Dynamic Couplings Of Unstable Hydrogen Flame Propagation And Explosion Pressure Evolution

The explosion pressure behavior is an important index of evaluating accident consequences and implementing safety protection measures.During gas explosion,the expanding flame tends to be unstable under flame instabilities,which will increase flame surface area and enhance explosion intensity.In the existing models of explosion pressure prediction,total combustion heat is assumed to release instantaneously or the expanding flame is assumed to move at a constant velocity,the unstable flame propagation is not considered and this directly results in the inaccurate evaluation of explosion pressure.In view of this,the experimental apparatus is designed and built to obtain flame morphologies and explosion pressure under flame instabilities.Using the experimental method,theoretical analysis and numerical simulation,this thesis is aimed at revealing the dynamic couplings of unstable flame propagation and explosion pressure evolution.The improved model of explosion pressure prediction is established based on the flame wrinkling factor.In addition,the explosion pressure behaviors of stoichiometric hydrogen-air mixture in different spatial scales are predicted theoretically.The main results and conclusions are as follows:(1)The effects of equivalence ratio,blend gas type,blend ratio and initial pressure on unstable flame propagation in the constant-pressure stage are revealed.As initial pressure increases,the destabilization effect or stabilization effect of thermodiffusive instability almost unchanges,the destabilization effect of hydrodynamic instability continues to enhance.On the lean and stoichiometric side,the destabilization effect of both thermodiffusive instability and hydrodynamic instability decreases monotonously as the methane and ammonia addition increases.The effective Lewis number of hydrogen-propane-air mixture undergoes the transition from Leeff<1.0 to Leeff>1.0,the destabilization effect of hydrodynamic instability decreases quickly and then almost unchanges with increasing propane addition.On the rich side,as the methane,propane and ammonia addition increases,the stabilization effect of thermodiffusive instability and destabilization effect of hydrodynamic instability are reduced gradually.(2)In the constant-volume stage,as the explosion pressure starts to rise,the destabilization effect or stabilization effect of thermodiffusive instability almost unchanges,the destabilization effect of hydrodynamic instability continues to enhance,which also results in experimental phenomena that the flame destabilization level is significantly aggravated.The characteristic length of Leeff<1.0 is closely connected with shortwave,the characteristic length of 1.0 and Leeff>1.0 is strongly dependent on computational domain.Besides,the burning rate and pressure rise rate could be increased by unstable flame propagation.As the methane,propane and ammonia addition increases,the enhancement effect of unstable flame propagation on burning rate and pressure rise rate is decreased monotonously.(3)The laminar burning velocity is a key parameter to quantitatively characterize burning rate and pressure rise rate.As the methane addition,propane addition and ammonia addition increases,the adiabatic flame temperature,thermal diffusivity,maximum mole fraction of active radicals are decreased monotonously,which contribute to the reduction of laminar burning velocity of hydrogen-methane-air,hydrogen-propane-air and hydrogen-ammonia-air mixture.In addition,the elementary reactions of generating and consuming H,O and OH radicals in the maximum rate are consisted of OH+H2=H+H2O、H+O2=O+OH、O+H2=H+OH and 2H+M=H2+M.(4)Considering the dynamic couplings of unstable flame propagation and explosion pressure evolution,the improved model of explosion pressure prediction is established based on the flame wrinkling factor,which is as follows:When the flame wrinkling factor is equal to 1.0,the above equation represents smooth flame model.When the flame wrinkling factor is assumed to increase exponentially((?)△=et and(?)△≤2.46),the above equation corresponds to wrinkled flame model.When the flame wrinkling factor is still 2.46,the above equation is called as turbulent flame model.Especially,the explosion pressure of stoichiometric hydrogen-air mixture will be underestimated and overestimated using smooth flame model and turbulent flame model,respectively.The explosion pressure evolution in different spatial scale could be reproduced satisfactorily using the wrinkled flame model.