Coupling characteristics of combustion-gas flows generated by two energetic materials in base bleed unit under rapid depressurization

Abstract

The present study is devoted to investigate the physical characteristics of the coupling combusting-gas flows generated by gun propellant and pyrotechnics along with the effect of initial pressure on the extension behavior of the igniter combustion jet under the rapid depressurization process that the BBU (base bleed unit) undergoes when a BBP (base bleed projectile) is flying out of the muzzle. A semi-closed bomb is designed to reproduce the depressurization process. The near-nozzle pattern evolution of the exhaust plume is observed by using an HSC (high-speed camera). Based on the experiment, a two-dimensional axisymmetric model is established including the high-resolution upwind scheme AUSM+ and the finite-rate chemistry model (no turbulence-chemistry interaction). The cell-centered FVM (finite volume method) is employed to simulate the coupling characteristics of the two high-temperature combustion gases under sharp depressurization. The results show that as the pressure drops rapidly, the exhaust plume gradually transforms from highly underexpanded supersonic flow into a subsonic flow, in where the wave structure converts from Mach reflection to regular reflection. The periodic shock diamond and rhombic flame string are formed, and finally the flame becomes continuous. In the subsonic flow, the radial heat convection and diffusion downstream of the flame in BBU are stronger than upstream. During the depressurization process, the influence of initial NPR (nozzle pressure ratio) becomes more significant. As the initial NPR increases, the igniter flame squashes downstream more slowly, and the heat loss of the gas expansion accelerates, thus the radial heat convection and diffusion in BBU decrease.