MID-INFRARED LASER ABSORPTION SPECTROSCOPY FOR CARBON OXIDES IN HARSH ENVIRONMENTS A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY R. Mitchell Spearrin September 2014 c Copyright by R. Mitchell Spearrin 2014 All Rights Reserved ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. (Professor Ronald K. Hanson) Principal Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. (Dr. Jay B. Jeffries) I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. (Professor Mark A. Cappelli) Approved for the University Committee on Graduate Studies iii iv Abstract Advancements in measurement science are presented regarding in situ laser-based detection of CO and CO2 in harsh combustion environments. Mid-infrared absorption sensing strategies, utilizing transitions in the 2.7 µm and 4.3 µm vibrational bands for CO2 and the 4.8 µm vibrational band for CO, were developed to enable sensitive measurements of temperature and carbon oxide concentrations in high-temperature gases. These new strategies (1) extend the utility of carbon oxide absorption sensing for hostile aeroengine applications and (2) offer significant improvements to previous methods for shock tube kinetics studies. The recent maturation of mid-infrared diode and quantum cascade lasers, combined with parallel progress in mid-infrared fiber optics, provides the platform from which the field-deployable sensors were designed. State-of-the art signal processing strategies, including calibration-free wavelength modulation spectroscopy, were implemented to tackle the thermo-mechanically harsh environments of a pulse detonation combustor and direct- connect scramjet. Time-resolved and spatially-resolved measurements of temperature and carbon oxide species concentrations were demonstrated to provide an in situ metric to evaluate combustion completion for engine development. For shock tube kinetics studies, a multi-band CO2 sensing strategy was developed to provide ppm-level species detection and highly-sensitive measurements of gas temperature with microsecond temporal resolution. v Acknowledgments I dedicate this dissertation to my late friend and undergraduate roommate Joseph Hanzich. He truly elevated my way of thinking and inspired me to maximize my personal potential. He was a major catalyst in my decision to pursue a PhD and I continue to find motivation in memory of his life. I would further like to recognize a number of individuals who made my PhD possible. First, I am indebted to my advisor, Professor Ron Hanson, for bringing me to Stanford and providing me ample opportunities to learn and grow through research. He has made me a better experimentalist and communicator, and I have truly been inspired by his passion for excellence. I would like to thank Dr. Jay Jeffries for sharing his expertise and for guiding many of the critical technical decisions made throughout the course of my PhD. He played a major role in my progression as a scientist, engineer, and project manager. I would also like to thank the many staff and students (current and former) who have provided me with day-to-day help in solving the numerous challenges which I encountered during my research. Chris Goldenstein, Wei Ren, Rito Sur, Vic Miller, Matt Campbell, Ian Schultz, and Marcel Nations amongst others have made contributions ranging from troubleshooting laser problems over lunch to finishing an off-site measurement campaign on my behalf at the birth of my first child. I am fortunate to have been surrounded daily by so many intelligent and gracious peers. Additionally, I’d like to thank the collaborators from other universities who enabled the exciting applications of my research. These include Professor Chris Brophy and Dave Dausen of the Naval Postgraduate School, and Professor Chris Goyne, Bob Rockwell, and Brian Rice of the University of Virginia. I must thank Professors Mark Capelli, Chris Edwards, and John Weyant of Stanford for taking the time to serve on my dissertation reading and/or oral committee. I also would like to recognize the primary sponsors of my PhD research: AFOSR and Dr. Chiping Li, NASA and Dr. Rick Gaffney, and ISSI and Dr. John Hoke. vi Moreover, I would be remissed not to recognize the many folks outside of Stanford who have en- abled me to pursue my doctoral degree. I would like to thank my parents, Raymond and Candience, for their unconditional support and confidence in me since the day I left home as the first person in my family to attend college. More importantly, I’d like to thank them for the principles they instilled in me during the many years before that day which have helped me succeed. My father taught me the value of honest, hard work and gave me a portion of his instinct for all things mechanical. My mother created a home of love and security, giving me the confidence to take risks and reach for lofty goals. I would like to thank the many other friends and family members who have provided encouragement and helped me keep perspective on life. Most importantly, I am forever grateful to my wife, Karen, who has supported my dreams and personal ambitions unwaveringly since we were married just prior to our time at Stanford. During our 4 years here, she has been the primary financial provider for our family while also dedicating more than her share of time both early morn- ing and night towards raising our now 2 year-old daughter Rachel. I am deeply appreciative of her patience during my many hours of work and her confidence in me to excel both professionally and personally. vii Contents Abstract v Acknowledgments vi 1 Introduction 1 1.1 Background . 1 2 CO2 TDL sensing for harsh engine environments 3 2.1 Introduction . 3 2.2 Absorption spectroscopy theory . 6 2.3 Sensor design . 7 2.3.1 Wavelength selection . 7 2.3.2 Laser modulation parameters . 11 2.3.3 Optical engineering . 12 2.4 Sensor development . 16 2.4.1 Measurements of key spectroscopic parameters . 16 2.4.2 Modulation-induced wavelength shift . 19 2.4.3 High gas density lineshape modeling . 21 2.5 High-temperature shock tube validation . 22 2.6 CO2 measurements behind detonation waves . 23 2.7 Summary . 24 3 CO sensing for detonation engines 26 3.1 Introduction . 26 3.2 Wavelength selection . 27 3.3 Sensor development . 29 3.3.1 Modulation depth optimization . 29 viii 3.3.2 Modulation-induced wavelength shift . 32 3.3.3 Spectral modeling . 32 3.3.4 Optical engineering . 37 3.4 CO measurements behind detonation waves . 39 3.5 Multi-species measurements in a PDE . 40 3.6 Summary of detonation engine research . 42 4 Ultra-sensitive CO2 diagnostic for kinetic studies 43 4.1 Introduction . 43 4.2 Spectroscopic framework . 45 4.3 Line selection . 46 4.4 Sensor development . 51 4.4.1 Light source selection . 51 4.4.2 Spectral modeling . 52 4.4.3 Optical setup . 55 4.5 Sensitive CO2 detection near 4.2 micron . 56 4.6 Cross-band CO2 thermometry . 57 4.7 Kinetics Summary . 59 4.7.1 Potential sensor improvements . 60 5 Spatially-resolved CO and CO2 sensing for scramjets 62 5.1 Introduction . 62 5.2 Methods . 63 5.2.1 Line Selection . 63 5.2.2 Laser absorption spectroscopy . 64 5.2.3 Optimization for non-uniform flows . 66 5.3 Experimental setup . 67 5.3.1 Optical hardware . 67 5.3.2 Facility interface . 69 5.4 Results . 70 5.4.1 Time-resolved measurements . 75 5.4.2 Spatially-resolved measurements . 76 5.5 Summary . 79 ix 6 Conclusion 80 6.1 Aeropropulsion research . 80 6.2 Shock tube kinetics research . 81 A Mid-infrared optics: practical issues 82 A.1 Laser output non-linearity . 83 A.2 Wavelength stability . 84 A.3 Susceptibility to back reflections . 85 A.4 Beam spatial mode quality . 85 A.5 Fiber-coupling . 87 A.6 Fiber transmission . 89 A.7 Multimodal dispersion in optical fibers . 90 A.8 Mid-infrared optical materials . 91 Bibliography 93 List of Tables −1 2.1 Collisional-broadening parameters for the R(26) CO2 line (E” = 273 cm in com- bustion exhaust gases. 19 3.1 Spectroscopic line assignments and collisional-broadening parameters for the CO lines of interest. Uncertainties for broadening measurements made in this work (i.e. CO–N2) are shown in parentheses. 34 4.1 Spectroscopic line assignments and modeling parameters for the CO2 lines of inter- est. Parameters taken from HITEMP 2010 except where measured (γAr, nAr). 54 5.1 Spectroscopic parameters of CO and CO2 transitions used in the scramjet sensor. 64 A.1 Mid-infrared optical material properties . 92 x List of Figures 2.1 Absorption line-strengths of CO2 and H2O at 2000 K based on the HITEMP database. 4 2.2 Absorption line-strengths of ν1+ν3 CO2 combination bands at 1000 K (HITEMP). 8 2.3 CO2 and H2O absorbance spectra simulations (air bath gas) near the peak of the R-branch in the ν1+ν3 band at an expected PDC temperature (1800 K), path-length (L = 4 cm) and composition for (top) P = 1 atm and (bottom) P = 5 atm. 9 −1 2.4 Pure CO2 absorbance spectra near 3733.48 cm at T = 1000 K and P = 40 torr, measured in a static cell and compared to a HITEMP 2010 simulation.