Explosion Characteristics And Flame Propagation Mechanism Of Hydrogen/Aluminum Dust Hybrid Mixtures

Dust explosions occur frequently in industrial production.Wet dedusting is widely used in the field of dust explosion protection.For aluminum dust with active chemical properties,when wet dedusting is used,damp aluminum dust may release hydrogen and induce hydrogen/aluminum dust hybrid explosions,which pose a threat to the dedusting process.Hydrogen/aluminum dust hybrid mixtures also exist in the environment of industrial production of alkyl aluminum and dehydrogenation of solid rocket propellant AlH3.The explosion theory of hydrogen/aluminum dust hybrid mixtures has not been systematically concerned.The obtained theories on the explosion of pure aluminum dust and pure hydrogen can not explain the two-phase cooperative mechanism of hydrogen/aluminum dust hybrid explosion process.Based on this,the explosion overpressure and flame propagation characteristics of hydrogen/aluminum dust hybrid mixtures were investigated.The main work and conclusions are as follows:(1)A visual closed cylindrical test platform for gaseous fuel/dust hybrid explosions was set up.The platform can synchronously collect the characteristic parameters and the flame propagation of flammable medium explosion.(2)The effects of hydrogen concentration and aluminum particle size on the maximum pressure rise Pex and the maximum rate of pressure rise(dP/dt)ex of the hybrid mixtures were studied.It was found that there were critical hydrogen concentrations that affected the changing trend of the Pex and(dP/dt)ex(the Pex corresponded to 4%,the(dP/dt)ex corresponded to 10%).When the hydrogen concentration was lower than the critical concentration,the Pex and(dP/dt)ex were more affected by the aluminum particle size.When the hydrogen concentration was higher than the critical concentration,the Pex and(dP/dt)ex were more affected by hydrogen concentration.A thermal energy equation suitable for describing aluminum particle combustion in hydrogen was established.The changing trend of energy curves of particles with different sizes were almost the same as the Pex and(dP/dt)ex curves obtained by experiments.(3)The variations of flame propagation velocity of hydrogen/aluminum dust hybrid mixtures were studied.The results showed that the flame propagation velocity accelerated at first and then decelerated.When the hydrogen concentration was higher than 15%,there were two peaks in the flame propagation velocity curves:the first peak was dominated by hydrogen combustion and the second peak was dominated by aluminum dust combustion.With the increase of hydrogen concentration,the maximum flame propagation velocity vmax changed.When the hydrogen concentration was less than 4%,the vmax of mixture was almost equivalent to that of pure aluminum dust;when the hydrogen concentration was between 4%and 20%,the vmax of the mixture was less than 3 times that of the pure aluminum dust;and when the hydrogen concentration was higher than 20%,the vmax of the mixture exceeded 110 m/s.(4)The flame structure of hydrogen/aluminum dust hybrid mixtures was studied.It was found that the flame microstructure of the hybrid mixtures was different from that of the asymmetric flame of pure aluminum dust.The micro-diffusion flame appeared in the flame front,and the classical asymmetric flame appeared in the combustion zone.The combustion velocity of the micro-diffusion flame was much higher than that of the asymmetric flame.In the process of flame propagation,when the hydrogen concentration was less than 20%,the flame presented a single thermally coupled front;when the hydrogen concentration was higher than 20%,the combustion showed obvious gas flame morphology,and the hydrogen flame front was separated from the aluminum dust flame front.(5)The explosion driving mechanism of hydrogen/aluminum dust hybrid mixtures can be divided into three stages:aluminum dust driven hybrid explosion stage(hydrogen concentration was less than 10%),the Pex,(dP/dt)ex,vmax were significantly affected by aluminum dust,the flame showed a heterogeneous combustion morphology;transition stage(hydrogen concentration was 10%-20%),the Pex,(dP/dt)ex,vmax were significantly affected by aluminum dust,the flame morphology changed from heterogeneous to homogeneous,and the flame propagation velocity curves began to show double peaks;hydrogen driven hybrid explosion stage(hydrogen concentration was higher than 20%),the Pex,(dP/dt)ex,vmax were significantly affected by hydrogen,the flame showed a homogeneous combustion morphology.(6)The morphology and composition differences between the explosion residues of hydrogen/aluminum dust hybrid mixtures and that of pure aluminum dust were studied.The existence of hydrogen aggravated the incomplete combustion of aluminum dust.The large incomplete combustion particles,small smooth particles and porous oxide layer appeared in the hybrid explosion residues of aluminum dust with median diameters of 56.18 ?m and 30.82 ?m.Incomplete combustion aggregates appeared in the hybrid explosion residues of aluminum dust with a median diameter of 5.07 ?m.Compared with the explosion residues of pure aluminum dust,Al(OH)3,AlO(OH)and other components were added.The newly formed residues and intermediate products accelerated the cooling of incomplete combustion particles and reduced the formation of flocculent alumina on the surface of aluminum particles.(7)According to Al(OH)3,AlO(OH)and other components in explosion residus,and based on the reaction kinetic mechanism of aluminum dust,a kinetic reaction model of hydrogen/aluminum dust hybrid explosion containing 23 kinds of species and 50 steps of reaction was established.Combined with flame microstructure,residus morphology and classical aluminum particle combustion theory,hydrogen/aluminum dust reaction mechanism and hybrid clouds combustion mode were established.The kinetic reaction mechanism,flame propagation mechanism and hydrogen-aluminum flame interactions of hydrogen/aluminum dust hybrid explosion were explained.