Investigation on spray characteristics and mixing mechanism of a backpressure-driven liquid jet in supersonic flows

Numerical and experimental investigations on spray characteristics and mixing mechanism of a backpressure-driven liquid jet in a tandem backward-facing step cavity were conducted in this study. The dynamic atomization process of a liquid jet driven by backpressure was accurately captured using a compressible two-phase flow large eddy simulation based on the Eulerian–Lagrangian approach. Fuel jet transport and fuel–air mixing with gas throttling were investigated systematically by comparing the influences of the mass fluxes of the gas throttling. The results indicate that, as the mass fluxes of the gas throttling increase, boundary layer separation occurs on the upper wall opposing the throttle slit, the upper wall opposite the injection section, and the bottom wall in sequence. The throttling shock wave gradually flows upstream, crossing the cavity, the backward-facing step, and the injection section as a result. The distance traveled forward is determined by the mass fluxes of the gas throttling. Fuel droplets in front of the throttling slit experience a “spray flash” phenomenon (it refers to the transient process in which the fuel spray moves forward from near the cavity to near the fuel injection position) under the action of the recirculation zone in the cavity. The streamwise velocity distribution of droplets shows a sharp mirror C-type distribution, but the Sauter mean diameter (SMD) distribution displays a circular mirror C-type distribution. The vertical velocity of droplets shows no characteristics of a uniform distribution. The SMD of droplets in the center of the spray is clearly larger than that at the edge of the spray, because small droplets with better followability enter the cavity in the recirculation zone of the cavity, and the SMD of droplets increases as the number of remaining large droplets in the main stream increases. Finally, the mixing enhancement mechanism of a backpressure-driven liquid jet in supersonic flows is mainly due to the combined effects of the throttle shock train and cavity-induced flow vortex.