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<p dir="ltr">Rotating detonation engines (RDEs) are a promising technology by which to increase the efficiency of propulsion and power generation systems. Self-sustained, rotating detonation waves within the combustion chamber provide a means for combustion to occur at elevated local pressures, theoretically resulting in hotter temperature product gas than a constant pressure combustion process could provide at equivalent operating conditions. Despite theoretical advantages of RDEs, the thermodynamic benefit has yet to be achieved in experimental applications. Additionally, much of the experimental work to date has been conducted at mean operating pressures lower than industrial applications will require, especially for rocket or gas turbine combustion environments. The sensitivity of these devices to operating pressure has made clear the importance of chemical reaction rates on the successful operation of these combustors. This work addresses critical risks associated with implementing this technology at flight-relevant conditions by advancing the understanding of deflagrative loss mechanisms on delivered performance and by investigating the coupling between chemical kinetic timescales and operating modes produced by the combustor.</p><p dir="ltr">A novel pressure measurement technique was developed in which the stagnation pressure of exhausting gas produced by the RDC is measured through quantification of the under-expanded exhaust plume divergence angle at megahertz-rates. Time-averaged stagnation pressure measurements obtained with this technique are shown to be within 1.5% of the equivalent available pressure (EAP) measured. Time-resolved stagnation pressure measurements produced by this technique provide a means to quantify the detonation cycle pressure ratio. It was shown that increasing the total mass flow rate through the combustor, therefore increasing the mean operating pressure, results in a decrease in both detonation wave velocities and detonation cycle stagnation pressure ratios.</p><p dir="ltr">Numerical modeling of detonations was conducted to understand the coupling of stagnation pressure ratios and wave speeds to deflagrative modes of combustion within rotating detonation combustors. Using the experimental measurements, it is shown that significant amounts of propellant combusts as a result of deflagration prior to (i.e., preburning) and after (i.e., afterburning) the detonation wave. Increasing the RDC operating pressure by 4x is shown to increase the amount of preburned propellant by 4.5x. Relevant chemical kinetic reaction rates of the conditions tested are modeled to increase by 4.5x as well, indicating that the increase in reactant preburning is the result of faster chemical kinetic timescales associated with higher pressure combustion. Results from this testing suggest an operating pressure upper limit for this combustor exists around 20 bar. At these conditions, chemical kinetic rates would be fast enough that deflagration would be the primary mode of combustion and the detonation would not exist. It is suggested that different injector or combustor designs might be able to extend operating limits, however it is unclear if there is a chemical kinetic limit at which no design would be able to overcome.</p><p dir="ltr">Despite significant amounts of deflagrative combustion within the RDC, the vacuum specific impulse produced by the RDC was shown to be between 95.0% and 98.5% of what an ideal deflagrative combustor could produce for most conditions. Given conventional rocket combustors typically operate at specific impulse efficiencies in the range of 90%-99%, it is noted that the RDC tested in this work has demonstrated, at the very least, equal performance to the current state of the art for rocket propulsion combustors while utilizing an effective combustor length (L*) of only 63 mm (2.5 inches). A detailed RDC performance model was developed which considered losses associated with deflagration (both preburning and afterburning) and incomplete combustion. Using measurements obtained from the experiment it is determined that incomplete combustion contributes a larger performance loss than the deflagration which occurs within the combustor.</p><p dir="ltr">A total of 17 parametric studies were conducted experimentally to evaluate the response of the RDC specifically to changes in the propellant chemical reaction timescales. Detonation wave arrival times ranged between 10 microseconds and 178 microseconds as a result of testing at ranges of operating pressures, equivalence ratios, and utilizing nine unique propellant combinations. It was shown that the wave arrival time is primarily a function of chemical kinetic timescales and injector mixing processes. A model using the injector momentum ratio and modeled deflagrative preheat times is shown to be able to closely predict experimentally obtained detonation wave arrival times.</p>

This investigation explores the detonation dynamics of a rotating detonation engine (RDE). Beginning with the general understanding and characteristics of hydrogen and compressed air as a detonation fuel source, this study further develops the experimental approach to incorporating a coal-based fuel mixture in an RDE. There is insufficient prior research investigating the use of coal as part of a fuel mixture and insignificant progress being made to improve thermal efficiency with deflagration. The U.S. Department of Energy's Office of Fossil Energy awarded the Propulsion and Energy Research Laboratory at the University of Central Florida a grant to lead the investigation on the feasibility of using a coal-based fuel mixture to power rotating detonation engines. Through this study, the developmental and experimental understanding of RDEs has been documented, operability maps have been plotted, and the use of a coal-based fuel mixture in an RDE has been explored. The operability of hydrogen and compressed air has been found, a normalization of all operable space has been developed, and there is evidence indicating coal can be used as part of a fuel mixture to detonate an RDE. The study will continue to investigate coal's use in an RDE. As the most abundant fossil fuel on earth, coal is a popular fuel source in deflagrative combustion for electrical power generation. This study investigates how the combustion of coal can become significantly more efficient.

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<div>Rotating detonation engines (RDEs) are one of the most promising options for improving combustor efficiency through a constant-volume combustion process. RDEs are characterized by continuous detonation propagation in an annular combustion chamber with an implicitly dynamic injection response. An additional benefit is the similarity of these devices to existing engine architectures. However, RDEs have yet to realize their thermodynamic and systemic advantages due the non-ideal physics of detonation in practical devices and the complex interactions between the detonations and the hydrodynamics of the reactants. The design of RDEs is heavily informed by experimental and simulation efforts, but simulations are expensive and often limited by the assumptions of the solver. Experiments have their own challenges; the dynamic reaction zone processes are difficult to examine experimentally in annular combustor geometry. Therefore, an RDE analogue, operating at near-atmospheric conditions with natural gas and oxygen, was developed that emulates the combustor geometry of an RDE in a linear channel that facilitates optical diagnostic capabilities. The experiment permits detailed characterization of the injection, mixing, and ignition processes in an RDE and provides a cross-platform comparison with simulation results, which are often two-dimensional or linear, 3-D domains.</div><div> </div><div>A unique phenomenon was discovered in this experiment, wherein a transverse combustion instability developed periodic, kilohertz-rate detonations through a non-linear amplification process. The behavior was highly repeatable and produced dominant cycle frequencies in two distinct regimes: 6-8 kHz and 10-11 kHz. An investigation of this phenomenon found that these cycle frequencies corresponded to natural dynamics in the oxidizer and fuel manifolds, respectively, and that the transition between regimes was facilitated by the injection pressure ratio between the oxidizer and fuel. This indicated that the injection hydrodynamics were being influenced by the manifold dynamics, and that the hydrodynamics played a key role in the amplification of the instability.</div><div> </div><div>The kinetic characteristics of the reactants were examined independently of the injection hydrodynamics as the second key component of the amplification process by altering the reactant chemistry. The combustion morphology was characterized against performance criteria to examine successful behavior. Results showed that cycle frequency and kinetic rates were directly proportional, and that non-linear growth of the flame was possible when the cycle frequency matched the dynamics supplied by the manifolds. When the cycle frequency exceeded the limits of the manifold dynamics, failure of the detonation behavior would occur. A computational analysis of the reactants was used to examine kinetic rate trends with variations in equivalence ratio, oxidizer dilution, and product gas recirculation.</div><div> </div><div>Particle image velocimetry (PIV) was performed on the experiment to study the flow structure of the injection process and the interactions with the detonation process. Time-averaged statistics showed that the detonation induced transverse perturbation to the flow, with varying strength and directionality with respect to the axial location of the shock. A correlation between this behavior and a reactivity gradient, linked to the local product gas residence time, was found. Analysis of the PIV images produced time-resolved measurements of the reactant fill, from which hydrodynamic timescales of the injection process could be obtained. Comparisons between the hydrodynamic and kinetic timescales created an operability map for the test condition which narrows the prediction of the product gas recirculation that occurs in the combustor.</div><div> </div><div>The experiments performed in this work has improved understanding of the dynamic injection that occurs during RDE operation. The self-excited generation of detonations through non-linear processes in this experiment brings to light important interactions between the combustor, injector, and manifolds that can improve, or hinder, the performance of RDEs.</div>

<p dir="ltr">Rotating detonation combustors are a developing technology with the potential to successfully integrate pressure gain combustion in to modern propulsion devices. Utilization of propagating detonation waves could increase combustion cycle efficiency and reduce combustor size, resulting in an overall increase in system range or payload-carrying capabilities. However, the sensitivity of rotating detonation combustor operation and performance to geometric features, such as injector configuration or chamber length, still needs to be characterized over a wide range of operating conditions. In addition, the hardware configuration that promotes easy ignition into a coherent detonation operating mode does not always maximize combustor performance, especially at low-loss conditions where feedback between chamber and manifold dynamics can exist. Therefore, a rotating detonation combustor with continuously variable geometry capabilities was designed in order to continuously vary any number of hardware design parameters during combustor testing. Not only does the variable geometry combustor enable rapid characterization of operability sensitivity with minimal hardware swaps, it also enables exploration of hysteresis in performance as the combustor is ignited in one configuration and transitioned to a different geometry while maintaining detonative operation.</p><p dir="ltr">The operability of the variable geometry rotating detonation combustor was first characterized with variable fuel injector location. Higher wave speeds were observed at injector locations closest to the oxidizer throat, with decreased wave speed and eventual transition to deflagrative operation occurring at locations farther downstream due to increasing momentum flux ratio. Variation in fuel injection location induced bifurcations in the number of waves, resulting in corresponding changes in wave speed and gross thrust. Hysteresis was observed in these quantities as the direction of injector translation was reversed. Active translation promoted detonative operation of the experiment at conditions and configurations that hitherto operated only in a deflagrative mode with fixed combustor geometry. </p><p dir="ltr">Sensitivity of rotating detonation combustor operation and performance to oxidizer injector pressure drop was characterized using continuous variation of the injector area during combustor operation. Propulsive performance of the combustor was evaluated using thrust and equivalent available pressure, relating them back to reactant supply pressures for assessment of combustor pressure gain. An effective reactant supply pressure was developed in order to combine contributions of both fuel and oxidizer manifold pressures to the total pressure of the system so that pressure gain could be accurately calculated. Pressure gain increased during a test as oxidizer injector area was increased and the corresponding manifold pressure was decreased. At larger injector areas, pressure gain decreased as the operating mode of the combustor transitioned from detonation to deflagration, concomitant with reduction of gross thrust. Modeling of injector recovery time revealed that the injector operated in both choked and unchoked regimes, which was used to explain detonation wave number transitions in the experiment. A broadened range of detonative operability enabled by active variation of combustor geometry resulted in higher performance with lower injector pressure drop.</p><p dir="ltr">Sensitivity of rotating detonation combustor operation and performance to combustor chamber length was characterized using continuous variation of the chamber length during combustor operation. Specific impulse of the combustor remained relatively constant as chamber length was decreased from its maximum values, proving the practicality of efficient packaging for rotating detonation combustors. A limiting chamber length at which combustion could not longer be supported within the chamber was found to exist for every operating condition, resulting in flame blow-out and performance degradation. Modeling of detonation fill height revealed that relatively low specific impulse measurements could be attributed to unburned reactants exiting the chamber, and a more efficient use of reactants was potentially the cause for improved performance at higher mass flow rates as detonation wave number increased and reactant residence time decreased.</p><p dir="ltr">This experiment and the associated analysis has helped further characterize rotating detonation combustor sensitivity to hardware design parameters. The continuously variable geometry capabilities enabled precise identification of geometric parameters that resulted in operating mode transitions. Analysis and modeling of the flow processes within the injector and chamber were used to help explain why these mode transitions occurred, and can be used for future rotating detonation combustor development.</p>

Earlier research has demonstrated that downstream of combustion in a rotating detonation engine, exhaust flow periodically reverses circumferential direction. For small periods, the circumferential flow reaches velocity magnitudes rivaling the bulk flow of exhaust, manifesting as a swirl. The minimization of this swirl is critical to maximizing thrust and engine performance for rocket propulsion. During this study, numerous nozzle contours were iteratively designed and analyzed for losses analytically. Once a nozzle was chosen, further losses were validated through computational fluid dynamics simulations and then tested experimentally. Three different configurations were run with the RDRE: no nozzle, a nozzle without a spike, and a nozzle with a spike. Images of the exhaust quality were recorded using OH* chemiluminescence in high-speed cameras. One camera was used to confirm the existence of a detonation and the frequency of detonation. The second camera is pointed perpendicular to the exhaust flow to capture the quality of exhaust. Quantitative results of the turbulent velocity fluctuations were obtained through particle image velocimetry of the side-imaging frames. All frames in each case were exported and converted to several time-averaged frames whereupon the time-averaged turbulent velocity fluctuation profiles could be compared between cases for swirl attenuation.

Notre étude vise à améliorer la compréhension des modes de rotation continue d’une détonation. Elle porte sur leur caractérisation dans une chambre annulaire de section,normale à son axe de révolution, constante ou linéairement croissante. Le principe de fonctionnement repose sur l’injection continue de gaz frais devant le front de détonation pour renouveler la couche réactive et entretenir sa propagation. Ce travail trouve son application dans le développement de systèmes propulsifs utilisant la détonation rotative comme mode de combustion (Rotating Detonation Engine, RDE). Nous avons conçu et réalisé un banc expérimental dont l’élément principal est une chambre annulaire de diamètre intérieur 50 mm, de longueur 90 mm et d’épaisseur 5 ou 10 mm. Elle peut être équipée de noyaux cylindrique ou conique, de longueurs comprises entre 12 mm et 90mm et, pour les cônes, de demi-angles au sommet compris entre 0± et 14.6±. Elle est alimentée par des injections séparées de carburant, l’éthylène, et d’oxydant, formé ici par un mélange d’oxygène et d’azote. Plusieurs concentrations d’azote ont été considérées de manière à étudier plusieurs détonabilités de mélange. La caractérisation des régimes de détonation, de leurs célérités et de leurs pressions, est fondée sur l’analyse de signaux de capteurs de pression dynamiques et sur des visualisations par caméras ultrarapides. Nos résultats expérimentaux détaillent la phase d’amorçage, les modes de combustion et leur stabilité. L’étude paramétrique, réalisée pour plusieurs détonabilités, débits massiques et géométries internes de la chambre, met en évidence que, si les deux premiers paramètres n’ont pas d’effet notable sur les célérités et les pressions des modes de détonation,la géométrie interne de la chambre joue, elle, un rôle majeur dans l’amélioration de ces caractéristiques, en particulier la diminution de la longueur du noyau et l’augmentation de sa conicité (de son demi-angle au sommet). Nous avons réalisé une étude numérique afin d’expliquer les déficits mesurés de célérité et de pression. Elle met en avant la dégradation des propriétés théoriques de détonation résultant de la dilution et du réchauffement des réactifs par les produits de détonation. Nous proposons également un calcul du rendement thermodynamique qui, à la différence de modélisations antérieures, prend en compte la structure d’une détonation rotative. Nous décrivons aussi un calcul de hauteur de front de détonation pour les modes et géométries de chambre considérés dans cette thèse. Notre étude démontre ainsi l’intérêt de futures recherches sur la géométrie interne des chambres annulaires à détonation rotative et sur la prise en compte des phénomènes à l’origine des pertes d’efficacité. / Our study aims at improving the understanding of how a detonation may continuously rotate. It is focused on rotation modes in an annular chamber with constant or linearly increasing normal section. The functioning principle is based on the continuous injection of fresh reactive gases so as to regenerate a reactive layer ahead of the detonation front and maintain sufficient conditions for detonation propagation. The main incentive of the work is the development of propulsive devices that use detonation as the combustion mode (Rotating Detonation Engine, RDE). We have designed and built an experimental test bench of which the main part is an annular chamber with inner diameter 50 mm length 90 mm, and thickness 5 or 10 mm. The chamber can be equipped with cylindrical or conical kernels with lengths ranging between 12 mm and 90 mm and, for the conical kernels, with the apex half-angles ranging between 0± and 14.6±. The fuel is ethylene and the oxidizer is a mixture of oxygen and nitrogen, and they are injected separately in the chamber. We have considered several nitrogen concentrations so as to vary the reactive mixture detonability. The characterizations of the detonation regimes, velocities and pressures are based upon the analyses of signals from pressure transducers and of direct light visualizations from high-speed cameras. Our experimental results detail the ignition phase, the combustion modes and their stability. We have carried out experiments with several detonabilities, mass-flow rates and kernel geometries. Our main finding is that modifying the kernel geometry, specifically decreasing the kernel length and increasing its conicity (the apex half-angle) significantly improve detonation velocities and pressures, unlike the first two parameters that have much lesser influences, in our conditions. We have conducted a numerical analysis that suggests that dilution and heating of the fresh gases by detonation products explain the measured deficits of pressure and velocity. We have presented a calculation of thermodynamic efficiency which, contrary to former modeling includes a more realistic structure of rotating detonation.We have proposed a calculation of detonation-front height for the rotation modes and the chamber geometries in this work. Our study thus demonstrates the interest in further research work on inner geometry of rotating-detonation chambers and on phenomena that may be responsible for efficiency losses.