Revisiting the minimum ignition energy of hydrogen–oxygen mixtures: Insights from experiment and theory

The minimum ignition energy (MIE) of hydrogen–oxygen mixtures, reported as approximately 1.2 μJ, is over an order of magnitude lower than that of hydrogen–air mixtures, indicating heightened ignition hazards. Despite its critical importance for hydrogen safety, the literature provides limited insight into the experimental determination and theoretical prediction of this value. This study investigates the MIE of hydrogen–oxygen mixtures near stoichiometric conditions through controlled spark ignition experiments. The quenching distance, defined as the minimum electrode gap enabling flame propagation, was determined through repeated ignition trials across varying hydrogen concentrations and gap distances. Logistic regression analysis was used to identify the gap corresponding to a 50% ignition probability, providing a statistically robust estimate of the critical flame kernel diameter. This experimentally derived quenching distance was interpreted as the minimum flame kernel size and used to calculate MIE values using two theoretical approaches: one based on the enthalpy required to heat the combustible mixture and another based on the mechanical work necessary to initiate flame expansion. The MIE values estimated using the work-based model aligned closely with the reference value of 1.2 μJ. Across hydrogen concentrations from 50% to 70%, the estimated MIE values ranged between 0.8 μJ and 2.4μJ, with the lowest MIE observed at 60% hydrogen concentration. These results underscore the importance of accurately determining flame kernel size and employing rigorous thermodynamic modeling to characterize ignition thresholds for hydrogen–oxygen mixtures. The findings provide critical insights for evaluating ignition hazards and contribute to the development of safer hydrogen energy systems.