Multistage combustion and reaction coupling of micro-sized aluminum particles in hot steam

• Identified four combustion stages of micron-sized aluminum in hot steam. • Coupled heterogeneous and homogeneous reactions reveal in-situ O 2 generation. • Equivalence ratio strongly affects oxide rupture and combustion pathways. • Higher T/P shorten ignition delay and boost aluminum-steam surface reactions. Aluminum particles, characterized by their high energy density and favorable reactivity, exhibit significant potential for clean energy applications and advanced propulsion systems. This study integrates heterogeneous and homogeneous reactions to comprehensively simulate the combustion of micron-sized aluminum particles in high-temperature, high-pressure steam under constant-volume conditions. Four distinct combustion stages—core melting, oxide layer rupture, core burning, and post-combustion interphase heat transfer—are identified. During the core burning stage, molten aluminum participates in heterogeneous reactions with steam (H 2 O), yielding molecular hydrogen (H 2 ) and alumina (Al 2 O 3 ). Simultaneously, a detailed analysis of the key reaction pathways reveals that vaporized aluminum engages in homogeneous gas-phase reactions that produce oxygen (O 2 ) through radical-mediated mechanisms (e.g., involving OH, HO 2 ). A quantitative assessment shows that the Al-O 2 heterogeneous reaction contributes up to about 8 % of the total aluminum consumption. Results further reveal that increasing equivalence ratio extends combustion duration substantially, particularly the stage of oxide layer rupture. At low equivalence ratios, rupture is driven mainly by heat convection from the hot steam, while at higher ratios, it is mainly driven by heat release from heterogeneous reactions. Elevated initial temperatures and pressures markedly shorten both the ignition delay and total combustion duration. These insights deepen the understanding of multiphase aluminum combustion in steam and provide guidance for advancing the design of metal-fueled propulsion systems.