Flame-Spreading and Combustion in Packed Beds of Propellant Grains

Abstract

This paper is concerned with the prediction of the pressure history during the process of flame-spreading and combustion of solid-propellant grains as would occur, for example in a gun cartridge. Solution of the governing conservation equations for the two-phase media requires the use of empirical relations to account for the physical processes of momentum and energy interaction between the solid grains and hot propellant gas. The results indicate the extreme importance of these interaction relations on the predictions of the pressure and velocity field. and at the other is a projectile which will begin to move at a given prescribed pressure. This 'piston then moves through a long length of channel of the same cross section as the cham- ber, in order that the flow processes following piston motion can be followed. A predescribed igniter mass flow provides gaseous products which, in turn, ignite the bed. Since solid propellants have high burning rates, the rate of gas generation within the chamber is high. Therefore, the pressure is rising rapidly during this flame-spreading process. The pressure rises enhances the pressure-dependent burning rate of the propellant, the result of which is a rapidly accelerating combustion process, with generally steep gradients along the length of the chamber. This gradient results in a high-velocity flow of the gases toward the piston end and into the unburned portion of the propellant. Hence, the flame-spreading and pressurization is rapid, and the entire burn is completed in a matter of milliseconds, if the ignition source is strong enough. Looking at flow process taking place, one may determine the effect on the particle bed. The hot igniter gases initially are driven into a quiescent bed of propellant particles. These ignite the particles nearest the igniter source. The mass generation and subsequent pressurization occur, and a flowfield develops within the chamber. The propellant par- ticles exert a drag force on the flowing gases and are ac- celerated. As shot-start pressure is approached, the entire bed usually has been set into motion.J Throughout the burn, the particles continue to decrease in size as they burn. Kuo and Summerfield2 have pioneered the analysis of packed-bed solid-propellant combustion.