Numerical investigation of upstream cavity enhanced combustion in a scramjet combustor

Cavities are widely used for flame-holding and ignition enhancement in scramjet engines. When used for this purpose the fuel injectors are commonly placed some distance upstream of or inside the cavity to allow entrainment of fuel into the cavity. Few efforts have investigated placement of the cavity upstream of the fuel injector, an arrangement which facilitates interaction between the cavity flowfield and the jet interaction around the injector. This study extends an earlier study on the mixing behaviour of a number of upstream cavity geometries to examine the combustion performance of these arrangements. Eight cavity geometries with length-to-depth (L/D) ratios ranging from 2.5-30 are numerically compared to a no-cavity baseline case for three thermal boundary conditions (isothermal 300K, isothermal 1800K and adiabatic), using hydrogen as fuel. The cavity geometries are found to increase combustion efficiency by up to 8%, with the increase larger at lower wall temperature conditions. The L/D = 15 cavity was found to perform well for all wall temperatures, while the performance of the L/D = 30 cavity varied the most with wall temperature. In addition, total pressure loss is seen to be very similar for the baseline and cavity cases, contrary to the trend observed when chemically frozen fuel was used. The flame center of mass is also farther away from the wall for the cavity cases, reducing wall temperature in the farfield. Wall heat flux was observed to increase near the cavity wall however, likely due to combustion in the cavity wall boundary layer. The study shows that the combustion performance trends are largely similar to the mixing performance trends, with the L/D = 15 and L/D = 30 cavities having the best and most temperature-dependent performance, respectively.