Spherical flame propagation Behaviors in Iron-Methane-Air mixtures

• Flame propagation of iron–methane–air mixtures was investigated experimentally. • Flame speed decreased near stoichiometric methane due to iron particle heat loss. • Under lean methane, iron oxidation supplied extra heat and enhanced propagation. • XRD showed Fe 2 O 3 formation under lean and FeO/Fe 3 O 4 under stoichiometric condition of methane-air. • Results clarify gas–solid interactions governing hybrid flame propagation. Recently, interest in metal powders such as iron has grown due to their high energy density and carbon-free combustion, making them promising sustainable energy carriers. However, their heterogeneous combustion and low burning velocity limit standalone use, and hybridization with gaseous fuels is often required to support ignition and stable propagation. Nevertheless, the interaction between gas and solid phases during ignition and flame propagation remains insufficiently understood. In this study, the flame propagation behavior of iron–methane–air hybrid mixtures was experimentally investigated under various methane equivalence ratios. Combustion tests were performed in a 4.2 L closed chamber equipped with a schlieren visualization system and a high-speed camera. Spherical iron powders were dispersed into methane–air mixtures and ignited by spark discharge, and the resulting flame radius and propagation speed were measured and compared with those of pure methane–air flames. Results show that near the stoichiometric methane condition, the flame propagation speed of hybrid mixtures decreased to roughly half that of methane–air flames, as the heat losses outweighed the heat released from iron combustion. In contrast, under lean methane conditions, the propagation speed exceeded that of methane–air flames, indicating additional heat release and volumetric expansion from iron combustion.