| In the field of industrial safety,flame acceleration(FA)and deflagration-todetonation(DDT)are common phenomena,but when the combustion state changes to detonation,more serious accidents will be caused.However,in the field of new detonation propulsion,people need to use detonation to achieve efficient propulsion,and deflagration to detonation is the simplest and most effective method.Therefore,it is of great significance to study the flame acceleration(FA)and deflagration to detonation transition(DDT)of combustible premixed gas in the flow field.Based on Open FOAM platform,RANS simulation is applied to the unsteady flow process in this paper.A density-based solver coupled with detailed chemical reaction mechanisms of 9 species and 21 steps and the SST k-? turbulence model is used to simulate the development process of the flame in the pipeline filled with hydrogen-air mixture according to the stoichiometric ratio.The effects of obstacle distribution and transverse concentration gradient of premixed gas on FA and DDT processes are investigated.The main research contents and conclusions are as follows:(1)Whether the initial flow field is homogeneous or has a transverse concentration gradient,the flame tip speed has experienced multiple "accelerationdeceleration-acceleration" periodic oscillations before detonation.The more homogeneous the premixed gas is,the more favorable the FA and DDT processes are.With the increase of transverse concentration gradient,the choking phenomenon becomes more obvious.(2)When obstacles are symmetrically distributed in the homogeneous field,DDT occurs first;When the concentration is inhomogeneous,the initiation is the earliest when the transverse concentration gradient is negative and the obstacles are staggered;When changing the uneven concentration gradient,the earliest detonation is the case where the hydrogen concentration is linearly distributed from 45% to 15% along the positive direction of the y axis.The more uniform the hydrogen distribution,the faster the initiation occurs.(3)For all the concentration cases,the detonation wave develops from the hot spot.There are two main initiation processes in the homogeneous case,one is the local spherical detonation caused by the coupling between the flame surface and the high pressure region,and the other one is the detonation caused by the high pressure in the unburned area caused by the interaction of multiple compression waves in front of the flame and the upper wall.The spherical detonation also appears in the case of inhomogeneity.In addition,they are two new initiation modes of double-wave collision and high pressure caused by reflection of shock wave on obstacles respectively.(4)Under the condition of homogeneous concentration field,the structure of the wave system formed by the detonation wave and the symmetrical obstacle is still symmetric,but when the detonation wave acts on the asymmetric obstacle,the asymmetric wave system is derived,and the unilateral wave system is generated when the obstacle is distributed on one side.For the uneven concentration field,no matter how the obstacles are distributed,a composite structure of oblique shock wave and curved surface detonation wave will eventually be formed,and the derived wave system structure is not symmetrical.The decoupling of the detonation waves that diffracted obstacles has also appeared to different degrees.Due to the action of the oblique shock wave and the boundary layer,vortex structure appears on the wall of the tube and develops shedding.(5)For the case of inhomogeneous,when the transverse concentration gradient is small,a composite structure of curved detonation wave and Mach Stem is eventually formed.When the transverse concentration gradient is large,the wave surface is composed of curved detonation and oblique shock.The transverse concentration gradient in the initial flow field is larger,the smaller the height of the stable detonation wave,the smaller the angle between oblique shock wave and wall surface. |
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