Laser-induced ignition and combustion of individual Mg particles under CO2 atmosphere

Abstract

Combustion studies of individual Mg particles under CO2 atmosphere are essential for the development of Mg/CO2 powder rocket engines for in-situ resource utilization (ISRU) technology on Mars. In this work, the ignition and combustion characteristics of individual micron-sized Mg particles prepared by gas-atomization method under CO2 atmosphere were investigated in detail. It was found that the ignition delay of Mg particles was determined by two factors, i.e., ambient pressure and particle size, and the former had a greater influence on the ignition delay of the particles. As the pressure increased, the ignition delay time of the particles decreased significantly. For particle sizes between 30 μm and 35 μm, the ignition delay times of Mg particles at 0.9 MPa and at 1.6 MPa were only ∼2 % and ∼1 % of that at atmospheric pressure. The ignition delay time of Mg particles increased linearly with the increasing particle size at both atmospheric pressure (0.1 MPa) and high-pressure (1.6 MPa) CO2. During melting and evaporation, the newly generated non-dense oxides grew unevenly along the surface of Mg particles, leading to inhomogeneity in the mechanical strength of the oxide layer. Fracture occurred preferentially at weak locations with low compressive strength limits, resulting in selectivity in the direction of the Mg vapor injection at the moment of shell breaking. Finally, it is proposed that the ignition and combustion of individual Mg particles should be dominated by a liquid-gas reaction and that pressure and particle size are crucial for the combustion performance of micron-sized Mg particles. Novelty and significance statementClassical understanding of Mg/CO2 combustion for in-situ resource utilization technology of Mars exploration relies on the gas-phase reaction, however, difficultly explaining the overall ignition and combustion evolution. A novel observation and recognition of traditional Mg/CO2 combustion reaction through self-built real-time microscope with high-speed cinematography and highly-focused inductive laser ignition, indicating that the oxides of individual Mg particles in CO2 atmosphere grow non-uniformly along the surface, which leads to the preferential shell breaking of Mg vapors at the weak points with lower mechanical strength and eventually causes the particles to be ignited. Highly temporally and spatially resolved visualized images, spectrum, temperature, and product SEM information firstly demonstrate that the ignition and combustion of individual Mg particles in CO2 are dominated by a liquid-gas reaction, experiencing the surface and gas-phase combustion and confirming that high pressure significantly improves the ignition performance of Mg particles and promotes gas-phase combustion in CO2.