A two-dimensional model of composite propellant flame structure and burning rate

A two-dimensional, steady state model of a burning composite propellant is developed to study the characteristics of the combustion process. The model geometry is a periodic sandwich unit with a gas phase coupled to a solid comprised of two oxidizer laminates separated by a fuel binder layer. Species and energy equations are solved in both phases separated by a free surface boundary. Simplified global chemical kinetics are used for the gas phase reactions and the condensed phase pyrolysis. Solutions to the model yield a unique value for the burning rate of the propellant in addition to descriptions of the flame structure and surface geometry. Analyses thus far have focused on variations in pressure, length scale, and propellant formulation. Results show that many of the trends observed experimentally for similar Ammonium Perchlorate/Hydroxyl Terminated Polybutadiene (AP/HTPB) sandwich configurations are replicated. Pressure variations have shown the AP to protrude above the fuel (HTPB) at low pressures, and to recess at high pressures. Formulation variants such as oxidizer loading and length scale show that large fuel widths result in a split flame structure with two leading edges (fuel protruding between) and narrow widths produce a merged flame with a recessed fuel section. Finally, the calculated pressure sensitivity of the burning rate (0.4 ~ 0.6) is consistent with experimental values for composite propellant combustion.