Molecular diffusion and phase stability in high-pressure combustion

The increased modeling complexity needed to simulate high-pressure combustion in rocket engines results in significant computational costs–costs which are not always justified for engineering applications. Multicomponent diffusion computations are at least 40% more expensive than the constant Lewis number diffusion assumption for the simplest hydrogen/oxygen combustion; the computational penalty increases rapidly with the number of species under consideration. The higher-fidelity diffusion modeling is justified in cryogenic fuel combustion if it affects the phase-stability of the propellants. We investigate the impact of the mixture averaged, multicomponent, and constant Lewis number diffusion models of a laminar counterflow flame at trans- and supercritical conditions for typical rocket propellants, namely: hydrogen, methane, and kerosene. Using vapor liquid equilibrium (VLE) theory, we show that, even in the limit cases, the mass diffusion model has a limited effect on the phase stability and pseudo-phase change of the propellants; the majority of the differential diffusion errors are located in the high temperature/ideal gas regions of the flame. The differential diffusion error due to real fluid thermodynamics is at most about 20% of the differential diffusion error which occurs in the ideal gas region of the flame. As a result, the impact of the differential diffusion remains similar to low-pressure combustion conditions, thus supporting the use of engineering-level simplifications for the simulation of these complex high-pressure reactive flows.