UCLA

With efficiencies in conventional rocket designs reaching the limit of theoretical possibility,there has been renewed interest in technologies which may be able to shift the boundaries of efficiency. One such technology is the rotating detonation rocket engine, which has the potential to create highly efficient engines in a small form factor. However, the detonation dynamics and complex flowfields inside the combustion chamber are greatly dependent on geometry; in particular, the downstream nozzle design affects dynamics inside the combustion chamber. In this work, high fidelity large eddy simulations of gaseous methane-oxygen rotating detonation rocket engines are presented for five engine configurations. The first simulation discussed is a validation case from the AIAA model validation in propulsion workshop. A laser model based on the Beer-Lambert law was developed for com- paring simulations with experimental laser absorbance measurements, and used to directly relate the simulation with experimental measurements of temperature, pressure, and CO column density in the exhaust of the engine. The analysis found that the simulation over- predicted pressure and thrust in the engine, as has been the case in other simulations of the engine, but that features in the exhaust flowfield closely matched experimental measurements. Close agreement between simulation and experiment was also seen in the measured CO mole fraction of the exhaust. The effect of adding a converging-diverging nozzle to a rotating detonation rocket engine was explored in the other four simulations, which consider an engine of two different lengths, with and without a constriction. The geometries matched experimental tests previously conducted at the Air Force Research Laboratory, and the operational modes attained in the simulations were found in all cases to directly relate to experimental observations. In the unconstricted geometries, flow in the chamber exceeded Mach 1 in pockets up- stream of the chamber exit. However, geometries with a diverging-converging nozzle directly followed the Mach-area relationship, with supersonic flow existing only in the diverging regions of the nozzle. This suggests a fundamental difference between the flowfield present in RDRE geometries with and without an area constriction, even though the constriction studied was gradual enough that no reflected shocks were observed traveling upstream. The formation enthalpy of the flow was measured inside the chamber for all configurations, and demonstrated that the difference in pressures and detonation structures associated with the chamber area constriction did not result in a significant change in the amount of energy released through combustion. Adding a constriction increased the average pressure of the combustion chamber, which would typically result in increased combustive energy release, but no associated release through combustion was observed. As such, although the use of a converging-diverging nozzle increased overall performance, the induced change in operating mode was detrimental to the extraction of energy from the flow. Changing chamber length was found to have little impact on the operation of an unconstricted rotating detonation rocket engine. However, changing the length of a chamber with a constriction resulted in a change in operating mode, and decrease in the strength of the counter-propagating waves. This suggests that, although unconstricted chamber geometries are likely optimized at short lengths, the length of the chamber is an important parameter to be considered when the engine utilizes a chamber area constriction.