Ignition and combustion characteristics of micron-sized Al-Li alloy particle in high-temperature gas flow

Abstract

Compared to pure aluminum particles, Al-Li alloy particles exhibit shorter ignition delay times and smaller combustion product sizes, making them a superior metallic additive for solid propellants. Therefore, this study experimentally and theoretically investigated the ignition and combustion characteristics of micron-sized Al-Li alloy particle. First, the ignition delay times of 8 μm Al-Li alloy particle over a wide range of temperatures were measured using a reflected shock tube. Second, theoretical models of ignition and combustion of micron-sized Al-Li alloy particle in high-temperature gas flow were developed, by considering comprehensive processes including convective heat transfer, radiative heat transfer, heterogeneous surface reactions, phase change, oxide layer rupture, diffusion-controlled combustion and micro-explosion. The ignition delay times and critical ignition temperature predicted by the model show good agreement with the experimental results. Detailed analysis reveals that micro-explosion can occur as the saturation vapor pressure of lithium exceeds the contact pressure at the Al-Li interface during combustion. Parametric studies further indicate that elevating ambient pressure increases the contact pressure at the Al-Li interface, thereby inhibiting micro-explosion. In contrast, raising ambient temperature increases the saturation vapor pressure of lithium, thus facilitating micro-explosion. Finally, an empirical formula was derived to predict the critical ambient pressure at which micro-explosion occurs in Al-Li alloy particle with 5 % lithium content.

Novelty and Significance Statement

In this study, both the ignition delay times and the critical ignition temperature of Al-Li alloy particle were measured using a reflected shock tube. Subsequently, theoretical models of ignition and combustion of micron-sized Al-Li alloy particle in high-temperature gas flow were developed, encompassing convective heat transfer, radiative heat transfer, heterogeneous surface reactions, phase change, oxide layer rupture, diffusion-controlled combustion and micro-explosion. Based on the model, the heat and mass transfer mechanism of Al-Li alloy particle during ignition and combustion was revealed, and particularly elucidating the micro-explosion mechanism as well as the effects of ambient pressure and temperature on micro-explosion. Finally, an empirical formula was proposed to predict the critical ambient pressure at which micro-explosion occurs in Al-Li alloy particle.