A Dynamics Study Of Hydrogen Detonation Propagation In Obstructed Channels

As the most promising new energy source for replacing traditional fossil fuels in the 21st century,hydrogen has received extensive attention internationally.However,hydrogen has also brought huge safety hazards during its use,transportation and storage because of the special properties of hydrogen,such as high diffusivity,low density,low ignition energy and wide flammability limit.The combustible mixture is easier to be produced once the occurrence of hydrogen leakage,and the explosion accidents will be formed if the ignition source exists.In the worst case,the explosion flame will be accelerated rapidly by different obstacles,and the more destructive detonation wave will be induced,which seriously threaten people's lives and property.Therefore,it is of far-reaching significance to systematically investigate the dynamics of hydrogen detonation propagation before its wide usage.In this study,the experimental investigation of the propagation mechanism of hydrogen detonation is performed in round and square detonation tubes,and the attention are paid to the cases of higher blockage ratios(>0.8)and higher thicknesses(>20 mm)orifice plates and irregular geometries(tube bundle and small-scale perturbation).PCB high-speed pressure transducers are used to record the time-of-arrival of a detonation wave,from which the average velocity of detonation propagation can be determined.The smoked foil technique is used to obtain the detonation cellular patterns.In addition,two-dimensional numerical simulation is employed to further explain the mechanism of detonation propagation.The research contents and results are as below:(1)The mechanism investigation of hydrogen detonation propagation in a tube with higher blockage ratio and thickness orifice plate.To understand the effects of the higher blockage ratio(BR)and thickness obstacle on the mechanisms of hydrogen detonation propagation,the propagation velocity and cellular structure of hydrogen detonation are measured experimentally in 90 mm inner diameter round tube and 300 mm side square channel,respectively.The results indicate that the critical range of hydrogen mole fraction for detonation propagation is gradually narrowed with the increases of BR.Moreover,the detonation wave is easier to be formed at the fuel-lean side when the blockage ratio of orifice plate is in the range of 0.802-0.96.This phenomenon can be due to the lower sound speed in combustion products at fuel-lean side.The critical mechanism of detonation failure can be quantified as d/?>1 and d/??>3 approximately.For the case of square tube,two various detonation ignition mechanisms are observed with the increases of orifice plate thickness.It can be named as symmetry ignition and asymmetry ignition mechanisms through comparing the detonation cellular characteristics.Based on the average geometry size reported by Dorofeev et al.in the study of detonation propagation limit,the geometry size which the effect of obstacle thickness can be considered is revised and written as:L'=(?+Deff)/2(1-deff/Deff).Near the limits,the critical mechanism of detonation propagation can be nearly quantified as L'/?<32 and L'/?I<37.34.(2)The mechanism investigation of hydrogen detonation propagation in a tube filled with higher BR's repeating orifice plates.The effect of the repeating orifice plates with greater BRs(0.7,0.8 and 0.93)on the detonation velocity and cellular structure is investegated in a 90-mm inner diameter tube.The results are consistent with previous studies obtained in the cases of smller BRs.That is,compared to the results obtained from a smooth tube,the average velocity determined from the tube filled with continuous orifice plates is lower,and the velocity can maintain a continuous and gentle decline with the decreases of initial pressure.The average speed can still maintain at about 0.6VCJ despite the detonation is failed completely.Two-dimension numerical simulation results further verify the experimental conclusion,and the same propagation regimes can be observed.A phenomenon of repeating detonation failure and re-initiation can be observed when a detonation wave propagates in a tube filled with continuous orifice plates.Thus,the speed obtained from the obstructed tube is the average value between the failed deflagration wave speed and re-initiated detonation wave speed,which is far smaller than the corresponding value in a smooth tube.In addition,it can be found that the occurrence probability of high reaction rate is higher in obstructed tube by calculating the probability density function of the reaction rate,which indicates although the average velocity in obstructed tube is decreased significantly,the local explosion centers with higher temperature can be produced balancing the negative effect from the repeating obstacles.Therefore,the average speed in obstructed tube can maintain a continuous and gentle decline.The critical condition of detonation propagation can be approximately quantified as d/?>1 and L/?>7?(3)The mechanism investigation of hydrogen detonation propagation in a tube with irregular geometry obstacles.The effect of tube bundle consisted by 20 mm outer diameter small pipes and small-scale perturbation created by 2 mm diameter metal rod on the detonation velocity and cellular structure is investigated.The effects of pipe bundles on the mechanisms of deflagration to detonation transition and detonation propagation limit and the roles of small-scale perturbation on detonation re-initiation are analyzed in detail.The results indicate that the bundle structures can significantly facilitate the detonation formation,the critical pressure can be decreased from 53 kPa in smooth tube to 12 kPa significantly,and the critical pressure of detonation formation closely depends on the position of the bundle geometry.The simulated results indicate that the mechanism of detonation formation in tube filled with pipe bundle structures can be due to the formation of local explosion centers induced by the shock reflection from the wall and the interaction between the jet flames.In addition,the small-scale perturbation can also significantly facilitate the detonation re-initiation by inducing the local explosion points,and the critical pressure can be decreased from 37 kPa in smooth tube to 25 kPa.Near the limits,the critical mechanisms of DDT and detonation propagation limit in the tube filled with pipe bundles and detonation re-initiation in the tube with a small-scale perturbation both can be quantified as DH/?>1 approximately.