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«Copyright by Ingmar Michael Schoegl The Dissertation Committee for Ingmar Michael Schoegl certifies that this is the approved version of the ...»

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Copyright

by

Ingmar Michael Schoegl

The Dissertation Committee for Ingmar Michael Schoegl

certifies that this is the approved version of the following dissertation:

Superadiabatic Combustion in

Counter-Flow Heat Exchangers

Committee:

Janet L Ellzey, Supervisor

Ofodike A Ezekoye

John R Howell

Philip S Schmidt

Oscar Gonzalez

Superadiabatic Combustion in

Counter-Flow Heat Exchangers

by

Ingmar Michael Schoegl, Dipl.-Ing.

Dissertation Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The University of Texas at Austin May 2009 To my family.

Acknowledgments There are many people who have been tremendously helpful and inspiring to me throughout the years. First and foremost, however, I want to thank my wife, Emily Blosser, for continuing support and encouragement. Also, I want to acknowledge my children – Conrad, Sebastian and Leo – who have been an inspiration and, while sometimes not being exactly helpful regarding the progress of my work, still greatly contributed by keeping my life in balance. I am deeply indebted to my parents, Kurt Schögl and Eva Neuwirth, as well as my entire family, who instilled me with a deep interest in discovery and learning.

This work would certainly not have been possible without the skillful guidance of my dissertation supervisor, Dr. Janet L. Ellzey, who has been an invaluable mentor.

Throughout the pursuit of my PhD degree, she has granted me the trust and freedom to investigate new ideas, thus teaching me lessons that are invaluable for conducting successful research. I also want to thank my dissertation committee members – Dr.

Ofodike A. Ezekoye, Dr. Philip R. Schmidt, Dr. John S. Howell and Dr. Oscar Gonzalez – for many valuable discussions and insights. Furthermore, Dr. Kovar has given me the opportunity to extend my knowledge beyond combustion science by engaging me in work on ceramics processing.

I want to thank all of my former and present colleagues in the laboratory for company and friendship throughout my time at UT; Ravi Dhamrat, Brian Vogel, Dan Vennetti, Matt Fay, Simone Marras, Mike Dixon, Brad Hull, Doyle Motes, Dan Leahey, Ryan Newcomb, Colin Smith and Jim Mikulak all have made life in the laboratory v interesting and enjoyable by engaging me in countless discussions.

During my timein Austin, I met a great number of people whose company I truly enjoyed and many of whom I now count among my best friends. While I am unable to name them all, I want to especially thank Mirko Gamba, with whom I have spent endless coffee breaks and evenings discussing research and many other aspects of life. Also, I want to thank Georg Stadler, who is one of the few people with whom I was able to share my thoughts on the unique perspective of Austrians living in the United States.

Last, but no least, I want to acknowledge the many sources of funding that have made this endeavor possible: the Department of Mechanical Engineering for giving me the opportunity to work as a Teaching Assistant and Assistant Instructor; the Cockrell School of Engineering, John and Mary Booker as well as C. W. W. Cook for their invaluable financial support; and The University of Texas at Austin for awarding me the Continuing Fellowship that has allowed me to fully focus on my research during the final stage of my PhD work. Also, this material is based on work supported by the National Science Foundation under Grant No. 0522640.

–  –  –

Syngas, a combustible gaseous mixture of hydrogen, carbon monoxide, and other species, is a promising fuel for efficient energy conversion technologies. Syngas is produced by breaking down a primary fuel into a hydrogen-rich mixture in a process called fuel reforming. The motivation for the utilization of syngas rather than the primary fuel is that syngas can be used in energy conversion technologies that offer higher conversion efficiencies, e.g. gas turbines and fuel cells. One approach for syngas production is partial oxidation, which is an oxygen starved combustion process that does not require a catalyst. Efficient conversion to syngas occurs at high levels of oxygen depletion, resulting in mixtures that are not flammable in conventional comvii bustion applications. In non-catalytic partial oxidation, internal heat recirculation is used to increase the local reaction temperatures by transferring heat from the product stream to pre-heat the fuel/air mixture before reactions occur, thus increasing reaction rates and allowing for combustion outside the conventional flammability limits. As peak temperatures lie above the adiabatic equilibrium temperature predicted by thermodynamic calculations, the combustion regime used for non-catalytic fuel reforming is referred to as ’superadiabatic’.

Counter-flow heat exchange is an effective way to to transfer heat between adjacent channels and is used for a novel, heat-recirculating fuel reformer design. An analytical study predicts that combustion zone locations inside adjacent flow channels adjust to operating conditions, thus stabilizing the process for independent variations of flow velocities and mixture compositions. In experiments, a reactor prototype with four channels with alternating flow directions is developed and investigated. Tests with methane/air and propane/air mixtures validate the operating principle, and measurements of the resulting syngas compositions verify the feasibility of the concept for practical fuel-reformer applications. Results from a two-dimensional numerical study with detailed reaction chemistry are consistent with experimental observations. Details of the reaction zone reveal that reactions are initiated in the vicinity of the channel walls, resulting in “tulip”-shaped reaction layers. Overall, results confirm the viability of the non-catalytic reactor design for fuel reforming applications.





–  –  –

2.1 Simulation results for rich premixed methane/air combustion at constant mass flow rate ρ u.............................. 33

2.2 Parameter values used for base cases in co- and counter-flow configuration. 38

–  –  –

4.1 Grid dependence of temperature deviations as function of local refinement levels in zones upstream and downstream of the reaction layer... 111

4.2 Comparison of dry species concentrations from numerical simulations to experimental results obtained for variations of φ and uin. Experimental values are given in parentheses....................... 122 B.1 Parameter fit of Eq. B.2 for methane/air mixtures............. 134 C.1 Uncertainties for gas chromatography system and calibration gases.... 146

–  –  –

2.1 Concept of combustion in heat exchanger: heat recirculation in co-flow configuration (a) and counter-flow configuration (b)............ 19

2.2 Channel setup (illustrated for co-flow configuration)............ 20

2.3 Solution domain is divided into segments between channel inlets/outlets and combustion zones (hatched areas): combustion zone in channel 1 left of combustion zone in channel 2 (a), combustion zone in channel 1 right of combustion zone in channel 2 (b), and combustion locations coincide and middle section collapses to zero length (c).......... 25

2.4 Relation between superadiabaticity and flow rate at different equivalence ratios..................................... 34 xv Quenching behavior: (a) model comparison at φ = 1.6; (b) effect of 2.5 change in equivalence ratio.......................... 37

Position of the combustion zone in co-flow configuration at φ = 1.6:

2.6 (a) flow rate as function of axial position; (b/c) temperature profiles for fast/slow solutions at specified locations.................. 39

2.7 Impact of model parameters on combustion characteristics in co-flow configuration: variation of (a) equivalence ratio φ; (b) inlet temperature T 0 ; (c) axial conduction parameter κ; (d) gas/wall heat transfer parameter µ. Solid lines correspond to the reference case (Fig. 2.6a)......... 42

2.8 Impact of model parameters on combustion characteristics in co-flow configuration: variation of (a) reactor geometry ; (b) external heat loss factor χ; (c) temperature of the environment T ∞ ; (d) radiative heat losses from reactor faces σ. Solid lines correspond to the reference case (Fig. 2.6a)................................ 43 Temperature profiles for channels in counter-flow configuration at φ = 2.9 2.0....................................... 45

2.10 Position of the combustion zone in channel 1 for counter-flow configuration at φ = 2.0: (a) flow rate as function of axial position; (b/c) temperature profiles for fast/slow solutions at specified locations..... 47

2.11 Stabilization of the combustion zone in counter-flow configuration: (a) increasing flow rate at φ = 2; (b) increasing equivalence ratio at u1 = 5. 48

2.12 Impact of model parameters on combustion characteristics in counterow configuration: variation of (a) equivalence ratio φ; (b) inlet temperature T 0 ; (c) axial conduction parameter κ; (d) gas/wall heat transfer parameter µ. Solid lines correspond to the reference case (Fig. 2.10a).. 50

–  –  –

3.1 Cross-sections of reactor prototype with two channels in counter-flow configuration.................................. 55

3.2 Temperature profile in 2-channel reactor at equivalence ratio 1.95 and inlet velocity 104.1 cm/s........................... 57

3.3 Operating range of 2-channel reactor..................... 59

3.4 Photographs of SiC tiles removed from two-channel prototype upon completion of the tests. Top and bottom of each tile show discoloring where tiles were inserted in grooves in the side walls (Fig. 3.1b).

Broken tiles in (a) and (d) are reassembled................. 60

3.5 Fuel reformer based on counter-flow heat exchange between channels with opposing flow directions......................... 62

3.6 Photographs of SiC tiles removed from 6th out of nine revision of the four-channel design, which show silver-colored graphite deposits. Left tile in (d) shows discoloring from previous test where tile was used in a different location............................... 64

3.7 Experimental apparatus consisting of counter-flow reactor, flow conditioning system (solid connectors) and data acquisition system (dashed connectors)................................... 65

3.8 Operating range of the fuel reformer. Hatched areas indicate parameter variations discussed in Sections 3.3.2 and 3.3.3.............. 70

3.9 Photographs of flames at reactor exits during stable operation and blowoff under fuel-rich operating conditions (φ ≥2.2).............. 71 xvii

3.10 Analytical predictions of gas and wall temperature profiles for variations of equivalence ratio (a) and inlet velocity (b). Gas temperatures for channel with opposite flow direction are not shown............ 72

3.11 Experimental results for wall temperature profiles along inner and outer channels. Contour plots illustrate dependence on equivalence ratio (a) and inlet velocity (b). Hatched areas represent flow straighteners at channel inlets (see Fig. 3.5)......................... 74

3.12 Syngas composition and temperature results as function of equivalence ratio (a) and inlet velocity (b)........................ 76

3.13 Operating range of the fuel reformer for C3 H8 /air mixtures. Shaded lines indicate parameter variations conducted for emissions analysis... 78

3.14 Concentration of major syngas components CO, H2, CO2 and O2 at different probe locations at u=125 cm/s and φ=2.4: variations of xposition (a) and y-position (b) of the sampling probe............ 81

3.15 Concentration of minor syngas components C2 H2, CH4, C2 H4 and C6 H6 at different probe locations at u=125 cm/s and φ=2.4: impact of xposition (a) and y-position (b) of the sampling probe............ 83

3.16 Concentration of major syngas components CO, H2, CO2 and O2, and maximum wall temperature: variation of inlet velocity at φ=2.4 (a);

variations of equivalence ratio at u=125 cm/s (b) and u=250 cm/s (c).. 84

3.17 Concentration of minor syngas components C2 H2, CH4, C2 H4 and C6 H6 :

variation of inlet velocity at φ=2.4 (a); variations of equivalence ratio at u=125 cm/s (b) and u=250 cm/s (c)................... 87

3.18 Distribution of H2 in products: (a) variation of velocity at φ=2.4; (b) variation of equivalence ratio at u=125 cm/s................ 90

3.19 Comparison of operating range for C3 H8 /air mixtures to results for CH4. Shaded lines indicate parameter variations conducted for emissions analysis..................................... 93 xviii

3.20 Comparison of experimental results for C3 H8 to results for CH4 : (a) variation of velocity at constant φ; (b) variation of equivalence ratio at u=125 cm/s................................... 94

3.21 Conversion efficiencies: (a) variation of velocity at constant φ; (b) variation of equivalence ratio at u=125 cm/s................ 97

3.22 Distribution of chemical input energy retained by product species in reforming of methane: (a) variation of velocity at φ=2.2; (b) variation of equivalence ratio at u=125 cm/s...................... 99

3.23 Distribution of chemical input energy retained by product species in reforming of propane: (a) variation of velocity at φ=2.4; (b) variation of equivalence ratio at u=125 cm/s...................... 100

4.1 Counter-flow reactor: (a) geometry; and (b) computational domain.... 107



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