Influence mechanism of water mist containing dimethyl methylphosphonate on hydrogen-air explosions

Hydrogen is a renewable, carbon-free energy carrier and an important chemical feedstock. However, its high burning velocity and low ignition energy render it more hazardous than conventional fuels. To effectively control the explosion intensity of hydrogen-air mixtures in confined spaces and elucidate the suppression mechanism of micron-sized water mist containing dimethyl methylphosphonate (O=P(CH3)(OCH3)2), a rectangular constant-volume combustion chamber was first constructed, and a schlieren optical system was employed to capture fine flame structures under the addition of the suppressant. Secondly, based on the kinetic models proposed by Jayaweera et al. and Jing et al., a coupled chemical kinetic mechanism for O=P(CH3)(OCH3)2 was developed and validated for accuracy. Lastly, the influence of O=P(CH3)(OCH3)2-containing fine water mist on flame instability structures, mean flame speed, explosion overpressure, and mean pressure rise rate was then investigated under different equivalence ratios, together with the chemical kinetic mechanism and pathways governing hydrogen-air deflagration suppression. Results indicate that water mist containing O=P(CH3)(OCH3)2 promotes the formation of cellular structures on the flame front, thereby inducing flame instability. At equivalence ratios of 0.8, 1.0, and 1.5, the O=P(CH3)(OCH3)2-laden water mist effectively reduces the average flame speed (with reductions ranging from 24.2% to 47.2%) and suppresses the formation of tulip flames, which are replaced by wrinkled flame structures. The mist suppresses the pressure rise rate by reducing the laminar flame speed, but simultaneously enhances flame instability, which tends to increase the pressure rise rate. The overall suppression performance (with pressure reduction ranging from 41.0% to 65.8%) results from the coupling of these two opposing effects. Additionally, the O=P(CH3)(OCH3)2-laden mist achieves effective explosion suppression by reducing the concentrations of H∙, O∙, and OH∙ radicals, with reductions exceeding 80%. The physical suppression arises from pre-flame cooling and dilution effects of the water mist, while the chemical suppression is attributed to the decomposition of O=P(CH3)(OCH3)2 into phosphorus-containing radicals such as HOPO∙, HOPO2∙, HPO2∙, PO(OH)2∙, and PO(H)(OH)∙. These species scavenge reactive H∙ and OH∙ radicals, promoting the formation of stable products like H2 and H2O, thereby interrupting the chain reactions in hydrogen-air explosions.