Study On The Propagation Characteristics Of Premixed Flame Of Hydrogen/Methane Deflagration In Ducts

Gas explosion is one of the most serious disasters in coal mine,which often caused heavy casualties and serious economic losses in china.To better prepare for such disasters,the technology of coal seam gas extraction has been emphasized by our country and the comprehensive utilization of the gas drained in coal mining has also been promoted.However,full exploitation of these fuels as effective alternatives could be limited by some of their properties,such as high ignition energy,low laminar flame speed and narrow flammability limits.One solution to these problems is adding hydrogen to the fuel gas.But adding hydrogen could change the basic combustion characteristics of hydrogen/methane mixtures,it can decrease the ignition energy,increase the laminar burning speed and extend the flammability limits,which greatly raises the fear of fire and explosion accident,thus it is essential to investigate the propagation characteristics of hydrogen/methane premixed flames.In the present paper,the deflagration characteristics of hydrogen/methane mixtures including flame structure,flame propagation speed and overpressure were collected,and both the experiments and numerical simulations were conducted to examine the propagation characteristics of hydrogen/methane premixed flames in ducts.The main achievements of this study are as follows:(1)Experimental study on the effects of hydrogen addition and duct dimensions change on propagation characteristics of premixed fame in open ducts was conducted.The results suggest that the addition of hydrogen to the methane/air mixtures could speed up the time of flame arrival to the vent and increased the maximum overpressure.The “tulip” flame disappeared with increasing cross sectional area and hydrogen fraction.Meanwhile,the tulip flame gradually appears and a more pronounced “tulip” flame is observed with increasing duct length.The maximum flame propagation speed occurs with an aspect ratio of 10 regardless of whether there was “tulip” flame formation.The maximum overpressure decreases with cross sectional area and increases with the length of the duct,whereas the corresponding time increases with cross sectional area and length of duct.(2)Experimental study on the propagation characteristics of premixed flame of hydrogen/methane deflagration in closed ducts was conducted and the effects of aspect ratio on premixed flame propagation characteristics were investigated.In these experiments,the classic “tulip” flame appeared in all configurations with all five hydrogen fractions.However,in the duct with the smallest aspect ratio,only the four earliest stages of the “tulip” flame were observed.And after the “tulip” flame formation,the flame propagated with different structure,e.g.,a quasi-plane flame,“tulip” flame and distorted “tulip” flame as the aspect ratio and hydrogen fraction increased.For all configurations,the flame speed dynamic is closely related to the pressure,they both significantly increase with increasing hydrogen,the oscillations appeared with the hydrogen addition and even get enhanced as the hydrogen fraction increased.(3)Experimental study on premixed flame propagated in obstructed duct was conducted and the effects of amount,position and blocking rate of obstacles on propagation characteristics of premixed hydrogen/methane flames were analyzed.When obstacle exists in the duct,there are similar trends in the flame evolution structure obtained from both equivalence ratios and hydrogen fractions of the same configuration.And when the obstacles mounted along the axis of the duct,for configurations with a number of obstacles greater than 2,there is negligible impact on the flame propagation and the maximum overpressure has a trend similar to that with the laminar flame speed.Meanwhile,both the overpressure and flame propagation speed varied with the blocking rate and position of obstacles.(4)Numerical study was performed by large eddy simulation to investigate the propagation characteristics of premixed hydrogen/methane flames in closed duct.It is found that the combustion models by Charlette and Zimont is able to well predict flame structure during the flame propagation,especially for the early stage.The interaction between flame front,the reverse flow and the vortex formed in the unburned and burned zone is key factor for the flame structure evolution during the process of flame propagation.For the fuel mixtures with ?=0.5,the error between the numerical prediction and experiment was minimal,but the opposite result appeared when ?=0 and 1.In general,for the fuel mixtures with ?=1,the differences between model calculation and predication were more marked than the mixtures with ?=0 and 0.5.The above research results have great theoretical significance and practical values for hydrogen/methane explosion prevention and control.