Ignition and combustion experiments on Mg/AP composite fuels in different reaction environments

Magnesium is considered a viable fuel for underwater propulsion owing to its ignitability and the exothermicity of the Mg-water reaction. In this experimental study, we examine the ignition and combustion of Mg/AP composite fuels in three different reaction environments: air, water vapor, and argon. We consider Mg/AP composite fuels containing different Mg loadings and particle sizes. Using thermogravimetry and time-resolved optical imaging, we analyze the thermal oxidation, ignition delay time, steady-state gas-phase combustion processes, and flame temperature of the composite fuels. Our findings show that a water vapor environment can lower the initial oxidation temperature of Mg by 120 °C relative to a baseline air environment. Laser ignition tests reveal that the combustion intensity of Mg/AP composite fuels rises as the Mg loading increases and as the Mg particle size decreases. The ignition delay time of the Mg/AP composite fuels is found to be longer in a water vapor or argon environment than in an air environment. Owing to the intense reaction of Mg in air, the flame temperature is found to exceed that in argon and water vapor environments. The latent heat of evaporation of water tends to lower the flame temperature in the water vapor environment. From these empirical observations, we propose a phenomenological mechanism for the combustion of Mg/AP composite fuels. Overall, this study shows that the Mg loading, Mg particle size, and environmental composition can have profound effects on the ignition and combustion behavior of Mg/AP composite fuels. These findings could be used to steer the development of water-reactive metallic fuels, with a view to improving the design of underwater propulsion systems. © 2022 The Combustion Institute