Ignition and combustion of Al and ammonium perchlorate-coated Al particles in a double oxidizing air–H2O–Ar atmosphere

Abstract

Aluminum is a promising energy-containing material that can satisfy both aerospace and underwater thermodynamic propulsion systems. A high-temperature pressurized combustion chamber and a CO2 laser were adopted to investigate the ignition and combustion characteristics of 2, 5, and 40 μm aluminum particles in a 1.0 MPa air–H2O–Ar mixed atmosphere (with varying O2 and H2O concentrations). The combustion-promoting effect of the ammonium perchlorate-coated modification (namely, the AP@Al sample) was evaluated. The results show that Al melts and agglomerates into millimeter-sized droplets (covered with a gas-phase flame) after ignition, and its nonhomogeneous combustion is dominated by diffusion. The flame propagation is greatly enhanced by the AP combustion-promoting effect of oxygen/heat supply and particle dispersion, resulting in a significant decrease in the ignition delay time (ti, approximately 55–85%) and combustion time (tc, approximately 32–96%), whereas an extraordinary increase in the maximum combustion temperature (Tmax, approximately 170–224 K) and pressure changes (approximately 1.7–8.5 kPa, approximately 10.6–136.5 kPa/s) are also observed. Excess water vapor inhibits the flame propagation for AP@Al, viz. the flame area and number of ejected burning particles is decreased. The main factor affecting the combustion temperature is the oxidant rather than the particle size, which may be related to the small difference in the active aluminum content of the samples and the diffusion-controlled combustion mechanism. Due to the advantage of O2 for oxidation and the diffusion rate, the gradual replacement of H2O by air (similar to conversion to a fuel-lean system) leads to a decrease in ti, whereas Tmax is increased for all the samples, and the tc and self-sustaining combustion time are increased for the Al samples, whereas tc is decreased for AP@Al samples for both the equal-molar oxidizing gas concentration and equivalence ratio of O2–H2O control groups in this work. Since O2 creates a new pathway for AlO production, the ignition and combustion performance of Al and AP@Al samples is significantly improved when H2O is replaced by a small amount of air.