Visualization of fuel–air mixing in an axisymmetric cavity-based scramjet combustor by 3-pentanone tracer planar laser induced fluorescence technique

Fuel–air mixing characteristics in an axisymmetric cavity-based scramjet combustor were investigated using the 3-pentanone tracer planar laser-induced fluorescence (PLIF) technique. The optically accessible scramjet combustor was deployed in a direct-connected rig at Mach 6 flight enthalpy. The mixture containing the air and the 3-pentanone vapor was used as a simulated gas fuel, and the fuel intensity distribution in streamwise and spanwise directions was visualized by the 3-pentanone PLIF technique. Distributions of the fuel edge were calculated and analyzed according to their variation due to the increase in fuel injection pressure. It was observed that most of the fuel was convected inside the cavity region by the vortex interaction of the jet with the cavity shear layer and the cavity recirculation zone. The extracted penetration heights based on the mean PLIF images were then observed to increase with the jet momentum flux ratio, and this function was expressed by a fitting curve. 3-pentanone PLIF results in three spanwise sections showed that the radial centroids of the ring-like PLIF profile were located inside the cavity region, and they approached the centerline gradually with increasing fuel injection pressures. The large eddy simulation results showed that counter-rotating vortex structures and the cavity recirculation zone enhanced the fuel–air mixing. The vortex pair induced by the fuel jet was found to improve the turbulent fluctuations inside the fuel plume and significantly alter the shape of the plume, whereas the cavity recirculation zone tended to transport more fuel from the shear layer inside the cavity region.