Computational Fluid Dynamics Modeling of Plasma Assisted Combustion in High Speed Flows

Hypersonic airbreathing propulsion has become a topic of greatly renewed interest in the last few years. However, the development of a practical scramjet engine has been limited by the problem of supersonic combustion (SSC), which is not only difficult to model, but also expensive and challenging to physically test. Recent advancements in the field have shown that plasma assisted combustion (PAC) can reduce ignition delay times and increase blowout limits by providing heat and radical species to the reacting flow. We develop a modeling scheme to simulate PAC via a novel plugin for the US3D CFD code. Our novel framework is validated using a close comparison of RANS simulations of two combustion modes, pre-cavity autoignition and cavity-stabilized ignition, with prior experimental results. We predict the plasma effect on extending operability limits of a cavity-stabilized, high-speed combusting flow, and assess its impact on combustion efficiency and system performance metrics. For the first time using CFD, we demonstrate that the addition of the thermal plasma allows for cavity ignition at lower stagnation temperatures, successfully increasing the critical inverse Damköhler number at which blowout occurs by as much as 15% on the lean side, and narrowing to 5% improvement towards stoichiometric. However, during autoignited combustion mode at high stagnation temperature, the plasma effect is marginal, with less than 1% improvement in stream thrust and combustion efficiency. These results suggest that PAC has a remarkable advantage in extending high speed combustor operability limits and thus vehicle flight envelope, but does not significantly contribute to performance enhancement under stable operation.