Effect of flow distortion on fuel/air mixing and combustion in an upstream-fueled cavity flameholder for a supersonic combustor

Abstract

This paper describes an experimental study of the effects of an incident shockwave on the flow field, fuel distribution and combustion within a cavity flameholder with upstream fuel injection. Two impingement locations are employed: (1) near the fuel injector (the so-called shock-on-jet case) and (2) on the cavity shear layer (the shock-on-cavity case). Shadowgraph is used to characterize the flow field. Air seeded with nitric oxide (NO) is used as the simulated fuel and the resulting planar laser-induced fluorescence (NO-PLIF) from NO molecules is used to characterize fuel/air mixing while planar laser-induced fluorescence of OH molecules to characterize the actual combustion process. The shadowgraph and NO-PLIF images are compared with a CFD (Computational Fluid Dynamics) solution of the Reynolds-averaged-Navier Stokes (RANS) for assessment and explanation of experimental results of non-reacting tests. The effect of the shock on the cavity shear layer is to control the fuel distribution within the cavity. The effect of the shock on the jet is to force the shear layer deep within the cavity, which results in higher fuel concentrations near the cavity centerline. The shock-on-cavity case causes the shear layer to separate upstream of the cavity. Mixing uniformity is enhanced by the increased breakup of the fuel plume. Combustion is stronger and more uniform with the shock impinging on the cavity, while it is limited to the edges of the cavity with shock impingement on the jet. The greater mixing afforded in the shock-on-cavity case reduces the fuel concentration near the centerline and allows stronger burning in the center of the cavity. Doubling the fuel injection momentum flux ratio does not strongly affect the pattern of fuel distribution in either case, but combustion in the shock-on-cavity case is reduced, because the fuel concentration at the centerline is high.