Effect of water vapor addition on explosion parameters and flammability limits of hydrogen-air

• Steam starts to highly influence H 2 -air–steam flammability limits above ∼ 45 °C. • 15.6 % steam content (at 55 °C) lowers explosion severity by 45.5 % for φ = 1.0. • Steam content narrows flammability limits, reaching non-flammable at 86 °C. This study addresses a critical gap in the literature by investigating the significant role of water vapor in mitigating the flammability and combustion severity of hydrogen-air mixtures, particularly at moderate initial temperatures. The research uses a 20-liter spherical vessel to investigate the impact of initial temperature and saturated water vapor addition on hydrogen-air mixture flammability limits at initial 1 bar. Results show that, for dry flammable mixtures, an increase in initial temperature leads to a reduction in both maximum pressure (P max ) and maximum rate of pressure rise ((dP/dt) max ), while the time to reach these values remains almost unaffected. The highest P max is recorded at a hydrogen concentration of 30 %, whereas the peak rate of pressure rise and shortest durations to achieve P max and (dP/dt) max occur at a 35 % hydrogen fraction. In the case of wet mixtures (hydrogen-air–water vapor), elevated initial temperatures increase saturation vapor pressure, further decreasing P max and (dP/dt) max , thus narrowing the flammability range. At an initial temperature of 86 °C, with a corresponding saturation water vapor pressure of 602 mbar, the mixture becomes non-flammable due to water vapor inerting. Similar to dry mixtures, critical hydrogen fractions for achieving maximum pressure and maximum pressure rise rate in wet mixtures are 30 % and 35 %, respectively. Additionally, the maximum pressure reduction percentages between wet and dry stoichiometric mixtures at 25, 35, 45, and 55 °C are 2.74 %, 4.19 %, 5.63 %, and 8.74 %, respectively. Corresponding reductions in the maximum pressure rise rate for wet versus dry stoichiometric mixtures at these temperatures are also 12.61 %, 19.25 %, 26.42 %, and 45.48 %.