Regression and Flame Structure in Cavity Flameholding Solid-Fuel Ramjet Fuel Grains

Introducing cavity flameholders into a solid-fuel ramjet fuel grain demonstrated increased fuel loading with sustained combustion in previously unfavorable geometries. Volumetric fuel loading improvements of up to 26% were demonstrated to sustain combustion. Regression patterns of cavity fuel grains are presented and show that the effect of implementing a cavity flameholder is to change the location of maximum regression and the reattachment point. The addition of a cavity flameholder does not appear to have a significant effect on combustion efficiency. However, it is noteworthy that longer cavities increased the chamber pressure above what was observed for a center-perforated fuel grains as a result of the increased mass addition and higher equivalence ratio associated with the higher regression rate. Large-eddy simulation computations were performed using a fourth-order discontinuous Galerkin finite element solver with a novel flamelet and progress variable formulation. The predictions agree well with the experiments and point to the increased heat transfer for longer cavities as the main flameholder mechanism. The larger heat feedback is supported by the formation of a stronger recirculation region, which leads to increased coherent fluctuations due to the transition between local and global instabilities.