Experimental Study Of Enhancing And Mitigating The Methane/Air Explosion By Ultrafine Water Mist

The risk of gas/dust explosion widely exists in the mining, transportation and utilization processes related to the flammable gas/dust such as coal mining, oil and gas transportation, grain processing and nuclear power etc. Especially the methane/air explosion in the coal mining occurred most frequently and caused huge loss to the process industry every year. For this reason, it is always a focus to study the explosion characteristics and develop the effective mitigation technologies of the gas/dust explosion.Considering the significant extinguishment performance of water mist when applied in the fire hazards field, besides, as an economic, water-saving, non-pollution and easily available resource, the ultrafine water mist technology has attracted many attentions for the gas explosion mitigation. The previous studies mainly focus on the influence of water mist on the gas explosion under the open conditions and the results indicate that the ultrafine water mist also has a mitigation effects on gas explosion. However, the practical explosion environments are more close to the confinement condition and the researches on the influence of ultrafine water mist on the gas explosion under the closed conditions are less reported. While the explosion enhancement after spraying is also lack of systematic investigation.Consequently, based on the entire sealed and visual requirements, a gas/dust explosion experimental system with ultrafine water mist was designed and established. The droplets’ Sauter Mean Diameters were measured by Phase Doppler Particle Anemometer (PDPA). And the propagation processes of explosion shock wave and flame were recorded by high speed camera and high frequency pressure sensor, respectively. Both single-fluid ultrafine atomizing nozzles and ultrasonic method were employed to study the influence of ultrafine water mist on the gas explosion at various concentrations in the entire sealed visual vessel. The experimental results indicate that both enhancement and mitigation effects will appear after spraying under the sealed explosion conditions.Firstly, the ultrafine water mist was generated by single-fluid ultrafine atomizing nozzles and its effects on CH4/Air explosions at the concentrations of8%,9.5%,11%and12.5%were studied. It is found that the maximum explosion overpressure, pressure rising rate, flame propagation velocity and flame brightness increased after spraying into the sealed vessel. The explosion parameters increased with the increasing of water amount and at last kept on a level higher than the dry experiments. By the analysis, the disturbance and turbulence due to the large and fast-moving droplets are supposed to be responsible for the explosion enhancing. After flame fronts reached to the top of explosion vessel, the flame in the full length would get brighter in the presence of ultrafine water mist which is called Secondary Reaction. Apart from the effects of turbulization, the reactions of steam methane reformation and water vapor decomposition may occur and generate other flammable gas (H2, CO) under the high temperature and pressure conditions in the sealed vessel, and the combustion of the new flammable gas would lead to the secondary increasing process of explosion flame.Secondly, the effects of ultrasonic water mist on CH4/Air explosions at the concentrations of6%,9%,11%and13%were experimentally studied in the sealed vessel. The ultrasonic water mist was generated in the explosion vessel directly. With the increasing amount of ultrasonic water mist, it was found that methane explosions at low concentration (6%) were mitigated absolutely, while the pressure parameters at the high concentration conditions (9%,11%and13%) increased slightly and decreased significantly until to a constant value. The maximum decreasements of△Pmax at the three high concentrations were21.1%,26.7%and22.9%, respectively, and the maximum decreasements of (dP/dt)max were71.7%,77.1%and52.0%, respectively. As a result of the inhomogeneous distribution of water vapor, flame cellular structures would appear after adding water mist and eventually caused the extinguishing of the flame in the burned zone before the flame fronts reached to the top of explosion vessel.It is concluded that whether ultrafine water mist plays a role of enhancement or mitigation on the methane/air explosion in the sealed vessel mainly depends on the following two points:a) Whether spraying process will cause disturbance and turbulence in the premixed mixtures or not. b) Whether the droplets" diameters are small enough so that there are large amount of water droplets could vaporize and absorb heat from the explosion shock wave and flame. Totally, the experimental results show that methane/air explosion in the sealed vessel could be effective mitigated by ultrasonic water mist, for the reason that there is no much significant turbulization generated by spraying and the droplets’diameters are enough small when applying ultrasonic atomizing method.