Mitigation of methane/air explosion in a closed vessel by ultrafine water fog

The mitigation effect of ultrafine water fog on the methane/air explosions with methane concentrations of 6%, 9%, 11% and 13% were experimentally studied in an entire closed visual vessel. The ultrafine water fog was generated in the vessel directly by ultrasonic atomization method. A high speed camera was used to record the flame propagation processes. The explosion flame evolution processes were discussed. The experimental results indicate that the maximum explosion overpressure (ΔPmax), the pressure rising rate ((dP/dt)max) and the flame propagation velocity decreased after adding water fog. The presentation of flame cellular structures after adding water fog and the stifling effect of water vapor caused the extinguishing of the flame in the burned zone and slowed down the flame propagation. The water fog could mitigate the methane explosion of low concentration (6%) absolutely. When applied at the high concentration conditions (9%, 11% and 13%), the water fog still presented a significant suppression effect. The maximum decreasements of ΔPmax under the three high concentration conditions with water fog were 21.1%, 26.7% and 22.9%, respectively, while the maximum decreasements of (dP/dt)max were 71.7%, 77.1% and 52.0%, respectively.