Analyzing Simulations of Flameholder Cavity Designs to Prevent Unstart in Scramjet Engines

Flameholder cavities are an essential part of scramjet engines. They provide extra time for air and fuel to mix, which creates a more reliable air-breathing engine. However, with the volatile nature of internal supersonic airflow, the design of the flameholder cavity is important for managing the back pressure at the engine's outlet to avoid unstart: a phenomenon at the inlet of the engine that restricts the delivery of oxygen to the combustion section. Excessive amounts of back pressure or strong shockwave interactions with the boundary layer can cause unstart. The shape of the flameholder cavity affects these components, so five cases were simulated in StarCCM+ using Reynolds Averaged Navier Stokes (RANS) model, specifically the SST turbulence model. The first case was the accepted shape for scramjet engines – a rectangular cavity with a ramp leading out of it toward the outlet. The next four cases were compared to the original: a rectangular-shaped cavity with no ramp, a circular-shaped cavity, and two triangular-shaped cavities with different depths. In each of the cases including the original, the residuals spiked multiple times, and back pressure was evident in the scalar scenes. The circular-shaped cavity had the most stable results along with the rectangular cavity with no ramp. Knowing which cavity geometries will positively affect airflow in scramjet engines will lead to less risk of unstart.