Study on the dehydrogenation, micro-explosion, ignition, and combustion of micro-sized AlH₃ particles in flat-flame hot exhaust

Aiming at the potential application in solid propellants or zero-carbon fuels, this work investigates the dehydrogenation, ignition, combustion, and micro-explosion behavior of the energetic aluminum hydride (AlH₃) particles (sized 20–240 μm) in the hot oxidative exhaust of a methane flat-flame with various temperatures of 1350–2100 K. Optical diagnostics, including high-speed photography, spectroscopy, and digital holography, show that AlH₃ particles typically undergo dehydrogenation, ignition, and self-sustained combustion stages. When the particles enter the hot exhaust and after a period of heating, the fast dehydrogenation occurs and a hydrogen flame with dimensions of approximately 2–3 mm forms around the particle, which persists for 3–4 ms. During the dehydrogenation stage, some particles exhibited micro-explosion behavior on a larger scale (5–10 mm). Under a typical environment temperature of 2100 K, the micro-explosion probability for particles with an average initial diameter of 50–150 μm ranged linearly from 52.7% to 5.5%. The three-dimensional trajectories of fragments after micro-explosion were reconstructed using high-speed digital holographic imaging coupled with a depth-of-field extension algorithm. Following dehydrogenation or micro-explosion, the particles ignited and transitioned into the combustion stage. The flame structure of remaining aluminum was fundamentally similar to that of pure aluminum particles of similar size, where a spherical flame was discernible, and an oxide cap formed through deposition on the particle surface. Spectral measurements indicated a blackbody radiation temperature of ∼3100 K. Finally, analysis of over 2000 original data points revealed that the AlH₃ particles can ignite at 1350 K under the present experimental conditions, significantly lower than that of traditional aluminum particles. The ignition and dehydrogenation delay times decreased by approximately 30% as the ambient temperature rose from 1350 to 2100 K, whereas they both exhibited a linear increase with initial particle diameter.