Numerical analysis of AP/HTPB composite propellant combustion under rapid depressurization

A two-dimensional steady/unsteady combustion model has been developed for ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/HTPB) composite propellant. An equivalent source term method is applied to describe the decomposition at the burning surface, and the combustion field in the gas phase is solved coupling with the thermal field in the solid phase. Firstly, a series of numerical simulations were carried out for steady combustion for pressure ranging from 0.2 MPa to 5.0 MPa. It is shown that the flame develops from a premixed structure at low pressure to a diffused one at high pressure. And the calculated burning rate agrees well with the experimental result. Then the unsteady combustion under rapid depressurization was predicted for the initial pressure of 5.0 MPa. The simulation results indicate that during the depressurization, the dominant factor of combustion turns from the diffusion flame to the AP decomposition flame. As the pressure decays, the heat release rate of the overall flame reduces and the flow rate of the combustion gas increases, with the out-of-phase blowing effect enhanced. And thus the heat feedback from the flame to the propellant burning surface is reduced. The higher the depressurization rate is, the faster the heat feedback and the surface temperature decrease.