Combustion and energy transport in the micro scale

COMBUSTION AND ENERGY TRANSPORT IN THE MICRO-SCALE LI JUN NATIONAL UNIVERSITY OF SINGAPORE 2009 COMBUSTION AND ENERGY TRANSPORT IN THE MICRO-SCALE LI JUN (B.Eng, M.Eng) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 Acknowledgements I would like to extend my sincere thanks to my supervisor, Professor CHOU Siaw Kiang from the Department of Mechanical Engineering, for his invaluable guidance and constant encouragement throughout this research project. To me, he is not only a knowledgeable scholar, but also an approachable mentor and a close friend. Without his support, patience and understanding, I definitely cannot make it through this long and tough yet enjoyable journey. I also express my heartfelt gratitude to my seniors, Dr. LI Zhiwang and Dr. YANG Wenming, for their insightful suggestions which are greatly helpful for me to advance my research in the past few years. I am also grateful to the lab officers, Mr. YEO, Mr. CHEW (from Thermal Process Lab) and Mr. TAN (from Energy Conversion Lab) for their kind assistance in the micro-combustion experiments. In addition, I wish to acknowledge the support from the technical staff of the Fabrication Support Centre (FSC) and the Advanced Manufacturing Laboratory (AML). I would also like to thank Mr. WANG Junhong from the Supercomputing and Visualization Unit (SVU) of NUS Computer Center for his professional advice regarding the use of UNIX system and the CFD software Fluent®. Last but not least, I take this opportunity to express my deepest gratitude to my parents for their unfailing love, unconditional sacrifice and steadfast support which are far more than I could ever hope for. To my beloved mom and dad, I will always owe every bit of my success and happiness. Li Jun Jan 20, 2009 -I- Table of Contents Table of Contents Acknowledgements . I Table of Contents .II Summary… VI List of Tables . VIII List of Captions of Figures IX List of Symbols XIV Chapter Introduction 1.1 Motivation . 1.2 Objectives and Scope of the Present Study 1.3 Organization of the Thesis Chapter Literature Review .8 2.1 Flame Quenching – Classical Definition 2.2 Micro-Scale Combustion – Fundamental Studies 11 2.2.1 Experimental Studies . 12 2.2.2 Numerical Investigations . 15 2.2.3 Analytical Models 19 2.3 Practical Micro-Combustors . 20 2.3.1 Swiss-Roll Micro-Combustors . 21 2.3.2 Cylindrical Tubes with Backward-Facing Steps 22 2.4 Chapter Summary . 23 Chapter Numerical Modeling of Premixed Hydrogen-Air MicroCombustion .24 3.1 Introduction . 24 3.2 Numerical Model 25 3.3 Results and Discussion . 29 3.3.1 Reference Case . 29 3.3.2 Combustor Size and Geometry 31 3.3.3 Inlet Velocity Profile 34 3.3.4 Slip-Wall Boundary Condition 40 - II - Table of Contents 3.4 Conclusion 45 3.5 Chapter Summary . 46 Chapter Numerical Modeling of Premixed Methane-Air MicroCombustion .47 4.1 Introduction . 47 4.2 Numerical Model 48 4.3 Results and Discussion . 51 4.3.1 Reference Case . 51 4.3.2 Combustor Size and Geometry 53 4.3.3 Inlet Velocity Profile 58 4.3.4 Slip-Wall Boundary Condition 63 4.4 Conclusion 64 4.5 Chapter Summary . 65 Chapter 1D Flame Model to Predict Flame Temperature in a MicroCombustor 66 5.1 Introduction . 66 5.2 1D Flame Model . 67 5.3 Reaction Zone Thickness (δr) . 74 5.4 Comparison with 2D Numerical Simulation 77 5.4.1 2D Numerical Simulation 77 5.4.2 Comparison between 1D Model and 2D Simulation . 79 5.5 Conclusion 85 5.6 Chapter Summary . 85 Chapter Investigation of Cylindrical Dump Micro-Combustors – Experiments 86 6.1 Introduction . 86 6.2 Experimental Set-up and Methodology 87 6.3 Results and Discussion – Connection Tube with din=2 mm . 92 6.3.1 Mean Wall Temperature 92 6.3.2 Emitter Efficiency 94 6.3.3 Flame Stabilization 97 6.4 Results and Discussion – Connection Tube with din=1 mm . 99 6.4.1 Transient Flame Behaviors (Combustors with d=2 mm) . 100 - III - Table of Contents 6.4.2 Mean Wall Temperature 108 6.4.3 Emitter Efficiency 110 6.4.4 Flame Stabilization 112 6.5 Conclusion 113 6.6 Chapter Summary . 114 Chapter Investigation of Cylindrical Dump Micro-Combustors – Numerical Modeling and Dimensional Analysis .115 7.1 Introduction . 115 7.2 Numerical Modeling . 118 7.2.1 Numerical Model and Boundary Conditions . 118 7.2.2 Numerical Results and Comparison with Experimental Data . 120 7.2.3 Some Discussion 127 7.3 Dimensional Analysis . 129 7.3.1 Mean Wall Temperature 129 7.3.2 Emitter Efficiency 132 7.3.3 Flame Stabilization 137 7.4 Conclusion 139 7.5 Chapter Summary . 140 Chapter Experimental Results on Planar Micro-Combustors .141 8.1 Introduction . 141 8.2 Combustor Design and Experimental Set-up 142 8.3 Pre-Experiment on Ignition 147 8.4 Effects of Combustor Configuration and Channel Width 149 8.4.1 Wall Temperature 152 8.4.2 Emitter Efficiency 157 8.5 Effects of Position of Porous Media (Lpm) 160 8.5.1 Wall Temperature and Emitter Efficiency . 160 8.5.2 Preheating by Combustor Wall 162 8.6 Conclusion 166 8.7 Chapter Summary . 168 Chapter Conclusions and Recommendations for Future Work .170 9.1 Summary of the Thesis . 170 9.2 Recommendations for Future Work . 172 - IV - Table of Contents References… 174 Appendix A Experimental Results of the Wall Temperature Measurement 183 Appendix B Uncertainty of Wall Temperature Measured by the Infrared Thermometer 192 Appendix C List of Publications during Ph.D. Study 194 -V- Summary Summary Micro-scale combustion offers both theoretical and practical challenges in relation to micro power generation systems. Therefore, the understanding of combustion and energy transport in the micro-scale is key to optimizing the design and operation of micro-combustors used in micro power devices. Common challenges encountered in micro-scale combustion include flame quenching, low efficiency, material failure and robustness. In this thesis, theoretical, numerical and experimental studies on microcombustion were carried out to investigate the fundamental issues such as flame temperature, flame stabilization, heat loss and heat recirculation through the combustor wall. The developed 2D numerical model (Chapters and 4) and 1D theoretical model (Chapter 5) served as the basis for the understanding of the premixed flames and heat transport in micro-combustors. With respect to the micro-thermophotovoltaic (TPV) system developed at the National University of Singapore (NUS) in particular, cylindrical micro-combustors with backward-facing steps (referred to as ‘dump microcombustors’) represent some favorable characteristics such as good control of the flame position, when they are used as a heat source and an emitter. For this particular configuration, the effects of combustor dimensions (combustor length, combustor diameter and step height) and flow conditions (flow velocity and fuel-air equivalence ratio) on the wall temperature distribution and radiation power were experimentally investigated (Chapter 6). In order to get deeper insights into the experimental results, both numerical modeling and dimensional analysis were performed (Chapter 7). In view of the advantage of having the thermal radiation flux normal to the receiver, that is, the PV cells, a new configuration, porous media combustion in a planar microcombustor, was developed and tested. To evaluate its performance as an emitter, - VI - Summary effects of flow conditions, channel width and position of the porous media inside the planar micro-combustor on the wall temperature distribution and radiation power were experimentally studied (Chapter 8). Based on the preliminary experimental results, this configuration was shown to offer a potential as a heat source for the micro-TPV system. In summary, the present study reaffirms the feasibility and potential in exploiting combustion in the micro-scale for various heat and power applications. - VII - List of Tables List of Tables Table 1.1 Energy density of various systems Table 2.1 Experimental studies on micro-combustion in micro-combustors with simple geometries 17 Table 2.2 Numerical simulations of micro-scale combustion in micro-combustors with simple geometries . 18 Table 3.1 Gas-phase reaction mechanism for H2-air combustion . 27 Table 4.1 Comparison of fuel properties between methane and hydrogen . 47 Table 4.2 Skeletal reaction mechanism for CH4-air combustion 49 Table 5.1 Properties of 1D adiabatic CH4-air premixed flames at p=1 atm and Tu=300 K . 77 Table 5.2 Summary of cp and kg used in this chapter 80 Table 6.1 Dimensions of the cylindrical dump micro-combustors . 91 Table 6.2 Flow conditions for the experimental study . 91 Table 6.3 Summary of the transient flame behaviors upon ignition in the d=2 mm micro-combustors . 104 Table 7.1 Dimensionless parameters of the cylindrical dump micro-combustors 135 Table 7.2 Comparison of the emitter efficiency . 135 Table 8.1 Results of the pre-experiment on ignition, H=1 mm and H=1.5 mm . 150 Table B.1 Uncertainty of wall temperature measurement by using the IR thermometer (d=2 mm, L=15 mm, u0=12 m/s and Ф=0.8) 193 - VIII - References [74] K.H. 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Thermal performance of a micro-combustor for micro-gas turbine system, Energy Conversion and Management, 48, pp. 1569-1578, 2007. - 182 - Appendix A Experimental Results of the Wall Temperature Measurement Appendix A Experimental Results of the Wall Temperature Measurement1 (a) Ф=0.6 (b) Ф=0.8 (c) Ф=1.0 (d) Ф=1.2 Figure A.1 Wall temperature, d=4 mm, din=2 mm, L=10 mm The origin of the abscissa corresponds to the exit of the micro-combustors (see Figure 6.2) - 183 - Appendix A Experimental Results of the Wall Temperature Measurement (a) Ф=0.6 (b) Ф=0.8 (c) Ф=1.0 (d) Ф=1.2 Figure A.2 Wall temperature, d=4 mm, din=2 mm, L=15 mm (a) Ф=0.6 (b) Ф=0.8 - 184 - Appendix A Experimental Results of the Wall Temperature Measurement (c) Ф=1.0 (d) Ф=1.2 Figure A.3 Wall temperature, d=4 mm, din=2 mm, L=20 mm (a) Ф=0.6 (b) Ф=0.8 (c) Ф=1.0 (d) Ф=1.2 Figure A.4 Wall temperature, d=3 mm, din=2 mm, L=10 mm - 185 - Appendix A Experimental Results of the Wall Temperature Measurement (a) Ф=0.6 (b) Ф=0.8 (c) Ф=1.0 (d) Ф=1.2 Figure A.5 Wall temperature, d=3 mm, din=2 mm, L=15 mm (a) Ф=0.6 (b) Ф=0.8 - 186 - Appendix A Experimental Results of the Wall Temperature Measurement (c) Ф=1.0 (d) Ф=1.2 Figure A.6 Wall temperature, d=3 mm, din=2 mm, L=20 mm (a) Ф=0.6 (b) Ф=0.8 (c) Ф=1.0 Figure A.7 Wall temperature, d=3 mm, din=1 mm, L=10 mm - 187 - Appendix A Experimental Results of the Wall Temperature Measurement (a) Ф=0.6 (b) Ф=0.8 (c) Ф=1.0 Figure A.8 Wall temperature, d=3 mm, din=1 mm, L=15 mm (a) Ф=0.6 (b) Ф=0.8 - 188 - Appendix A Experimental Results of the Wall Temperature Measurement (c) Ф=1.0 Figure A.9 Wall temperature, d=3 mm, din=1 mm, L=20 mm (a) Ф=0.6 (b) Ф=0.8 (c) Ф=1.0 Figure A.10 Wall temperature, d=2 mm, din=1 mm, L=10 mm - 189 - Appendix A Experimental Results of the Wall Temperature Measurement (a) Ф=0.6 (b) Ф=0.8 (c) Ф=1.0 Figure A.11 Wall temperature, d=2 mm, din=1 mm, L=15 mm (a) Ф=0.6 (b) Ф=0.8 - 190 - Appendix A Experimental Results of the Wall Temperature Measurement (c) Ф=1.0 Figure A.12 Wall temperature, d=2 mm, din=1 mm, L=20 mm - 191 - Appendix B Uncertainty of Wall Temperature Measured by the Infrared Thermometer Appendix B Uncertainty of Wall Temperature Measured by the Infrared Thermometer The wall temperature of the micro-combustors (both the cylindrical and the planar) was measured by the non-contact IR thermometer which has the accuracy up to ±(0.3%T+1) K. Prior to the wall temperature measurement, the wall emissivity was determined by using a series of thermal melt crayons accurate to 1% of their predefined melting points. In this case, the uncertainty of the wall temperature actually depends on the accuracy of both the thermometer and the crayons. According to Moffat [101], when the result X is calculated from a set of variables Xi, i=1,2,3,…,N, that is X=X(X1,X2,X3,…,XN), the root-sum-square (RSS) uncertainty, δX is given by ⎡ N ⎛ ∂X ⎞ ⎤ δ X = ⎢∑ ⎜ δ Xi ⎟ ⎥ ⎢⎣ i =1 ⎝ ∂X i ⎠ ⎥⎦ 1/ (B.1) where δXi is the uncertainty of the variable Xi. Thus, the relative uncertainty of wall temperature can be written as follows, δ Tw Tw ⎡⎛ ∂T δε ⎞ ⎛ ⎞ ⎤ w = ⎢⎜ ⎟ + ⎜ 0.3% + ⎟ ⎥ Tw ⎠ ⎥ ⎢⎣⎝ ∂ε Tw ⎠ ⎝ ⎦ 1/ (B.2) where ε is the wall emissivity. From Eq. (B.2), it can be seen that the relative uncertainty is also a function of the wall temperature. A sample calculation is carried out based on the d=2 mm micro-combustor (L=15 mm) under the flow conditions of u0=12 m/s and Ф=0.8, as shown in Table B.1. - 192 - Appendix B Uncertainty of Wall Temperature Measured by the Infrared Thermometer Table B.1 Uncertainty of wall temperature measurement by using the IR thermometer (d=2 mm, L=15 mm, u0=12 m/s and Ф=0.8) Position Tw (oC) ∂Tw ∂ε δ Tw 695.5 130 1.40% 702.5 130 1.39% 712.5 130 1.38% 724 140 1.46% 735 140 1.45% 748 140 1.43% 761.5 130 1.32% 773 140 1.40% 782 140 1.38% 10 788.5 150 1.47% 11 788.5 150 1.47% 12 782.5 150 1.47% 13 760 140 1.41% 14 710.5 130 1.38% 15 641 12 1.38% Tw - 193 - Appendix C List of Publications during Ph.D. Study Appendix C List of Publications during Ph.D. Study 1. J. Li, S.K. Chou, Z.W. Li and W.M. Yang. A comparative study of H2-air premixed flame in micro combustors with different physical and boundary conditions, Combustion Theory and Modelling, 12, pp. 325-347, 2008. 2. J. Li, S.K. Chou, W.M. Yang and Z.W. Li. Experimental and numerical study of the wall temperature of cylindrical micro combustors, Journal of Micromechanics and Microengineering, 19, 015019 (11pp), 2009. 3. J. Li, S.K. Chou, Z.W. Li and W.M. Yang. Development of 1D model for the analysis of heat transport in cylindrical micro combustors, Applied Thermal Engineering, 29, pp. 1854-1863, 2009. 4. J. Li, S.K. Chou, G. Huang, W.M. Yang and Z.W. Li. Study on premixed combustion in cylindrical micro combustors: transient flame behavior and wall heat flux, Experimental Thermal and Fluid Science, 33, pp. 764-773, 2009. 5. J. Li, S.K. Chou, W.M. Yang and Z.W. Li. A numerical study on premixed micro combustion of CH4-air mixture: effects of combustor size, geometry and boundary conditions on flame temperature, Chemical Engineering Journal, 150, pp. 213-222, 2009. 6. J. Li, S.K. Chou, Z.W. Li and W.M. Yang. A potential heat source for the microthermophotovoltaic (TPV) system, Chemical Engineering Science, 64, pp. 32823289, 2009. 7. J. Li, S.K. Chou, Z.W. Li and W.M. Yang. Characterization of wall temperature and radiation power through cylindrical dump micro-combustors, Combustion and Flame, accepted for publication, 156, pp. 1587-1593, 2009. 8. J. Li, S.K. Chou, Z.W. Li and W.M. Yang. Experimental investigation of porous media combustion in a planar micro-combustor, Fuel, accepted for publication, in press. 9. J. Li, S.K. Chou, Z.W. Li and W.M. Yang. Development of 1D model to predict the flame temperature in cylindrical micro combustors, Heat Transfer Engineering, accepted for publication, in press. - 194 - Appendix C List of Publications during Ph.D. Study 10. S.K. Chou, J. Li, Z.W. Li and W.M. Yang. Microscale combustion: progress and challenges. In: The Sixteenth Australasian Fluid Mechanics Conference, 2007, Gold Coast, Australia. 11. S.K. Chou, W.M. Yang, J. Li and Z.W. Li. Heat flux through the wall of a micro combustor for heat and power applications. In: The Seventh High Temperature Air Combustion and Gasification International Symposium, 2008, Phuket, Thailand. - 195 - [...]... generators Epstein and Senturia [1] proposed the concept of the micro heat engine’ in 1997 as one solution to meet this demand Thereafter, a series of combustion- based micro power generators have been prototyped, including the micro gas-turbine [2], the micro- thermoelectric device [3] and the micro- thermophotovoltaic (TPV) system [4] Such work was mainly motivated by the fact that hydrogen and most hydrocarbon... on this point will be given in Chapter 5 Therefore, an improved model (by taking account of heat recirculation) is needed Working as the energy source and the emitter in the micro- TPV system, the cylindrical micro- combustor plays an important role in determining the electrical energy output A high and uniform wall temperature is favorable for the TPV generator The physical dimensions such as the combustor... hydrogen-air and methane-air mixtures in the micro- combustors; (2) improve the 1D flame model by including the effects of heat recirculation to predict the flame temperature in the cylindrical micro- combustors; (3) investigate the effects of the combustor dimensions and flow conditions on the wall temperature distribution of the cylindrical micro- combustors; (4) analyze the experimental results using both the. .. distance in such a manner that as the pressure increases the quenching distance decreases [23] The fourth factor is the wall temperature Increasing the wall temperature will reduce the heat loss from the flame, therefore implying larger quenching distance This explains why the classical definition of ‘quenching distance’ becomes inapplicable in the context of heated wall It is pointed out that the term... originally at the rim of the tube can travel backwards to the inside of the tube After the burner is removed, the combustion wave can exist independently, even though the inner diameter is smaller than the quenching distance Similarly, the wall-flame interaction was investigated by Ju and Xu [36] Their theoretical analysis showed that with the decrease of the channel width, there exist two distinct... detailed in the respective chapters when they are used 1.3 Organization of the Thesis This chapter gives an overview of the micro -combustion research and the motivation of the present study, followed by the objectives and scope of this thesis Chapter 2 presents a comprehensive review of the literature dealing with key issues and findings related to micro -combustion Chapter 3 details the development of the. .. at the external (non-insulated) wall, W/m2 qx Volumetric heat loss in the axial direction, W/m3 Qc Convective heat loss at the external wall within the portion ‘L’ (Figure 6.2), W Qe Energy contained in the exhaust, W Qf Heat loss through the flanges (applicable to the planar micro- combustors), W Q gw Heat exchange between the gases and combustor wall within the length of Δx, W Qin Energy input by the. .. summarizes the major contributions of the present study and proposes some recommendations for the future work -7- Chapter 2 Literature Review Chapter 2 Literature Review Ever since the concept of the micro heat engine’ was proposed, increasing interests have been spurred in developing micro power devices, mainly due to their potential for achieving higher energy density over the existing batteries The practical... Review and the Peclet number was found to be around 30-50 There are other simple models [21-23] dealing with the heat loss induced flame quenching, with the expressions of the quenching distance explicitly given A common feature of these theoretical models is that the wall temperature is assumed to be cold (~298K) in considering that the quenching process is complete in a very short time, and therefore the. .. Present Study Based on the challenges and issues described in the previous section, the overall aim of this study is to develop an in- depth understanding of combustion in the micro- scale leading to better design and control of micro- combustors for various heat and power applications Therefore, the objectives of the present study are: (1) develop a numerical model to study the premixed combustion flows of . theoretical and practical challenges in relation to micro power generation systems. Therefore, the understanding of combustion and energy transport in the micro-scale is key to optimizing the design and. COMBUSTION AND ENERGY TRANSPORT IN THE MICRO-SCALE LI JUN NATIONAL UNIVERSITY OF SINGAPORE 2009 COMBUSTION AND ENERGY TRANSPORT IN THE MICRO-SCALE. through the combustor wall. The developed 2D numerical model (Chapters 3 and 4) and 1D theoretical model (Chapter 5) served as the basis for the understanding of the premixed flames and heat transport