Study On Flame Quenching And Secondary Ignition Of Premixed Flame Through Narrow Channels In A Closed Vessel

Flame suppression is a crucial requirement in a variety of fields,including industry,the military,and public safety.In order to stop flame propagation or flammable gas explosions,flame arrester and flame-proof enclosure are widely used in the chemical industry.Flames quenching occurs when it pass through channels.In this paper,quenching and secondary ignition characteristics of a premixed flame passing through a tiny slit and porous slits in a closed pipe were investigated.Many researchers have conducted extensive research on the properties of flame propagation through channels,but the majority of them examine flame quenching and secondary igniting independently,and there are still a great number of key issues that need to be resolved.In this study,flame quenching and secondary ignition are viewed as the entire process of flame propagation via slits,and the flame behavior travelling through various slits was investigated using experimental and numerical simulation techniques.The main contents and innovative are as follows:(1)The quenching and secondary ignition behavior of a methane-air premixed flame passing through a narrow parallel channel was studied to analyze the effects of initial pressure,pre-combustion pipe length,and obstacles on critical gap opening.As the initial pressure increases,the critical gap opening decreases,because the increase in pressure increases the combustibility of combustible gas.Lengthening the precombustion pipeline will reduce the critical gap opening,as flame development over a longer line will produce a stronger pressure wave.And the combustible gas is easier to ignite after being compressed by the pressure wave.Obstacles increase the turbulence intensity of the flame,thus increasing the critical gap opening.In order to calculate the critical gap opening,we proposed that Dp=ktoSf,k,t0 and Sf are constant,the time when the hot burnt gas passes through the slit and the speed when the flame reaches the slit,respectively,and the range of k is from 0.111 to 0.266.(2)The quenching and secondary ignition properties of the flame passing through a circular slit were investigated,and the critical diameters under the influence of varying equivalence ratio,initial pressure,and obstacle were determined.Results demonstrated that the critical diameter grows as the equivalent ratio increases.The critical diameters are 6.0,8.0,and 9.2 mm when the equivalent ratio is 0.7,1.0,and 1.3,respectively.This is because the jet is formed when premixed flame passes through a small slit in a closed pipe,accompanied by a velocity gradient.Thus,flame stretch rate will strengthen the impact of thermal mass diffusion on flame quenching.When the Le number is greater than 1.0,the flame temperature decreases,which leads to flames being easier to quench.When the Le number is less than 1.0,as the flame stretch rate increases,the flame temperature increases,resulting in the flames becoming more difficult to quench.After the addition of obstacle,the critical diameters decrease with the increase of the equivalent ratio,reaches the minimum value of 9.0 mm at the equivalent ratio of 0.9,and then increases with the increase of the equivalent ratio.Obstacle increases turbulence intensity,inhibiting the role of thermal mass diffusion in stretching.By introducing new parameters δ1,the thermal quenching boundary layer model was modified to make the calculated critical diameter more consistent with the experimental value.(3)The quenching and secondary ignition properties of the hydrogen-air premixed flame through porous slits were explored,and the flame propagation characteristics at various ignition positions were obtained.The experiment revealed that the ignition process differs between the upper and lower limits.At the upper limit,it is hot burnt gas secondary ignition.Detached flame ignition occurs at the lower limit,revealing the effect of thermal mass diffusion on flame extinguishment.(4)The quenching behaviors of flame through channels were also studied by numerical simulation.The results show that when turbulence intensity is low,the effect on quenching length is not obvious.When the turbulence intensity is greater than 10%,increasing the intensity will reduce the quenching length significantly.The quenching length decreases with the increase of Le number as Le number is less than one.When the Le number is greater than 1,the thermal mass diffusion coefficient has almost no effect on the quenching length.When flame passes through porous slits,there are three stages in the process,as follows.I)when the flames just enter the slits,the propagation speeds in the channels of the three slits are the same.2)the temperature of the middle slit rises,and its flames move faster as flames on both sides transfer thermal to the middle slit.The propagation distance of the intermediate flame is increasing with respect to the propagation distance of the flame on both sides.3)the distance between the flames in the middle slit and the flames in the slits of both sides remains within a certain range.They move this way until they pass through the slits.At the same time,the reaction rate in the middle slit decreases gradually,similar to flames from singlehole slit,influenced by the wall surface.The phenomenon is also appeared in the experiment,which shows that the flame can pass through slits at the sides but cannot pass through the middle slit when it is at the lower limit.