The Influence of Progress Variable Selection in the Evolution-Variable Manifold Framework

The simulation of high-speed propulsion systems is limited by the cost of finite-rate chemistry. The Evolution-Variable Manifold (EVM) approach reduces this cost by precomputing a library of states based on batch reactors for use at simulation runtime. A progress variable is used to track the extent of reaction towards equilibrium. In hydrogen combustion, the heat release within these batch reactors can be obtained through both oxidation and recombination processes. We examine four candidate progress variables to assess their ability to capture both of these modes in high-speed combustion: water vapor, heat release, reaction entropy, and a two-phase progress variable. Advantages and limitations for these tabulation choices are identified. Preliminary steady simulations of a jet in supersonic crossflow are used to assess these approaches for autoignition-driven flows. We find progress variables based on heat release may be unsuitable for this regime, while ones based on reaction entropy slightly underpredict the temperature rise compared to conventional and two-phase progress variables. Future work will investigate further modification and optimization of progress variables within the EVM framework and determine their influence on the predicted heat release within cavity-stabilized combustors.