FLOW AND HEAT TRANSFER ANALYSIS ON

SPIRAL COUNTER FLOW HEAT

RECIRCULATING BURNER

MUHAMMAD FIRDAUS BIN OTHMAN

A project report submitted in partial fulfillment of the

requirements for the award of the degree of

Master of Engineering (Mechanical)

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

JULY 2006

iii

To my beloved wife, Nina and

newborn princess Miza

iv

ACKNOWLEDGEMENT

I would like to thank many people for their help in completing this project,

but my foremost appreciation goes to my main thesis supervisor, Dr, Mazlan Abdul

Wahid for his guidance and thoughts throughout the hardship. I am also very

thankful to my co-supervisor Dr. Kahar Osman for the helpful suggestions and

advices during the project duration. Their continuous support, words of

encouragement and wisdom has lead to the completion of such a piece.

I am also taking this opportunity to recognize the companionship and

motivation from my fellow postgraduate friends in particular, Redhwan, Fuad and

Kamaruzaman that drives me towards completing the project. Also not to forget my

office colleagues for their understanding on how important for me to do what I have

to do.

Finally, my sincere and deepest appreciation to my wife, Nina for her love,

understanding, patience and sacrifices that need to be accommodated behind such

work. To my newborn daughter, Miza, you are the limelight of my life which hope to

turn as a strength for me to pursue further in life. Also, I am grateful to my parents,

siblings and in-laws family members on their non-exhaustive support for me to

achieve goals that I have planned for. Thank you all.

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ABSTRACT

In order to establish a performance characterization of spiral counter flow

heat recirculating burner, an understanding of the chemical kinetics, heat transfer and

flow dynamics aspects of the combustor needs to be developed. Research would be

focusing on the micro-scaled square spiral counter flow configuration which is also

known as the “Swiss roll” micro-combustor with propane-air mixture as the case

study. A two-dimensional Computational Fluid Dynamics (CFD) with propane-air

premixed stoichiometric numerical based model is being adopted and focus of the

study would be in observing the thermal characteristic (i.e. heat recirculation rate) of

the combustor. This is the parameter that characterizes the preheat energy obtained

by the incoming reactants through combustion process and its high energy post-

combustion products. The performance of the combustor with respect to the thermal

characteristic is being analyzed at a range of 40<Re<1000 steady, laminar and

incompressible fluid flow velocity profile. In addition, a parametric study on

identifying the effect of different geometrical aspect ratio between channel wall

thickness to channel width is also being envisaged. Analysis has shown that a heat

recirculation method has managed to produce an excess enthalpy flame beyond the

adiabatic flame temperature which leads towards extending the limitation of the

combustion process feasibility in a miniaturization of a combustor system.

vi

ABSTRAK

Bagi mewujudkan ciri prestasi penunu aliran bertentangan yang berpusar

dengan peredaran semula haba, pemahaman dari aspek kinetik kimia, pemindahan

haba dan aliran dinamik pembakar perlu dibangunkan. Penyelidikan ini memberi

fokus terhadap penunu aliran bertentangan yang berpusar dengan peredaran semula

haba yang berbentuk segi empat dan berskala mikro yang juga dikenali sebagai

pembakar berskala mikro “Swiss roll” dengan bahan bakar campuran propana-udara

sebagai kes kajian. Kajian melibatkan model berangka pracampur stoikiomterik

propana-udara didalam dua dimensi dengan menggunakan kaedah dinamik bendalir

berkomputer (CFD) dimana fokus kajian tertumpu kepada ciri terma (kadar

peredaran haba) pembakar. Parameter ini menggambarkan tenaga prapanas yang

diperolehi oleh bahan bakar sebelum bertindak balas melalui proses pembakaran

dimana perolehan tenaga tinggi ini berpunca daripada hasil pembakaran bahan bakar

itu sendiri. Prestasi pembakar melalui ciri terma ini dianalisa pada keadaan bendalir

yang tak boleh mampat, mantap dan lamina dimana profil halaju sekitar

40<Re<1000. Selain daripada itu, kajian dalam mengenal pasti kesan perubahan

geometri diantara ketebalan dinding saluran dan lebar saluran terhadap ciri terma ini

turut dijalankan. Analisa telah membuktikan bahawa kaedah peredaran semula haba

berjaya menghasilkan lebihan entalpi yang melebihi suhu nyala adiabatik dimana ia

menjurus kepada memanjangkan had kebolehlaksanaan proses pembakaran didalam

pengecilan sistem pembakar.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION

ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES ix

LIST OF FIGURES x

LIST SYMBOLS/ABBREVIATIONS xiv

LIST OF APPENDICES xvi

1 INTRODUCTION 1 1.1 Background 1

1.2 Scope of the Study 3

2 LITERATURE REVIEW 5 2.1 “Swiss roll” Experiment by Sitzki et al. (2001) 5

2.2 Numerical “Swiss roll” Model by Chen et al. (2004) 6

2.3 Exprimental and Numerical Model by Shinoda et al. (1998)

7

2.4 “Swiss-roll” Combustor with Catalyst Experiment by Ahn et al. (2005)

9

2.5 Numerical Model by Hua et al. in Micro-scaled Combustor (2005)

10

viii

3 METHODOLOGY 13 3.1 Analysis Framework 13

3.2 Model Validation 14

3.3 Numerical Modeling 16

3.3.1 Governing Equations 17

3.3.2 FLUENT Numerical Solver 19

3.3.3 Model Geometry and Mesh 20

3.3.4 Boundary Conditions 21

3.3.5 Reaction at Combustor 23

3.3.6 Heat Recirculation Rate 23

4 RESEARCH FINDINGS 25 4.1 Validation Model 25

4.2 Heat Recirculation Rate Analysis 28

4.2.1 Effects from Inlet Velocity 29

4.2.2 Effects from Geometrical Aspect Ratio 33

5 RECOMMENDATIONS AND CONCLUSIONS 37

REFERENCES 39 Appendix A 41-46

ix

LIST OF TABLES

TABLE NO. TITLE PAGE

1.1

Propane limiting combustible properties

4

4.1 Enthalpy from variation of inlet velocity

29

4.2 Heat recirculating rate for variation of inlet velocity

30

4.3 Enthalpy from variation of aspect ratio @ Re=1000

33

4.4 Rate of heat recirculation from variation of aspect ratio @ Re=1000

33

4.5 Enthalpy from variation of aspect ratio @ Re=400

34

4.6 Rate of heat recirculation from variation of aspect ratio @ Re=40

35

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LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1

Concept of heat recirculation

2

1.2 “Swiss roll” configuration

3

2.1 Macro-scale combustor

5

2.2 Micro-scale combustor

6

2.3 Four passes of heat exchanger with one combustor pass

7

2.4 Heat recirculating rate and thermal efficiency

8

2.5 Independence of heat recirculation rate on equivalence

ratio (Re=180, l/d=7.5)

8

2.6 Dependence of heat recirculation rate on Re (Ø=0.25,

l/d=7.5)

9

2.7 Dependence of heat recirculation rate on aspect ratio

(Ø=0.25, Re=180)

9

2.8 Schematic diagram of micro-scale combustion chamber

10

xi

2.9 Distribution of hydrogen conversion rate along central axis

with various inlet diameter (D) in adiabatic wall

11

2.10 Contour of water production rate along central axis at inlet

diameter (a): 0.4mm, (b): 0.2mm, (c): 0.1mm, (d): 0.05mm

under adiabatic wall

11

2.11 Contour of temperature along central axis of the chamber

at inlet diameter (a): 0.4mm, (b): 0.2mm, (c): 0.1mm,

(d): 0.05mm under adiabatic wall

12

2.12 Temperature distribution along the outer wall of the

chamber with different wall conductivity properties

12

3.1 Case study parameter

14

3.2 Ahn et al. [5] extinction limit map for the Inconel “Swiss

roll” of propane-air mixture

16

3.3 Variation of equivalence ratio at extinction limit with Re

by Chen et al. [3]

16

3.4 Numerical model process flow in FLUENT

17

3.5 Segregated solver scheme process flow

19

3.6 Model drawing in GAMBIT

20

3.7 Meshing in GAMBIT

21

3.8 Boundary conditions parameter at the outer-most wall

21

xii

3.9 Inlet velocity boundary conditions with propane-gas

mixture ratio definition

22

3.10 Heat transfer modes in the channel

22

3.11 Sample of reaction phenomenon setup in FLUENT

23

3.12 Heat recirculation rate definition

24

4.1 Validation model results

26

4.2 Temperature profile for Re=1000

26

4.3 Temperature profile for Re=400

27

4.4 Temperature profile for Re=100

27

4.5 Temperature profile for Re=40

27

4.6 Reactant channel zones

28

4.7 Heat recirculation rate profile with respect to incoming

reactant channels

30

4.8 Combustion flame temperature of 3000K at inlet Re=40

31

4.9 Combustion flame temperature of 3660K at inlet Re=200

31

4.10 Combustion flame temperature of 3520K at inlet Re=1000

31

4.11 Shinoda et al. [4] configuration

32

xiii

4.12 Heat recirculation rate profile with respect to aspect ratio at

Re=1000

34

4.13 Heat recirculation rate profile with respect to aspect ratio at

Re=40

35

xiv

LIST OF SYMBOLS/ABBREVIATIONS

A - Surface area

CFD - Computational fluid dynamics

cp - Specific heat at constant pressure

D - Binary diffusion coefficient

D - diameter

d - Channel width

EFAB - Electrochemical fabrication

h - Coefficient of thermal convection

hfo - Enthalpy of formation

i - Species count

k - Coefficient of thermal conductivity

L - Thickness of channel wall

l - Channel length

MEMS - Micro-electro-mechanical system

m’’ - Mass flux

m’’’ - Mass production rate per unit volume

N - Number of passes

p - Pressure

q - Heat transfer rate

Re - Reynolds number

T - Temperature

t - Time

V - Velocity vector

Y - Mass fraction of species

α - Thermal radiation absorptivity

ε - Thermal radiation emissivity

xv

Ø - Equivalence ratio

ρ - Density

υ - Kinematic viscosity

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A

Sample analysis steps in FLUENT

41

CHAPTER 1

INTRODUCTION

1.1 Background

Evolution of micro-electro-mechanical system (MEMS) development has lead

to increasing demands on the essential need to search for a micro-power generation

devices that has such a high power density parameter. Battery technology currently

has a limited power density to cater for enhancement of MEMS industry power

requirement whereby it could be noted that an alkaline battery would only has a

specific energy of 0.6 kJ/g and lithium ion battery has an energy storage density of

around 1.2 kJ/g [1].

Understanding on this limitation has motivated research activities to look for an

micro-scale combustion-based power generation device which is understood that

hydrocarbon fuels has a high energy storage capacity that is typically in the range of

45 kJ/g [2]. That is why most vehicles such as automobiles, aircraft and even rocket

are relying on the combustion process to propel. Thus, taking the micro-combustor as

the alternative solution would lead to a longer operating time of the MEMS devices

or a lighter system with the same amount of power capability from those battery

solutions. In addition, since it is a refueling based system, a waste disposal will not

be an issue [3].

Based on conventional combustion theory, there is a major limitation with

regards to the geometrical miniaturization allowable factor namely the quenching

2

distance. This is due to the fact that flames will extinguish upon entering a narrow

passageway. In addition, with such miniaturization process, it would lead to a high

ratio of surface area to volume. Such a ratio value would prone for a significant heat-

loss behavior of the combustor that leads to affect the combustion stability parameter

such as the flammability limits or also known as extinction limit and the reaction rate

(i.e. ignition energy and time) for self-sustained combustion or decreases the

combustion efficiency itself.

In lieu with the limitation imposed by the combustion theory with respect to the

micro-scale combustor system, a heat recirculation mechanism is adopted as to

effectively recirculate the thermal energy from such a high temperature burnt gas

(exhaust heat) to the low temperature fresh fuel-air mixture which substantially

extend the extinction limit of the hydrocarbons[3]. It is understood that combustion

process would experience a finite heat loss that leads of not be able to achieve an

adiabatic flame temperature that governs the limitation on the combustion process

feasibility. Transferring the exhaust heat from the combustion product would

increase the reactants total enthalpy which leads to achieving excess enthalpy flame

that is beyond the adiabatic flame temperature. This excess enthalpy flame would

then extend the limitation on the flammability limit of the hydrocarbon reactants.

Figure 1.1 Concept of heat recirculation

With the energy optimization strategy is in place, a limitation on the

quenching distance has been put up to the test beyond the conventional limit where it

is known that quenching distance need to be at least larger than an adiabatic flame

3

thickness which is then dependent on the combustion reaction process. This

combustion reaction process in turn has a dependency on the thermal condition of the

chamber. Thus, a heat recirculation has substantial benefit of allowing a combustion

process to exist at a very low thermal power to attain similar combustion process

performance with the non-preheated reactants or better reaction process at similar

thermal ignition condition alternately [5].

Looking from a compact and effective heat exchanger design perspective, a

spiral, “Swiss roll” with counter flow heat recirculation configuration has been taken

by most researchers as the selected configuration to be implemented as the micro-

combustor whereby this set of heat exchanger would ensure the post-combustion

heated energy is transmitted to the very beginning of the reactants flow passage.

Figure 1.2 “Swiss roll” configuration

1.2 Scope of the Study

In characterizing the optimal design performance of a spiral counter flow heat

recirculating burner, there are three (3) major perspectives that need to be accounted

which are the chemical kinetics, the heat transfer and the flow dynamics. Based on

the past studies on “Swiss roll” configuration, a lot of effort has focused on analyzing

the chemical kinetics and associated heat transfer aspect of it. However, none has

indulged in terms of characterizing the thermal performance by observing the heat

recirculating effectiveness which leads to broaden the range of quenching distance

and equivalence ratio spectrum limit as reported from past studies. Below is the

Reactants Exhaust gas

Combustion region

4

conventional limitation on the propane chemical kinetics properties without any pre-

heating treatment:

The principle objective of the research is to study the relationship between

geometry and performance of spiral counter flow heat recirculating burner. As heat

recirculation is the main driver that established such “excess enthalpy” state to the

incoming reactants, it is the author’s motivation to study this parameter and adopt it

as the performance measurement by following an approach adopted by Shinoda et al.

[4] in characterizing the heat-recirculating ceramic burner . Thus, a fractional heat

recirculation rate is taken as the criteria that define the thermal performance of the

“Swiss roll” combustor that is being analyzed within the study rather than

temperature profile due to its non-representative in characterizing the heat

recirculation index [4]. Only one type of fuel being considered that is a propane-air

premixed mixture as to have means of validating the results with respect to the

experimental work done by Ahn et al [5]. This parameter is studied with respect to

different steady, laminar, incompressible fluid flow profile with geometrical

manipulation of the aspect ratio between wall thickness to the channel width and to

observe the extent of number of spiral loop limitation for effective heat recirculation

towards the incoming reactants. In addition, the study is limited to 2-D computational

approach with conduction, convection and radiation of heat transfer modes are being

considered. Having to understand the heat recirculation profile with respect to

different geometrical configuration (i.e. number of spiral loops and aspect ratio) with

different fluid flow properties would establish a platform that leads towards design

optimization of the combustor for future development.

Table 1.1 : Propane limiting combustible properties Flammability Limits Quenching Distance (mm)

Fuel Lean Limit

(Ø min)

Rich Limit

(Ø max)

Stoichiometric

Mass Air-Fuel Ratio

For Ø=1 Absolute

Minimum

Propane

(C3H8) 0.51 2.83 15.6 2 1.8

REFERENCES

[1] Hua, J., Wu, M. and Kumar, K. Numerical Simulation of the Combustion of

Hydrogen-air Mixture in Micro-scaled Chambers. Part I: Fundamental Study.

Chemical Engineering Sciences, 2005. 60: 3497-3506.

[2] Sitzki, L., Borer, K., Schuster, E., Ronney, P. and Wussow, S. Combustion in

Microscale Heat-recirculating Burner. The 3rd Asia-Pacific Conf. on Combustion.

June 24-27, 2001. Seoul, Korea.

[3] Chen, M. and Buckmaster, J. Modelling of Combustion and Heat Transfer in

“Swiss roll” Micro-scale Combustors. Combust. Theory Modelling, 2004. 8: 701-

720.

[4] Shinoda, M., Maihara, R., Kobayashi, N., Arai, N. and Churchill, S.W. The

Characteristics of a Heat-recirculating Ceramic Burner. Chemical Engineering

Journal, 1998. 71: 207-212.

[5] Ahn, J., Eastwood, C., Sitzki, L. and Ronney, P. Gas-phase and Catalytic

Combustion in Heat-recirculating Burners. Proc. Combust. Inst., 2005. 30: 2463-

2472.

[6] Hua, J., Wu, M. and Kumar, K. Numerical Simulation of the Combustion of

Hydrogen-air Mixture in Micro-scaled Chambers. Part II: CFD Analysis for a

Micro-combustor. Chemical Engineering Sciences, 2005. 60: 3507-3515.

[7] Turns, S.R. An Introduction to Combustion: Concepts and Applications. 2nd. ed.

Singapore: McGraw-Hill. 2000

[8] Wahid, M.A. Combustion Process. Class Notes (SMJ 5493). Universiti Teknologi

Malaysia; 2005

41

[9] Yuasa, S., Oshimi, K., Nose, H. and Tennichi, Y. Concept and Combustion

Characteristics of Ultra-micro Combustors with Premixed Flame. Proc. Combust.

Inst., 2005. 30: 2455-2462.

[10] Spadaccini, C.M., Zhang, X., Cadou, C.P., Miki, N. and Waitz, I. A. Preliminary

Development of a Hydrocarbon-fueled Catalytic Micro-combustor. Sensors and

Actuators, 2003. 103: 219-224.

[11] Shan, X.C., Wang, Z.F., Jin, Y.F., Wu, M., Hua, J., Wong, C.K. and Maeda, R.

Studies on a Micro Combustor for Gas Turbine Engines. J. Micromech.

Microeng., 2005. 15: S215-S221.

[12] Ronney, P.D. Analysis of Non-adiabatic Heat-recirculating Combustors.

Combust. and Flame, 2003. 135: 421-439.

[13] Leach, T.T. and Cadou, C.P. The Role of Structural Heat Exchange and Heat

Loss in the Design of Efficient Silicon Micro-combustors. Proc. Combust. Inst.,

2005. 30: 2337-2444.

[14] Fluent Inc. Fluent Manual. Lebanon, N.H (USA). 2003.