The mathematical model presented in this work considers a two-dimensional sandwich configuration of AP-HTPB based composite propellant exposed to hot gas with gas phase reactions for ignition. Temperature dependent thermo-physical properties of condensed and gas phases have been used in the ignition simulation. Five gas-phase reactions are assumed to govern gas-phase heat generation. The specific objectives of the present investigation are to study the effect of initial gas temperature, initial gas composition and pressurization rate on the time required for the inert heating during ignition transient of HTPB-AP propellants and also on the propellant surface heat flux and temperature distributions. The gas-phase pressure is assumed to vary linearly with time and is uniform in a small region adjacent to the solid propellant.. The computer program developed for simulation is first validated with the experimental data available in the literature for rapid pressurization rates (varying from 20 to 300 GPa/s) for AP-PBAA propellant. Simulation is then carried out for AP-HTPB propellant by conducting parametric studies on the onset of ignition. The onset of thermal runaway of surface temperature during ignition transient is assumed to occur when the temperature near AP-HTPB interface reaches 623K which is the decomposition temperature of AP. This decomposition triggers a rapid thermal runaway of surface temperature at the interface setting the stage for sustaining the ignition process. Pressurization rates, initial reactant mass fractions and initial gas temperatures are varied from 0.5 to 2GPa/s, from 0.9 to 0.1 and from 800 to 1300K respectively. Simulation results show that as the pressurization rate and initial gas temperatures increase, the time delay for the thermal runaway decreases and surface heat flux increases. There exists an optimum time delay for a specific initial oxidizer to fuel initial mass fraction ratio of the gas phase for a given pressurization rate and initial gas temperature.
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