Flame Propagation Mechanism In Ethylene/polyethylene Hybrid Explosions

With the increase of the production capacity of global polyethylene,the risk of ethylene/polyethylene hybrid explosion increases sharply.To predict the explosion consequences accurately and develop the effective protection measures essentially,it is vitally important to understand flame propagation characteristics in hybrid explosions adequately.It is a multiphase transient dynamic process of explosion when dust clouds coexisted with flammable gas.However,the essential issues about the law of development in hybrid explosions are not well acknowledged,especially for hybrid explosion flame propagation characteristics and flame propagation mechanisms.As a consequence,the flame propagation dynamic processes in ethylene/polyethylene hybrid explosions were studied in this study.The physical model of flame propagation structures in ethylene/polyethylene hybrid explosions were established,and flame propagation mechanisms in the hybrid explosions were revealed.The main conclusions are as follows:(1)With the decrease of particle size,or the increase of ethylene concentration,the minimum explosible concentrations(MEC)of ethylene/polyethylene hybrid mixtures were gradually decreased;However,because of severe agglomeration between small polyethylene particles,the MEC of ﹤75 μm polyethylene hybrid mixture was higher than that of the 75–100 μm polyethylene hybrid mixture.Flame front of hybrid mixture with polyethylene particles diameter ﹤ 75 μm was consisted of the floating flame characterized by a discontinuous structure.The structure continuity of flame front were increased with increasing particle size distribution.It was also found that the flame front became dim and discontinuous with the increase of ethylene concentration.(2)With increasing ethylene concentration,or decreasing polyethylene particle size,flame propagation velocity of hybrid explosions increased.Flame propagation velocity increased and then decreased with increasing dust concentration.The fluctuations of instantaneous flame propagation velocities dramatically decreased as ethylene concentration increased.It was indicated that flame propagation mechanism transited from the devolatilization-controlled regime to the kinetics-controlled regime with the increase of ethylene concentration.(3)The maximum flame temperature of hybrid mixture with small polyethylene particles(< 100 μm)increased and then decreased with increasing dust concentration under the condition of low ethylene concentration;however,the maximum flame temperature of hybrid mixture with large polyethylene particles did not change significantly.With the increase of ethylene concentration,the maximum flame temperature of hybrid explosion increased.Moreover,the effect of particle size distribution on the maximum flame temperature decreased.(4)A physical model of ethylene/polyethylene hybrid flame propagation mechanism was developed to reveal hybrid flame propagations.Hybrid flame was divided into unburnt zone,small particle pyrolysis zone,preheated zone,premixed gas flame zone,large particle pyrolysis zone,and dust flame zone.In addition,hybrid explosions of ethylene/polyethylene dust mixtures could be divided into four different regimes,which included no-explosion,synergistic explosion,gas-driven explosion and dust-driven explosion.In the condition of low ethylene concentration,the large particle pyrolysis zone and part of the premixed gas flame zone were covered by the dust flame zone.At the same time,flame propagation of hybrid explosion was driven by the dust explosion.With the increase of ethylene concentration,the premixed flame zone was separated gradually and an independent premixed flame was existed.The regime of hybrid explosion was transited to the gas-driven explosion,even the ethylene concentration was lower than its lower flammability limit(LFL).