Supersonic Mixing and Combustion in a Cavity Flameholder

Efficient mixing and flame stabilization remain central challenges to supersonic combustion due to their relatively long time scales with respect to residence time. Cavity flameholders can aid such efforts, but can also introduce losses, and the entrainment and flameholding mechanisms remain incompletely described. We study these in detail by numerical simulations of the Illinois Arc-heated Combustion Tunnel (ACT) II facility, where a round underexpanded ethylene jet issues horizontally into a cavity under a $M = 3$ oxidizer freestream. During sustained combustion, a flame stabilizes around the jet and acts as a heat source on the freestream. Results are compared against wall-pressure measurements and shock-angle imaging from corresponding experiments. We focus on the relative effects that mean versus turbulent and inert versus burning flow have on mixing and entrainment and, in the sustained-flame case, the relative effects of flow versus chemical reactions via local Damköhler-number statistics. Particularly, we assess how recirculation and turbulence affect jet-flame structure and stabilization.