Dynamic combustion of solid propellants: effects of unsteady condensed phase degradation layer

A numerical model is developed for pressure- and/or radiation-driven dynamic combustion of solid propellants. The model relaxes the two key assumptions invoked in the classical, quasisteady, homogeneous, one-dimensional flame (QSHOD) model, namely, the quasisteady gas-phase (QSG) and quasisteady condensed phase degradation layer (QSC). First, the proposed non-QSC model is validated with respect to previous results of radiation-driven burning. The model is further validated for steady burning results by comparing with experimental data. Then, the effects of relaxing the QSC assumption on dynamic burning predictions are investigated under conditions of 1) an oscillating and 2) an exponentially decreasing pressure. It is confirmed that relaxing the assumptions of both QSC and QSG are equally important for computing the unsteady burning characteristics of solid propellants. For pressure-driven burning relaxing the QSC assumption results in a destabilizing effect on the frequency response function. The predicted response function magnitude is quantitatively compared with experimental data for double base propellants. The comparison seems to be better with a value of condensed phase activation energy higher than that suggested by A. A. Zenin ("Thermophysics of Stable Combustion Waves of Solid Propellants," Nonsteady Burning and Combustion Stability of Solid Propellants, edited by L. De Luca, E. W. Price, and M. Summerfield, Vol. 143, Progress in Astronautics and Aeronautics, AIAA, Washington, DC, pp. 197–231). Results of unsteady simulations of burning under a rapid depressurization transient exhibit salient features of combustion recovery and extinction observed in experiments. The predicted critical depressurization rate is found tomarkedly decrease (by a factor of four in the particular case simulated) when the QSC assumption is relaxed