There always exists the possibility that acoustic combustion-instabilities take place in the combustion chamber of a solid-propellant rocket motor. This is a phenomenon that the energy-gain of a standing acoustic wave in the combustion chamber exceeds the energy-loss of that wave, the pressure amplitude of that wave increases because of the surplus energy, and this increase causes the unstable combustion of the propellant. The response function of a solid propellant indicates the inclination of the propellant to cause the acoustic combustion-instability. Therefore, those engaged in the engineering of the solid-propellant rocket must clearly understand the meaning of the response function and roughly know about the experimental methods for its measurements. This article gives them necessary knowledge. First, fundamentals of acoustics are described. Next, the expression of the coefficient of the exponential increase of the acoustic pressure amplitude caused by the interaction of oscillatory pressure with oscillatory velocity of gas particles at the propellant surface [Culick and Yang, Progress in Astronautics and Aeronautics, Vol. 143, pp. 719-747] is presented. Then, the relation between this coefficient and the response function is explained. Last, the experimental methods for the measurements of the response function are presented.
Water mist has been recognized as an alternative for halogenated hydrocarbon fire suppressants. In this study, the effect of water mist on the methane/air counterflow diffusion flame was investigated numerically. The numerical simulation was performed using OPPDIF code in CHEMKIN package, modified to include the evaporation process of water mist. When the water mist evaporates completely in the flame zone, the evaporation process itself scarcely affects the critical stretch rate at extinguishment. The flame temperature is slightly affected by thermal effect, and it decreases with increase of the amount of water mist added. The simulated stretch rates at extinguishment are in good agreement with previous experiments in the range of the mass fraction of water mist Y0 < 0.060. The largest effect of water mist on stretch rate at extinguishment is attributed to sensible heat, followed by latent heat and chemical reaction. The chemical effect is rather small, but not negligible as compared to the dilution and thermal effects. The chemical effect is attributed to the decrease of radical species in chain branching reactions. The suppression effectiveness of water mist is better than IG-55, IG-541, N2 and CO2.