Study A of Plasma Ignition Enhancement for Aeroramp Injectors in Supersonic Combustion Applications
by
Scott D. Gallimore
Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University In partial fulfillment of the requirements for the degree of DOCTOR of PHILOSOPHY in Mechanical Engineering
APPROVED: _____Dr. Walter F. OBrien_____ Dr. Walter F. OBrien
Dr. Joseph A. Schetz
Dr. Joseph A. Schetz
Dr. Robert J. Mahan
Dr. Robert J. Mahan
Dr. Uri Vandsburger
Dr. Uri Vandsburger
Dr. Lourdes Q. Maurice
Dr. Lourdes Q. Maurice
May 2001 Blacksburg, Virginia
A Study of Plasma Ignition Enhancement for Aeroramp Injectors in Supersonic Combustion Applications
by Scott D. Gallimore Committee Chair: Dr. Walter F. O'Brien Mechanical Engineering Department
(ABSTRACT)
The main goal of this project was to investigate the mixing and chemical phenomena associated with the integration of a low-power, uncooled plasma torch into a fuel injector array. The potential application was for an integrated scramjet igniter/injector, with the hope of producing superior mixing and flameholding performance for supersonic combustion applications. To create a knowledge base for integration, several key investigations were made of the anode material, anode geometry, and spectrographic analysis of different light hydrocarbon fuels and inert feedstocks, all aimed at increasing the ignition potential of the plasma torch. Investigations of the anode material demonstrated the molybdenum provided longer lifetimes than either pure copper or tungsten-copper anodes. In addition, geometric studies of the anode revealed that anodes with short constrictor lengths and sonic exit nozzles provided superior ignition performance based on higher transfer rates of thermal energy from the arc to the feedstock. This resulted in the production of higher hydrogen atom
concentrations within the plasma jet. Spectrographic observation of the plasma jets revealed that methane, ethylene, propylene, and propane plasmas all contain excited atomic hydrogen, a radical known to participate in important chain-branching combustion reactions. Based on the knowledge gained, and encouraging results, a candidate scramjet igniter and flameholder was designed. The design was observed to exhibit a synergistic effect between the plasma igniter and fuel injector in that the fuel injector provides not only a subsonic region for plasma ignition, but also lifts the combustion enhancing radicals out into the fuel-air stream by means of counter-rotating vortices. Furthermore, under the conditions tested, increases in plasma torch power produced an exponential increase in the intensity of downstream products, indicating an enhancement effect. Based upon these observations, the integrated igniter/injector design is expected to perform well in supersonic combustion applications.
Acknowledgements
First thanks must go to God, for getting me through this project with only minor psychological damage. His hand was involved daily. Without His support I would have never made it to the point I am at today. I would also like to thank my parents, brothers, and sister, who were always there for me. Trips back home were like visiting an oasis and made the work here seem so far away that I could truly relax. On the academic side of things, I am especially grateful to Dr. Walter F. OBrien, my advisor. He was especially insightful and guided the project with a proficiency that only comes with years of experience. He provided me with the freedom to explore creative avenues, but also made sure to channel that motivation towards worthy goals. I would also like to thank him for the multiple opportunities he provided for me to teach. I found these times perhaps the most rewarding of all my duties here at Tech. I am also extremely appreciative of Dr. Joseph Schetz, who was constantly involved in the project and always on the lookout for new opportunities through which we could make our work public. His results-or-die vision always brought out my best performance. I would also like to thank Dr. Uri Vandsburger, Dr. Robert Mahan, and Dr. Lourdes Maurice for sacrificing their time to serve on my committee. Finally, my lab partner Lance Jacobsen must be given credit. He worked extremely hard to make this project a success and always strived to produce the highest quality results. His selfless sacrifice of time to collect data that he would not even use was especially appreciated. Without the help of the AOE and ME machines shops, this project would never have been a success. Special thanks must go to Bruce of AOE and Bill of ME, who machined electrode after electrode and injector after injector. They were both especially helpful each time we had a new emergency that had to be taken care of right now. Certainly the turbolab guys must be given thanks: Peter, Keith, Matt, Wayne, Grant, John, Mac, Drew, Alexandre, and Joe (evil!). These guys made the days sitting in front of a computer a little more enjoyable with their shall we say antics. However, a special award is reserved for Karl. Guten Tag Herr Sheldon. And lastly, how could I forget Chip? Dont forget your fireboots!
______________________________________________________________________________________ The plasma igniter/aeroramp integration was initially developed under subcontract to, and in cooperation with, Phoenix Solutions Co. under an Air Force SBIR Phase II plasma igniter program with the Air Force Research Laboratory. A Study of Plasma Ignition Enhancement for Aeroramp Injectors in Supersonic Combustion Applications
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Table of Contents
Abstract ii Acknowledgments iii List of Figures viii List of Tables xii Nomenclature xiii Motivation xv Introduction 1 Chapter 1: The Role of Plasma Igniters and Fuel Injectors in Hypersonic Technology 6 1.1: Development of Hypersonic Technology 7 1.2: Identified Difficulties with Supersonic Combustion 11 1.3: Mixing and Flameholding Methods in Supersonic Flow 13 1.3.1: Flameholding by Means of Passive Mixing Devices 13 1.3.1.1: Cavities 14 1.3.1.2: Rearward-Facing Steps 15 1.3.1.3: Ramp Injectors 16 1.3.1.4: Lobe Mixers 17 1.3.1.5: Wedges 18 1.3.2: Flameholding by Means of Active Mixing Devices 19 1.3.3: Recent Fuel Injector Developments 19 1.3.4: Development of Aeroramps 21 1.4: Development of Plasma Torches for Supersonic Combustion 24 1.4.1: Chemical Processes Governing the Effectiveness of Plasma Torches 26 1.4.2: Demonstrations of Plasma Torch Enhancement for High-speed Flows 28 1.4.3: Other Methods of Ignition in High-Speed Flows 30 1.4.4: Recent Advances in Plasma Torch Aided Supersonic Combustion 31 1.5: Attempts at Injector/Igniter Integration 32 1.6: Key Issues Related to Current Ignition/Injection Systems 34 1.7: Proposed Advances 35 Chapter 2: The Virginia Tech Plasma Torch, 3rd Generation (VTPT-3) 36 2.1: The Anode and Cathode 37 2.2: The Torch Body 38 2.3: The Micrometer Drive Assembly 39 2.4: The Bolt and Body Insulators 40 2.5: The Flow Swirler and Support Rod 40 2.6: Design Issues 41 Chapter 3: Experimental Setup and Equipment 43 3.1: Lab Equipment 43 3.1.1 The Flow System 43 3.1.1.1: The Torch Feedstock System 43
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3.1.1.2: The Injector Fuel System 44 3.1.2: The Power System 44 3.1.3: The Data Acquisition System 44 3.1.3.1: The Spectrometer 45 3.1.3.2: Temperature Probes 45 3.1.3.3: High-Speed Cameras 45 3.1.3.4: Other Data Acquisition Equipment 46 3.1.4: Other Laboratory Equipment 46 3.1.4.1: The Supersonic Tunnel 46 3.1.4.2: Positioning Stages 47 3.1.4.3: Argon Laser 48 3.1.4.4: Spectral Band-pass Filters 48 3.2: Experimental Procedures 49 3.2.1: Plasma Torch Setup 49 3.2.2: Spectroscopic Analyses 50 3.2.2.1: Basic Spectroscopic Studies 50 3.2.2.2: Spatial Spectroscopic Studies of the Plasma Jet 50 3.2.2.3: Spatial Spectroscopic Studies of the Plume 51 3.2.3: Total Temperature Sampling 52 3.2.4: Stereoscopic Investigations 53 3.2.5: Shadowgraphs 53 3.2.6: Surface Oil Flow 54 3.2.7: High-Speed Digital Photography 55 3.2.8: Filtered Photography 55 3.2.9: Video Observation 55 Chapter 4: Anode Material Selection and Design Issues 56 4.1: Material Review and Selection Criteria 57 4.2: Test Procedure 58 4.3: Results and Discussion 59 4.3.1: Quiescent Electrode Erosion Results 60 4.3.2: Stereoscopic Investigation of the Anodes and Cathodes 61 4.3.2.1: Tungsten-Copper 61 4.3.2.2: Molybdenum 62 4.3.2.3: Copper 64 4.3.2.4: Investigations of the Cathode 65 4.3.3: The Anode Attachment Point Model 66 4.3.4: Crossflow and Anode Exit Geometry Effects on Arc Attachment Point 70 4.3.4.1: Arc Attachment for Normal Injection 71 4.3.4.2: Arc Attachment for Transverse Injection 72 4.3.5: The Effect of Air and Nitrogen Plasmas On Molybdenum 74 4.4: Conclusions 75 Chapter 5: Spectroscopic Studies of Hydrocarbon, Air and Nitrogen Plasmas 78 5.1: Experimental Procedures 79 5.2: Results and Discussion 80 5.2.1: Methane 80
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5.2.1.1: Hydrocarbon Fragments 81 5.2.1.2: Hydrocarbon-Air Molecules 83 5.2.1.3: Excited Air Molecules 85 5.2.2: Ethylene 86 5.2.3: Propane and Propylene 86 5.2.4: Nitrogen and Air 88 5.2.5: Discovery and Analysis of Molybdenum Nitride 90 5.2.6: H-C2 Line Ratios 94 5.3: Conclusions 95 Chapter 6: Geometric Studies of the Plasma Torch Anode 96 6.2: Test Procedure 97 6.2.1: Anode Exit Geometry 97 6.2.2: Studies of Nozzles, Throat Lengths, and Injection Angle 98 6.2.3: Stereoscopic Investigation of Anode Wear 99 6.3: Results and Discussion 100 6.3.1: Studies of the Anode Diverging Section 100 6.3.1.1: Results of the Power Requirements 100 6.3.1.2: Results of the Protection Studies 101 6.3.2: Geometric Study of Nozzles, Throat Lengths and Injection Angle 103 6.3.2.1: Results for the 90, 2.54-mm Throat Length, Sonic Anode 111 6.3.2.2: Results for the 90, 3.12-mm Throat Length, Sonic Anode 117 6.3.2.3: Results for the 90, Supersonic Anode 119 6.3.2.4: Results for the 60, 2.54-mm Throat Length, Sonic Anode 122 6.3.2.5: Results for the 60, 3.12-mm Throat Length, Sonic Anode 125 6.3.2.6: Results for the 60, Supersonic Anode 130 6.4: Conclusions 132
Chapter 7: Plasma Torch Operational Characteristics in Quiescent and Supersonic Environments 135 7.1: Experimental Setup 136 7.2: Results and Discussion 137 7.2.1: Quiescent Experimental Results 137 7.2.1.1: Spectral Measurements of the Plasma Jet Centerline 137 7.2.1.2: Two-Dimensional Ion Concentration Profiles 141 7.2.1.3: High Speed Visualization 143 7.2.2: Supersonic Experimental Results 147 7.2.2.1: Methane in Supersonic Flow 148 7.2.2.2: Ethylene in Supersonic Flow 156 7.2.2.3: Nitrogen in Supersonic Flow 159 7.4: Conclusions 166
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Chapter 8: Comparisons of CFD Calculations and Measurements for a Sonic Methane Plasma Jet in a Supersonic Crossflow 169 8.1: Experimental Conditions 169 8.2: Computational Methodology 170 8.2.1: Jet and Boundary Conditions 170 8.2.2: Grid Generation 171 8.2.3: Reaction Mechanism 173 8.3: Results and Discussion 173 8.3.1: Sources for Error 179 8.4: Conclusions 180 Chapter 9: The Results of the Integrated Design 182 9.1: Experimental Setup 184 9.2: Results and Discussion 185 9.2.1: Methane-Ethylene Experiments 186 9.2.2: Nitrogen-Ethylene Experiments 195 9.2.3: Ethylene-Ethylene Experiments 204 9.2.4: Air-Ethylene Experiments 206 9.2.5: Electrode Emission 210 9.3: Conclusions 211 Chapter 10: Conclusions and Recommendations 214 10.1: Recommendations References 220 Appendix A: A Review of Plasma Spectroscopy 226 Appendix B: Topics Related to Plasma Dynamics 234 Appendix C: Relationships for Momentum Flux Ratio, Mass Flowrate, and Chamber Pressure 254 Appendix D: Selected Excerpts from Numerical Model for Plasma Jet Injection in a Supersonic Crossflow 258 Vita 300
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List of Figures
1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28) 29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39) 40) 41) Figure 1.1: Approximate Performance Levels of Various Classes of Engines (Curran, 1999) 7 Figure 1.2: The Dual-Combustor Ramjet Concept (Waltrup et al., 1996) 9 Figure 1.3: A Concept Picture of the X-30 10 Figure 1.4: Dual Cavities for Flameholding and Mixing (Yu et al., 1998) 15 Figure 1.5: Compression and Expansion Ramps (Stouffer et al., 1993) 17 Figure 1.6: Illustration of Swept and Unswept Ramp Injectors 17 Figure 1.7: Squared-off Lobe Mixer (Marble et al., 1990) 18 Figure 1.8: Comparison of a Physical Ramp and Aeroramp Injector (Fuller et al., 1996) 22 Figure 1.9: A Plasma Torch Design for Combustion Enhancement of IC Engines (Weinberg, 1978) 25 Figure 1.10: Direct Arc Injection Method (DAIM) 32 Figure 2.1: Schematic of VTPT-3 37 Figure 2.2: Plasma Torch Electrodes 38 Figure 2.3: Plasma Torch Body 39 Figure 2.4: The Micrometer Drive Assembly 39 Figure 2.5: Bolt and Body Insulators 40 Figure 2.6: Flow Swirler and Support Rod 41 Figure 3.1: Tunnel Schematic Showing Injector, Torch and Equipment Locations 47 Figure 3.2: Velmex Positioning Stages 48 Figure 3.3: The Measurement Locations of Centerline, Exit, and Two-Dimensional H-Profiles 51 Figure 3.4: Spectrometer Setup for Plume Studies 52 Figure 3.5: Laser-Shadowgraph Setup Schematic 54 Figure 3.6: Surface Oil Flow Setup 54 Figure 4.1: Stereoscopic Photographs of Worn Tungsten-Copper Anodes 62 Figure 4.2: Stereoscopic Photographs of Worn Molybdenum Anodes 63 Figure 4.3: Stereoscopic Photographs of Copper-1 64 Figure 4.4: Stereoscopic Photograph of Copper-3 65 Figure 4.5: Microscopic Photographs of Cathode Tip Wear 66 Figure 4.6: Results of Analytical Arc Diameter Analysis 70 Figure 4.7: A Molybdenum Anode in Crossflow after 50+ Runs 72 Figure 4.8: Figure 4.8: A Schematic of an Anode Designed for Angled Injection 73 Figure 4.9: Wear on Copper Anodes for 45 Transverse Injection 74 Figure 4.10: Molybdenum Anodes Showing Erosion from Air Plasma 75 Figure 5.1: Methane Spectrogram 81 Figure C2 5.2: Swan Band Systems 82 Figure 5.3: CH Bands and H Line near 430 nm 83 Figure 5.4: CN Violet Band Systems 84 Figure 5.5: NH Bandheads at 336 nm 85 Figure 5.6: N2 Second Positive System 86 Figure 5.7: Spectrogram of Ethylene Plasma 86 Figure 5.8: Spectrograms of Propane and Propylene Plasmas 87 Figure 5.9: Spectrogram of Nitrogen Plasma 88
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Figure 5.10: Neutral and Charged Nitrogen Peaks 89 Figure 5.11: Spectrogram of Air Plasma 89 Figure 5.12: A Comparison of Air and Nitrogen Plasma Spectra 90 Figure 5.13: Molybdenum Nitride Peaks at 524.02 and 536.16 nm 91 Figure 5.14: Comparison of N2 Plasma Spectra with Copper and Molybdenum Anodes 92 Figure 5.15: Photographs of Molybdenum Nitride Deposits 94 Figure 5.16: The Affect of Torch Power on H-C2 Ratio 95 Figure 6.1: Anode Exit Geometries 98 Figure 6.2: Anode Designs for the Nozzle, Throat Geometry and Injection Angle Study 99 Figure 6.3: Power Relationships for Different Anode Half Angles 101 Figure 6.4: High-speed Photographs of Torch Operation in a Mach 3.0 Crossflow 102 Figure 6.5: Surface Oil Flow Photographs of Torch Operation in a Mach 3.0 Crossflow 102 Figure 6.6: An Example of Spectral Exit Profile 103 Figure 6.7: Comparison of Exit Profiles for Sonic Designs (H Line, qt=1.17) 105 Figure 6.8: Comparison of Exit Profiles for Supersonic Designs (H Line, qbar=1.17) 106 Figure 6.9: Centerline Temperature Profiles Comparing Power 107 Figure 6.10: Plot of Maximum Total Temperature Ratio versus Power 108 Figure 6.11: Penetration Height versus Power 109 Figure 6.12: Comparison of Exit Profiles for Different Momentum Flux Ratios 110 Figure 6.13 2D H-Line Profiles for Various Powers and Momentum Flux Ratios 112 Figure 6.14: Centerline Temperature Measurements for 90, 0.100 Sonic Anode 113 Figure 6.15: 2D Temperature Profiles Comparing Power and Momentum Flux Ratio Variations 115 Figure 6.16: Close-up Photograph of 90, 0.100 Throat Length Sonic Anode 116 Figure 6.17: Centerline Temperature Measurements for 90, 3.12 mm Sonic Anode 117 Figure 6.18: Photograph of the 90, 3.12 mm Throat Length, Sonic Anode 118 Figure 6.19: Centerline Temperature Measurements for the 90 Supersonic Design 119 Figure 6.20: 2D Temperature Profile for the 90 Supersonic Design 120 Figure 6.21: Photographs of the 90 Supersonic Anode 121 Figure 6.22: 2D H-Line Profile for 60, 0.100 Throat Length Anode 122 Figure 6.23: Centerline Temperature Measurements for the 60, 2.54 mm Sonic Anode 123 Figure 6.24: 2D Temperature Profile for 60, 2.54 mm Throat Length Anode 123 Figure 6.25: Anode Wear on the 60, 2.54 mm Throat Length, Sonic Anode 124 Figure 6.26: Comparison of Exit Profiles for Different Momentum Flux Ratios 125 Figure 6.27: 2D H-Line Profiles for Various Momentum Flux Ratios 126 Figure 6.28: Centerline Temperature Measurements for 60 Sonic Long Throat 127 Figure 6.29: 2D Temperature Profiles of the 60, 3.12 mm, Sonic Throat Anode 128
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78) Figure 6.30: Anode Wear on the 60, 3.12 mm Throat Length, Sonic Anode 129 79) Figure 6.31: Centerline Total Temperature Measurements for the 60 Supersonic Anode 130 80) Figure 6.32: 2D Temperature Profile for 60 Supersonic Anode 131 81) Figure 6.33: Close-up Photograph of 60, Supersonic Anode 131 82) Figure 6.34: Close-up Photograph of 60, Supersonic Anode 132 83) Figure 7.1: Radical Concentrations Along the Centerline of the Plasma Jet 138 84) Figure 7.2: H Intensity Variations Along the Jet Centerline 139 85) Figure 7.3: Average Rate of Photon Flux Along Jet Centerline 140 86) Figure 7.4: H Intensity Variations for Changing Flowrate 141 87) Figure 7.5: Ion Concentration Profiles 142 88) Figure 7.6: High-Speed Filtered Photographs 144 89) Figure 7.7: A Graphical Representation of the Arrowhead Theory 144 90) Figure 7.8: OH Visualization of Plasma Jet 145 91) Figure 7.9: High-speed Photographs of CN within the Plasma Jet 146 92) Figure 7.10: H Filtered Photograph Down Axis of Jet 146 93) Figure 7.11: High-speed Photographs of the Plasma Jet 147 94) Figure 7.12: Electrode Emission in Mach 3.0 Crossflow (VTPT-2) 148 95) Figure 7.13: Plasma Torch Electrode Emission in a Mach 2.4 Crossflow 149 96) Figure 7.14: Methane Combustion Plumes in a Mach 2.4 Crossflow 150 97) Figure 7.15: Spectrogram of Methane Flame Plume 151 98) Figure 7.16: Shadowgraphs for a Momentum Flux Ratio of 0.80 153 99) Figure 7.17: Shadowgraphs for a Momentum Flux Ratio of 1.15 154 100) Figure 7.18: Shadowgraphs for a Momentum Flux Ratio of 2.30 155 101) Figure 7.19: Diagram of Surface Oil-Flow Patterns 156 102) Figure 7.20: Surface Oil Flow Pictures (Methane) 156 103) Figure 7.21: Intensity of H Line versus Power for Ethylene 157 104) Figure 7.22: Ethylene Centerline Temperature Profiles 158 105) Figure 7.23: Two-Dimensional Temperature Profile for Ethylene 159 106) Figure 7.24: Ethylene Flame Plume in Mach 2.4 Crossflow 159 107) Figure 7.25: Spectral Intensity of N2 Molecule at 380.5 nm 160 108) Figure 7.25: Spectrogram of Nitrogen Plume 161 109) Figure 7.26: Spectral Peaks of NO2 in the Yellow-Orange Region 161 110) Figure 7.27: Color Variations in the Nitrogen Plasma Jet 162 111) Figure 7.28: Nitrogen Plume Variations with Increasing Power 163 112) Figure 7.29: A Nitrogen Plume with a Layered Structure at 1250 Watts 164 113) Figure 7.30: NO Distribution of Nitrogen Plume 1.25 cm Downstream of Torch Exit 164 114) Figure 7.31: Centerline Temperature Profiles for Nitrogen 165 115) Figure 7.32: Two-Dimensional Temp Profiles for Nitrogen 166 116) Figure 8.1: Illustration of Computational Grid Location 171 117) Figure 8.2: The Computational Domain and Block Structure 172 118) Figure 8.3: Centerline Total Temperature Profile Comparison for 1500 W 174 119) Figure 8.4: Complete Total Temperature Profile Comparison for 1500 W 175 120) Figure 8.5: Shock Structure Comparison for 1500 W and a qt of 1.18 175 121) Figure 8.6: Mass Fractions of Some Major Species 176 122) Figure 8.7: Mass Fraction Plots at 49.6 deq Downstream of Torch Exit 177
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Figure 8.8: Air Mass Fraction Plots at 49.6 deq Downstream of Torch Exit 178 Figure 9.1: The Experimental Setup 185 Figure 9.2: The Lifting Effect of an Upstream Aeroramp 186 Figure 9.3: Filtered Photographs Showing the Effect of Aeroramp Lifting on Plasma Species 186 Figure 9.4: Spectrogram of Excited Ethylene/Methane Fragments by Methane Plasma Interaction 187 Figure 9.5: Changes of the Spectral Profiles of C2 Line with Increasing Power for Methane Plasma 188 Figure 9.6: A Comparison of Fuel vs. No Fuel on the C2 Line Intensity for Various Powers 189 Figure 9.7: The Exponential Dependence of the Average C2 Line Intensity on Torch Power 190 Figure 9.8: C2 Line Profiles for 2500 W and Various Injector Momentum Flux Ratios for Methane Plasma 191 Figure 9.9: The Effect of Increasing the Fuel Mass Flowrate on the Intensity of C2 Line 192 Figure 9.10: Distribution of the Maximum C2 Line Intensity versus Distance from Torch 192 Figure 9.11: Filtered Photographs of Methane Plasma Jet for 2000 W 193 Figure 9.12: CH Profile Variations with Increasing Power 194 Figure 9.13: CH Profile Variations with Increasing Injector Mass Flowrate 194 Figure 9.14: 2D Temperature Profile of Methane Plume at 2000 W 195 Figure 9.15: A Spectrogram of Excited Ethylene and Nitrogen Species by Interaction with a Nitrogen Plasma Jet 196 Figure 9.16: Profiles Showing the Effect of Arc Gap on CN Distribution for Nitrogen Plasma 197 Figure 9.17: CN Line Maxima for Two Arc Gaps 198 Figure 9.18: A Comparison of Fuel Mass Flowrate Effects on CN Line Intensity 198 Figure 9.19: Radical Profiles Near Torch Exit 200 Figure 9.20: Filtered Photographs for Nitrogen-Ethylene at 2000 W 201 Figure 9.21: Photographs Demonstrating the Effect of Increasing Power on the CH Profile 202 Figure 9.22: Evidence of Fuel Interaction with Nitrogen Plasma Jet 203 Figure 9.23: 2D Temperature Profile for Nitrogen Plume at 2000 W 204 Figure 9.24: Changes of the Spectral Profiles of C2 Line ...