Propagation and Structure of Hybrid Aluminum–hydrogen–Air Detonations: A Computational Approach

Hybrid detonations involving metallic fuels and gaseous reactants exhibit complex multiphase coupling mechanisms that remain poorly understood despite their potential to extend detonation limits and improve pressure-gain combustion performance. This study investigates aluminum–hydrogen–air hybrid detonations using a fully compressible, multiphase Eulerian–Eulerian framework with adaptive mesh refinement. The objective is to quantify the effects of aluminum particle size on detonation stability, propagation speed, and internal structure. Simulation results suggest that particle heat release overlaps with the gas-phase reaction zone for sub-micron particles (0.1 µm), leading to strongly coupled hybrid detonations with regular cellular structures and velocities near 2090 m/s. As particle size increases, the reactive coupling weakens and the detonation transitions toward decoupled or failed regimes. Intermediate particle sizes (3–6 µm) delineate the transition between reactive and inert behavior, governed by competition between aluminum ignition delay and gas-phase induction time. A detailed grid-independence analysis confirms that sub-micron-particle detonations require five to six refinement levels (△x ≋ 2.5 µm) for quantitative convergence, while coarse meshes overpredict detonation velocity by up to 2.4%. The converged fine-grid results are consistent with previous hybrid detonation studies. Crucially, small additions of reactive particles enable self-sustaining detonations in otherwise non-detonable lean hydrogen mixtures (ϕH2 = 0.25). The findings highlight fundamental scaling relations between particle size, reaction timescales, and multiphase energy coupling, offering new insights for pressure-gain propulsion systems and energetic formulations employing reactive metals. This work is positioned as a numerical sensitivity and mechanism-exploration study rather than a closure model for aluminum combustion in detonative flows.