Unsteady chemical kinetics behavior of AP/HTPB propellant with micro-scale model

The heterogeneous AP/HTPB propellant is a mixture of periodical units comprised of two AP (ammonium perchlorate, NH₄ClO₄) particles separated by a HTPB (hydroxyl-terminated polybutadiene) layer. So, the flame structure and the respond of the burning rate of such a material is resulted by an incredibly complex set of concurrent reactions taking place in the gas, liquid, and solid phases of a heterogeneous mixture, especially under unstable environment conditions. This article starts with a one-dimensional unstable chemical kinetics model to fix the respond of the burning rate to unstable conditions. Then, a two-dimensional micro-scale combustion model of ‘sandwich’ AP/HTPB propellant under unsteady-state conditions is explored to investigate the characteristic of microflame structure of this material and parameter distributions. It is demonstrated that for the periodic sandwich geometry a decrease in the environment pressure from 3 MPa to 2 MPa can sharply decrease the transient burning rate that is lower than the steady-state burning rate under corresponding pressure. Moreover, the temperature of the burning surface decreases when the depressurization process proceeds and the extinguishment will be considered to occur as the temperature of the burning surface is less than critical extinguishment temperature of 700 K. It is obtained from the two-dimensional, unsteady-state model that, when the depressurization rate is higher, the temperature of the burning surface decreases more sharply and the extinguishment phenomenon may occur.