lecture 12: shock tube applications with lasers...lecture 12: shock tube applications with lasers 1....

Lecture 12: Shock Tube Applications with Lasers

1. Laser Absorption Theory

2. Survey of Capabilities

3. Kinetics Applications:Rate Constant MeasurementsMulti-Species Time-Histories

4. New Species Diagnostics

1. Laser Absorption Theory• Governing Equation: Beer‐Lambert Law

/0 = exp(‐Slu () Xspecies Ptotal L)

• Quantitative absorption requires database for S, • What species have been measured?

Line LineStrength shape

I0

ILASER

L

2

2. Survey of Capabilities:Species and Wavelengths

UltravioletCH3 216 nmNO 225 nmO2 227 nmHO2 230 nmOH 306 nmNH 336 nm

Spectra‐Physics 380

VisibleCN 388 nmCH 431 nmNCO 440 nmNO2 472 nmNH2 597 nm HCO 614 nm

First use of tunable dye lasers in shock tubes (1982)

InfraredCO 2.3 mH2O 2.5 mCO2 2.7 mFuel 3.4 mNO 5.2 mC2H4 10.5 m

3

Coherent MIRATi‐Sapphire Ring Laser

Ultra‐fast lasers used to extend UV tuning range (2009)



InfraredCO 2.3 mH2O 2.5 mCO2 2.7 mFuel 3.4 mNO 5.2 mC2H4 10.5 m

4


NovaWave Mid‐IR Laser

InfraredCO 2.3 m, 4.6 mH2O 2.5 mCO2 2.7 m, 4.3 mFuel 3.4 mNO 5.2 mC2H4 10.5 m

New lasers allow simple access to mid‐IR (2007‐10)



5


InfraredCO 2.3 m, 4.6 mH2O 2.5 mCO2 2.7 m, 4.3 mFuel 3.4 mNO 5.2 mC2H4 10.5 m

How sensitive are laser absorption diagnostics in shock tubes?



6


2. Laser Absorption Yields High SensitivityRepresentative Detection Limits: Large Molecules

• Large molecules: 10‐100’s ppm

1000 1500 2000 2500 3000

0.01

0.1

1

10

100

1000CO2

CH3

C2H4M

inim

um D

etec

itivi

ty [p

pm]

Temperature [K]

1atm,15cm,1MHz

H2ONH2

7

sub‐ppmsensitivity for OH, CH, CN

2. Laser Absorption Yields High SensitivityRepresentative Detection Limits: Diatomic Molecules

• Diatomic molecules @ 1500K:‐ sub‐ppm detectivity for UV absorbers‐ ppm detectivity for IR absorbers

1000 1500 2000 2500 3000

0.01

0.1

1

10

100

1000

CO

OH

Det

ectio

n Li

mit

[ppm

]

Temperature [K]

1atm,15cm,1MHz

CH

CN

8

3. Kinetics Applications

1. Foundation Fuel KineticsOH+H2=H2O+H

2. Methyl Ester KineticsME+OH=ProductsMethyl Formate pyrolysis

3. Butanol Isomer Kineticsn‐Butanol pyrolysist‐butanol+OH using isotopic labeling

9

3.1 Foundation Fuel Kinetics:

• H + O2 = OH + O• H2O2 + M = OH + OH + M• OH + H2O2 = H2O + HO2

• OH + HO2 = H2O + O2

• HO2 + HO2 = H2O2 + O2

• CH3 + HO2 = Products • C2H4 + M = C2H2 + H2 + M• OH + H2 = H2O + H

10

• Recent Elementary Rate Constant Measurementsusing Shock Tube/Laser Absorption Methods

3.1 Foundation Fuel Kinetics

1. Design: use sensitivity analysis to kinetically isolate reactions

2. Execute: use shock tubes for step heating and laser absorption for species detection

Example: H + O2 OH + O

• Accurate k values critical to combustion modeling

k

How Do We Measure Individual Reaction Rates?- Two-Step Process

11


• H2O time‐history provides precise determination of k1

Shock Conditions: 1472 K, 1.8 atm, 0.1%O2/0.9%H2/Ar

H2O Sensitivity H2O Time‐History

12

• Example: H + O2 OH + ORate Measurement using H2O Laser at 2.55 mm


13

Arrhenius Plot: H + O2 OH + O

Arrhenius Plot: H + O2 OH + O

Modern laser methods significantly reduce measurement uncertainty!

k1=1.04x1014 exp(‐7705/T) {1=5%}

14



fast upon shock heating

TBHP: tert‐butylhydroperoxide

Motivation: Large uncertainty in kOH+H2 gives large uncertaintyin modeled H2/air flame speeds

Experimental Strategy: Direct determination of rate constant• Pseudo‐first order experiment• Fuel in excess• TBHP used as a prompt OH precursor

‐ Useful T range (850 to 1350 K) ‐ Pioneered by Bott and Cohen (1984), also used at Argonne

Acetone

+ +

15

• Elementary Reaction Rate Determination:OH + H2 H2O + H


• High SNR data, ppm sensitivity• kbest‐fit determined within 3‐5%

16

• Representative OH Absorption Data


• Very low overall uncertainty in k: ±17% (2)

kBest‐Fit

17

• Arrhenius Plot: OH + H2 H2O + H


• Excellent agreement with previous work,but uncertainty in k substantially reduced

0.4 0.6 0.8 1.0 1.21E11

1E12

1E13

2500K

kBest-Fit

833K1000K1250K

k 1 [cm

3 mol

-1 s

-1]

1000/T [1/K]

Current Study Krasnoperov and Michael (2004) Michael and Sutherland (1988) (revised Kc) Davidson et al. (1988) (revised Kc) Frank and Just (1985) Oldenborg et al. (1992)

1667K

18

• Comparison with Past Work: OH + H2 H2O + H

3.2 Methyl Ester Kinetics

1. Rate Constant Measurements– OH + Methyl Formate– OH + Methyl Acetate– OH + Methyl Propanoate– OH + Methyl Butanoate

2. Methyl Formate Pyrolysis– Multi‐species Data

Conducted as first‐order experiments, with TBHP as source of OH

MF, CH3OHCO, CH4, CH2O

19


• Low scatter data with ± 25% overall uncertainty• Good agreement with modified SAR

(Structure Activity Relationship)

0.7 0.8 0.9 1.0 1.1 1.2

1E12

1E13

833K909K1000K1111K1250K

MButanoate MPropanoate MFormate MAcetate

Lines: Modified SAR (SAR x 0.75)

k [c

c/m

ol/s

]

1000/T [1/K]

Methyl Ester + OH = Products

1429K

+/-25%

MB

MP

MA

MF

20

• Summary: OH+Methyl Esters Products


• Shock tube/laser absorption rate 2‐4x faster than calculation

• Confirms continuing value of high–accuracy experimental data

0.7 0.8 0.9 1.0 1.1 1.21E11

1E12

1E13

+/-25%

Tan et al. (2012)

833K909K1000K1111K1250K

k

[cc

/mol

/s]

1000/T [1/K]

Methyl Formate + OH = Products

1429K

Current Study

21

• Comparison with Recent Quantum Calculations:Methyl Formate +OH Products


• Laser data successfully tracks all major contributors to O‐atom balance• Atom balance provides important new validation tool for chemical models

@ t = 300s

% Oxygen balance:5.5% in CH3OCHO34.8% in CH3OH44.9% in CO5.8% in CO2

7.2% in CH2OTotal: >98%

22

22

• Methyl Formate Pyrolysis Kinetics:Multi-Species Laser Absorption Data Can ProvideNear-Complete Oxygen Balance

3.3 Butanol Isomer Kinetics

1. n‐Butanol Pyrolysis:Challenge: Complex PathwaysSolution: Multi‐Species Time‐Histories

OH, CO, CH4, H2O, C2H4

2. tert‐Butanol + OH ProductsChallenge: Multiple Reaction Sites,

OH ReformationSolution: Isotopic Labeling

23

• Two Studies of Butanol Kinetics


• Challenges: Multiple Reaction Sites, OH Reformation

• Solution: Isotopic labeling provides strategy to identify OH attack sites

24

• OH+ tert-Butanol ProductsUse of Isotopic Labeling


16

16

16

16

16 16 16

16

16

16

• Measurement of OH removal by t‐C4H9

16OH affected by OH reformation pathway

• Measurement gives

k1b+ k1a (1‐BR2)

= (k1b+k1a )(1‐BR1BR2)

t‐C4H916OH + 16OH

OH reformation! 25

• OH+ tert-Butanol ProductsConventional Experiment using t-C4H9

16OH


16

16

16

18

18

18

18

18 18

16

18

16

• Measurement of OH removal by t‐C4H9

16OH affected by OH reformation pathway

• Measurement gives

k1b+ k1a (1‐BR2)

= (k1b+k1a )(1‐BR1BR2)

• Measurement of 16OH in t‐C4H9

18OH gives (k1a+k1b) with no 16OH reformation

t‐C4H9O18H + 16OH

No 16OH reformation! 26

• OH+ tert-Butanol ProductsConventional Experiment using t-C4H9

16OH


0 20 40 60 80

10

20

30

500 ppm t-butan18ol29 ppm tbhp

Measurement 18k' = 1.08x10-11

1.218k' 0.818k'

OH

Mol

e Fr

actio

n [p

pm]

Time [s]

• Slow removal of OH during 16O butanol pyrolysis• Faster removal of OH during 18O butanol pyrolysis

• How do the rates compare?

0 20 40 60 80

5

10

15202530

OH

Mol

e Fr

actio

n [p

pm]

500 ppm t-butan16ol17 ppm tbhp

Measurement 16k' = 2.18x10-12 1.216k' 0.816k'

time [s]

16O‐Butanol 18O‐Butanol

1020 K1.22 atm

1020 K1.22 atm

27

• OH Absorption Data for tert-Butanol (O16 & O18)

ktotal = (k1a+k1b)Derived from t‐C4H9

18OH

0.8 0.9 1.0 1.1

10-12

10-11

1000K

k1a+k1b

Sarathy et al. (2012)

Net

OH

dec

ay ra

te

(cm

3 /mol

ecul

e/s)

1000/T

1250K

28

• OH + tert‐Butanol (O16 & O18)Net OH Decay Rate due to Reaction with t‐Butanol


(k1a+k1b) (1‐BR1BR2)Derived from t‐C4H9

16OH

• first direct determination of overall rate (k1a+k1b)• Ratio (18O/16O expts.) gives (1‐BR1BR2) = 0.2• Since BR1 = 0.95, then BR2 = 0.8 (±0.05)

0.8 0.9 1.0 1.1

10-12

10-11

1000K

k1a+k1b

Sarathy et al. (2012) (k1a+k1b) (1-BR1BR2)

Sarathy et al. (2012)

Net

OH

dec

ay ra

te

(cm

3 /mol

ecul

e/s)

1000/T

1250K

ktotal = (k1a+k1b)Derived from t‐C4H9

18OH

29

• OH + tert-Butanol (O16 & O18)Net OH Decay Rate due to Reaction with t-Butanol



Current Work:– Alkyl radicals, e.g., C2H5

– Alkenes, Alkynes, e.g. C3H6, C2H2

– Aldehydes, e.g., CH2O, CH3CHO

Example: Aldehydes

30


31

Motivation: Aldehydes provide critical information about first stages of hydrocarbon oxidation, especially oxygenated fuels

H

HC=O C=O

H

CH3

Challenge: Overlapping absorption spectraStrategy: Three colors (1 and 2 in IR, and 3 in UV)

1,2

IR absorption

3

Formaldehyde Acetaldehyde Reflectedshock wave

Detectors

Three Lasers

UV absorption

31

• Aldehyde Diagnostics


Measured Absorbance Time‐Histories Aldehyde Concentration Time‐Histories

• Recovered dCH2O/dt & plateau mole fractions: Success!

• Shock heat trioxane/acetaldehyde mixture• Observe formaldehyde formation• Recover correct CH2O/CH3CHO concentrations

32

• Validation Experiment: Simultaneous Measurement ofKnown CH2O & CH3CHO Concentrations

Next: Three Lectures onLaser-Induced Fluorescence (LIF)

Two level model More complex models Applications

33

Summary:Laser absorption & shock tubes are a frontier for combustion kinetics

lecture 12: shock tube applications with lasers...lecture 12: shock tube applications with lasers 1....

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