Entropy waves produced in oscillatory combustion of solid propellants

Abstract

The dynamic response of a flat solid-propellant flame to an oscillating pressure field was studied experimentally in a window burner fitted to a T-tube rocket motor that served as a pressure oscillator. The burned gas temperature was measured as a function of time (or phase) during a pressure oscillation, and also as a function of distance from the surface. Such instantaneous measurements of gas temperature, when coordinated with simultaneous measurements of pressure, provide a measure of the entropy content of each element of gas as it flows away from the flame. Since the entropy content of each element of gas is nearly conserved as it flows along, an entropy wave train is formed. It is possible to,make deductions regarding the physics of the dynamic burning process by comparing the magnitude and phase of the observed entropy waves with the theoretical values predicted on the basis of a particular flame model. In general, the results show that the temperature of the gas flowing from the combustion zone responds neither isentropically nor isothermally to the pressure. Such responses were assumed in previous publications on the subject. However, on the basis of the KTSS model published recently, the magnitude and phase of the entropy wave are expected to vary in a more complicated form with the imposed frequency and the propellant properties. The observed waves reported in this paper tend to support these KTSS theoretical expectations.