Experimental Study On The Suppression Of Premixed Flame Propagation In The Duct With Pellets Bed

With the development of the economy,fossil fuels shortage,global warming,environmental pollution,and environmental degradation have become more and more serious.So there is an urgent need to develop alternative clean energy sources.Among alternative fuels,combustible gases(methane,natural gas,hydrogen,etc.)have attracted much attention and favor,have been widely used in industrial production and household sectors.However,combustible gas is flammable and explosive.In the actual process,combustible mixture may form in leaky pipelines.In short,there is a serious risk of accidental fire and explosion,which will always cause destructive disasters.Therefore,from a safety point of view,studying the early propagation dynamics of premixed combustible gas-air flames in the duct will help to understand and master the propagation behavior and development mechanism of the premixed flame more comprehensively.At the same time,it is of great significance to explore how to suppress flame and control explosion for preventing or reducing combustible gas fires and explosions.In this paper,combustible gases with different reactivity are selected,and the experimental system and experimental method procedures of premixed flame propagation and suppression are adopted to study the suppression effect of the pellets bed on the premixed flame propagation in the duct.A variety of influencing factors are considered in this study,including premixed gases with different equivalence ratios(fuel-lean,stoichiometric and fuel-rich conditions),premixed flames with different propagation speeds(low-speed and high-speed),and different combinations of pellets beds(single-size and multi-size combined pellets bed),different pellets materials(ceramic pellets and stainless pellets),etc.In the experiments,a high-speed schlieren photography system is used to record the flame propagation behavior(including flame shape and flame velocity),and pressure transducers are used to track the pressure buildup inside the duct.The aim of this paper is to fully understand the effects of the pellets bed on the dynamic propagation characteristics of the premixed flame under different equivalence ratios in the closed duct.It is of great significance to explore the effective suppression method and guide the safety design of spherical suppression materials in engineering applications for process safety researchers and engineers.First of all,premixed methane-air and hydrogen-air are choosed as experimental objects.The effect of the single-size ceramic pellets on the low-speed and high-speed premixed flame propagation in the duct is analyzed in detail.The experimental study of premixed methane-air flame shows that the single-size ceramic pellets are advantageous for the suppression effect on such low-speed propagation premixed flames in the duct.The flame can be quenched by the pellets with a smaller diameter(3 mm)for a wide equivalence ratios range from 0.8 to 1.2.For larger pellet diameter(5 or 10 mm),flame extinction also occurs in fuel-rich mixtures.In addition,the flame front is disturbed significantly when the flame can survive through the pellets bed,especially for larger porosity.The disturbance can lead to flame enhancement(such as second speed peak).Moreover,the pressure dynamics is closely related to the flame propagation in this study,depending on whether the flame quenches in the pellets bed.And the peak pressure in the duct decreases to a low value range of about 0.161～0.202 MPa(critical quenching peak pressure)in the experimental cases with the flame quenching.Then,the experimental study of premixed hydrogen-air flame shows that the evolutions of the tulip flame phenomenon are similar in all cases for the fuel-rich mixture.But the experimental formation time of tulip flame is earlier than the theoretical values at Φ=5.6.It is because that the larger overall thermodynamic index(ε)can result in greater positive effect on the laminar flame speed,which will speed up the formation of the tulip flame.At the same time,ceramic pellets mainly exhibit a significant suppression effect on the flame velocity and pressure at the fuel-rich condition.However,flame acceleration phenomenon will occur during the interaction between the flame and the pellets bed,which mainly happens at Φ=1.6(with larger initial laminar flame speed),In addition,the large diameter pellets bed(d=5 mm,10 mm)show enhancement effect on the flame velocity and pressure in fuel-lean mixture.Whereas the presence of the d=3 mm pellets bed presents better suppression effects on the premixed hydrogen-air flame propagation than the large diameter pellets bed(d=5 mm,10 mm)in this experimental study,especially near the stoichiometric condition.Subsequently,the pellets of different diameters are combined as the pellets bed.It is to explore the effect of the multi-size combined pellets bed on the suppression of the premixed flame propagation in the duct.The experimental results show that the multi-size combined pellets beds exhibite a better suppression effect than the single-size combined pellets beds on the flame velocity and pressure of the premixed methane-air flame propagation in the duct at the fuel-lean and stoichiometric condition.The other multi-size combined pellets beds can all quench premixed methane-air flames except for the pellets bed of d=5&10 mm(with the largest average void diameter).While for the premixed hydrogen-air flame propagation,the unevenness of the voids in the pellets bed will promote flame acceleration and pressure rise in the duct.In other words,the combined pellets beds show an enhancement effect on the premixed hydrogen-air flame propagation at this time.However,this enhancement effect will be weakened as the hydrogen concentration increases.Especially under the condition of stoichiometric ratio,the combined pellets bed of d=3&5&10 mm shows a trend of suppression on the propagation of premixed hydrogen-air flame.In the fuel-rich mixture,all the combined pellets bed can quench the premixed methane-air flame.The critical quenching peak pressure in the duct is in the low range of about 0.15～0.20 MPa when the flame extinguishes.However,a higher heat transfer rate to the unburned mixture will also cause a higher flame velocity in the upstream duct during the flame extinguishing process.For premixed hydrogen-air flames in the fuel-rich condition,the combined pellets beds begin to show the suppression effect on the flame velocity and the pressure in the duct.When the flame is located at about 180 mm from the ignition position,it starts to maintain a low speed until it touches the pellets bed.In addition,the upstream flame velocity is declined to about 50%and the pressure peak is attenuated by about 56%at Φ=5.6,compared to the case with no pellets.At the same time,the premixed hydrogen-air flame quenching occurs with the d=3&5&10 mm pellets bed at Φ=5.6.Finally,the difference in the suppression effect of the pellets materials(metal and non-metal)on the premixed flame in the duct is compared.In the experiments,the stainless pellets and ceramic pellets are used to carry out the experimental study.The main analysis aiming at d=3 mm pellets bed(single-size pellets bed)and d=3&5&10 mm pellets bed(multi-size combined pellets bed).It is found that for the metal pellets bed,both d=3 mm pellets bed and d=3&5&10 mm pellets bed have suppression effect on the flame velocity and pressure in the fuel-rich condition.Wherein,the metal pellets exhibits a suppression effect on the flame propagation inside the pellets bed at a high equivalence ratio(Φ>1.6),and shows an enhancement effect at a low equivalence ratio(Φ<1.6).The comparative study of metal pellets and ceramic pellets shows that metal pellets(stronger thermal conductivity)do not exhibit a better suppression effect than ceramic pellets in the fuel-lean and stoichiometric conditions.It is due to the enhancement effect of heat loss.However,the suppression effect of heat loss is dominant in the fuel-rich condition.Therefore,the suppression effect of metal pellets on flame propagation and pressure is better than that of ceramic pellets,including the flame velocity in the upstream duct,the average flame velocity in the pellets bed,the flame velocity distribution in the downstream duct,and the pressure in the duct,etc.At the same time,as the hydrogen concentration increases,the better suppression effect of the metal pellets becomes more obvious.