Study On The Pressure Characteristics And Flame Dynamics Of Vented Hydrogen Deflagrations

Hydrogen is an important clean energy to solve the problems of fossil fuel exhaustion and environmental pollution,and is also an important medium to achieve the goal of‘carbon neutrality and carbon reduction’.Hydrogen has characteristics of low ignition energy,wide combustion limit,fast diffusion rate and laminar combustion rate,and therefore,hydrogen is prone to cause leakage,fire and explosion accidents.In recent years,hydrogen explosion accidents have sounded the alarm for the rapid development of hydrogen energy industry,so solving the hydrogen safety problem is the premise for the wide application of hydrogen energy.Hydrogen explosion venting is an important method to reduce explosion overpressure in confined space,reduce the damage of buildings and equipment and prevent people from being injured.However,the current research on vented hydrogen explosions mainly focused on the internal overpressure of fuel-lean hydrogen-air mixture.Vented hydrogen explosion with fuel-rich concentration was scarcely investigated,especially for the external pressure characteristics while it is an important research object in vented hydrogen explosion.In this paper,vented hydrogen explosions from fuel-lean concentration to fuel-rich concentration were experimentally and numerically investigated.Experimental platform was set up using pressure sensors,high-speed camera and other equipment.A numerical simulation platform was established based on fluid mechanics equations,standard RANS k-εtwo equations turbulence model and the modified Weller combustion model.The effects of venting parameters on internal and external overpressure and flame dynamic behavior were investigated.The currently available engineering models of overpressure prediction for vented hydrogen explosion were tested using experimental data and a new model was established.Firstly,the overpressure and flame behavior of vented hydrogen explosion with concentration 18%-50%were studied in this paper.Then the comparison between the experimental and numerical data was carried out.Comparison results show that the predicted flame behavior and external overpressure histories agree well with the experimental data.However,the predicted internal overpressure curves agree well with experimental data only for 14%-18%and 34%-50%hydrogen-air mixtures.In the study of the mechanism of different overpressure peaks,it is found that the maximum internal overpressure is induced by the vent film rupture for front ignition,while it is caused by the external explosion for center and back ignitions.In addition,with the increase of hydrogen concentration,the overpressure first increases and then decreases,and its maximum value appears at hydrogen concentration of 34%for front and center ignitions,while the maximum overpressure reaches at hydrogen concentration of 46%for back ignition.The external overpressure has only one overpressure peak generated by the leading pressure wave for front ignition,while the overpressure peak generated by the external explosion occurs when the hydrogen concentration reaches 18%and 14%for the center and back ignition,respectively.The maximum internal and external overpressure shows the same variation with the increase of hydrogen concentration for different ignition locations.With the increase of volume obstruction ratio,the maximum internal overpressure increases at the hydrogen concentration of 22%-38%,but changes slightly when the hydrogen concentration equals to 18%and 42%.The maximum external overpressure increases with the hydrogen concentration increase from 18%to 34%hydrogen concentration and has little change at hydrogen concentration of 38%and 42%.Secondly,the effects of vent area（expressed as the dimensionless vent coefficient K_v）and vent burst pressure on vented hydrogen explosion were experimentally and numerically studied.With the decrease of vent area,the internal overpressure increases for different ignition positions.The external overpressure increases for front ignition,while it firstly increases and then decreases for center and back ignitions and reaches the maximum value when K_v=17.36 and K_v=8.65,respectively.With the increase of vent burst pressure,the maximum internal and external overpressure increase linearly for different ignition positions.The maximum internal overpressure for back ignition is significantly larger than those for front and center ignitions,while the latter two are approximately the same.The maximum external overpressure increases with the increase of vent burst pressure for front and center ignitions.However,the maximum overpressure appears at the vent burst pressure P_v=57.3 k Pa for center ignition.Finally,some currently available models,such as NFPA68,Molkov（Best-fit）,Molkov（Conservative）and Wen&Sinha models were tested by full-size experimental data.According to the reference test data and the quasi-laminar flame velocity formula,the critical radius R₀ of cell structure instability and the corresponding quasi-steady laminar flame velocity S₀ of premixed hydrogen-air flame in the Wen&Sinha model are modified.The modified model can quickly predict the maximum overpressure with an error of less than 40%for vented hydrogen explosion under the condition of the hydrogen concentration from fuel-lean concentration（14%）to fuel-rich concentration（50%）.Moreover,the maximum overpressure of vented hydrogen explosions with volume blockage ratio of 0-24%at the hydrogen concentration less than 26%can be predicted well by modified model.