Aluminum combustion modeling in solid propellant environments

Abstract

An improved numerical model of aluminum particle combustion has been developed based on previous modeling work done at Brigham Young University. Thermochemical calculations have been performed to determine the range of temperatures, pressures, and species distributions resulting from the combustion of various formulations of rocket propellants. The previouslydeveloped model has been modified to take into account such species distributions at high temperatures and pressures. Kinetic rate equations are introduced for the homogeneous reaction of COZ and Hz0 with ahuninum The model has produced results which are consistent with experimental data from many authors. In addition, the model has been used to predict alumirmm combustion rates for the range of conditions predicted by thermochemical calculations to be present inside the rocket motor, for which there is little or no experimental data. The effects of less reactive species such as HCl and HZ have been included for pressures up to 70 atm and temperatures up to 4000 K. Introduction Aluminum powder is presently used, and has been used for many years, as an ingredient to increase the specific impulse of solid rocket propellants because of the large amount of heat generated during the aluminum oxidation reaction. This occurs when aluminum reacts with the available Hz0 and CO, (the major oxidizing species) produced from a burning solid propellant In solid propellants, aluminum is used in quantities of lo-20% by mass, and the particles are typically 20-30 microns in diameter. During heat-up, these particles may melt and coalesce into larger agglomerates, ranging from 100-200 microns in size. It is very useful to be able to predict the time required for the aluminum particles to burn once they are ignited. Empirical correlations may be used, but data collected from rocket motors themselves are virtuahy non-existent because of the harshness of the motor environment Most empirical correlations have therefore been derived from carefully controlled lab experiments. However, these experiments differ greatly from the conditions in a rocket motor, and while such experiments are informative, it can be difficult to extrapolate their results to the conditions of a rocket motor. Computer modeling of ahnninq combustion thus becomes an attractive alternative. Background When aluminum ignites, the heat.of reaction is so great that the aluminum boils and thus remains at about 2800K (at 1 atm). The outward flux of gaseous ahuninum causes a flame zone to form at about 2-4 times’ the diameter of the particle, in a manner similar to that of a burning hydrocarbon droplet. In this thune zone a homogeneous reaction takes place between the ahnninq and, available oxidizer(s). Figure 1 shows a representation of the vaporphase aluminum combustion process. In the flame zone, the oxidized products consist of gas-phase AlxO, species such as AlO. Figure 1. Representation of the vapor-phase aluminum combustion process.’ There is no gas phase Al203 since Al203 dissociates at its boiling point However, there is usually not enough heat for all the oxide species to remain in the gas phase, so some A&O, species condense and associate to form