Combustion of bimodal aluminum particles and ice mixtures

The combustion of aluminum with ice is studied using various mixtures of nano- and micrometersized aluminum particles as a means to generate high-temperature hydrogen at fast rates for propulsion and power applications. Bimodal distributions are of interest in order to vary mixture packing densities and nascent alumina concentrations in the initial reactant mixture. In addition, the burning rate can be tailored by introducing various particle sizes. The effects of the bimodal distributions and equivalence ratio on ignition, combustion rates, and combustion efficiency are investigated in strand experiments at constant pressure and in small lab-scale [1.91 cm (0.75 in.) diameter] static firedrocket-motor combustion chambers with center-perforated propellant grains. The aluminum particles consisted of nanometer-sized particles with a nominal diameter of 80 nm and micron-sized particles with nominal diameters of 2 and 5 μm. The micron particle addition ranged from 0% to 80% by active mass in the mixture. Burning rates from 1.1 (160 psia) to 14.2 MPa (2060 psia) were determined. From the small scale motor experiments, thrust, C*, Isp, and C* and Isp efficiencies are provided. From these results, mechanistic issues of the combustion process are discussed. In particular, overall lean equivalence ratios that produce flame temperatures near the melting point of alumina resulted in considerably lower experimental C* and Isp efficiencies than equivalence ratios closer to stoichiometric. The infstitution of micron aluminum for nanometer aluminum had little effect on the linear burning rates of Al/ice mixtures for low-mass infstitutions. However, as the mass addition of micron aluminum increased (e.g., beyond 40% 2-μm aluminum in place of 80-nm aluminum), the burning rates decreased. The effects of bimodal aluminum compositions on motor performance were minor, although the experimental results suggest longer combustion times are necessary for complete combustion.