laser color is specified as wavelength λ (µm) units and laser frequency ν in wavenumber

(cm-1) units, which is related to the temporal frequency ν~ in units of Hz; e.g.,

c/~1 νλν == . When absorbance levels exceed unity, the sensitivity of the measurement

is reduced by the exponential nature of Beer’s law and the absorption is termed optically

thick. The design fuel concentration was stoichiometric (fuel-air equivalence ratio Ф =

1) and the design pathlength (for an optical spark plug) was 1.2 cm (a double-pass

configuration of a 0.6 cm physical dimension). The sensor wavelengths were restricted

to regions with α < 1. The gas temperature is inferred from the ratio of two transmission

measurements at different wavelengths:

)(),,(

),,(

ln

ln

2

1

2

1

20

10

TfLnpT

LnpT

I

I

I

I

i

i ≈==

−

−

λσλσ

αα

λ

λ (2.2)

11

The sensitivity of the absorbance ratio to temperature depends on the difference in

the variation of the absorption cross sections with temperature at the two wavelengths. In

general, the maximum temperature sensitivity occurs when the absorption cross section

increases with temperature at one wavelength and decreases with temperature at the other,

though there are advantages to choosing one of the two lines to be temperature insensitive.

The sensitivity of the absorbance ratio to temperature is given by:

[ ]2

2

21

12

),,(

),,(),,(

),,(),,(

)(

pT

dT

pTdpT

dT

pTdpT

dT

Tfd

λσ

λσλσ

λσλσ −

= (2.3)

The needed temperature-dependent cross section data are available for iso-octane [21]

and selected gasoline blends [15].

In addition to temperature sensitivity, the wavelengths were chosen for similar

detection signal-to-noise ratio (SNR) and this criterion is expressed by constraining the

ratio of absorbance 0.2 < α1/α2 < 5. The selected wavelengths (laser colors) were selected

to minimize interference from other species; and therefore strong transitions of water

vapor (expected in the intake air) were avoided. In addition, practical fuels can thermally

decompose into products with structured absorption (e.g., methane and ethylene) and

strong transitions of these species were also avoided. Finally, anticipating the

simultaneous production of both wavelengths by difference-frequency-generation (DFG)

in a single periodically poled lithium niobate (PPLN) crystal, the maximum difference

between λ1 and ( λ2 is restricted.

Using these criteria the mid-IR laser frequencies of 2951 cm-1 and 2970.5 cm

-1

were selected for iso-octane. Fig. 2.1 illustrates the temperature-dependent absorption

spectrum of iso-octane, and these two selected wavelengths are indicated in the figure by

12

arrows. Wavelength selection for gasoline depends on fuel blend and specific values

must be considered for each blend. The variation of the gasoline infrared absorption

spectrum with composition and the temperature dependence of example gasolines are

discussed in detail in Klingbeil et al. [15]. For this gasoline blend the wavelength

selections were 2951.0 cm-1 and 2970.5 cm

-1, but we emphasize that other sensor

wavelengths can be optimal for other gasoline blends. Note that the current gasoline

measurements used sample 7 of [15], and the temperature-dependent absorption-cross-

section spectrum for that gasoline blend was reported in Fig. 10 of [15]. The analysis of

the composition of this selected blend of premium gasoline was reported in the context of

twenty other gasoline samples. The reader is referred to [15] for further discussion of the

variation of absorption cross section with gasoline composition.

2900 2920 2940 2960 2980 30000

1x105

2x105

3x105

4x105

ν2ν1

2970.5 cm-1

Cross Section (cm2/mol)

Frequency (cm-1)

230 K

774 K

2951.0 cm-1

Figure 2.1: Absorption cross section for iso-octane in the region of the C-H stretching vibrational

transitions for temperatures of 230K and 775K; wavelengths selected for temperature and

concentration noted.

13

2.2.2 Two-Color mid-IR absorption sensor

Mid-IR laser light was generated using difference-frequency-generation (DFG) of a near-

IR pump with a near-IR signal laser in a PPLN (periodically-poled lithium niobate)

crystal. Mid-IR generation by DFG has been used previously for absorption sensing

(reviewed in [22]), and the DFG laser architecture used here is similar to that used by

Richter et al. [23]. For simultaneous temperature and hydrocarbon concentration, two

mid-infrared laser frequencies, ν1 and ν2, are required. For this sensor, two near-IR pump

lasers (ν1,pump and ν2,pump) were combined with a single near-IR signal laser (νsignal); the

overall architecture is illustrated in Fig. 2.2. Two distributed-feedback (DFB) single-

mode diode lasers (Toptica) were used as pump lasers (approximately 10 mW output

power); the signal laser was a single-mode DFB C-band telecommunications laser (NEL)

that was amplified in erbium-doped fiber to approximately 500 mW. Outputs from the

two pump lasers and the signal laser are combined with a polarization-maintaining fiber

coupler and directed through a temperature-controlled ridge-waveguide PPLN crystal

(NEL). The output from the PPLN included two mid-infrared laser frequencies (ν1 and

ν2), which were fiber coupled and used for absorption measurements in heated cells and

shock-heated gases to test and validate sensor performance. The MIR output power was

typically 50 µW.

Intensity modulation of the mid-IR and detection at the modulation frequency

provided insensitivity to the thermal background seen by the detector as well as any

variation of this background with crank angle. Sinusoidal modulation of the near-IR

pump-laser intensity produced sinusoidal modulation of the intensity of the mid-IR

difference-frequency output, and this intensity modulation was isolated with lock-in

14

detection. The two pump lasers were modulated at different frequencies (f1 and f2). The

frequency difference allowed the infrared transmission to be collected by a single

detector (thermoelectrically cooled InAs) and demultiplexed by the lock-in into two

signals proportional to the transmitted intensity at ν1 and ν2. Absorption of the mid-IR at

ν1 and ν2 was observed by reduction in the lock-in signals at f1 and f2 with and without

fuel present.

ν1,pump

ν2,pump

νsignal EDFA

f1

f2 PPLN

T control

Optical fiber 2-color DFG

output

ν1=ν1,pump-νsignalν2=ν2,pump-νsignal

ν1,pump

ν2,pump

νsignal EDFA

f1

f2 PPLN

T control

Optical fiber 2-color DFG

output


2-color DFG

output


Figure 2.2: Sensor architecture showing the DFG scheme for simultaneous production of two

mid-IR laser frequencies centered at ν1 and ν2 with intensity modulation at f1 and f2; EDFA

denotes the erbium-doped fiber amplifier used to increase the intensity of the telecom laser.

2.2.3 WMS-1f model for species with broad spectral features

Modulation of the injection current to the pump laser diode produced the desired intensity

variation of the mid-IR output. However, this injection current modulation also produced

synchronous wavelength modulation, and quantitative absorption measurements required

the wavelength-modulation contribution to the mid-IR intensity signals at f1 and f2 to be

characterized. Therefore a methodology for wavelength-modulation spectroscopy by

absorbing hydrocarbons and hydrocarbon blends was developed and is discussed here in

the context of correcting the modulated-intensity absorption measurement. The

measurement technique and this wavelength-modulation correction were validated in a

15

heated static cell and shock-heated flows for a pure hydrocarbon species (iso-octane) and

a fuel blend (gasoline).

The mid-IR laser frequencies ν1(t) and ν2(t) are a function of time with a

sinusoidal modulation at the modulation depths a1 and a2. The time-varying laser

frequency ν(t) is distinguished from the center frequency of the modulating laser ν by

the overstrike bar; similarly the average intensity is denoted I (to distinguish this value

from the time-varying intensity I(t). Using this notation, the sinusoidal modulation at

frequency fj of the mid-IR DFG output laser frequency can be written as:

)2cos()( tfat jjjj πνν += (2.4)

where jν denotes the center laser frequency and the subscript j denotes the specific laser

frequency 1ν or 2ν and aj is the modulation depth of the jth-laser frequency. Each laser’s

intensity varies with the sinusoidal injection current modulation:

[ ])2cos(1)( 000 φπ ++= tfiItI jjjj (2.5)

where jI0 is the average laser intensity, ji0 is the intensity amplitude at 1fj normalized

by jI0 , and φ is the wavelength-modulation/intensity-modulation phase shift. The non-

linear component (2f) of the laser intensity was assumed negligible in this analysis. In

general, the phase shift φ varies with modulation frequency and depends on the

architecture (and heat-transfer) of the specific laser device; for the work here the

approximation of πφ = introduced negligible errors, as determined by the agreement

between measured and known concentration and gas temperature. The assumption of

πφ = is the standard for most diode laser wavelength modulation spectroscopy literature

(e.g., see Philippe et al. [24]).

16

Although small hydrocarbon species such as methane have discrete absorption

spectra in the region of the C-H stretch near 3.4 µm with rotationally resolved transitions,

larger hydrocarbon species do not have discrete structure in their absorption spectra at

atmospheric pressure, but instead have broad, blended features as exemplified in the

spectrum of iso-octane in Fig. 2.1. The modulation amplitude was relatively small

compared to the width of these absorption features, which allows for a simplified solution

for the wavelength-modulation correction of the intensity-modulation 1f signal even

when there is significant absorbance.

The maximum modulation depth a of the injection-current-tuned DFB pump lasers

was only a few wavenumbers (< 3 cm-1 for the Toptica pump lasers used here). In this

small range the cross section varied linearly with laser frequency as long as 1ν and 2ν

were not selected within ±a of an inflection point of the cross section. Examination of

the absorption cross sections of gasoline blends, typical gasoline constituents, and most

polyatomic hydrocarbon fuel species shows this approximation could be used for a wide

range of wavelengths. For example, Fig. 2.3 shows the absorption cross section of iso-

octane in the region of the two selected laser wavelengths 11 /1 νλ = and

22 /1 νλ = . For

the region jj a±ν the cross section is well approximated by a linear function

( )ν

σννσ

d

d j

jjj −+ . The time-varying laser transmission can now be rewritten using the

sinusoidal frequency modulation in Eqn. (2.4) and Beer’s law at jν :

( )[ ])2cos(,exp)(

)()(

0

tfaTLntI

tIt jji

j

t

jπνστ

νν +−=

=

17

( )

+−= )2cos(

)(exp tf

d

TdaTLn j

j

jji πν

σσ (2.6)

The total transmitted intensity of the two mid-IR laser frequencies ( 1ν and 2ν ) becomes:

( )[ ]

+−++== ∑∑

==)2cos(

)()(exp)2cos(1)()( 00

2

1

2

1

tfd

TdaTLntfiItItI j

jjjijjj

jjj π

ν

σσππ (2.7)

Although Eqn. (2.7) was solved without an optically thin assumption, for the work

presented here two simplifying assumptions were made because the modulation depth

was relatively small. First, for the values of aj < 3 cm-1 used, the oscillating part of the

absorbance was small, 1)2cos()(

<<tfd

TdLan j

jji π

ν

σ. The 1f signal for each laser was

isolated using a two-channel lock-in tuned to f1 and f2, and the magnitude of the signal is

given in Eqn. (2.8), which was derived by expanding Eqn. (2.7) and collecting the 1f

terms:

[ ]

+⋅−=0

001

)(

1)(exp2 i

d

TdLan

TLniIG

R

jji

jijjj

jfν

σ

σ (2.8)

where Gj is the optical-electrical gain of the detection system for laser wavelength νj. In

the absence of absorption the magnitude of the 1f signal is 2

000

1

jjj

f

iIGR

j= . The small

modulation depth also led to the second assumption that

18

( )

≈+

dv

Td

i

Lan

i

d

TdLan

j

j

jj

j

jji )(

exp100

σν

σ

, leading to an effective cross section defined

as: ν

σσσ

d

Td

i

aTT

j

j

j

jeffectivej

)()()(

0

, −= (2.9)

which allowed the ratio of the magnitude of 1f signals with absorption at νj to those

without absorption to be given by:

[ ])(exp ,01

1TLn

R

R

effectiveji

jf

jfσ−= (2.10)

This simplified model for the 1f signal can be used to correct the temperature-dependent

absorption cross section using three parameters to describe the influence of modulation:

the modulation depth aj [cm-1], the slope of the absorption cross section with laser

frequency,ν

σ

d

Td j )(, and the intensity-modulation amplitude of the laser, i0j, normalized

by the average laser intensity at the laser center frequency, jI0 . Eqn. (2.7) was solved

numerically and the results were compared with results using the simplified WMS-1f

absorption model given in Eqn. (2.10), showing differences less than 1%. It is

straightforward to test the numerical solutions using Eqn. (2.7) with the analysis of the

absorption determined from Eqn. (2.10) for other applications with different absorption

spectra or different modulation parameters of the laser hardware. When the wavelength-

modulation correction can be expressed as a linear correction to the absorption cross

section, a table of σeffective(T) can be constructed using Eqn. (2.9) and subsequent data

reduction becomes extremely rapid.

19

2940 2945 2950 2955 29600

10

20

30

40

a1

Iso-octane

ν1 = 2951 cm

-1Cross Section (m2/m

ol)

Freqeuncy (cm-1)

σ2

2960 2965 2970 2975 29800

10

20

30

40

a2

ν2 = 2970.5 cm

-1

Cross Section (m2/m

ol)

Freqeuncy (cm-1)

σ2

Iso-octanea b

2940 2945 2950 2955 29600

10

20

30

40

a1

Iso-octane

ν1 = 2951 cm

-1Cross Section (m2/m

ol)

Freqeuncy (cm-1)

σ2

2960 2965 2970 2975 29800

10

20

30

40

a2

ν2 = 2970.5 cm

-1

Cross Section (m2/m

ol)

Freqeuncy (cm-1)

σ2

Iso-octanea b

Figure 2.3: Iso-octane absorption cross section in the region near 1ν = 2951 cm-1 (panel a) and

2ν = 2970.5 cm-1 (panel b); note that the variation of the cross section with laser frequency is

nearly linear over the range ±a.

2.3 Experiments to test sensor performance

The two-color fuel and gas temperature absorption sensor was tested with known

mixtures of iso-octane or gasoline in a nitrogen diluent in a heated static cell for 300 ≤ T

≤ 600 K and behind reflected shock waves for 600 ≤ T ≤ 1100 K. Fig. 2.4 illustrates the

experimental setup. The output from the fiber-coupled laser was collimated and directed

through known gas mixtures either in the heated cell or through the reflected-shock

region of a shock tube (2 cm from the end wall). Experiments in the heated cell were

limited to temperatures less than 600 K to avoid thermal decomposition of the iso-octane

during the residence time. For higher temperatures (up to 1100 K), the shock tube

provided the rapid heating and the short test times needed to avoid thermal

decomposition during the 2 ms test times. The light transmitted though the heated gas

Frequency (cm-1

) Frequency (cm-1

)

20

mixture was focused onto a thermoelectrically cooled InAs detector. The detector signal

was split into lock-in amplifiers to isolate the 1f signals at f1 and f2. The baseline

intensity signals, R01f1 and R

01f2, were measured in the evacuated cell (or shock tube), and

the signals, R1f1 and R1f2, were measured after adding the gas mixture into the heated cell

(shock tube).

2-color,

Intensity

modulated

mid-IR

DFG

f1

f2

gas

mixtures

in heated

cell or

shock tube

Detector

f1lock-in

f2lock-in

time-resolved laser intensity

@f1 @f2

2-color,

Intensity

modulated

mid-IR

DFG

f1

f2

gas

mixtures

in heated

cell or

shock tube

Detector

f1lock-in

f2lock-in

time-resolved laser intensity

@f1 @f2

Figure 2.4: Experimental arrangement for absorption measurements of iso-octane and gasoline

versus temperature with a two-color, intensity-modulated sensor in either a heated cell or across a

shock-heated flow; the transmitted light is focused onto a single detector where two lock-ins

demultiplex the signals at the modulation frequencies f1 and f2.

2.3.1 Details of gas handling and experimental apparatus

The gas-handling manifold and heated cell have been described previously [15, 21], and

will only be briefly described here. Iso-octane has sufficient vapor pressure (10 Torr at

19.2 °C and 40 Torr at 45 °C), that vapor from a liquid sample was either used neat or

manometrically mixed with nitrogen. The mixing manifold and gas handling lines were

heated (~100 °C) to avoid wall losses. For gasoline measurements, liquid gasoline was

injected via a septum into an evacuated mixing tank and then diluted with nitrogen. The

mole fractions of the gas mixtures were verified with FTIR measurements. The heated

21

cell with an optical path of 20.6 cm was constructed of stainless steel with sapphire

windows, and was uniformly heated in a temperature-controlled furnace. Type K

thermocouples are mounted on the optical cell to monitor temperature.

Mixtures of iso-octane or gasoline diluted in nitrogen were heated by a gas-driven

incident shock and subsequent reflected shock, providing nearly stationary, isothermal

gas mixtures for test times of 1-2ms. The shock tube facility is described in detail in 3.3

(experimental setup in chapter 3).

2.3.2 Measurements of iso-octane absorption for WMS-1f model

validation

A combination of heated-cell and shock tube measurements of iso-octane were made to

validate the expression in Eqn. (2.9) for an effective absorption cross section, which

accounts for the wavelength modulation produced by injection-current modulation of the

laser intensity. The sensor wavelengths were set to 1ν = 2951 cm-1 and

2ν = 2970.5 cm-1

by adjusting the signal laser to 6514.66 cm-1 and pump lasers to 9493.97 and 9464.32

cm-1. The output colors of the two-color, intensity-modulated, DFG laser were intensity

modulated with f1 = 42.5 kHz and f2 = 75 kHz. When the intensity modulations of the

two pump lasers were adjusted to optimize the 1f lock-in signals at f1 and f2, modulation

depths a1 = 2.7 cm-1 and a2 = 2.95 cm

-1 were measured. The absorption cross section,

)(Tσ , was determined from earlier FTIR measurements [21], and plotted as a function of

temperature as a dashed line in Fig. 2.5 for the two laser center frequencies, 1ν and

2ν .

22

The slope of the absorption cross section ν

σ

d

Td j )( determined from these earlier

temperature-dependent FTIR measurements [21] was used to calculate the effective

absorption cross section σeffective(T) and plotted as a solid line in Fig. 2.5 as a function of

temperature for the two laser center frequencies, 1ν and 2ν . The significant difference

between the σeffective(T) and )(Tσ illustrates the influence of the wavelength modulation of

±a about the laser center frequency. Note that σeffective(T) contains the sensor-specific

modulation parameters. Measured absorption data of iso-octane were used to test this

model.

Static cell measurements were made at 296 K, 426 K, and 576 K with an iso-octane

partial pressure ~0.6 Torr, which resulted in absorbance ranging from ~0.25 to ~0.7. In

the shock tube, a mole fraction of 0.00726 iso-octane in nitrogen was used for five

reflected shock temperatures between 618 and 1086 K. The effective cross section was

calculated from the measurements by:

0

1

1

1ln

f

effective

i f

R

n L Rσ

=

(2.11)

These data are shown as points in Fig. 2.5 and are in excellent agreement with the

model results shown as the solid line; RMS deviations of the measurements from the

model are 0.8% for 1ν = 2951 cm

-1 and 1.3% for

2ν = 2970.5 cm-1. The shock tube

experiments were conducted for a range of post-reflected-shock pressures (1-3 atm),

although pressure dependence was not expected. The shock tube data were single-shot

with a 10 kHz bandwidth, while cell measurement data were averaged for 1 second with

the same lock-in bandwidth. For species with broad absorption features like the

23

hydrocarbon fuels in the C-H stretching region, the pressure dependence of the

absorption cross section can be neglected over a wide range of pressures [15, 21].

Temperature was inferred from the ratio of the absorption using σeffective(T), and

these measured values are plotted versus the known temperature in Fig. 2.6. The RMS

error between the laser absorption measured temperature and the actual temperature was

4% over the range between room temperature up to 1086 K. The largest difference was

observed at the highest temperature of 1086 K, and we speculate that this difference

resulted from two factors: the possible decomposition of iso-octane during the shock tube

test time at this high temperature and the extrapolation of the temperature-dependent

FTIR cross section data measured at 800 K to a value at 1086 K needed to reduce these

measurements. Note the FTIR temperature range was limited by the window seals of the

heated cell.

Once temperature has been measured, Eqn. (2.10) was used to calculate the iso-

octane concentration and the differences between measured and prepared mixture values

are plotted in Fig. 2.7. Again the relatively large deviation for the highest temperature

point was attributed to possible fuel decomposition and the extrapolation of the 800 K

FTIR data to 1086 K; if this highest temperature value is ignored the RMS deviation

between measured and prepared iso-octane mole fraction was ~4.7%. The good

agreement of the sensor measurements of iso-octane for σeffective(T), gas temperature and

mole fraction serve to validate the sensor performance from room temperature to 1100 K

(although this upper temperature value required iso-octane cross section data to be

extrapolated from 800 K).

24

400 600 800 10000

20

40

60

Cross Section [m2/mol]

Temperature [K]

σ(T) for ν1=2951 cm

-1

σeff(T) for modulation about ν

1=2951 cm

-1

σeff heated cell measurements

σeff shock tube measurements

400 600 800 10000

20

40

60

80

100

Cross Section [ m

2/mol]

Temperature [K]

σ(T) for ν2=2970.5 cm

-1


2=2970.5 cm

-1



a b

400 600 800 10000

20

40

60

Cross Section [m2/mol]

Temperature [K]

σ(T) for ν1=2951 cm

-1


1=2951 cm

-1



400 600 800 10000

20

40

60

80

100

Cross Section [ m

2/mol]

Temperature [K]

σ(T) for ν2=2970.5 cm

-1


2=2970.5 cm

-1



a b

Figure 2.5: Measurements of σeffective(T) for iso-octane (panel a for 1ν = 2951 cm-1; panel b for

2ν = 2970.5 cm-1) in a heated cell (diamonds) and shock-heated gases (squares); the absorption

cross section )(Tσ at the center frequency of the modulated laser was taken from FTIR

measurements and the effective cross section including wavelength modulation ±a was calculated

from Eqn. 2.9.

300 600 900 1200

300

600

900

1200

Measured Temperature (K)

Known Temperature (K)

Known Temperature

Heated cell

Shock tube

Iso-octane

Figure 2.6: Two-color absorption measurements of iso-octane temperature. (Note shock tube

data are a single-point result acquired at 10 kHz)

25

200 400 600 800 1000 12000.00

0.25

0.50

0.75

1.00

1.25

heated cell

shock tube

Xmeasured / X

prepared

Temperature (K)

Iso-octane

Figure 2.7: Two-color absorption measurements of iso-octane mole fraction as a function of

temperature. (Note shock tube data are single point at 10 kHz)

Several uncertainties contribute to the determination of iso-octane temperature and

mole fraction with uncertainties in single-species cross-section data [15] having the

largest effect. Error analyses yielded an uncertainty estimate of 3% for the iso-octane

temperature and 6% for the mole fraction measurement. This assessment does not include

possible uncertainties associated with fuel decomposition and the extrapolation of the 800

K FTIR data to 1086 K.

2.3.3 Measurements of gasoline vapor mole fraction and

temperature

Mid-IR absorption of gasoline was previously studied by us as a function of gasoline

blend composition [15]; that work published temperature-dependent absorption cross

26

section data for a premium gasoline without ethanol additives (labeled as sample 7).

Measurements of this gasoline sample are made here using the two-color, mid-IR DFG

laser. Four measurements were conducted in the heated cell for temperatures between

300 and 600K. The gasoline partial pressure was varied from 3 to 9 Torr for relatively

constant absorbance values near 1. A mixture of gasoline in nitrogen with a mole

fraction of 0.00979 was prepared for shock tube experiments with reflected-shock

temperatures between 533 K and 1052 K. The temperature determined by the sensor is

compared with the known temperature in Fig. 2.8, and the RMS deviation between the

measured temperature and actual temperature is 5% over the range from room

temperature up to 1052 K. Again, the FTIR cross section data was extrapolated above

800 K for the values of )(Tσ needed for σeffective(T). The relatively large deviation for the

highest temperature point was attributed to possible fuel decomposition and the

extrapolation of the 800 K FTIR data to 1052 K. The RMS deviation is reduced to 1.4 %

if the value at 1052 K is ignored. The error bars in Fig. 2.8 and 2.9 included estimates of

the uncertainties in the gasoline absorption cross-section model developed by Klingbeil et

al. [15]. Error analyses yielded an uncertainty estimate of 5% for the gasoline

temperature and 10% for mole fraction measurement. This does not include uncertainties

associated with possible fuel decomposition and the extrapolation of the 800 K FTIR data

to 1052 K.

The ratio of the gasoline mole fraction measured by the absorption sensor to that

prepared is plotted versus temperature in Fig. 2.9. The largest deviation is again at the

highest temperature where the fuel starts to decompose and the absorption cross section

has been extrapolated. Even including this data point, the RMS deviation is only 8%

27

over the entire temperature range, which reduces to 4% if the data point at 1052 K is

excluded.

300 600 900 1200

300

600

900

1200

Measured Temperature (K)

Known Temperature (K)

Known Temperature

Heated cell

Shock tube

Gasoline

Figure 2.8: Two-color absorption measurements of gasoline temperature

200 400 600 800 1000 12000.00

0.25

0.50

0.75

1.00

1.25

Xmeasured / X

prepared

Temperature (K)

heated cell

shock tube

Gasoline

Figure 2.9: Two-color absorption measurements of gasoline mole fraction as a function of

temperature

28

Chapter 3

Interference-Free Mid-IR Laser

Absorption Detection of Methane

3.1 Introduction

Methane detection schemes using laser absorption have been developed previously for

both the mid- and near-infrared regions of the methane absorption spectrum. Past studies

of diode-laser-based detection of methane include the use of a liquid-He-cooled, lead-salt

diode laser at 7.4 µm [27], semiconductor diode lasers at 1.65 µm [28-35], a recently

developed mid-IR difference-frequency-generation (DFG) system at 3.2~3.4 µm [36-44],

and a quantum cascade laser at 8.1 µm [45-46]. However, these detection strategies were

not applied for the detection of methane in the presence of strong absorption interference

29

from other hydrocarbon species. During the decomposition of n-heptane, for example,

other hydrocarbon species are produced, including C2H4, C2H6, C3H6, and C4H8, all with

relatively strong absorption features near 3.4 µm (all share transitions of the fundamental

C-H vibration that occurs at this wavelength). However, of these species, only methane

has a structurally resolved absorption spectrum; the other potentially-interfering species

have primarily broad, unresolved absorption spectra in this wavelength region.

Here we developed a novel, mid-IR scanned-wavelength laser absorption

diagnostic for time-resolved, interference-free, absorption measurement of methane

concentration. The scanned-wavelength diagnostic uses a differential absorption (peak

minus valley) scheme that takes advantage of the spectral structure difference between

methane and other hydrocarbon products. This differential absorption scheme is easier to

implement than the WMS technique [33-35, 44, 46]. In the WMS technique, one needs

to know the exact line shape function including the pressure broadening coefficient as

well as the line strength to simulate the absorption. But in the peak minus valley scheme,

one can directly get differential absorbance by capturing peak and valley feature in one

scan, without required knowledge of the line shape function. Tunable mid-IR light was

generated using a difference-frequency-generation (DFG) laser and scanned over a peak

and valley (2938.24 – 2938.01 cm-1) structure of methane absorption spectrum with 50

kHz scanning frequency. This diagnostic is particularly suited to methane concentration

measurements in the presence of other n-alkane decomposition species.

30

3.2 Development of interference-free methane detection

scheme

3.2.1 FTIR measurements of absorption spectra of hydrocarbon

species

Implementation of the scanned-wavelength methane diagnostic requires an accurate

understanding of the mid-IR absorption features of methane and other potential

interfering species. To determine which potentially-interfering species could be

important, a simulation of the time-histories of the decomposition products in n-heptane

pyrolysis was performed using a recently developed n-alkane reaction mechanism,

Sirjean et al./JetSurf 1.0 (2009) [47]. The results are shown in Fig. 3.1. During n-heptane

pyrolysis, methane (CH4) and ethane (C2H6) are the two major alkane products, ethylene

(C2H4), propene (C3H6) and butene (C4H8) are the three major alkene products, and

acetylene (C2H2) is the major alkyne product. Absorption spectra of these key alkanes

and alkenes, i.e., methane, ethane, ethylene, propene and butene, acquired in our FTIR at

774K, are illustrated in Fig. 3.2 (acetylene does not absorb in this wavelength region). A

Nicolet 6700 FTIR spectrometer was used to measure spectra from 2.9 to 4 µm with a

resolution of 0.06 cm-1. At high temperature, methane has a structured and resolved

absorption spectrum different in character from the other hydrocarbon species. Because

of this, we are able to assume that the absorption cross-sections of these other

decomposition products remain constant over the very narrow frequency interval (0.2~0.3

cm-1) that will be used to detect methane. This assumption is confirmed in the detailed

view of Fig. 3.2 shown in Fig. 3.3.

31

Detailed spectra of two promising choices, A and B, for wavelength pairs to detect

methane are shown in Fig. 3.4 and 3.5 along with the HITRAN modeling [48] of the

methane cross-section at 405 K. The methane absorption spectrum has a well-defined

peak and valley level of cross-section (for both wavelength features A and B) over the

0.23 cm-1 frequency spacing whereas the absorption level of other hydrocarbon species

stays constant due to their broad absorption spectral features.

\

0 1 2 3 4 51E-3

0.01

0.1

C2H6

C3H6

C2H2

CH4

H2

Mole Fraction

Time [ms]

C2H4

Figure 3.1: Simulation of major species time-histories during n-heptane pyrolysis behind a

reflected shock wave using the Sirjean et al./JetSurF 1.0 (2009) mechanism. Initial conditions:

1% C7H16 in argon, 1636K, 1.327 atm.

32

2900 2920 2940 29600

10

20

30

C2H4

C3H6

C4H8

C2H6C

ross section [m

2/mol]

Frequency [cm-1]

B

A

CH4

Figure 3.2: FTIR measurement of the absorption cross-section of methane and larger

hydrocarbon species. Heated FTIR cell conditions: 0.4-6% species in argon, 774K, 1 atm.

2936 2937 2938 29390

10

20

30

C2H

4

C3H

6

C4H

8

C2H

6

Valley of wavelength feature A

Cross section [m

2/mol]

Frequency [cm-1]

Peak of wavelength feature A

CH4

Figure 3.3: Detailed view of absorption cross-section measurement near wavelength feature A.

Heated FTIR cell conditions: 0.4~6% species in argon, 774K, 1 atm.

33

2936 2937 2938 29390

20

40

60

80

Valley of wavelength feature A

Frequency [cm-1]

Cross section [m

2/m

ol]

Peak of wavelength feature A

Figure 3.4: HITRAN modeling of methane cross-section near the wavelength of feature A.

Simulation conditions: 405K, 1 atm.

2916 2917 29180

20

40

60

80

Valley of wavelength feature B

Frequency [cm-1]

Cross section [m

2/mol]

Peak of wavelength feature B

Figure 3.5: HITRAN modeling of methane cross-section near the wavelength of feature B.

Simulation conditions: 405K, 1 atm.

34

3.2.2 Absorption theory and the differential absorption scheme

We present here a short summary of the theoretical details of the differential absorption

(peak minus valley) scheme. In the absence of light losses from absorption of liquid and

scattering by particles, the transmission of monochromatic light at wavelength λ through

a uniform gaseous species is described by Beer’s law as shown in equation (2.1).

Other species can produce interference absorption, or extinction can also be caused

by liquid droplets, soot or other particulates. Additional terms are added to equation (2.1)

to describe the transmitted light intensity with interfering hydrocarbon absorption and

extinction (all other interference from absorption or scattering)

extinctionnhydrocarboCHI

Iταα ++=

−

4

0

ln , (3.1)

where 4CHα is the vapor-phase absorption by methane, nhydrocarboα is the sum of the

vapor-phase absorption by other hydrocarbon species, and extinctionτ represents total

interference extinction (absorption and scattering) from all other sources.

We selected peak and valley wavelengths where the differential cross-section of

methane is maximum, and where the differential cross-section of other hydrocarbon

species is negligible; ),(4 peakCH να >> ),(4 valleyCH να , ),(),( valleynhydrocarbopeaknhydrocarbo νν αα ≈ .

We have assumed that the total extinction from all other sources, extinctionτ is independent

of wavelength, or that the wavelength dependence of extinctionτ over the 0.23 cm-1 peak

and valley spacing is negligible. The sum of the differential cross-sections of other

hydrocarbon species is also negligible. The two extinction measurements can thus be

subtracted as in equation (3.2).

35

( ) ( )),(),(

,,

,0,0

44

44

lnln

peakCHpeakCH

valleyextinctionnhydrocarboCHpeakextinctionnhydrocarboCH

valleypeakI

I

I

I

νν

νν

νν

αα

τααταα

−≈

++−++=

−−

−

(3.2)

The methane mole fraction can thus be inferred from the total differential absorption

without any interference from other vapor phase hydrocarbons and any extinction from

other sources using equation (3.3),

RT

LP

I

I

I

I

totvalleyCHpeakCH

valley

valley

peak

peak

CH ⋅⋅−

−

=)(

ln

),(),(

),(

),(0

),(0

),(

4

44 νν

ν

ν

ν

ν

σσχ (3.3)

if the methane differential cross-section, σpeak minus valley = σCH4(ν,peak) - σCH4(ν,valley) is known.

3.2.3 Wavelength selection criteria

Wavelength selection rules for two-wavelength temperature and concentration

measurements of hydrocarbons have been reported [49]. However, these criteria were

generally developed for species with broad, unresolved absorption spectra (e.g., n-

heptane). In this work, we reconstruct these criteria for species with a resolved,

structured absorption spectrum that have absorption interference from other species with

broad, unstructured absorption spectra.

Five selection rules are used.

Selection rule 1: wavelengths that are accessible by the laser. The DFG laser used

in this work can be tuned over a wide (100 nm) range (3330 to 3430 nm) by using

different signal lasers. Each signal laser is combined with the pump laser to

generate a central target wavelength with a 24-25 nm tuning range.

36

Selection rule 2: maximum high-temperature absorption cross-section for methane.

The absorption cross-section at the peak wavelength should provide the strongest

absorption possible to increase diagnostic sensitivity.

Selection rule 3: maximum differential absorption of methane. The two

wavelengths (peak and valley wavelengths) should be chosen to maximize the

differential absorption cross-section of methane (σpeak minus valley).

Selection rule 4: minimum peak minus valley wavelength spacing. The spacing

between peak and valley wavelengths should be minimized to ensure that the

differential cross-sections of other hydrocarbon species at the peak minus valley

wavelength are negligible.

Selection rule 5: minimum interference from H2O and CO2. The wavelength pairs

(peak and valley wavelengths) for the feature were selected to avoid any water,

carbon dioxide absorption interference, species that will occur in air and as

combustion products.

Table 3.1 summarizes the properties of the two line features, A and B, which were

considered in this study. Using these rules, wavelength feature A was selected as the

optimal choice, primarily because of the higher peak methane absorption at lower

temperatures (i.e., 774 K). For methane detection studies at higher temperatures (i.e., to

2000 K) wavelength feature B would be a slightly better choice, because of the higher

peak methane absorption in the temperature range.

37

Table 3.1: Two wavelength pair candidates for differential absorption (peak minus valley)

scheme for methane detection.

3.3 Experimental setup

Shock tubes and laser absorption provide an attractive method to study high-temperature

gas-phase chemistry in a controlled and uniform manner. We first describe the DFG

laser operation, and then the shock tube operation.

Mid-IR laser light near 3.4 µm was generated using difference-frequency-

generation (DFG) of a near-IR signal and a near-IR pump laser combined in a PPLN

crystal. This mid-IR generation method was first used by Chen et al. (2007) [50]; we

employed a similar system first described by Richter et al [51]. The signal laser, a

wavelength-tunable, fiber-coupled DFB laser, is amplified in a Yb/Er fiber amplifier.

The pump laser has a fixed wavelength (1064 nm). Outputs from the pump laser and the

signal laser are combined with a polarization-maintaining fiber combiner and connected

through a temperature-controlled periodically-poled lithium niobate (PPLN) crystal.

PPLN is phase-matched for maximum power output of mid-IR light (approximately ~170

µW). The overall architecture is illustrated in Fig. 3.6.

Wavelength pairs of A Wavelength pairs of B

38

Figure 3.6: Experimental schematic of laser system used to generate tunable mid-IR radiation

near 3.4 µm.

For the methane concentration measurement, one near-IR signal laser was current-

modulated at a 50 kHz scanning frequency with a 0.5 Vp-p scanning amplitude using a

function generator (Stanford Research Systems Model DS 345). A sawtooth signal from

the function generator was connected to ILX Lightwave Model LDX-3620 laser diode

current controller to modulate the near-IR signal laser wavelength. An ILX lightwave

Model LDT-5910B temperature controller was used to control the signal laser

temperature. The radiation from this modulated near-IR signal laser was combined with

that from a near-IR pump laser to generate modulated mid-IR light which covered the

peak and valley wavelengths (2938.01 and 2938.24 cm-1) in each scan. The differential

absorption (peak minus valley) scheme with a 50 kHz scanning frequency enables

methane vapor concentration measurement with 20 µs time resolution in the presence of

39

strong absorption interference from other hydrocarbon species and extinction from other

sources.

The shock tube used consists of a 13.7 m long, stainless steel tube with an inner

diameter of 15.24 cm, separated into two sections (driver and driven sections) by a

polycarbonate diaphragm and closed at both ends. To perform an experiment, the

following procedure is followed. The driven section is filled with the desired test gas

mixture at a low pressure (P1=0.02-0.26 atm). The driver section is filled to a high

pressure with helium, bursting the separating diaphragm. A shock wave is formed and

propagates into the driven section. This incident shock wave compresses and heats the

test gas mixture behind it, increasing the pressure and temperature to P2 and T2. The

incident shock wave reaches the end wall and is reflected back through the driven section.

The test gas mixture is further compressed and stagnated by this reflected shock wave to

a higher final pressure and temperature P5, T5. For the work in this chapter, 2% methane

in argon was used.

The reflected shock conditions were calculated based on incident shock speeds

using a chemically frozen shock code (FROSH) and species thermodynamic data from

Kee et al [52]. Incident shock speeds were determined using shock arrival times

measured using five piezo-electric pressure transducers (PZT) located over the last part of

the driven section and four interval counters (Fluke PM6666).

The optical arrangement used for the high-temperature cross-section measurement

in this chapter and methane concentration measurements in chapter 4 and 5 is illustrated

in Fig. 3.7. The light from the DFG laser is directed through a ZnSe wedge and is split

into two beams: a reference beam and a transmitted beam. The transmitted beam was

40

pitched through two opposing calcium fluoride (CaF2) windows located 2cm from the

shock tube end wall. Two cryogenically cooled indium antimonide (InSb) detectors (IS-

2.0, Infrared Associated, Inc.) with 2 mm x 2 mm active area and 1.2 MHz bandwidth

were used for common mode rejection (CMR) to reduce laser intensity noise. Common

mode rejection (CMR) was used to reduce the influence of laser intensity fluctuations,

and using this method a 0.3% absorbance detection limit over 5 ms was achieved. Two

irises and one optical bandpass filter, Spectragon 3390-040nm (60nm FWHM, 3390 nm

center wavelength) were used to minimize thermal emission from the shock-heated gas

mixture.

Figure 3.7: Experimental arrangement for laser absorption measurements in a shock tube.

41

3.4 Results and discussion

3.4.1 Cross-section measurements

3.4.1.1 Methane

High-temperature measurements of the absorption cross-section of methane and

other interfering hydrocarbon species were acquired behind reflected shock waves in the

shock tube. Methane cross-sections at both peak and valley wavelengths was measured

over a temperature range of 1000K to 2000K and pressures of 1.3 to 5.4 atm.

Methane cross-sections of wavelength pair for A (2938.24, 2938.01 cm-1) are

shown in Fig. 3.8 and 3.9; measurements for wavelength pair for B (2917.64, 2917.45

cm-1) are shown in Fig. 3.10 and 3.11. For both line pairs, the differential cross-section

(σpeak minus valley) of methane shows strong exponential temperature dependence. A

generalized expression for the differential cross-section as a function of pressure and

temperature was derived based on measured differential cross-section for each line pair.

For line pair A the expression is:

76.0

0

32.3

00),(

=P

P

T

TPT σσ

atmP

KT

molm

809.1

1010

/39.11

0

0

2

0

=

=

=σ

For line pair B the expression is:

93.0

0

31.3

00),(

=P

P

T

TPT σσ

atmP

KT

molm

810.1

1149

/873.9

0

0

2

0

=

=

=σ

These expressions agree with the measured values with an RMS deviation of 4.1% for

line pair A and 3.8% for line pair B.

42

1200 1600 20000

3

6

9

12

15

σpeak σvalley σpeak-valley

Cross section [m

2/mol]

Temperature [K]

Figure 3.8: Measured cross-section of methane at peak and valley wavelengths for wavelength

pair A and its differential cross-section (σ peak minus valley) for pressure of 1.3 to 1.8 atm.

1200 1600 20000

3

6

9

12

15

(P = 1.3~1.8 atm)

(P = 2.0~3.0 atm)

(P = 3.7~5.4 atm)

Cross section [m

2/mol]

Temperature [K]

Figure 3.9: Measured pressure dependence of the differential cross-section (σpeak minus valley) for

methane for wavelength pair A.

43

1200 1600 20000

3

6

9

12

15

σpeak σvalley σpeak-valley

Cross section [m

2/mol]

Temperature [K]

Figure 3.10: Measured cross-section of methane at peak and valley wavelengths for wavelength

pair B and its differential cross-section (σpeak minus valley) for pressure of 1.3 to 1.8 atm.

1200 1600 20000

3

6

9

12

15

(P = 1.3~1.8 atm)

(P = 1.9~2.7 atm)

(P = 3.6~5.4 atm)

Cross section [m

2/mol]

Temperature [K]

Figure 3.11: Measured pressure dependence of the differential cross-section (σpeak minus valley) for

methane for wavelength pair B.

44

3.4.1.2 Interfering species

The cross-sections of potentially interfering hydrocarbon species (C2H4, C2H6, C3H6, and

C4H8), and carbon dioxide (CO2) and water (H2O) were measured behind reflected shock

waves at over pressures from 0.7 to 1.4 atm for both the peak and valley of wavelength

pair A. These measurements are shown in Fig. 3.12. Differential cross-sections for these

species over the wavelength pair A are very small. A quantitative comparison of

differential cross-sections (σpeak minus valley) of methane, other hydrocarbon species, carbon

dioxide and water at 1200K for the wavelength pair A is illustrated in Fig. 3.13.

Similarly, the differential cross-sections for interfering species over the wavelength pair

B are also very small as shown in Fig. 3.14 and 3.15. The differential cross-sections of

other interference species are negligible for both wavelength pairs, compared to that of

methane, confirming our assumption that the absorption spectra of interference species

are sufficiently constant over the 0.2 cm-1 wavelength interval of interest in the present

study. Carbon dioxide and water have almost no absorption at both peak and valley

wavelength.

500 1000 1500

0

3

6

9

12

CO2

H2O H

2O CO

2

C2H

6

C4H

8

C3H

6

σpeak σvalley

Cross section [m

2/mol]

Temperature [K]

C2H

4

45

Figure 3.12: Measured absorption cross-sections of interfering hydrocarbon species for the peak

and valley wavelengths for wavelength pair A and its differential cross-section (σpeak minus valley).

σpeak and σvalley are almost identical in all cases at both temperature.

1E-3

0.01

0.1

1

10

CO2

H2O

C4H8

C3H6

C2H6

C2H4

CH4

Figure 3.13: A comparison of measured differential cross-sections (σpeak minus valley) at wavelength

pair A for methane and other interfering species at 1200K.

1000 1200 1400 1600

0

3

6

9

12

C2H

6

C4H

8

C3H

6

σpeak σvalley

Cross section [m

2/mol]

Temperature [K]

C2H

4

CO2

H2O H

2O CO

2

Figure 3.14: Measured absorption cross-sections of interfering hydrocarbon species for the peak

and valley wavelengths for wavelength pair B and its differential cross-section (σpeak minus valley).

σpeak and σvalley are almost identical in all cases at both temperature.

46

1E-3

0.01

0.1

1

10

H2OCO

2C4H8

C3H6C

2H6

C2H4

CH4

Figure 3.15: A comparison of measured differential cross-sections (σpeak minus valley) at wavelength

pair B for methane and other interfering species at 1200K.

3.4.2 Demonstration of methane time-history measurements

The interference-free methane detection scheme can be used to provide methane

concentration time-history data during the decomposition of a large hydrocarbon species

(e.g., n-heptane). These data can be used to test and refine current reaction mechanisms

for these hydrocarbons as will be described in chapter 4 and 5. In this section, one

example case from chapter 4 is analyzed to demonstrate how this methane detection

scheme is used to acquire methane concentration time-history behind reflected shock

waves during hydrocarbon fuel decomposition. Fuel pyrolysis experiments are usually

conducted at temperatures of 1200-1600 K, and in this high temperature range the

wavelength pair B is more sensitive than the wavelength pair A, due to its slightly higher

47

differential absorption cross section, as described in Figure 3.8 and 3.10. Therefore, the

methane time-histories in chapter 4 and 5 were all measured by using the methane cross-

sections acquired at wavelength pair B.

The time–resolved total absorbance measurements for wavelength pair B during

n-heptane pyrolysis with 20 µs time resolution are plotted in Fig. 3.16. The conversion

process from the raw absorbance to methane concentration is described in detail in

section 4.3.1.

0.0 0.5 1.0 1.50.00

0.05

0.10

0.15

0.20

Peak

Valley

Peak-Valley

1% C4H

10 / Ar

1597 K, 1.44 atm

Absorbance

Time [ms]

Figure 3.16: Total absorbance time-history of n-heptane pyrolysis at the peak and valley

wavelengths for wavelength pair B and the differential absorbance. Initial reflected shock

conditions: 1597 K, 1.44 atm, 1% n-heptane in argon.

Some further insight into the difference between the model and measurement in these

experiments can be derived from the total valley absorbance. The difference between the

total valley absorbance and that attributed to methane absorbance, i.e. the “residual

valley” can be associated with the sum of the absorbance of the interfering species (i.e.

48

C2H4, C2H6, C3H6, C4H8 and others). However, the simulated concentrations of the

higher alkenes (e.g. pentene and hexene) and other interfering species are negligible

compared to those of smaller hydrocarbons (methane to butene) considered here.

Fig. 3.17 shows the predicted absorbance (for the experiment of Fig. 3.16) based on

the measured cross sections and simulated concentrations using the Wang et al.

[63]/JetSurf 2.0 mechanism. Ethylene likely provides the largest fraction of the plateau

absorbance. However, other minor species strongly contribute to the rapidly changing

early-time absorbance. The measurements of ethylene concentration time-histories

during n-heptane pyrolysis are described in chapter 4. The Wang et al. [63]/JetSurf 2.0

mechanism accurately predicts ethylene plateau yields under the same temperature and

pressure condition. Hence, the differences between the measured and simulated total

valley wavelength absorbances most likely can be attributed to imprecise predictions of

the time-histories of other intermediate products, in particular the higher alkenes (propene

and butene) or smaller alkanes (e.g. ethane). This demonstration confirms the value of

both the peak-minus-valley methane measurements and the residual valley absorption

measurements in providing uniquely useful information for validating and refining n-

alkane or other bio-fuel reaction mechanisms.

49

0.0 0.5 1.0 1.50.00

0.05

0.10

0.15

C3H

6

n-C7H

16 C4H

8 C2H

6C

2H

4

Residual valley data

Absorbance

Time [ms]

Sum of simulated residual valley

Figure 3.17: Simulated and experimental residual valley wavelength absorbance (i.e., the total

valley less contributions from methane) time-histories during n-heptane pyrolysis. Initial

conditions: 1597K, 1.44 atm, 1% n-heptane in argon.

50

Chapter 4

Decomposition Species Measurements

during n-Alkane Pyrolysis

4.1 Introduction

Detailed n-butane and n-heptane oxidation mechanisms have been extensively

developed by many groups [57-61]. The n-butane mechanism of Marinov et al. [60] was

developed to model rich sooting premixed n-butane flames and was validated for burner

measurements of C3H4, C4H2, C4H4, C4H6, C4H8, C5H6, C6H5CH3, and higher aromatics.

The detailed n-heptane mechanism of Curran et al. [61] was developed (and updated

[62]) and validated by comparison with experiments in plug-flow and jet-stirred reactors,

shock tubes and rapid compression machines over an initial pressure range from 3 to 50

51

atm, and temperatures from 650 to 1200 K. These experiments included the

measurements of CO, O2, CH2O, CH3CHO, C2H5CHO, C7H14, and C7H14O. [61]. The

mechanism of Wang et al. [63], JefSurF 2.0, is a detailed mechanism developed for the

combustion of jet fuel surrogates. The model was validated against laminar flame speeds,

ignition delay times, species profiles behind reflected shock waves, and jet-stirred and

flow reactor data to describe the pyrolysis and oxidation kinetics of all n-alkanes up to n-

dodecane and mono-alkylated cyclohexanes at high temperatures.

Recently, Oehlschlaeger et al. [64] measured the high temperature rate constants

of the two n-butane decomposition reactions, (1) n-C4H10 � n-C3H7 + CH3 and (2) n-

C4H10 � C2H5 + C2H5, in a shock tube by monitoring CH3 radicals in n-butane oxidation.

The overall n-heptane decomposition rate combining three major decomposition

pathways, (1) n-C7H16 � p-C4H9 + n-C3H7, (2) n-C7H16 � C5H11-1 + C2H5 and (3) n-

C7H16 � C6H13-1 + CH3 was also measured by Davidson et al. [65] in a shock tube by

monitoring CH3 radicals in n-heptane oxidation. C2H4 time-histories were recently

measured behind reflected shock waves in the pyrolysis of n-heptane by Pilla et al. [66].

However, the temperature and C2H4 absorption coefficient were assumed to be constant

during pyrolysis in that study. An improved gasdynamic/kinetic model that accounts for

changes in temperature due to the endothermic nature of the pyrolysis process has now

been developed. A mid-IR scanned-wavelength laser absorption diagnostic for

interference-free CH4 detection was recently developed by Pyun et al. [67] as described

in chapter 3, however, information on CH4 concentrations during n-alkane pyrolysis has

not previously been available. In this study, to extend the kinetic database for n-butane

and n-heptane, we measured CH4 and C2H4 concentration time-histories behind reflected

52

shock waves during the high temperature pyrolysis of n-butane and n-heptane. The

changes in temperature and pressure during fuel pyrolysis were calculated using a

constant volume gasdynamic model and the recent chemical mechanism by Wang et al.

[63]. The calculated pressure and temperature profile and the temperature-dependent

absorption coefficients were used to infer CH4 and C2H4 time-histories.

4.2 Laser absorption diagnostics

4.2.1 Methane detection

As described in chapter 1 in detail, a mid-IR, scanned-wavelength laser absorption

diagnostic for interference-free CH4 detection was used to measure CH4 concentration

time-histories. The scanned-wavelength diagnostic uses a difference frequency

generation (DFG) laser and a differential absorption scheme that takes advantage of the

spectral structure difference between CH4 and other hydrocarbon products near 3.43 µm

where fundamental C-H stretching vibrational bands are located. This CH4 diagnostic

offers 20 µs time resolution with a detection limit of ~200 ppm for a path length of 15.24

cm at 1500 K and 1.5 atm.

4.2.2 Ethylene detection during n-alkane pyrolysis (one

wavelength)

C2H4 was measured using a fixed-wavelength absorption scheme at 10.532 µm with a

CO2 laser. The P(14) line of the CO2 laser at 10.532 µm line has been used previously to

53

measure C2H4 concentrations [66,70] because it is coincident with the strong Q-branch of

the ν7 ethylene band [70]. The C2H4 mole fraction in a given experiment is determined

using Beer’s law: )exp( 42

42

0

LRT

P

I

IHC

HCtotal σχ

−=

where I/I0 is the fractional transmission, L the path length, and σC2H4 is the C2H4

absorption cross-section at 10.532 µm. Since the values of σC2H4, L and total pressure,

Ptot, were measured or known, the mole fraction of C2H4 could be inferred from the

measurement of the fractional transmission, I/I0. The minimum detection limit for CO2

laser diagnostics using a common mode rejection scheme is approximately 100 ppm C2H4

at 1500 K and 1.5 atm.


4.3.1 Species concentration time-histories and gasdynamic models

As described in 4.2.2, species concentration time-history was inferred from the raw

absorbance signal using Beer’s law, which describes the species mole fraction as a

function of temperature, pressure and its absorption coefficient. The changes in

temperature and pressure caused by chemical reactions associated with fuel pyrolysis

slightly change the species absorption coefficients that are temperature dependent. To

determine the appropriate temperature profile to use in the conversion of absorbance to

mole fraction, two different gasdynamic models were used in the simulation: constant

volume and constant pressure. The time-varying temperature, pressure and species

absorption coefficient during fuel pyrolysis was discussed by Pyun et al. [69].

54

Fig. 4.1 and 4.2 show the calculated pressure and temperature changes during 1%

n-butane pyrolysis using two different chemical mechanisms by Wang et al. [63] and

Marinov et al. [60] with the constant volume and the constant pressure gasdynamic

models. Fig. 4.1 also includes the measured pressure in the shock tube experiments. The

Wang et al. [63] mechanism predicts 7.3% change between initial pressure and the

pressure at 1 ms, and the Marinov et al. [60] mechanism predicts 7.5% pressure change,

based on the constant volume gasdynamic model. Notably, the measured pressure trace

falls more slowly than the models. The constant volume gasdynamic model with the

mechanisms by Wang et al. [63] and Marinov et al. [60] predicts 9.2% and 9.3%

temperature drop in 1ms, respectively. Meanwhile, the constant pressure gasdynamic

model with the mechanisms by Wang et al. [63] and Marinov et al. [60] predict

temperature drops of 7.1% and 7.3%, respectively. As confirmed above, the calculated

pressure and temperature profiles are more strongly dependent on the gasdynamic models

than the chemical mechanisms for n-butane pyrolysis.

Fig. 4.3 and 4.4 show the calculated pressure and temperature changes during 1%

n-heptane pyrolysis using the different chemical mechanisms by Wang et al. [63] and

Curran et al. [62] with the constant volume and the constant pressure gasdynamic models.

Fig. 4.3 also includes the measured pressures in the shock tube experiments. The Wang

et al. [63] mechanism predicts 8.9% change between initial pressure and pressure at 1 ms

and the Curran et al. [62] mechanism predicts 9.8% pressure change based on the

constant volume gasdynamic model. Note that the rapid fall in pressure suggested by the

models is not captured by the measured pressure trace. The constant volume gasdynamic

model with the mechanisms by Wang et al. [63] and Curran et al. [62] predicts 12.0% and

55

12.9% temperature drop at 1ms, respectively. Similarly, the constant pressure

gasdynamic model with the mechanisms by Wang et al. [63] and Curran et al. [62]

predict temperature drops of 9.9% and 10.5%. The temperature and pressure drop of n-

heptane pyrolysis is slightly higher than that of n-butane due to the higher energy

absorbed during the endothermic decomposition reactions with the larger n-heptane

molecule.

It is desirable to ensure that the species concentrations inferred from measured

raw absorbance do not change significantly regardless of which chemical mechanisms

and gasdynamic models are used to simulate temperature and pressure during n-butane

and n-heptane pyrolysis. The highest temperature cases of n-butane (1565 K) and n-

heptane (1597 K) pyrolysis experiments that show the largest difference between

chemical mechanisms and gasdynamic models are analyzed in Table 4.1 and 4. 2.

For n-butane pyrolysis, Table 4.1 shows the measured mole fractions of CH4 and

C2H4 at 1ms inferred using the T, P values from the mechanisms of Wang et al. [63] and

Marinov et al. [60] with the constant volume and the constant pressure gasdynamic

models. The differences between measured mole fractions from the mechanisms by

Wang et al. [63] and Marinov et al. [60] with the identical gasdynamic model are

negligible: less than 0.5% for the constant volume and 0.6% for the constant pressure

gasdynamic models. However, the difference between measured mole fractions from the

constant volume and constant pressure gasdynamic models is up to 2.7%, which is

somewhat smaller than the detection limit of our laser diagnostics.

For n-heptane pyrolysis, Table 4.2 shows the measured mole fraction of CH4 and

C2H4 at 1ms inferred using the T, P values from the mechanisms by Wang et al. [63] and

56

Curran et al. [62] with the constant volume and the constant pressure gasdynamic models.

The differences between measured mole fractions derived from the mechanisms by Wang

et al. [63] and Curran et al. [62] are relatively higher, up to 5.4% in the highest

temperature case where the temperature and pressure change is maximized. However,

this maximum difference (~1000ppm C2H4) is comparable to our detection limit of the

laser diagnostics.

Even though the pressure and temperature changes during high concentration fuel

pyrolysis are significant, up to 12 % in the highest temperature cases, the convolution of

these two effects (temperature and pressure change) and the temperature-dependent

absorption coefficients made the inferred mole fractions almost indistinguishable for four

different combinations of the chemical mechanisms and the gasdynamic models. A

shock tube is neither a perfect constant volume nor a perfect constant pressure system,

but it has been widely assumed to be a constant volume system, and our long-time

pressure data seem to be closer to the constant volume prediction. Hence, the Wang et al.

[63] mechanism with the constant volume gasdynamic model was selected for use in

calculating temperature and pressure profiles and to infer mole fractions of CH4 and C2H4

during n-butane and n-heptane pyrolysis in this study.

57

0.0 0.5 1.00.0

0.5

1.0

1.5

2.0

Pressure (atm

)

Time [ms]

Wang et al. CV

Marinov et al. CV

CP

Measured pressure

1% C4H

10 / Ar

1565 K, 1.45 atm

Figure 4.1: Simulated pressure profiles with the mechanisms of Wang et al. [63] and Marinov et

al. [60] with the constant volume and the constant pressure gasdynamic models. Initial

conditions: 1% n-butane in argon, 1565 K, 1.45 atm.

0.0 0.5 1.0 1.51350

1400

1450

1500

1550

16001% C

4H

10 / Ar

1565 K, 1.45 atm

Temperature (K)

Time [ms]

Wang et al. CV

Wang et al. CP

Marinov et al. CV

Marinov et al. CP

Figure 4.2: Simulated temperature profiles with the mechanisms of Wang et al. [63] and Marinov

et al. [60] with the constant volume and the constant pressure gasdynamic models. Initial

conditions: 1% n-butane in argon, 1565 K, 1.45 atm.

58

0.0 0.5 1.00.0

0.5

1.0

1.5

2.0

Pressure (atm

)

Time [ms]

Wang et al. CV

Curran et al. CV

CP

Measured pressure

1% C7H

16 / Ar

1597 K, 1.44 atm

Figure 4.3: Simulated pressure profiles with the mechanisms of Wang et al. [63] and Curran et al.

[62] with the constant volume and the constant pressure gasdynamic models. Initial conditions:

1% n-heptane in argon, 1597 K, 1.44 atm.

0.0 0.5 1.0 1.51350

1400

1450

1500

1550

16001% C

7H

16 / Ar

1597 K, 1.44 atm

Temperature (K)

Time [ms]

Wang et al. CV

Wang et al. CP

Curran et al. CV

Curran et al. CP

Figure 4.4: Simulated temperature profiles with the mechanisms of Wang et al. [63] and Curran

et al. [62] with the constant volume and the constant pressure gasdynamic models. Initial

conditions: 1% n-heptane in argon, 1597 K, 1.44 atm.

59

Table 4.1: Measured mole fraction of CH4 and C2H4 in the pyrolysis of 1% n-butane in argon at

1ms derived using different chemical mechanisms and gasdynamic models.

Table 4.2: Measured mole fraction of CH4 and C2H4 in the pyrolysis of 1% n-heptane in argon at

1ms derived using different chemical mechanisms and gasdynamic models.

4.3.2 Sensitivity analysis and updated chemical mechanisms

4.3.2.1 n-Butane pyrolysis

The CH4 and C2H4 concentration time-histories were measured in mixtures of 1% n-

butane in argon at temperatures from 1254 to 1565 K and pressures from 1.45 to 1.64 atm.

Sensitivity analysis in Fig. 4.5 and 4.6 shows that CH4 and C2H4 concentration are

sensitive to many different reactions, labeled here: 1, 2, 3, 4, 5, 6, and 7 (reaction 1, 2 and

3 were not modified in the Wang et al. [63] mechanism, while the reactions 4, 5, 6 and 7

were updated or adjusted to fit measured CH4 and C2H4 time-histories as will be

described in this section). Reaction (1) (CH4 + H � CH3 + H2) was reviewed extensively

over the temperature ranges from 350 K to 2500 K by Baulch et al. [71], combining 10

sets of measurements and 5 sets of reviews and evaluations. This evaluated rate k1 by

Baulch et al. [71] is within 7% error with the one used in the Wang et al. [63] mechanism

60

between 1200 K and 1600 K. The C2H6 decomposition reaction (Rxn. (2): C2H6 � CH3

+ CH3) is relatively well established by Oehlschlaeger et al. [72] and Kiefer et al. [73].

Both CH4 and C2H4 concentration time-histories are sensitive to Reaction (3), the H-

abstraction from n-butane (Rxn. (3): nC4H10 + H � pC4H9 + H2). However, the

adjustment of this rate k3 was not necessary to fit measured CH4 and C2H4 time-histories.

The major decomposition reactions, Rxn. (4) and Rxn. (5), are important initiation

steps in the n-butane chemical mechanisms. These two n-butane decomposition rates, k4

(Rxn. (4): nC4H10 � nC3H7 + CH3) and k5 (Rxn. (5): nC4H10 � C2H5 + C2H5) were

recently measured by Oehlschlaeger et al. [64]. RRKM model were fit from 1320 to

1600 K to describe the measured decomposition rates with the following parameters:

k∞,4(T)=4.28 1014exp(-35180/RT) s

-1, k0,4(T)=5.34 10

17exp(-21620/RT) cm

3mol

-1s-1

and Fcent,4(T)=0.28 exp(-T/1500 K) for Rxn. (4), k∞,5(T)=4.28 1014exp(-35180/RT) s

-1,

k0,5(T)=5.34 1017exp(-21620/RT) cm

3mol

-1s-1 and Fcent,5(T)=0.28 exp(-T/1500 K) for

Rxn. (5) [64].

In an effort to generate an improved reaction mechanism for n-butane pyrolysis,

the n-butane decomposition rate values from Oehlschlaeger et al. [64] were used in the

Wang et al. [63] mechanism. In addition to these measured n-butane decomposition rates,

two other reactions with large sensitivities were adjusted to improve agreement between

the simulations and the measured CH4 and C2H4 time-histories. The two additional

modified reactions were CH3-abstraction and H-abstraction from n-butane; here

improved fits to the measurements were achieved by: increasing k6 (Rxn. (6): nC4H10 +

CH3 � sC4H9 + CH4) by a factor of 2 and decreasing k7 (Rxn. (7): nC4H10 + H � sC4H9

+ H2) by a factor of 2, within the estimated uncertainty of these reaction rate values.

61

Increasing k6 by a factor of 2 improved the predicted CH4 plateau level by 6 to 25%,

while decreasing k7 by a factor of 2 improved the predicted C2H4 plateau level by 10 to

17%. The two n-butane decomposition rates measured by Oehlschlaeger et al. [64] and

two adjusted rates (i.e., k4, k5, k6 and k7) were incorporated into the original Wang et al.

[63] mechanism to form the modified Wang et al. [63] mechanism for n-butane. All

reactions that were modified in the Wang et al. [63] mechanism are shown in Table 4.3.

No. Reaction Modifications to the modified Wang et al.

mechanism for n-butane

4 nC4H10 � nC3H7 + CH3 Modified with the measured rate by Oehlschlaeger et al. [64]

5 nC4H10 � C2H5 + C2H5

6 nC4H10 + CH3 � sC4H9 + CH4 Increased by a factor of 2

7 nC4H10 + H � sC4H9 + H2 Decreased by a factor of 2

Table 4.3: All reactions that were modified in the Wang et al. [63] mechanism to form the

modified mechanism of Wang et al. [63] for n-butane.

0.0 0.5 1.0 1.5

-0.4

0.0

0.4

(4) nC3H

7+CH

3=C

4H

10

(5) C2H

5+C

2H

5=C

4H

10

(6) C4H

10+CH

3=sC

4H

9+CH

(7) C4H

10+H=sC

4H

9+H

2

(1) CH4+H=CH

3+H

2

(2) CH3+CH

3=C

2H

6

(3) C4H

10+H=pC

4H

9+H

2

2

3

6 7

5

4

1

1% n-C4H

10 / Ar

1450 K, 1.5 atm

CH

4 Sensitivity

Time [ms]

Figure 4.5: CH4 sensitivity for n-butane pyrolysis using the Wang et al. [63] mechanism. Only

the seven reactions with the largest sensitivities are shown; of these, only the four reactions in

highlighted boxes, Rxn. 4-7, were varied in this study. Initial conditions: 1% n-butane in argon,

1450 K, 1.5 atm.

62

0.0 0.5 1.0 1.5

-0.4

0.0

0.44

5

3

7

(3) C4H

10+H=pC

4H

9+H

2

(4) nC3H

7+CH

3=C

4H

10

(5) C2H

5+C

2H

5=C

4H

10

(7) C4H

10+H=sC

4H

9+H

2

1% n-C4H

10 / Ar

1450 K, 1.5 atm

C2H

4 Sensitivity

Time [ms]

Figure 4.6: CH4 sensitivity for n-butane pyrolysis using the Wang et al. [63] mechanism. Only

the four reactions with the largest sensitivities are shown. Initial conditions: 1% n-butane in argon,

1450 K, 1.5 atm.

4.3.2.2 n-Heptane pyrolysis

The CH4 and C2H4 concentration time-histories were measured in mixtures of 1% n-

heptane in argon at temperatures from 1190 to 1597 K and pressures from 1.44 to 1.69

atm. Sensitivity analysis in Fig. 4.7 and 4.8 shows that CH4 and C2H4 concentrations are

sensitive to many different reactions including Rxns. (1) and (2) described in the previous

section and six other reactions related to n-heptane decomposition (reactions 1, 2 were

not modified in the Wang et al. [63] mechanism, while the rates for reactions 8, 9, 10, 11,

12 and 13 were updated or adjusted to fit measured CH4 and C2H4 time-histories as will

be described in this section)..

The three major decomposition pathways of n-heptane pyrolysis reactions are (8),

(9) and (10): Rxn (8): nC7H16 � pC4H9 + nC3H7, Rxn. (9): nC7H16 � C5H11-1 + C2H5

63

and Rxn. (10): nC7H16 � C6H13-1 + CH3. The overall n-heptane decomposition rate

constants (ktot = k8 + k9 + k10) were recently measured by Davidson et al. [65] near 1.8

atm. Their expression for the overall decomposition rate is ktotal = 9.00 1014exp(-

67300/RT) s-1 [65]. Each n-heptane decomposition reaction was then specified using the

the n-heptane branching ratio estimated by Babushok et al. [74] at 0.1 MPa. The

modeled n-heptane branching ratio at temperatures from 1200 to 1700 K is shown in Fig

4.9. In addition to the measured n-heptane decomposition rates, two other reactions with

large sensitivities were adjusted to improve the simulations to the measured CH4 and

C2H4 time-histories. As will be described below, improved fits to the measurements were

achieved by decreasing k11 (Rxn. (11): nC7H16 + CH3� C7H15-3 + CH4) and k12 (Rxn.

(12): nC7H16 + CH3 � C7H15-2 + CH4), both by a factor of 2. These changes are within

the expected uncertainties for these reaction rate constants. Increasing both of k11 and k12

by a factor of 2 resulted in the reduction of the predicted CH4 plateau level by 14 to 43%.

However, this modification did not make notable changes of the predicted C2H4 time-

histories. Finally, the rate constant of the H-abstraction from ethylene, Rxn. (13)

(C2H4+H � C2H3 + H2) was updated with the estimated value by Baulch et al. [71],

combining 7 sets of measurements and 2 sets of reviews and evaluations to fit especially

the high-temperature C2H4 time-history at 1597 K. The C2H4 sensitivity analysis at this

temperature is shown in Fig. 4.10.

The measured n-heptane decomposition rates, modeled n-heptane branching ratio

and three adjusted rates (k8 to k13) were incorporated into the original Wang et al. [63]

mechanism to form the modified mechanism of Wang et al. [63] for n-heptane. All

reactions that were modified in the Wang et al. [63] mechanism are shown in Table 4.4.

64

No. Reaction Modifications to the modified Wang et al.

mechanism for n-heptane

8 nC7H16 � pC4H9 + nC3H7

Modified with the measured rate by Davidson et al. [64]

and the modeled branching ratio by Babushok et al. [74] 9 nC7H16 � C5H11-1 + C2H5

10 nC7H16 � C6H13-1 + CH3

11 nC7H16 + CH3� C7H15-3 + CH4 Decreased by factor of 2

12 nC7H16 + CH3 � C7H15-2 + CH4

13 C2H4+H � C2H3 + H2 Modified with the estimated rate by Baulch et al. [71]

Table 4.4: All reactions that were modified in the Wang et al. [63] mechanism to form the

modified mechanism of Wang et al. [63] for n-heptane.

0.0 0.5 1.0 1.5

-0.4

0.0

0.4

10 9

12111

2(8) pC

4H

9+nC

3H

7=nC

7H

16

(9) C5H

11-1+C

2H

5=nC

7H

16

(10) C6H

13-1+CH

3=nC

7H

16

(11) nC7H

16+CH

3=C

7H

15-3+CH

4

(12) nC7H

16+CH

3=C

7H

15-2+CH

4

(1) CH4+H=CH

3+H

2

(2) CH3+CH

3(+M)=C

2H

6(+M)

8

1% n-C7H

16 / Ar Jetsurf 2.0 Mech.

1450 K, 1.5 atm

CH

4 Sensitivity

Time [ms]

Figure 4.7: CH4 sensitivity for n-heptane pyrolysis using the Wang et al. [63] mechanism. Only

the seven reactions with the largest sensitivities are shown; of these, only the five reactions in

highlighted boxes, Rxn. 8-12, were varied in this study. Initial conditions: 1% n-heptane in argon,

1450 K, 1.5 atm.

65

0.0 0.5 1.0 1.5

0.0

0.2

14 10

(14) nC7H

16+H=C

7H

15-2+H

2

9

8

(8) pC4H

9+nC

3H

7=nC

7H

16

(9) C5H

11-1+C

2H

5=nC

7H

16

(10) C6H

13-1+CH

3=nC

7H

16

1% n-C7H


1450 K, 1.5 atm

C2H

4 Sensitivity

Time [ms]

Figure 4.8: C2H4 sensitivity for n-heptane pyrolysis using the Wang et al. [63] mechanism. Only

the four reactions with the largest sensitivities are shown. Initial conditions: 1% n-heptane in

argon, 1450 K, 1.5 atm.

5.5 6.0 6.5 7.0 7.5 8.0 8.50

2

4

60.1 MPa

Relative rate constant

(relative to Rxn. (8))

10000/T [1/K]

(8) nC7H

16=pC

4H

9+nC

3H

7

(9) nC7H

16=C

5H

11-1+C

2H

5

(10) nC7H

16=C

6H

13-1+CH

3

Figure 4.9: Modeled branching ratio of n-heptane decomposition from Babushok et al. [74].

66

0.0 0.5 1.0 1.5-0.1

0.0

0.1

(13)

(7)

(6)

(5)

(5) pC4H

9+nC

3H

7=nC

7H

16

(6) C5H

11-1+C

2H

5=nC

7H

16

(7) C6H

13-1+CH

3=nC

7H

16

(13) C2H

4+H=C

2H

3+H

2

1% n-C7H


1600 K, 1.44 atm

C2H

4 Sensitivity

Time [ms]

Figure 4.10: C2H4 sensitivity for n-heptane pyrolysis using the Wang et al. [63] mechanism. Only

the four reactions with the largest sensitivities are shown. Initial conditions: 1% n-heptane in

argon, 1600 K, 1.44 atm.

4.3.3. Measured methane and ethylene time-histories

4.3.3.1 n-Butane pyrolysis

Fig. 4.11 and 4.12 show the measured CH4 time-histories during 1% n-butane pyrolysis at

lower temperatures of 1254 and 1375 K and at higher temperatures of 1482K and 1565K

respectively. The mechanism of Wang et al. [63] performs significantly better than that

of Marinov et al. [60] for predicting CH4 time-histories in the two high-temperature cases.

The modeled CH4 concentration from the Wang et al. [63] mechanism is higher than the

measured concentration by 5% and 7% at 1ms for 1482 K and 1565 K respectively while

the Marinov et al. [60] mechanism is higher by 24% and 28%.

67

At the lowest temperature, the simulated CH4 time-histories from the mechanisms

by Wang et al. [63] and Marinov et al. [60] are almost indistinguishable. The Marinov et

al. [60] performs better than the Wang et al. [63] only at 1375 K. The modified

mechanism of Wang et al. [63] for n-butane significantly improved the simulated CH4

time-histories, fitting the data very well in all cases ranging from 1254 K to 1565 K. The

modeled CH4 concentrations from the modified Wang et al. [63] mechanism for n-butane

are within 2%, 8%, 1% and 7% of the measured concentration at 1ms, and within our

detection uncertainties in each case from 1254 K to 1565 K.

The measured C2H4 time-histories during 1% n-butane pyrolysis at the same

temperatures as CH4 are shown in Figs. 4.13 and 4.14. In all cases, the Marinov et al.

[60] mechanism showed faster C2H4 production rate than the Wang et al. [63] mechanism.

The mechanism of Marinov et al. [60] performs better than Wang et al. [63] for

simulating C2H4 time-histories in the two low temperature cases. The modeled C2H4

concentration from the Marinov et al. [60] mechanism is lower than the measured

concentration at 1ms by 24% and 12% for 1254 K and 1375 K, respectively, while the

Wang et al. [63] mechanism is lower by 45% and 30%. At higher temperatures, the

modeled C2H4 plateau-levels from both mechanisms are not much different from each

other. The modeled C2H4 concentrations using the Marinov et al. [60] mechanism are

lower than the measured concentration by 12% at 1 ms for both 1482 K and 1565 K, and

the Wang et al. [63] simulation is lower by 16% and 17%.

Finally, the modified mechanism of Wang et al. [63] for n-butane significantly

improved the simulated C2H4 time-histories, fitting the data very well in all cases ranging

from 1254 K to 1565 K. The modeled C2H4 concentrations from the modified

68

mechanism for n-butane at 1ms are different from the measured data by 1%, 7%, 1% and

1.5%.

Several uncertainties contribute to the uncertainty in the determination of CH4 and

C2H4 time histories. The uncertainties in scanned laser intensity noise and absorption

coefficients were found to be the major sources of uncertainty for CH4 time-histories, and

the uncertainties in absorption coefficients have the largest effect for C2H4 time-histories.

Other contributions to uncertainties in the CH4 and C2H4 time histories are temperature

(±1%) and pressure (±0.5%). Error analyses yielded uncertainty estimates of 8.5% for

CH4 and 5% for C2H4 time histories at 1.5 ms for 1565 K.

0.0 0.5 1.0 1.50.000

0.002

0.004

1375 K, 1.58 atm

Current study

Marinov et al.

Wang et al.

Modified Wang et al. (n-butane)

CH

4 Mole fraction

Time [ms]

1% C4H

10 / Ar

1254 K, 1.64 atm

Figure 4.11: Comparison of measured CH4 time-histories at two low temperatures with the

mechanisms by Wang et al. [63], Marinov et al. [60] and the modified Wang et al [63] for n-

butane. Initial conditions: 1% n-butane in argon, 1254 K, 1.64 atm and 1375 K, 1.58 atm.

69

0.0 0.5 1.0 1.50.000

0.002

0.004

1482 K, 1.50 atm

Current study

Marinov et al.

Wang et al.


CH

4 Mole fraction

Time [ms]

1% C4H

10 / Ar

1565 K, 1.45 atm

Figure 4.12: Comparison of measured CH4 time-histories at two high temperatures with the



0.0 0.5 1.0 1.50.000

0.005

0.010

0.015

0.020

1375 K, 1.58 atm

Current study

Marinov et al.

Wang et al.


C2H

4 Mole fraction

Time [ms]

1% C4H

10 / Ar

1254 K, 1.64 atm

Figure 4.13: Comparison of measured C2H4 time-histories at two low temperatures with the



70

0.0 0.5 1.0 1.50.000

0.005

0.010

0.015

1482 K, 1.50 atm

Current study

Marinov et al.

Wang et al.


C2H

4 Mole fraction

Time [ms]

1% C4H

10 / Ar

1565 K, 1.45 atm

Figure 4.14: Comparison of measured C2H4 time-histories at two high temperatures with the



4.3.3.2 n-Heptane pyrolysis

Fig. 4.15 and 4.16 show the measured CH4 time-histories during 1% n-heptane pyrolysis

at lower temperatures of 1190 and 1350 K and at higher temperatures of 1450 K and

1597 K, respectively. The mechanism of Wang et al. [63] performs better than that of

Curran et al. [62] for predicting CH4 time-histories in all cases except for the 1450 K case.

The modeled CH4 concentration from the Wang et al. [63] mechanism is higher than the

measured concentration by 8%, 16%, 19% and 27% at 1ms for four different

temperatures, in the order of the lowest to the highest, while the Curran et al. [62]

mechanism is lower by 40% and 23% and higher by 2% and 55%.

71

The modified mechanism of Wang et al. [63] for n-heptane significantly improved

the predicted CH4 time-histories at all temperatures, fitting the measured ones very well

in three cases ranging from 1190 K to 1450 K. The modeled CH4 concentrations from

the modified mechanism by Wang et al. [63] for n-heptane at 1ms are within 5%, 0% and

1% of the measured concentrations, comparable with our detection uncertainty in each

case from 1190 K to 1450 K.

As described for n-butane pyrolysis experiments, similar error analyses yielded

uncertainty estimates of 8.5% for CH4 and 5% for C2H4 time histories at 1.5 ms for 1597

K.

0.0 0.5 1.0 1.50.000

0.002

0.004

1350 K, 1.50 atm

Current study

Curran et al.

Wang et al.

Modified Wang et al.

CH

4 Mole fraction

Time [ms]

1% C7H

16 / Ar

1190 K, 1.69 atm


mechanisms by Wang et al. [63], Curran et al. [62] and the modified Wang et al [63] for n-

heptane. Initial conditions: 1% n-heptane in argon, 1190 K, 1.69 atm and 1350 K, 1.50 atm.

72

0.0 0.5 1.0 1.50.000

0.002

0.004

0.006

1450 K, 1.47 atm

Current study

Curran et al.

Wang et al.


CH

4 Mole fraction

Time [ms]

1% C7H

16 / Ar

1597 K, 1.44 atm




The measured C2H4 time-histories during 1% n-heptane pyrolysis at the same

temperatures as CH4 are shown in Fig. 4.17 and 4.18. The mechanism of Wang et al. [63]

performs better than that of Curran et al. [62] for predicting C2H4 time-histories in the

two high temperature cases. The modeled C2H4 concentrations by the Wang et al. [63]

mechanism at 1ms are within 3% of the measured concentrations for 1450 K and 1597 K,

while the Curran et al. [62] mechanism is lower by 10% and 9%. However, the

mechanisms of Wang et al. [63] and Curran et al. [62] perform similarly for C2H4

concentration in the two low temperature cases of 1190 K and 1350 K, as shown in Fig.

4.17.

73

0.0 0.5 1.0 1.50.00

0.01

0.02

0.03

1350 K, 1.50 atm

Current study

Curran et al.

Wang et al.


C2H

4 Mole fraction

Time [ms]

1% C7H

16 / Ar

1190 K, 1.69 atm




0.0 0.5 1.0 1.50.00

0.01

0.02

0.03

1450 K, 1.47 atm

Current study

Curran et al.

Wang et al.

Modified Wang et al. (n-heptane)

C2H

4 Mole fraction

Time [ms]

1% C7H

16 / Ar

1597 K, 1.44 atm




74

The modified Wang et al. mechanism for n-heptane significantly improved the fit

of the predicted C2H4 time-histories in all cases except for the lowest temperature of 1190

K. The modeled C2H4 concentrations from the modified Wang et al. mechanism for n-

heptane at 1ms are different from the measured ones by 25%, 2%, 0% and 0%, in each

case from 1190 K to 1597 K.

4.4 Conclusions

The high-temperature pyrolysis of n-butane and n-heptane was studied behind reflected

shock waves by measuring time-histories of CH4 and C2H4 in mixtures of 1% fuel in

argon at temperature of 1200-1600 K and a pressure near 1.5 atm. It was shown that the

inferred species mole fractions from measured raw absorbance data were not a strong

function of the chemical mechanisms and gasdynamic models used to simulate

temperature and pressure during n-butane and n-heptane pyrolysis. The Wang et al. [63]

mechanism with a constant volume gasdynamic model was used to calculate temperature

and pressure profiles and to infer mole fractions of CH4 and C2H4. The measured species

time-histories were compared with the several decomposition mechanisms for both n-

butane and n-heptane: Marinov et al. [60] for n-butane, Curran et al. [62] for n-heptane,

and Wang et al. [63] commonly for n-butane and n-heptane.

For n-butane, the two measured n-butane decomposition rates by Oehlschlaeger et

al. [64] were incorporated into the mechanism of Wang et al. [63]. Two additional rates

were also adjusted for CH3-abstraction and H-abstraction from n-butane to form a

modified Wang et al. mechanism for n-butane. For n-heptane, the overall n-heptane

decomposition rate measured by Davidson et al. [65] was incorporated into the Wang et

75

al. mechanism and the two CH3-abstraction reactions from n-heptanes were also adjusted

and the H-abstraction reaction from ethylene was finally updated to form the modified

mechanism of Wang et al. for n-heptane. Those mechanistic changes are listed in the

table 4.3 and 4.4. The modified mechanisms of Wang et al. for n-butane and n-heptane

were improved and agree with the measured time-histories of CH4 and C2H4 reasonably

well for both n-butane and n-heptane pyrolysis.

76

Chapter 5

Decomposition Species Measurements

during DME Pyrolysis

5.1 Introduction

There have been early studies of DME pyrolysis in static and flow reactors [75-

78] that measure DME time-histories at low temperature. DME oxidation studies have

also been performed in flow reactors to develop DME oxidation models [79-81, 83].

Shock tube studies of DME oxidation have involved measurements of ignition delay

times by Pfahl et al. [84], Dagaut et al. [85], and Cook et al. [86]. Based on the result of

these studies, several DME oxidation mechanism have been proposed [81-83, 85]. In this

paper we shall concentrate on the DME oxidation mechanisms developed by Curran et al.

77

[82] and Zhao et al. [83]. The Curran et al. [82] mechanism has been continuously

updated, with Curran et al. [82] as the current version. Recently, Cook et al. [87]

measured the high-temperature rate constant of two DME reactions, (1) DME + Ar �

CH3 + CH3O + Ar and (2) DME + OH � CH3OCH2 + H2O in a shock tube by

monitoring OH radicals. A shock tube study of DME pyrolysis by Hidaka et al.

monitored major decomposition products [88], however, the measurements were made by

gas chromatography and the data were not time-resolved.

The mole fraction time-histories for CO, CH4, C2H4, H2, CH2O, C2H6 and DME at

reflected shock conditions of 1450 K, 1.5 atm, and 2% DME/Ar, simulated using the

Curran et al. [82] mechanism, are shown in Fig. 5.1. Here we focus on CO, CH4 and

C2H4. As shown in Figs. 5.2, 5.3 and 5.4, the simulated time-histories from the Zhao et al.

[83] mechanism significantly differ from these.

0.0 0.5 1.00.00

0.01

0.02

0.03

0.04

C2H

6

CH2O

DME

C2H

4

CH4

CO

Mole fraction

Time [ms]

2% DME / Ar

1450K, 1.5 atm

H2

Figure 5.1: Simulations of species time-histories during DME pyrolysis behind a reflected shock

wave using the Curran et al. [82] mechanism. Initial conditions: 2% DME in argon, 1450 K, 1.5

atm.

78

0.0 0.5 1.0 1.50.000

0.002

0.004

0.006

0.008

CO m

ole fraction

Time [ms]

Curran et al.

Zhao et al.

0.5% DME / Ar

1450 K, 1.5 atm

Figure 5.2: Simulations of the CO time-history during DME pyrolysis behind a reflected shock

wave using the Curran et al. [82] and the Zhao et al. [83] mechanisms. Initial conditions: 0.5%

DME in argon, 1450 K, 1.5 atm.

0.0 0.5 1.0 1.50.000

0.002

0.004

0.006

0.0081% DME / Ar

1450 K, 1.5 atm

CH

4 Mole Fraction

Time [ms]

Curran et al.

Zhao et al.

Figure 5.3: Simulations of the CH4 time-history during DME pyrolysis behind a reflected shock

wave using the Curran et al. [82] and the Zhao et al. [83] mechanisms. Initial conditions: 1%


79

0.0 0.5 1.0 1.50.000

0.002

0.004

0.006

0.0082% DME / Ar

1450 K, 1.5 atm

C2H

4 M

ole Fraction

Time [ms]

Curran et al.

Zhao et al.

Figure 5.4: Simulations of the C2H4 time-history during DME pyrolysis behind a reflected shock

wave using the Curran et al. [82] and the Zhao et al. [83] mechanisms. Initial conditions: 2%


Clearly, the differences in these simulations, particularly for CH4 and C2H4, are

large. To help resolve these differences, we have measured the time-histories of CO,

CH4 and C2H4 during DME pyrolysis behind reflected shock waves.

5.2 Laser absorption diagnostics

5.2.1 Methane detection

The mid-IR scanned-wavelength laser absorption diagnostic for interference-free CH4

detection was used as described in 4.2.1 and Chpater 1.

5.2.2 Ethylene detection during DME pyrolysis (two wavelengths)

80

C2H4 was measured using a two-wavelength absorption scheme (10.532 µm and 10.675

µm) with a CO2 laser that allowed the rejection of DME absorption interference that

occurred at these wavelengths. The 10.532 µm line has been used previously to measure

C2H4 concentrations because it has significant absorption due to the strong Q-branch of

the ν7 ethylene band, which has a strong overlap with the P(14) line of the CO2 laser at

10.532 µm [70]. The 10.675 µm P(28) line also was previously suggested to reject

absorption interferences from other small alkenes (C3H6, C4H8) from jet fuel

decomposition because these alkenes have nearly identical absorption coefficients at the

two wavelengths, while the differential absorption of C2H4 is significant [90]. As noted

in Fig. 5.1, CO, CH4, C2H4, CH2O, C2H6 and DME are the major decomposition species

that occur during DME pyrolysis. However, while both C2H4 and DME have absorption

at these two wavelengths, CO and CH2O do not have absorption lines in the 10 µm region

and both CH4 and C2H6 have negligible absorption at 10.532 µm [66]. C2H4 mole

fraction could thus be calculated using a two-wavelength scheme, allowing for

contributions for the two species, C2H4 and DME. The minimum detection limit with this

scheme is approximately 250 ppm C2H4 at 1500 K and 1.5 atm.

A short summary of the theoretical details of the two-wavelength scheme is presented

here. In the absence of light losses from absorption of liquid and scattering by particles,

the transmission of monochromatic light at wavelength λ through a uniform gaseous

species is described by Beer’s law as shown in Eqn. (2.1). Additional terms are added to

equation (2.1) to describe the transmitted light intensity with C2H4 and DME absorption.

DMEHCI

Iαα +=

−

42

0

ln (5.1)

81

where 42HCα is the vapor-phase absorption by ethylene, DMEα is the vapor-phase

absorption by DME. We have two equations with two unknowns (χC2H4, χDME).

( )

( )DMEmDMEHCmHCmDMEmHC

m

DMEmDMEHCmHCmDMEmHC

m

RT

PL

I

I

RT

PL

I

I

χσχσαα

χσχσαα

µµµµ

µ

µµµµ

µ

675.10,675.10,675.10,675.10,

675.100

532.10,532.10,532.10,532.10,

532.100

424242

424242

ln

ln

+=+=

−

+=+=

−

(5.2)

The ethylene mole fraction can thus be inferred from solving those two equations above

and can be simplified to equation (5.3).

−

−

=mHCmDMEmHCmDME

m

mDME

m

mDME

HC

I

I

I

I

PL

RT

µµµµ

µ

µ

µ

µ

σσσσ

σσ

χ532.10,675.10,675.10,532.10,

675.100

532.10,

532.100

675.10,

4242

42

lnln

(5.3)

High-temperature measurements of the absorption cross-sections of ethylene at 10.532

µm were acquired by Ren et al. [70], while those at 10.675 µm were measured in this

study, and these are shown in Fig. 5.5. The measured DME cross-sections at both 10.532

µm and 10.675 µm for the temperature conditions of 2% DME pyrolysis experiments of

this study are also shown in Fig. 5.5. Small differences in the DME cross-sections at the

two wavelengths are not negligible (~8%), justifying the use of equation (5.3).

82

1300 1400 1500 16000

5

10

15

σDME, 10.532µm σDME, 10.675µm σethylene, 10.532µm (Ren et al.)

σethylene, 10.675µm

Cross Section [m

2/mol]

Temperature [K]

Figure 5.5: Measured cross-sections of DME and ethylene at 10.532 µm and 10.675 µm.

5.2.3 Carbon monoxide detection

A direct absorption strategy with a fixed-wavelength using a quantum cascade laser

(QCL) was used to measure CO concentration time-histories. The center of the R(13)

transition line at 2193.359 cm-1, in the fundamental rovibrational band around 4.56 µm,

was excited to maximize absorption sensitivity and minimize the absorption interference

from H2O and CO2, resulting in ppm-level CO detectivity in the reflected shock

experiments. The line-strength and collision self-broadening parameter for the R(13)

transition were taken from HITRAN database, and the collisional broadening parameter

of CO in argon was measured in our shock tube facility in the temperature range of 1000-

1800 K as described by Ren et al. [89].

83


5.3.1 Species concentration time-histories and gasdynamic models

The time-varying temperature, pressure and species absorption coefficient during fuel

pyrolysis are discussed in detail in 4.3.1.

Fig. 5.6 and 5.7 show the calculated pressure and temperature changes during

0.5% DME pyrolysis using two different chemical mechanisms from Curran et al. [82]

and Zhao et al. [83] with the constant volume and the constant pressure gasdynamic

models. The Curran et al. [82] mechanism predicts 0.1% change between initial pressure

and pressure at 1 ms and the Zhao et al. [83] mechanism predicts 0.4% pressure change

based on the constant volume gasdynamic model. The Curran et al. [82] mechanism with

the constant volume and the constant pressure models predict the same temperature drop

of 1.4% in 1 ms, while the Zhao et al. [83] mechanism predicts a 0.8% and 0.9%

temperature drop in 1 ms with the constant volume and the constant pressure models,

respectively. However, because of the convolution of these two effects (temperature and

pressure change), the calculated CO mole fractions, shown in Fig. 5.8, are

indistinguishable for the two models.

Aside from the simulated species concentrations, it is necessary to ensure that the

species concentration inferred from measured raw absorbances does not change

considerably, regardless of which chemical mechanisms and gasdynamic models are used

to simulate temperature and pressure during DME pyrolysis. The temperature-dependent

absorption coefficients were also used to infer species concentrations because the

changes in temperature and pressure caused by chemical reactions associated with fuel

pyrolysis slightly change the species absorption coefficients. Table 5.1 shows the

84

measured mole fraction of CO, CH4 and C2H4 at 1ms inferred using the T, P values from

the mechanisms of Curran et al. [82] and Zhao et al. [83], with the constant volume and

the constant pressure gasdynamic models. The differences between measured mole

fractions from the constant volume and the constant pressure gasdynamic models are

negligible: less than 0.3% for the Curran et al. [82] and 0.7% for the Zhao et al. [83]

mechanism. The difference between the mechanisms from Curran et al. [82] and Zhao et

al. [83] for inferring mole fraction is relatively higher, up to 7% in the highest

temperature case of 2% DME pyrolysis where the temperature and pressure change is

maximized. However, this difference is smaller than our detection resolution of the laser

diagnostics for C2H4 measurement. We have elected to utilize the Curran et al. [82]

mechanism with the constant volume gasdynamic model to calculate temperature and

pressure profiles and to infer mole fractions of CO, CH4 and C2H4 during DME pyrolysis.

The Curran et al. [82] mechanism was selected because it performs significantly better

than the Zhao et al. [83] mechanism for predicting the measured CH4 and C2H4 time-

histories as will be described in 5.3.3.2 and 5.3.3.3.

85

0.0 0.5 1.0 1.51.44

1.46

1.48

1.500.5% DME / Ar

1542 K, 1.47 atm

Pressure (atm

)

Time [ms]

Curran et al. CV

Zhao et al. CV

CP

Figure 5.6: Simulated pressure profiles with the mechanisms of Curran et al. [82] and Zhao et al.

[83] with the constant volume and the constant pressure gasdynamic models . Initial conditions:

0.5% DME in argon, 1542 K, 1.47 atm.

0.0 0.5 1.0 1.51500

1520

1540

15600.5% DME / Ar

1542 K, 1.47 atm

Temperature (K)

Time [ms]

Curran et al. CV

Curran et al. CP

Zhao et al. CV

Zhao et al. CP

Figure 5.7: Simulated temperature profiles with the mechanisms of Curran et al. [82] and Zhao et


conditions: 0.5% DME in argon, 1542 K, 1.47 atm.

86

0.0 0.1 0.2 0.3 0.40.000

0.002

0.004

0.0060.5% DME / Ar

1542 K, 1.47 atm

Mole fraction

Time [ms]

Curran et al. CV

Curran et al. CP

Zhao et al. CV

Zhao et al. CP

Figure 5.8: Simulated CO time-histories with the mechanisms by Curran et al. [82] and Zhao et


conditions: 0.5% DME in argon, 1542 K, 1.47 atm.

Table 5.1: Measured mole fraction of CO, CH4 and C2H4 in the pyrolysis of 0.5%, 1% and 2%

DME in argon at 1ms from different chemical mechanisms and gasdynamic models.

87

5.3.2 The DME decomposition reaction rate constant

CO, CH4 and C2H4 concentration time-histories in high-temperature DME pyrolysis are

sensitive to many different reactions, as shown in Fig. 5.9, 5.11 and 5.12. However, all

three species are strongly sensitive to the DME decomposition rate constant k1

(CH3OCH3 � CH3 + CH3O). This rate was recently measured using OH laser absorption

in a shock tube by Cook et al. [87]. The rate was measured at different pressures from

0.6 to 11.5 atm and temperatures from 1350 to 1800 K, and the measured rates and

RRKM model were fit using the Troe falloff form over a range of pressures [87].

However, at temperatures from 1350 to 1600 K, the RRKM modeling at 1.5 atm

underpredicts the measured data by Cook et al. [87] at pressures from 1.4 to 1.6 atm as

shown in Fig. 5.13. Therefore, in this study, the decomposition rate k1 was reassessed by

fitting a limited set of Cook et al. [87] data, covering temperatures from 1349 to 1572 K.

Using this limited data set, a reassessed, best-fit DME decomposition reaction k1 under

the experimental conditions of the present study is k1=1.63 1012exp(-58460/RT) s

-1, still

in good agreement with Cook et al. [87]. Two new mechanisms were generated: the

modified Curran et al. [82] and the modified Zhao et al. [83] that include this reassessed

DME decomposition rate and no other changes.

88

0.0 0.5 1.0-1

0

1

2

CH2O+H<=>HCO+H

2

CH3OCH

3+H<=>CH

3OCH

2+H

2

CH3OCH

3(+M)<=>CH

3+CH

3O(+M)

CO sensitivity

Time [ms]

0.5% DME / Ar

1450 K, 1.5 atm

Figure 5.9: CO sensitivity for DME pyrolysis using the Curran et al. [82] mechanism. Only the

three reactions with the largest sensitivities are shown. Initial conditions: 0.5% DME in argon,

1450 K, 1.5 atm.

0.0 0.5 1.0

0.0000

0.0001

0.0002

0.0003

CH2O+CH

3 <=> HCO+CH

4

CH3OCH

3+CH

3<=>CH

3OCH

2+CH

4

CH

4 ROP (mole/cm

3sec)

Time [ms]

1% DME / Ar

1450 K, 1.5 atm

Figure 5.10: CH4 ROP for DME pyrolysis using the Curran et al. [82] mechanism. Only the two

reactions with the largest ROPs are shown. Initial conditions: 1% DME in argon, 1450 K, 1.5 atm.

89

Figure 5.11: CH4 sensitivity for DME pyrolysis using the Curran et al. [82] mechanism. Only the

three reactions with the largest sensitivities are shown. Initial conditions: 1% DME in argon, 1450

K, 1.5 atm.

0.0 0.5 1.0

0

1

2

2% DME / Ar

1450 K, 1.5 atm

CH3OCH

3+H=>CH

3OCH

2+H

2

C2H

6+H=>C

2H

5+H

2

CH3OCH

3(+M)<=>CH

3+CH

3O(+M)

C2H

4 Sensitivity

Time [ms]

Figure 5.12: C2H4 sensitivity for DME pyrolysis using the Curran et al. [82] mechanism. Only

the three reactions with the largest sensitivities are shown. Initial conditions: 2% DME in argon,

1450 K, 1.5 atm.

0.0 0.5 1.0-0.5

0.0

0.5

1.0

1.5

1% DME / Ar

1450 K, 1.5 atm

CH3OCH

3(+M)<=>CH

3+CH

3O(+M)

C2H

6(+M)<=>CH

3+CH

3(+M)

CH3OCH

3+CH

3<=>CH

3OCH

2+CH

4

CH

4 Sensitivity

Time [ms]

90

5.5 6.0 6.5 7.0 7.5

103

104

105

106

k [1/s]

10000/T [1/K]

Cook et al. data, 1.4-1.6 atm

Current RRKM model (Cook et al.), 1.5 atm

Cook et al. data fit in this experimental range

Figure 5.13: The current measured DME decomposition rate constant (k1) and RRKM model

from Cook et al. [87], and the fit to lower temperature shock tube data.

5.3.3 Measured species time-histories and comparison with

modeling

5.3.3.1 Carbon monoxide time-history measurements

CO time-histories were measured during the pyrolysis of 0.5% DME in argon

mixtures at temperatures from 1352 to 1542 K and pressures from 1.47 to 1.60 atm.

Sensitivity analysis in Fig. 5.9 shows that CO concentration is sensitive mainly to three

different reactions: (1) DME � CH3 + CH3O, (2) CH2O + H � HCO + H2 and (3) DME

+ H � CH3OCH2 + H2. The rate k2 was measured directly by Friedrichs et al. by

monitoring formaldehyde using 174 nm absorption for temperatures from 1510 to 1960 K

[91], and their k2 rate is within 3% of the one used in the Curran et al. [82] mechanism.

91

The rate k3 in the recent review of Baulch et al. [71] was based on many different

experimental results, and is not significantly different from the one used in the Curran et

al. [82] mechanism, within 15% error; and the Zhao et al. [83] mechanism used the same

k3 rate as Curran et al. [82]. As discussed in 5.3.2, the measured DME decomposition

rate (k1) was reassessed in this experimental range and that was incorporated into the two

major mechanisms of Curran et al. [82] and Zhao et al. [83] to form the modified

mechanisms. The modified Curran et al. [82] mechanism predicts lower CO

concentrations at all temperatures, and in particular, shows very good agreement with the

experiment at 1352 K, but not at 1445 and 1542 K. Several uncertainties contribute to

the determination of CO time histories. The contributions to uncertainties in CO time-

histories are absorption coefficient (±4%), temperature (±1%) and pressure (±0.5%).

Error analyses yielded uncertainty estimates of 4.2% for CO time histories.

0.0 0.5 1.00.000

0.002

0.004

0.006

Mole Fraction

Time [ms]

Measured CO concentration

Curran et al.

Modified Curran et al.

Zhao et al.

Modified Zhao et al.

0.5% DME / Ar

1352 K, 1.60 atm

Figure 5.14: Comparison of measured CO time-history with the modified and unmodified DME

decomposition mechanisms from Curran et al. [82] and Zhao et al. [83]. Initial conditions: 0.5%


92

0.0 0.1 0.2 0.30.000

0.002

0.004

0.0060.5% DME / Ar

1445 K, 1.51 atm

Mole Fraction

Time [ms]


Curran et al.


Zhao et al.





0.00 0.05 0.10 0.150.000

0.002

0.004

0.006

0.0080.5% DME / Ar

1542 K, 1.47 atm

Mole Fraction

Time [ms]


Curran et al.


Zhao et al.





93

5.3.3.2 Methane time-history measurements

The CH4 time-histories were measured in mixtures of 1% DME in argon at temperatures

from 1291 to 1494 K and pressures from 1.48 to 1.55 atm. The rate of production

analysis in Fig. 5.10 shows that CH4 is formed in two major pathways: (4) DME + CH3

� CH3OCH2 + CH4 and (5) CH2O + CH3 � HCO + CH4. Sensitivity analysis in Fig.

5.11 shows that the CH4 concentration is sensitive to many reactions, but is most strongly

sensitive to three reactions: (1) DME � CH3 + CH3O, (4) DME + CH3 � CH3OCH2 +

CH4 and (6) C2H6 � CH3 + CH3. Aside from the DME decomposition reaction (R1), the

reaction of fuel with methyl radical (R4) is also important for describing fuel

consumption and CH4 time-history. Prior experimental studies on reaction (4) are all

restricted to low temperatures (<1000) [75-78], and there is significant difference

between k4 values used in the mechanisms of Curran et al. [82] and Zhao et al. [83]. The

k4 value of Zhao et al. is higher than that of Curran et al. by a factor of up to 3.5 when it

is extrapolated to temperatures of 1200 - 1600 K. The C2H6 decomposition reaction (R6)

is relatively well-established by Oehlschlaeger et al. [72] and Kiefer et al. [73].

As shown in Fig. 5.17, 5.18 and 5.19, the mechanism of Curran et al. [82]

performs significantly better than that of Zhao et al. [83] in all cases ranging from 1291 K

to 1494 K. As described above, the major contributor to the difference between the

predicted CH4 time-histories using Curran et al. and Zhao et al. is the difference between

the modeled rate coefficients of the reaction of fuel with methyl radical (R4) used in

these two mechanisms. Therefore, the k4 rate of Curran et al. more accurately describes

the CH4 removal than the Zhao et al. value in these experiments.

94

0.0 0.5 1.0 1.50.000

0.005

0.0101% DME / Ar

1291 K, 1.55 atm

Measured CH4 concentration

Curran et al.


Zhao et al.


Mole Fraction

Time [ms]

Figure 5.17: Comparison of measured CH4 time-history with the modified and unmodified DME

decomposition mechanisms from Curran et al. [82] and Zhao et al. [83]. Initial conditions: 1%


0.0 0.5 1.0 1.50.000

0.005

0.010

0.0151% DME / Ar

1359 K, 1.52 atm


Curran et al.


Zhao et al.


Mole Fraction

Time [ms]




95

0.0 0.5 1.0 1.50.000

0.005

0.010

0.0151% DME / Ar

1494 K, 1.48 atm


Curran et al.


Zhao et al.


Mole Fraction

Time [ms]




The difference between simulated CH4 concentration from the modified and

unmodified Curran et al. [82] mechanism is almost negligible especially for the two

lower temperature cases in Fig. 5.17 and 5.18. At the higher temperature of 1494 K, the

modified Curran et al. [82] mechanism predicts the plateau level of CH4 concentration

slightly higher than the unmodified Curran et al. [82] mechanism.

As described in chapter 4, the uncertainties in scanned laser intensity noise and

absorption coefficients were found to be the major sources of uncertainty for CH4 time-

histories. Error analyses yielded uncertainty estimates of 7% for CH4 time histories at 1.5

ms for 1494 K.

96

5.3.3.3 Ethylene time-history measurements

The C2H4 time-histories were measured in mixtures of 2% DME in argon at

temperatures from 1354 to 1509 K and pressures from 1.45 to 1.53 atm. The sensitivity

plot of C2H4 in Fig. 5.12 is as complex as that of CH4. However, C2H4 concentration is

most sensitive to three different reactions: (1) DME � CH3 + CH3O, (3) DME + H �

CH3OCH2 + H2 and (7) C2H6 + H � C2H5 + H2. As discussed in 5.3.3.2., the

mechanisms of Curran et al. [82] and Zhao et al. [83] used the same k3 rate which is not

significantly different from the one from the fitting based on many different experimental

results at up to 1528 K by Baulch et al. [71]. As shown in Fig. 5.20, 5.21 and 5.22, the

mechanism of Curran et al. [82] performs significantly better than that of Zhao et al. [83]

for predicting C2H4 time-histories in all cases ranging from 1354 K to 1509 K. The

modified Curran [82] mechanism predicts C2H4 time-histories slightly lower than the

unmodified Curran et al. [82] mechanism by 14%, 5% and 8% at 1 ms in the cases of

1354 K, 1429 K and 1509K respectively. The modified Curran et al. [82] mechanism fits

the measured C2H4 time-histories better than the unmodified one at the lowest and

highest temperature cases in Fig. 5.20 and 5.22. However, the difference between the

predicted C2H4 mole fraction by two different mechanisms (unmodified and modified) is

smaller than the uncertainty bounds of the C2H4 laser measurement. The uncertainties in

laser intensity noise at two different wavelengths and absorption coefficients were found

to be the major sources of uncertainty for C2H4 time-histories. Error analyses yielded

uncertainty estimates of 9.5% for C2H4 time histories at 1.5 ms for 1509 K.

97

0.0 0.5 1.0 1.50.000

0.002

0.004

0.006

0.0082.0% DME / Ar

1354 K, 1.53 atm

Measured C2H4 Concentration

Curran et al.


Zhao et al.


Mole Fraction

Time [ms]

Figure 5.20: Comparison of measured C2H4 time-history with the modified and unmodified DME



0.0 0.5 1.0 1.50.000

0.002

0.004

0.006

0.0082.0% DME / Ar

1429 K, 1.49 atm


Curran et al.


Zhao et al.


Mole Fraction

Time [ms]




98

0.0 0.5 1.0 1.50.000

0.002

0.004

0.006

0.0082.0% DME / Ar

1509 K, 1.45 atm


Curran et al.


Zhao et al.


Mole Fraction

Time [ms]




5.4 Conclusions

High-temperature dimethyl ether (DME) pyrolysis was studied behind reflected shock

waves by measuring time-histories of CO, CH4 and C2H4 in mixtures of 0.5%, 1%, and

2% DME in argon, respectively at temperature of 1300-1600K and pressures near 1.5 atm.

It was shown that the inference of species mole fractions from measured raw absorbance

data was not a strong function of the chemical mechanisms and gasdynamic models used

to simulate temperature and pressure during DME pyrolysis. The Curran et al. [82]

mechanism with a constant volume gasdynamic model was used to calculate temperature

and pressure profiles, needed to infer mole fraction of CO, CH4 and C2H4 from

absorbance data. The measured species time-histories were compared with the two major

DME decomposition mechanisms: Curran et al. [82] and Zhao et al. [83]. The

99

mechanism of Curran et al. [82] performed significantly better than that of Zhao et al.

[83] for predicting the time-histories of CH4 and C2H4. A further update to the

mechanism was made by incorporating the recent DME decomposition rate by Cook et al.

[87]. This modified Curran et al. mechanism agrees with the measured time-histories of

CH4 and C2H4 reasonably well in all cases of experiments and also agrees well with the

time-histories of CO at low temperature (<1400K).

100

Chapter 6

Summary and Future Work

6.1 Summary

6.1.1 Two-color absorption sensor for fuel and temperature sensing

A two-color, mid-IR absorption sensor was developed and demonstrated for simultaneous

measurements of gas temperature and species concentration for iso-octane and gasoline.

The mid-IR light needed was produced by a novel, two-color MIR laser source based on

DFG of near-IR pump and signal diode lasers. Each of the output wavelengths was

intensity modulated to enable frequency demultiplexing of the two colors of transmitted

light and avoid interference from thermal background. Injection-current modulation of

the two DFG pump lasers produced simultaneous modulation of the laser frequency. The

broad, blended absorption features of hydrocarbons in the C-H stretching region near 3.4

101

µm enable a simplified model of the wavelength-modulation contribution to the measured

1f signals. This simplified model results in an effective absorption cross section that

depends on the cross section of the target hydrocarbon and parameters specific to the

laser modulation. Measurements of iso-octane were performed in the controlled

environment of a heated cell and in shock-heated gas mixtures to validate this simplified

WMS signal model over the range from 300-1100 K.

Demonstration measurements of simultaneous gas temperature and fuel

concentration were performed for iso-octane and a premium blend of gasoline.

Quantitative measurements rely on temperature-dependent gasoline cross section data.

For the range from 300 < T < 800 K the sensor measurements of temperature had an

RMS deviation of only 2.5% for iso-octane and 1.4% for gasoline; for fuel concentration

measurements the RMS deviation was only 5% for iso-octane and 4% for gasoline.

Extrapolation of the FTIR-based temperature-dependent cross-sections, from 800 to 1100

K, provided the needed database for higher-temperature measurements. However, for the

extended temperature range of these measurements, 300 < T < 1100 K, the RMS

deviation roughly doubles from values measured only over the range 300 < T < 800 K,

because little information on the high-temperature behavior of the cross-section is

available.

Measurements in shock-heated gases confirmed the 10 kHz measurement

bandwidth (10 µs time response), which is sufficient for crank-angle resolution at 1600

rpm. The sensor design presented has good potential for crank-angle-resolved

measurements in production IC-engines.

102

6.1.2 Interference-free mid-IR laser absorption detection of

methane

A novel, mid-IR scanned-wavelength laser absorption diagnostic was developed for time-

resolved, interference-free, absorption measurement of methane concentration time-

histories in the presence of other hydrocarbon species. The differential absorption (peak

minus valley) scheme was based on the absorption spectral structure difference between

methane and other hydrocarbon products from n-heptane decomposition. This

measurement technique can be generalized to measure concentration of other

hydrocarbon species with discrete, structured absorption spectra in the presence of

interference from other species with broad, unresolved absorption spectra.

6.1.3 Decomposition species measurements in n-alkane pyrolysis

CH4 and C2H4 concentration time-histories were measured behind reflected shock waves

during the pyrolysis of two n-alkanes: n-butane and n-heptane. Experiments were

conducted at temperatures of 1200-1600 K and at pressures near 1.5 atm, with fuel

concentrations of 1% in Ar. A mid-IR scanned-wavelength laser absorption diagnostic

with a difference frequency generation (DFG) laser near 3.43 µm was used to measure

CH4 concentration time-histories. C2H4 was measured using a fixed wavelength

absorption scheme at 10.532 µm with a CO2 laser. The mechanism of Wang et al. [63]

with a constant volume gasdynamic model was used to calculate temperature and

pressure profiles and to infer the mole fractions of CH4 and C2H4. The measured CH4

and C2H4 time-histories in n-butane pyrolysis were compared to simulations based on the

103

comprehensive n-alkane mechanism by Wang et al. [63] and the detailed n-butane

mechanism by Marinov et al. [60]. Based on these comparisons, the n-butane

decomposition rates measured by Oehlschlaeger et al. [64] were incorporated into the

Wang et al. mechanism and two additional butane abstraction reaction rate constant

adjustments were also made to form the modified Wang et al. mechanism for n-butane.

The measured CH4 and C2H4 time-histories during n-heptane pyrolysis were also

compared to simulations based on the mechanisms by Wang et al. [63] and Curran et al.

[62]. The overall n-heptane decomposition rate measured by Davidson et al. [65] was

incorporated into the Wang et al. mechanism, and the two CH3-abstraction reactions from

n-heptane were adjusted and the H-abstraction reaction from ethylene was updated to

form the modified Wang et al. mechanism for n-heptane.

Using these modified mechanisms the agreement between simulation and

experimental time-histories of CH4 and C2H4 were both significantly improved for n-

butane and n-heptane pyrolysis.

6.1.4 Decomposition species measurements in DME pyrolysis

High-temperature dimethyl ether (DME) pyrolysis was studied behind reflected shock

waves by measuring time-histories of CO, CH4 and C2H4 in mixtures of 0.5%, 1%, and

2% DME in argon respectively. Experiments were conducted at temperatures of 1300-

1600K and pressures near 1.5 atm. A direct absorption strategy with a fixed wavelength

(2193.359 cm-1) using a quantum cascade laser (QCL) was used to measure CO

concentration time histories. A mid-IR scanned-wavelength laser absorption diagnostic

with a difference frequency generation (DFG) laser near 3.43 µm was used to measure

104

CH4 concentration time histories. C2H4 was measured using a two-wavelength

absorption scheme at 10.532 µm and 10.675 µm with a CO2 laser. The mechanism of

Curran et al. [82] with a constant volume gasdynamic model was used to calculate

temperature and pressure profiles and to infer the mole fractions of CO, CH4 and C2H4.

The concentration time-histories of CO, CH4 and C2H4 were all found to be strongly

sensitive to the DME decomposition rate k1 (CH3OCH3 � CH3 + CH3O) which was

recently measured by Cook et al. [87]. This measured k1 value was incorporated into two

major DME decomposition mechanisms of Curran et al. [82] and Zhao et al. [83]. The

modified Curran et al. mechanism was found to predict the time histories of CH4 and

C2H4 significantly better than the modified Zhao et al. mechanism.

6.2 Future work

Future work could be focused on the second motivation of this dissertation: the

understanding and refining of chemical reaction models of fuel decomposition. In order

to achieve this goal, two different types of work are needed.

First, there is a need to develop laser diagnostics for the detection of other small

hydrocarbon species, such as, C2H6, C3H6 and C4H8 to get additional species time-

histories data beyond that currently acquired, of CO, CH4 and C2H4, during fuel

decomposition. Experiments were tried to measure those small hydrocarbon species in

3.4 µm region, where the fundamental C-H stretching vibrational bands are located.

However, all of these species commonly have strong absorption at this wavelength region

and the multi-wavelength strategy for multi-species in this wavelength is not sensitive

enough to differentiate those hydrocarbon species. C3H6 and C4H8 also have absorption

105

at 10.5 µm region, but their absorption spectra are not covered by the conventional CO2

laser which was used for C2H4 detection in this study. CO2 isotope lasers or other

quantum cascade lasers can possibly be built and customized to cover 10 µm region, apart

from 3.4 µm region.

Second, specific rate constants are needed for further improvement of fuel

decomposition mechanisms. For example, a direct study of the rate constants for the

reaction of DME with H atom (Rxn. (3)) and DME with methyl radical (Rxn. (4)), may

improve the DME decomposition mechanisms by reducing the number of unknown rate

constants of reactions important in decomposition.

106

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mid-ir laser-based diagnostics for hydrocarbon fuel … · at the beginning of the engine cycle....

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