Inlet Distortion Effects on Fuel Distribution and Ignition in Scramjet Cavity Flameholder

The effects of inlet distortion and shockwave interaction on fuel-air distribution and ignition in a cavity-based flameholder with supersonic flow were investigated using nanosecond-gated laser-induced breakdown spectroscopy ( n-LIBS). Experiments were conducted in a Mach-3 freestream direct-connect-type wind tunnel with direct cavity fueling. Ethylene, C2H4, was used as the fuel for all testing. Inlet distortion was created using a wedge on the top wall of the tunnel, which formed an oblique shock. The distortion wedge was positioned at two distinct locations such that the shock intersected 1) the boundary layer just upstream of the cavity or 2) the shear layer above the cavity flameholder. N-LIBS with direct spectrum matching was implemented for quantitative measurements of gas density and fuel mole fraction throughout the cavity in both pre-ignited and ignited conditions for three different cavity flow patterns ( two distortion cases and no distortion). For all flow patterns fuel mole fraction was seen to increase in the cavity between the nonignited case and the ignited case, due to changes in air entrainment from the freestream. Distortion was seen to affect the cavity shear layer and thus the fuel distribution within the cavity, as well as local gas density.