thermal and fluid flow analysis of swirling flameless combustion...

THERMAL AND FLUID FLOW ANALYSIS OF SWIRLING FLAMELESS

COMBUSTION

RAID ABID ALWAN

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Doctor of Philosophy (Mechanical Engineering)

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

MARCH 2016

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Dedicated to my beloved mother,

to the most precious persons in my life, my beloved wife Saja,

my sweetheart beautiful daughter Qamar and my darling son Mohammed.

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ACKNOWLEDGEMENT

Thanks to ALLAH, the Most Gracious, the Most Merciful, the Most

Bountiful who gave me the courage and patience to accomplish this research work.

Without his help and mercy, this would not have come into reality.

I would like to deeply express my gratitude for the help and support from my

supervisor, Associate Prof. Dr. Mazlan Abdul Wahid and my co-supervisor Dr.

Mohd Fairus Mohd Yasin on their fascinating guidance, encouragement, and

valuable comments throughout the research work. I was fortunate to be one of their

graduate students. Their experience and creativity gave me great profit for carving

my future career.

I want to acknowledge fellow colleagues at HiREF laboratory who have been

helpful and friendly, never fail to make the lab a pleasant place to work in. Many

thanks to Dairobi and Rosli for sharing valuable technical skills and knowledge. In

addition, special thank for their help to Prof. Dr. Arkan Al Taie and Dr. Abuelnuor. I

am grateful to the Iraqi Ministry of Electricity for the financial support and for

granting me the study leave to complete the doctoral degree.

Finally, but not the least, my greatest thanks from my heart to my family for

giving the unlimited supports and patience to complete my study. I would never ever

forget their sacrifice that they have done for me. I appreciate the sacrifice of my wife,

father, father-in-law and mother-in-law in helping me financially and morally to

finish my study.

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ABSTRACT

Flameless combustion is a novel combustion mode that is also to achieve ultra-low emissions of NOx and CO while producing a uniform temperature distribution and a stable combustion. In this work, a newly designed Internally Preheated Swirling Flameless Combustor (IPSFC) has been developed at the High Speed Reacting Flow Laboratory (HiREF), Faculty of Mechanical Engineering, Universiti Teknologi Malaysia (UTM) to achieve high performance combustion and low emission. The study examines the sequential development of a low emission swirling flameless vortex combustor operating from a thermal load of 7 kW to 15 kW. The swirling flameless combustor has been modified to include preheating where the fresh air passes through a helical tube that is fixed inside the combustion chamber before being injected into the flameless combustor for some experiments. The objective of this study is to investigate in detail the role of air inlet geometry with and without air preheating on the performance of the swirling flameless combustion without the addition of diluted gas. Investigation on the effect of multiple air–fuel injection configuration found that the case of SFR42 to be the best configuration for optimum flameless combustion performance. SFR42 is a swirling combustor with 4 inlets of tangential air and 12 inlets axial air with 11 inlets coaxial fuel. The lowest NOx and CO emissions are observed at the equivalence ratio of 0.8 with the value of 4 ppm and 24 ppm, respectively. In general temperature uniformity which is an important characteristic of flameless combustion is observed to vary from 0.03 to 0.06 at the different equivalence ratio. This work also demonstrated the achievement of swirling flameless combustion with and without preheated tangential air. Overall, preheated air has contributed to the increase of 5% thermal efficiency compared to the non-preheated case at the expense of 4 ppm maximum increment of NOx emission. In this thesis some simulation study is also performed to investigate the detail flow field inside the swirl combustor. The numerical investigation confirms the experimental finding on the outstanding performance of SFR42 configuration. It is found that in this configuration the bulk swirling motion was produced in the combustor for good mixing between fuel and oxidizer which in turn lead to complete combustion at low peak temperature. This results in the combustion process with low emission.

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ABSTRAK

Pembakaran tanpa api adalah mod pembakaran baru yang boleh menghasilkan

pelepasan NOx dan CO rendah di samping mendapatkan taburan suhu yang seragam dan pembakaran yang stabil. Dalam kajian ini, Internally Preheated Swirling Flameless Combustor (IPSFC) telah direka dan dibangunkan di Makmal Aliran Tidakbalas Berkelajuan Tinggi (HiREF), Fakulti Kejuruteraan Mekanikal, Universiti Teknologi Malaysia (UTM) bagi menghasilkan pembakaran yang berprestasi tinggi dengan pelepasan emisi yang rendah. Kajian ini memfokuskan pelepasan pembakar berpusar yang rendah beroperasi dari beban haba 7 kW kepada 15 kW. Pembakar berpusar tanpa api telah diubahsuai untuk prapemanasan di mana udara segar melalui satu tiub heliks yang diletakkan di dalam kebuk pembakaran lalu memanaskan udara tersebut sebelum dimasukan kedalam kebuk pembakaran dalam beberapa eksperiment. Objektif kajian ini adalah untuk menyiasat secara terperinci peranan geometri salur masuk udara dengan dan tanpa prapemanasan udara ke atas prestasi pembakaran berpusar tanpa api tanpa penambahan gas sebagai pencair. Siasatan ke atas kesan konfigurasi suntikan berganda udara-bahan api mendapati bahawa kes SFR42 adalah konfigurasi terbaik untuk prestasi pembakaran optimum. SFR42 adalah pembakar berpusar dengan 4 salur masuk udara tangen dan 12 salur masuk udara paksi dengan 11 salur masuk bahan api sepaksi. Pelepasan NOx dan CO paling rendah direkodkan pada nisbah kesetaraan 0.8 dengan masing-masing bernilai 4 ppm dan 24 ppm. Secara umum keseragaman suhu yang merupakan satu ciri penting dalam pembakaran tanpa api didapati berubah dari 0.03-0.06 pada nisbah yang kesetaran berlainan. Kajian ini juga menunjukkan pembakaran berpusar tanpa api dengan dan tanpa udara tangen yang diprapanaskan telah dicapai. Secara keseluruhan, udara yang diprapanaskan telah menyumbang kepada peningkatan sebanyak 5% kecekapan haba berbanding dengan kes udara yang tidak diprapanaskan tetapi dengan peningkatan pelepasan NOx maksima settingi 4 ppm. Dalam tesis ini beberapa kajian simulasi dijalankan untuk menyiasat medan aliran terperinci dalam pembakar pusaran. Kajian numerikal didapati menghalkan penemuan yang hampir sama dalam mempastkan prestasi tinggi yang dicapai oleh eksperiment pada konfigurasi SFR42. Dalam konfigurasi ini sebahagian besar gerakan berpusar dihasilkan dalam pembakar dan ini membantu pencampuran yang baik antara bahan api dan pengoksida yang seterusnya membawa kepada pembakaran lengkap pada suhu puncak rendah. Keadaan tersebut seterusnya menyebabkan proses pembakaran berlaku dengan pelepasan emisi yang rendah.

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TABLE OF CONTENTS

CHAPTER

1

TITLE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS

LIST OF SYMBOLS

LIST OF APPENDICES

INTRODUCTION

1.1 Research background

1.2 Problem Statement

1.3 Research Objectives

1.5 Significance of Research

1.6 Research Scope

1.7 Thesis Outline

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5

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2

LITERATURE REVIEW

2.1 Introduction

2.2 Pollutant Formation

2.2.1 Nitrogen Oxides Formation

2.2.1.1 Thermal (Zeldovich) Mechanism

2.2.1.2 Prompt (Fenimore Mechanism)

2.2.1.3 N2O Intermediate Mechanism

2.2.1.4 Fuel NO

2.2.1.5 Fuel NO2 Formation

2.2.2 CO Formation

2.3 NOx Abatement Strategies

2.3.1 Injection of Diluents

2.3.2 Exhaust Gas Recirculation (EGR)

2.3.3 Exhaust Clean-up

2.3.4 Lean Premixed Combustion

2.3.5 The Swirling Generation

2.3.5.1 Turbulent Vortex Flow

2.3.6 Asymmetric Vortex Combustion

2.4 Flameless Combustion

2.4.1 Development of Flameless Combustio

2.4.2 Principles of Flameless Combustion

2.4.3 Flameless Combustion Investigation by

Various Researchers

2.4.4 Swirling Flameless Combustor Considerations

2.4.4.1 Reactive Gas Recirculation

2.4.4.2 Swirling Flows

2.4.4.3 Thermal Intensity

2.4.5 Benefits of flameless Combustion

2.4.5.1 NOx Reduction

2.4.5.2 Energy Saving

2.4.5.3 CO2 Reduction

2.4.5.4 Noise Reduction

2.4.6 Preheated Air in NOx Emission under

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3

4

Flameless Combustion

2.4.7 Effect of Diluent Gas

2.5 Numerical Studies in Flameless Combustion

2.6 Summary

RESEARCH METHODOLOGY

3.1 Introduction

3.2 Research Methodology Flowchart

3.3 Experimental Setup of Swirling Flameless Combustion

System

3.3.1 Development of Asymmetric Swirling


3.3. 2 The Combustion Air Supply System

3.3.3 The Fuel System

3.3.4 Experimental Measurement

3.3.4.1 Flow Measurement

3.3.4.2 Temperature Measurement

3.3.4.3 Exhaust Gas Measurement

3.3.5 Combustion Observation

3.4 Cases Investigated

3.5 Thermal Efficiency

3.6 Combustible Mixture and Preparation

3.7 Experimental Procedure

3.8 Computational Study of Swirling Flameless

3.8.1 Grid Independence Test

3.8.2 Computational Cases of Vortex , Swirl and

Swirling Flameless Combustion

RESULTS AND DISCUSSION

4.1 Introduction

4.2 Experimental Swirling Combustion

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4.2.1 Effect of Air and Fuel Entry on Swirling

Combustion

4.2.2 Effect of Axial Air Flow on Swirling

Combustor

4.2.3 Validation of the Computational Study on

Swirling Combustion

4.2.3.1 Temperature Distribution

4.2.3.2 Velocity and Swirl Number

4.3 Swirling Flameless Combustion

4.3.1 Effect of Axial to Tangential Air on Swirling


4.3.1.1 Temperature Distribution and

Exhaust Emissions


4.3.2 Effect of Location Axial Air Entry on

Swirling Flameless Combustion

4.3.2.1 Temperature Distribution

4.3.2.2 Exhaust Gas Analysis

4.3.2.3 Computational Study on the Effect

of Axial Air Entry on Swirling


4.3.3 Effect of Preheat Tangential Air on Swirling


4.3.3.1 Temperature Distribution and

Exhaust Emissions


4.3.4 Effect of Fuel Type on Swirling Flameless

Combustion

4.4 Visual Observation

4.5 Thermal Efficiency

CONCLUSIONS AND RECOMMENDATIONS FOR

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FUTURE RESEARCH

5.1 Conclusion

5.2 Suggestions for Further Study

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REFERENCES

APPENDICES A-C

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168-177

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LIST OF TABLES

TABLE NO

2.1

2.2

3.1

3.2

3.3

3.4

3.5

3.6

3.7

4.1

4.2

4.3

4.4

4.5

4.6

TITLE

Various names given to flameless combustion

Contribution to total NO form different mechanisms

Natural gas composition

Nominal accuracies for gas analyzer

Experimental parameters used to investigate swirling

combustion.

Experimental parameters used to investigate swirling

flameless combustion.

Boundary conditions

Numerical parameters for investigating vortex and

swirling combustion.

Numerical parameters for investigating Swirling


Different cases of asymmetric vortex depending on the

location of air and fuel entry.

Asymmetric and swirling vortex combustion cases.

Recirculation factor with different cases SA4F4,

SA4F10, and SA10F11

Different cases depend on the ratio of axial to tangential

air entry

Recirculation factor with different cases SFR4, SFR43,

and SFR41

Different cases at varying axial air locations with

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4.7

4.8

4.9

4.10

4.11

equivalence ratio (= 0.5-1.2).

Temperature uniformity ratio for different cases

(different axial air location).

Recirculation factor with different cases SFR4, SFR2,

and SFR42.

Different cases with and without preheated tangential air

at equivalence ratio (0.5-1.2).

NOX measured of flameless combustion with various

configurations combustion.

Summary of the results of thermal efficiency at

equivalence ratio = 1.0.

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LIST OF FIGURES

FIGURE NO.

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

2.10

2.11

2.12

2.13

2.14

2.15

TITLE

NOx formation

NOx abatment strategies

Internal EGR system

External EGR system

Lean premixed combustor

Tangential entry swirling generator

Guide vanes swirling generator

Jet flow (a) low swirl (Sn˂ 0.4) and (b) high swirl

(Sn˃0.6)

Qualitative representation of typical profiles of (a)

tangential and (b) axial velocities in a confined vortex

flow possessing central recirculation zone.

Schematic of the asymmetric vortex combustor reported

by Gabler

Schematic of the asymmetric vortex combustor reported

by Saqr

Center recirculation zone (CRZ)

Photographs of the reaction zone in the HiREF flameless

furnace, (a) represent visible flame and (b) flameless

modes

Flameless oxidation burner

The stability limits are A. stable flame zone; B. instability

zone; C. flameless combustion zone

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2.16

2.17

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.1

4.2

4.3

4.4

4.5

Shown are (a) Different combustor geometries, (b) NOx

emissions for different combustors, and (c) CO emissions

for different combustors

Temperature of the FLOX combustor with time

Flowchart of the research methodology

Experimental set-up for swirling flameless combustion

Schematic of the IPSFC combustor showing (a) isometric

view, and (b) side view.

Internally air preheated system

Shown are locations of air and fuel in swirling combustor

for (a) case SA4F4, (b) case SA6F4 , (c) case SA8F4 , (d)

case SA4F10, (e) case SA6F10, (f) case SA8F10 and (g)

case SA10F11.

Shown are locations of air and fuel in swirling flameless

combustor for (a) case SFR4, (b) case SFR2 and (c) case

SFR42

Schematic of the swirling flameless combustor isometric

view.

Grid independence test.

Computational domain of swirling vortex combustor

Temperature distribution along the central axis of

chamber at different inlet conditions of asymmetric

vortex combustor

Average NOx emission at different inlet conditions for

asymmetric vortex combustor at stoichiometric

equivalence ratio.

Average oxygen concentration at different inlet

conditions for asymmetric vortex combustor at

stoichiometric equivalence ratio.

Average CO emission at different inlet conditions for

asymmetric vortex combustor at stoichiometric

equivalence ratio.

Temperature distribution along the central axis of

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4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

chamber for cases SA4F4, SA4F10 and SA10F11 in

swirling combustion.

Effects of the equivalence ratio on NOx emissions for

cases SA4F4, SA4F10 and SA10F11 in swirling

combustion.

Effects of the equivalence ratio on O2 concentrations for


combustion.

Effects of the equivalence ratio on CO emissions for


combustion.

Comparison of experimental and numerical data on

temperature distribution along the central axis.

Temperature Contour on plane (A) and plane (B) with

iso-surfaces of the maximum temperature in the domain

to represent flame structure for Cases SA4F4, SA4F10

and SA10F11.

The contours of velocity distribution in the chamber for

Cases SA4F4, SA4F10, and SA10F10: left on plane (A)

and right on plane (B).

The velocity vector fields in the chamber for Cases

SA4F4, SA4F10, and SA10F10: left on plane (A) and

right on plane (B).

Swirl number variation along the central axis.

Chamber temperature as a function of time from

conventional combustion (A) to flameless combustion (B)

for Φ =1 in swirling flameless combustion of case SFR4.

Temperature distribution along the central axis in cases

SFR4, SFR41 and SFR42 at stoichiometric equivalence

ratio.

Effect of axial to tangential air ratio on NOx emissions

for swirling flameless combustion for cases SFR41,

SFR4, and SFR43 at different equivalence ratios.

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105

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4.17

4.18

4.19

4.20

4.21

4.22

4.23

4.24

4.25

4.26

4.27

4.28

4.29

4.30

Effect of axial to tangential air ratio on CO emissions for

swirling flameless combustion for case SFR43, SFR4,

and SFR41 at different equivalence ratios.

Effects of the equivalence ratio on NOx emissions for

swirling flameless combustion Case SFR4 and swirling

combustion Case SA10F11.


swirling flameless combustion Case SFR4 and swirling

combustion Case SA10F11.

Predicted tangential velocity profiles at different axial

distances with case SFR4.





Swirl number variation along the central axis for cases

SFR4, SFR43, and SFR41.


Cases SFR4, SFR43, and SFR41: left to right on plane

(A), (B), and (C), respectively.

The contours of vector velocity distribution in the

chamber for Cases SFR4, SFR43, and SFR41: left to right

on plane (A), (B), and (C), respectively.

Temperature distribution along the central axis at

different equivalence ratios in case SFR4.






stoichiometric equivalence ratio for different cases SFR4,

SFR2, and SFR42.

Effects of the equivalence ratio on NOX emissions for

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4.31

4.32

4.33

4.34

4.35

4.36

4.37

4.38

4.39

4.40

4.41

4.42

4.43

swirling flameless combustion for case SFR4, SFR2, and

SFR42.

O2 concentrations emissions with equivalence ratio for

swirling flameless combustion for cases SFR4, SFR2, and

SFR42.

Effects of the equivalence ratio on CO2 emissions for


SFR42.



SFR42.

Comparison of experimental and numerical data on

temperature distribution along the central axis.

The contours of temperature distribution along the central

axis distribution in the chamber for cases SFR4, SFR2,

and SFR42: left on plane (A) and right on plane (B).

(a) The contours of O2 mass fraction and (b) CO2 mass

fraction distribution along the plane A of the chamber for

cases SFR4, SFR2, and SFR42, respectively.

(a) The contours of NOx mass fraction and (b) CO mass

fraction distribution along the plane A of the chamber for

cases SFR4, SFR2, and SFR42, respectively.





Swirl number variation along the central axis


Cases SFR4, SFR2, and SFR42: left to right on plane (A),

(B), and (C), respectively.

The velocity of vortex core region in the chamber for

cases SFR4, SFR2, and SFR42

The velocity streamline fields in the chamber for Cases

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4.44

4.45

4.46

4.47

4.48

4.49

4.50

4.51

4.52

4.53

SFR4, SFR2, and SFR42: left to right on isometric, front

view and top view respectively.

The axial velocity fields in the chamber for Cases SFR4,

SFR2, and SFR42: left to right on plane (A), (B) and (C)

respectively.

Temperature distribution along the central axis for case

with and without preheat tangential air at stoichiometric

equivalence ratio.

Average NOX concentrations plotted against equivalence

ratio for cases with and without preheated tangential air,

PSFR4 and SFR4, respectively.



PSFR2 and SFR2, respectively




Average CO concentrations plotted against equivalence










distances, (a) for case PSFR4 (with preheated tangential

air) and (b) for case SFR4 (without preheated tangential

air).


distances (a) for case PSFR2 (with preheated tangential

air) and (b) for case SFR2 (without preheated tangential

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4.55

4.56

4.57

4.58

4.59

4.60

4.61

4.62

air).


distances with (a) for case PSFR42 (with preheated

tangential air) and (b) for case SFR42 (without preheated

tangential air).

Swirl number variation along the central axis for cases

SFR4, SFR2, and SFR42 (without preheated tangential

air) and cases PSFR4, PSFR2, and PSFR42 (with

preheated tangential air) at stoichiometric equivalence

ratio.

The velocity of vortex core region in the chamber for

Cases SFR4, SFR2, and SFR42 (without preheated

tangential air) and cases PSFR4, PSFR2, and PSFR42

(with preheated tangential air) at stoichiometric

equivalence ratio.


stoichiometric equivalence ratio.


ratio with different types of fuel.

Average O2 concentrations plotted against equivalence




Photographs of propane flames at different O2

concentrations.

Photographs of natural gas flames at different O2

concentrations

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LIST OF ABBREVIATIONS

NOX - Nitrogen Oxide

CO - Carbon Monoxide

CO2 - Carbon Dioxide

N2O - Nitrous Oxide

O2 - Oxygen

N2 - Nitrogen

LPG - Liquefied Petroleum Gas

NG - Natural Gas

CH4 - Methane

ppm - Part Per Million

CFD - Computational Fluid Dynamics

FLOX - Flameless Oxidation

HiTAC - High Temperature Air Combustion

MILD - Moderate or Intense Low Oxygen Dilution

LNI - Low NOX Injection

FODI - Fuel/Oxidant Direct Injection

IPFC - Internal Preheated Flameless Combustion

COSTAIR - Continuous Staged Air Combustion

EGR - Exhaust Gas Recirculation

Rtu - Temperature Uniformity Ratio

LHV - Lower Heating Value

HRS - High Cycle Regenerative Combustion System

CGRI - Canadian Gas Research Institute Burner

IFRF - International Flame Research Foundation

SCFH - Standard Cubic Feet Per Hour

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LIST OF SYMBOLS

T - Temperature (K ,° C)

- Equivalence ratio

MF - Mass flowrate of fuel

MA - Mass flowrate of air

ME - Mass flowrate of exhaust gas

- Mass flowrate

Ta - Air temperature

- Average temperature within the combustion chamber

Qin - Input power

Qout - Output power

- Thermal efficiency

Cp - Higher specific heat ∆T - Difference of temperature between two points

HF - Final enthalpy

HO - Initial enthalpy A,a - Air

F,f - Fuel

Ø - Diameter

ρ - Density v - Volume

xxiii

LIST OF APPENDICES

APPENDIX

A

B

C

TITLE

Experimental procedure for the flameless combustion

Calibration sheet of apparatus

List of publications

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CHAPTER 1

INTRODUCTION

1.1 Research Background

Heat and power, which are integral parts of our daily lives, are generally

produced via combustion processes. Excessive heat production and elevated global

warming are consequences of the process. Global warming is a topic of great

importance. According to the Intergovernmental Panel on Climate Change, averaged

over all land and ocean surfaces, temperatures warmed roughly 0.85 degrees Celsius

from 1880 to 2012 (IPCC 2013). Due to the gradual increase in the average

temperature of the earth, a permanent change of the earth's climate is expected. This

change poses a significant threat to human civilization. Combustion of fossil fuels

which is the primary cause of global warming. Fossil fuels are hydrocarbons, primarily

coal, fuel oil or natural gas, formed from the remains of dead plants and animals. The

fossil fuels is the main source of energy production in the world. It is related to the

emissions of potential pollutants and green-house gases. Hence, reducing combustion

emissions and improving the thermal efficiency of combustion systems are critical

challenges in designing thermal energy and power systems. Several combustion

techniques were developed to reduce emissions of pollutant gases.

2

Flameless combustion is one of these techniques. The main feature of flameless

combustion is the absence high temperature of flame front. The reaction takes place

uniformly throughout the combustion chamber well below the dissociation

temperature of N2, hence minimizing NOx formation. This form of combustion

features a low concentration of oxygen around 5% (Lezcano 2013). Therefore, the

ignition process of flameless combustion is characterized by slower chemical reaction

rates, uniform temperature distribution, wider reaction zones, and invisible flame. The

main operation principle for this technique lies in the concept of exhaust gas and heat

recirculation. The heat from the exhaust gases is used to raise the temperature of the

oxidant stream while, the exhaust gases are used to dilute the oxidant stream and hence,

reduce the oxygen concentration to maintain low temperature in the combustion zone.

As a result of this technique, less NO is formed (Dally, Riesmeier et al. 2004). The

name flameless refers to a negligible visible signature from the flames as compared to

conventional ones.

The recirculation of flue gas means that combustion products are recirculated

and mixed with fresh fuel and air streams. It is a key parameter in flameless

combustion. Recirculation is divided into two categories: internal and external. The

former category depends on burner design, while the latter is based upon the returned

flue gas to the combustor by an external pipe. In the internal, the flue gases are

circulated back to the combustion due to the burner aerodynamics. Recirculation and

superior mixing of air and fuel are highly significant in combustion processes.

Common procedures are used to create the recirculation and stabilization of the

combustion during swirl flow. This plays a critical role to recirculate the section of the

hot combustion product back toward the flame origin. Several approaches are used for

that process; the generation of swirling combustion by a tangential air entry in a

cylindrical combustor is one of them.

Swirling flameless combustion is employed in a direct injection of both air and

fuel, without any need for a flame stabilizer. To generate an auto swirling process, the

air is injected tangentially and axially to impart swirling without the use of swirlers

that are typically used in conventional combustors. Benefits of flameless combustion

3

technology are the ultra-low pollutant emissions, homogeneous temperature inside the

combustion chamber, and stable combustion.

Flameless is a promising combustion technology that can achieve the

combination of high efficiency and ultra-low emissions. It is based on mixing of fuel

and oxidizer and high flue gas recirculation. The separated fuel has high momentum,

and air flows entrain the flue gas through internal recirculation. Thus, the oxygen

concentration in the combustion zone is diluted. This leads to a more distributed heat

release rate of the chemical energy, thus avoiding the high peak temperatures and

reducing the pollutant emissions.

1.2 Problem Statement

In the nowadays strict emission regulations, environmental issues of power

generation play an important role in the economic viability of modern power plants.

To reduce harmful emissions, the current trend is to design industrial combustion

devices that operate with high efficiency and low emissions. One of these harmful

emissions is NOx from combustion processes. One of the most effective method of

reducing NOx is the design and implementation of flameless combustion. To achieve

flameless combustion, there are two fundamental requirements to be achieved

(Wünning and Wünning 1997, Lille, Blasiak et al. 2005).

1. The combustion temperature within the chamber should be greater than the

auto ignition temperature of the mixture (E.g., 800 °C for natural gas/air)

(Wünning and Wünning 1997, Cavigiolo, Galbiati et al. 2003).

2. The flue-gas recirculation ratio (Kv) between fuel, oxidizer and diluted gas

(flue gas, N2 or CO2) should be greater than ‘three’ (>3) (Wünning and

Wünning 1997, Flamme 2001).

4

A key point necessary for efficient design of a flameless combustor is to ensure good

mixing between the incoming fresh fuel/air mixture and the re-circulated hot burnt

gases. Adequate and fast mixing between the injected air and the internally re-

circulated hot reactive gases to form hot and diluted oxidant is critical for flameless

combustion, followed by rapid mixing with fuel. Many researchers used diluted gases

like CO2 and/or N2 that are injected into the combustion chamber to achieve high

circulation required for flameless combustion. Swirl is a phenomenon known to help

combustion. Many researchers used vanes or tangential entry to generate swirl. Some

researcher proposed introduction of swirl with tangential entry using asymmetric

vortex combustor. In this study asymmetric vortex combustion (AVC) concept is used

as the basic design for the flameless combustion system (Saqr 2011). However the

(AVC) by Saqr has some problem regarding the low temperature region near the

center. In vortex combustion, where there are no axial air such as in AVC, the process

of flame stabilization and mixing in AVC is concentrated near the circumferential wall

of the combustor. The temperature at the center of the combustor is to be relatively

low compared with wall combustor. This is justified by the presence of CRZ which

entrains ambient air into the combustor (Saqr 2011). This situation is not good for

flameless combustion. The axial component of the reacting flow within the combustor

is essential, and this is normally achieved by introducing a swirl motion generated by

the interaction between axial and tangential air velocity components.

This study focuses on a new combustor that solves the problem by achieving a

swirling flameless combustion with high recirculation, and without gas dilution. A

modified design is proposed for a flameless combustor with a high recirculation for

fast mixing. This combustor configuration adopts the previous asymmetric vortex

geometry proposed by Saqr (Saqr 2011). The concept of vortex flame provides flame

stability by stabilizing the reaction zone on the boundary of a forced vortex field, which

allows rapid mixing between air and fuel upstream of the reaction zone. Therefore, the

vortex flame demonstrates the visual characteristics of a premixed flame, although it

is a non-premixed flame. Therefore, the stability is radically enhanced while avoiding

the typical drawbacks of premixed flames.

5

This involves the investigation of the effects of burner geometry, air preheating

and swirling on flameless combustion efficiency as well. Current research focuses

also on the development of combustion technology, aiming at reducing NOx emission

while increasing combustion efficiency.

1.3 Research Objectives

This study focuses on the newly designed asymmetric swirling flow

combustion system. The objectives of this present work are:

1. to analyze experimentally the effects of multiple air-fuel injection

configurations on swirling flameless.

2. to evaluate the effect of air preheating on thermal efficiency of

swirling flameless combustion.

3. to investigate numerically the gas recirculation and fuel-air mixing

characteristics for different flow configuration swirling flameless

combustion.

1.4 Significance of Research

In this study a new design of a flameless combustor called the Internally

Preheated Swirling Flameless Combustor (IPSFC) has been proposed, developed,

fabricated and investigated. This study has practical applications in production

industries in general. It also has a significant contribution in solving the problem of

global warming and reduction of depletion of ozone layer. It also contributes in

supporting the relevant industries such as gas turbines, to obtain more efficient

combustion process. A new combustor configuration is designed to achieve a swirling

6

flameless combustion without the addition of gas dilution (e.g N2, CO2 etc.). To the

best of author’s knowledge, the present study is the first study of its kind, in swirling

flameless combustion.

1.5 Research Scope

The research scope covers the design and manufacture of a laboratory scale

flameless combustor called HiREF Internally Preheated Swirling Flameless

Combustor (IPSFC-HiREF). Computational and experimental approaches are used in

this research. The experimental component is focused on the characteristics of swirling

flameless combustion under different locations of axial air inlets. Three types of

hydrocarbon gases are utilized: natural gas, propane, and diluted methane. Nitrogen

oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2) emissions, and oxygen

(O2) concentrations are measured in the exhaust gas during the experiments. The effect

of axial to tangential air ratio induction on the swirling flameless combustion

performance for natural gas is studied. Effects of the equivalence ratios on the pollutant

emissions are determined. These equivalence ratios are in the range between 0.5 and

1.2. The effect of diluted methane (CO2 diluted) on the swirling flameless combustion

process is also investigated. Numerical investigations are performed to calculate the

recirculation ratio and swirling number of the combustor in each case study. In the

study, swirling flameless combustion was investigated for two cases: with and without

tangential air preheating.

7

1.6 Thesis Outline

This thesis consists of five chapters. Chapter one introduces the present study.

Chapter two is a twofold literature review covering: (i) pollutant emissions with an

emphasis on nitrogen oxides (NOx), and (ii) a review of past and current studies on

flameless combustion. In chapter three, the design criteria for swirling flameless

combustion are listed with emphasis on reactive gas recirculation and recirculation

enhancement, swirl generation, and mixing fuel and air. Chapter 3 also presents a

discussion of the equipment, as well as the methodology employed in all phases in the

present study. A thorough discussion of the results then follows in chapter four. The

thesis finishes with chapter five, which contains the conclusion that precedes

recommendations for future research based on our findings.

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