Investigation on the Mechanism of Flame Acceleration and Deflagration to Detonation Transition of Combustible Gases
Author:Zhao Yong Yao
Combustible gas explosion is a kind of catastrophic accident which is often encountered in industrial production and human life.In the process of explosion,the high temperature and high pressure are not only a threat to the safety of life,but also cause great damages to industrial production.It seriously affects the harmony and stability of society.The process of gas explosion involves flame acceleration,deflagration to detonation,and explosion accident which often occurred in complex environment,resulting the explosion process complicated.Thus its physical and chemical mechanism have not been known clearly.Therefore,investigating the mechanism of flame acceleration and deflagration to detonation transition mechanism is beneficial to enrich the basic theories of combustion and detonation,and provide an important theoretical direction to the explosion prevention and control.In this thesis,the explosion process of combustible gases was studied by numerical simulations and experiments.The numerical simulation consists of direct numerical simulation and large eddy simulation.The experimental part includes the gas explosion in small scale and large scale long straight pipe.We analyzed the influence of wall heat loss on flame acceleration and deflagration to detonation,the propagation of explosion wave in large scale complex environment,and the influence of the obstacle on flame propagation.The main researches are as follows:(1)We established reactive flow N-S governing equations with viscous diffusion and heat conduction,and given heat loss function of the wall with non-adiabatic conditions.The governing equations were combined with a high precision WENO numerical scheme,and considered the wall heat loss conduction,to numerical simulation the flame acceleration and DDT.(2)By studying the effect of wall heat loss on flame acceleration and deflagration to detonation,it is found the wall heat loss causes the flame acceleration decrease,deflagration to detonation distance and time prolong,and the detonation wave propagation velocity decreases;wall heat loss making the energy loss to wall weakens the combustion products propagates upstream,while promotes reverse flow,thus reduces the flame acceleration rate;wall heat loss delay pressure wave forming leading shock wave,which weaken the interaction between flame and pressure wave;because the interaction between pressure and boundary layer,in the boundary layer the unburned gas can form ultrafast flame in front of the flame in the adiabatic pipe,while there is no ultrafast flame in the boundary layer in the heat loss pipe but there is a phenomenon of preignition,which triggers deflagration to detonation.(3)Based on the established N-S equations,the LES equations are obtained by spatial filtering.Combining the subgrid scale turbulent kinetic energy transport equation,the effect of subgrid scale is closed.The chemical reaction kinetic model were investigated which include the flamelet model,fold flame surface model and thickened flame model;the WENO scheme and the TVD-Runge-Kutta scheme are used,we finally realize to simulate the flame acceleration and DDT in large scale area by LES.(4)By studying the effect of channel width on deflagration to detonation,it is found flame can rapidly accelerates and translates to detonation in channel with millimeter width.The DDT distance increases with the increase of the channel width;when the width is less than that of the combustible gas detonation cell size,no obvious cellular structure can be seen,and detonation wave velocity is less than CJ velocity.With the increasing of the width,three stages of the process of DDT become more obvious,the detonation wave velocity is close to the CJ value,and the cellular structure can be seen.(5)The main causing disaster factors of gas explosion in mine is high velocity air flow,high temperature,and high pressure combustion products.The gas explosion hazard distance is far greater than that of the gas accumulation zone,when accumulated zone length is 14 m,the overpressure is still more than 0.5 bar at the position of 360 m far from the left end,and the gas flow speed is more than 100 m/s,enough to cause damage to the human body.With the increase of the length of the gas accumulation zone,the maximum explosion overpressure,the gas flow speed and the flame propagation distance are increase,and the ratio of the length of the flame zone to that of premixed gas is 5-7.(6)For the gas explosion in small scale channel,the whole process of DDT was studied by numerical simulation.By coMParing the numerical simulation and experimental results,the validity of the numerical simulation is verified.In the small scale channel,the interaction of flame and pressure wave and flow field promotes the flame acceleration;the subsequent interaction of flame and shock wave makes the pressure increases,the local explosion appeared in the boundary layer resulting in detonation transition,and the retonation phenomenon.Numerical simulation results also revealed the evolution of the flame front.(7)For combustible gas explosion in large scale long straight pipe,experimental results and numerical simulation results were coMPared.It is found without obstacles,the flame can not transform into a detonation wave and its speed increased first and then decreased.With obstacles in the pipe,the flame acceleration rate increases and the detonation wave can form due to the obstacles.When the flame through the obstacles,the gas flow speed increases,flame front becomes jet shape and accelerates forward,the unburned gas interact with obstacles,inducing flame front turbulent increasing the flame surface area,to further accelerate the flame propagation;as the pressure waves reflect between obstacles,there appears adverse pressure gradient,and the flame velocity decreases,so the final flame velocity exhibits oscillatory.