Heat transfer effects in nano-aluminum combustion

Nano-aluminum combustion remains poorly understood, as evidenced by recent paradoxical measurement results. While nanoscale Al is often assumed to burn in the kinetic limit, optical temperature measurements of nano-aluminum particles combusting in a heterogeneous shock tube at pressures near 20 atm showed peak temperatures above 3000 K in ambient air at 1500 K. This temperature overshoot and the simultaneous burning times measurements greater than 100 ìs cannot be described from an energy balance considering continuum heat transfer losses from the particle. A conservative estimate assuming all reaction energy is used for sensible heating of the aluminum particle requires complete combustion in less than 1 ìs to reach peak temperatures of 3000 K suggesting deviation from continuum mechanics. Similar heat transfer effects are seen in laser induced incandescence (LII) experiments of various nano-particles. In the shock tube and LII experiments the particle Knudsen number is near unity and non-continuum heat transfer effects and slip conditions can be expected. Previous work by Igor Altman suggests a formula which predicts an upper limit on the energy accommodation coefficient (EAC) which characterizes the energy transfer between a gas particle and surface. A simple model considering surface combustion and an energy balance using various heat transfer loss mechanisms shows that an upper limit of 0.005 for the EAC predicted by Altman's theory agrees well with experimental data. An EAC of 0.0035 produces a best fit of the thermal profile for varying nano-aluminum particle sizes and predicts the temperature overshoot at high pressures. The decrease in the EAC for nano-aluminum has significant implications in the application and modeling of nano-aluminum used in explosives and solid rocket motors where ideally the particle transfers energy to the surrounding atmosphere. In such cases of low EAC radiation plays a more significant role in particle heat transfer.