Rotating Detonation Combustor Mechanics A Dissertation submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Aerospace Engineering and Engineering Mechanics of the College of Engineering and Applied Science April 23, 2018 by Vijay G Anand B.E. Aeronautical Engineering, Anna University, 2013 Dissertation Committee: Dr. Ephraim Gutmark (Advisor) Dr. Shaaban Abdallah Dr. Mark Turner Abstract Recent years have witnessed a notable increase in endeavors resorted to investigating unsteady combustion/pressure processes that offer a prospective increase in stagnation pressure due to a more efficient combustion of fuel. One such pressure gain combustion (PGC) concept is a rotating detonation combustor (RDC). RDCs make use of a rotating detonation wave that travels circumferentially about a hollow or annular chamber at kilohertz frequencies, continually combusting the supplied reactants without the need for more than one initial ignition event. Due to its simplicity in design, which can be integrated into existing systems’ architecture, and the lack of moving mechanical components, RDCs are at the forefront of PGC research. The current dissertation deals with the basic mechanics of these combustors. Specifically, the diverse modes of detonative operation in annular and hollow combustor configurations are experimentally studied, and the variables dictating these modes are extracted. The question of what exactly constitutes a rotating detonation combustor is answered, by “converting” a conventional atmospheric deflagrative hollow combustor into an RDC. Further, based on this demonstration, the numerous kinships between RDC operation and decades of observations pertaining to high frequency combustion instabilities in rocket engines are presented and discussed. It is argued that most of the poorly understood phenomena of high frequency instabilities can be explained by detonation-based physics. Finally, evidence is presented that suggests rotating detonations to be type of near-limit detonation behavior. The findings of this study are proposed to be useful for the three different communities of RDC research, rocket engine instabilities and fundamental detonation physics. ii Acknowledgements First and foremost, I thank my mother and father for all that they have done for me. It is fair to say that nurture is as important as nature in an individual’s growth, and they have contributed to my current standing in quantitatively equal, but qualitatively different ways. The sacrifices rendered by my family are recognized, and will be remembered. Second, I extend my warmest regards to my advisor, Dr. Ephraim Gutmark, for providing the perfect combination of guidance and directives, and for respecting me as a researcher. Having an in-phase relationship with one’s advisor is an element of probabilistic slimness, and I was fortunate enough to experience the same over my years in graduate school. I also thank all the members of my committee and every staff in my lab that I have interacted with through the years — they have supported this endeavor. Thirdly, I cannot emphasize enough the ways I have changed over my stay here in Cincinnati. In that regard, I am thankful for my colleagues from the lab, who collectively made me feel like I had a long bridge to gap (mostly unintentionally). As the saying goes, if one is the smartest person in the room, one is in the wrong room; I routinely found myself in the right room, especially during my first few years in the lab. Adversity is a beautiful thing. In the same vein, a special shout out is in order for the detonation engines research team of which I am a part of. For reasons unknown, every single member that has entered and exited this group appear to possess an uncanny affinity for extreme political inappropriateness and heightened on-point humor — for this I truly am thankful for. I am especially grateful to Andrew St. George for building the foundations of our group, and for helping me acquire my data through all those weary, wintery, sub-zero nights. I could not have done what I have, in the time that I did, if not for him. Last, but most definitely not the least, I am grateful to the United States of America. I know for a fact, anecdotally and otherwise, that hard work and intellect can only get one so far in life. The x-factor is the privilege of opportunity, and for that I owe this country one. iv “A hundred years from now, people will look back on us and laugh. They'll say, 'You know what people used to believe? They believed in photons and electrons. Can you imagine anything so silly?' They'll have a good laugh, because by then there will be newer, better fantasies... And meanwhile, you feel the way the boat moves? That's the sea. That's real. You smell the salt in the air? You feel the sunlight on your skin? That's all real. Life is wonderful. It's a gift to be alive, to see the sun and breathe the air. And there isn't really anything else.” ― Michael Crichton, in The Lost World, 1995 v Preface The current PhD thesis deals with the specifics of rotating detonation combustor mechanics. In particular, various modes of operation of the device are analyzed, predominantly experimentally. The following articles have resulted from this work, and constitute it: 1. Anand, V., Gutmark E. An extensive review of rotating detonation combustors, with parallels to rocket engine instabilities. Progress in Energy in Combustion Science (Invited – To be submitted). 2. Anand, V., St. George, A., Driscoll, R., Gutmark, E.: Investigation of rotating detonation combustor operation with H2-Air mixtures. Int. J. Hydrogen Energy. 41, 1281–1292 (2016) 3. Anand, V., St. George, A., Driscoll, R., Gutmark, E.: Characterization of instabilities in a Rotating Detonation Combustor. Int. J. Hydrogen Energy. 40, 16649–16659 (2015). 4. Anand, V., St. George, A., Driscoll, R., Gutmark, E.: Analysis of air inlet and fuel plenum behavior in a rotating detonation combustor. Exp. Therm. Fluid Sci. 70, 408–416 (2016). 5. Anand, V., St. George, A., Driscoll, R., Gutmark, E.: Longitudinal pulsed detonation instability in a rotating detonation combustor. Exp. Therm. Fluid Sci. 77, 212–225 (2016). 6. Anand, V., St. George, A., Gutmark, E.: Amplitude modulated instability in reactants plenum of a rotating detonation combustor. Int. J. Hydrogen Energy. 42, 12629–12644 (2017). 7. Anand, V., St. George A., Farbos De Luzan, C., Gutmark, E.: Rotating detonation wave mechanics through ethylene-air mixtures in hollow combustors, and implications to high frequency combustion instabilities. Exp. Therm. Fluid Sci. 92, 314–325 (2018). 8. Anand, V., Gutmark E. Rotating Detonations vs. Spinning Detonations: Similarities and Differences. AIAA J. (Accepted – 2018). 9. Anand, V., Farbos De Luzan C, Babu L, St. George A, Driscoll R, Gutmark E. On Mean Pressure Shifts and Chugging Oscillations in Back-pressurized Rotating Detonation Combustors (Awaiting clearance for submission). vi 10. Anand, V., St. George A, Jodele J, Knight E, Gutmark E. The Origins of Wave Directionality, Chaotic Propagation and Onset Time after Ignition in a Rotating Detonation Combustor (Awaiting clearance for submission). In addition to the above, the following publications were also authored during the duration of the program, but are not a part of the current thesis: i. Anand, V., St. George A, Driscoll R, Randall S, Gutmark EJ. Statistical Treatment of Wave Instability in Rotating Detonation Combustors. 53rd AIAA Aerosp. Sci. Meet., Kissimmee, Florida, (2015). ii. Anand, V., St. George A, Gutmark E. Hollow Rotating Detonation Combustor. 54th AIAA Aerosp. Sci. Meet., San Diego, California, (2016). iii. Anand, V., Jodele J, Knight E, Gutmark E. Dependence of Pressure, Combustion and Frequency Characteristics on Valved Pulsejet Combustor Geometries. Flow, Turbulence and Combustion (Accepted - 2017). iv. Anand, V., Glaser, A., Gutmark E. Acoustic Characterization of Pulse Detonation Combustors. AIAA J. (Accepted - 2018). v. Anand, V., Gutmark E. Rotating Detonation Combustor Research at the University of Cincinnati. Flow, Turbulence and Combustion (Invited - Accepted - 2018). vi. Anand, V., Gutmark E. Types of low frequency instabilities in a rotating detonation combustor. Active Flow and Combustion Control. Book Chapter. (Invited – Submitted - 2018). Over the last two years, three PhD and two Master’s students have graduated from the Detonation Engine Test Facility in the Gas Dynamics and Propulsion Laboratory at the University of Cincinnati. Since the facility used has remained unaltered during this time, its minutiae will not be detailed in the current thesis for the sake of brevity. The author directs the readers to consult the following theses for detailed information on the same: a. St. George, A., Development and Testing of Pulsed and Rotating Detonation Combustors. PhD Thesis (2016). vii b. Driscoll, R., Investigation of Sustained Detonation Devices: the Pulse Detonation Engine- Crossover System and the Rotating Detonation Engine System (2016). c. Wilhite, JM., Investigation of Various Novel Air-Breathing Propulsion Systems (2016). d. Knight, E., Effects of corrugated outerwall on rotating detonation combustor behavior (2018). In addition, to avoid the time-consuming process of reorienting the published/accepted journal articles into a traditional thesis structure, the current document resorts to adopting the format of a PhD by publication. In this regard, the