Study On The Characteristics Of Propagation Distance Of Deflagration To Detonation Transition In Circular Tubes

Recent studies show that premixed combustible media deflagration can develop spontaneously into detonation.That is,the deflagration to detonation transition?DDT?phenomenon occurs.However,the distance of the free development of DDT is still unclear.In addition,the length-diameter ratio required for free development of DDT are large.For large-diameter pipes,to study detonation propagation through experimental methods,a large pipe length is necessary which has higher requirements to experimental pipeline.The obstacle has been proved to be able to realize deflagration acceleration propagation.Therefore,the accelerating effects of obstacles on the transition from deflagration to detonation are also worthy of attention.Given this,a experiment platform of gaseous DDT is built based on two experimental conditions as the study project.The two experimental conditions are respectively the cases that deflagration transforms into detonation spontaneously in a circle tube,and DDT occurs accelerated by the spiral tube obstacle.The variation run-up distance of DDT under different parameters such as initial pressure and argon dilution concentration is studied systematically.The main work and conclusions are as follows:?1?The experimental device of gaseous DDT in circular tubes was built,and the characteristics of the propagation velocity varied with distance in the whole transition process from deflagration acceleration to C-J stable detonation were studied.?2?Typical combustible mixtures growth process of steady detonation under free acceleration under different initial pressures?absolute pressure 10¹00 kPa?was experimentally studied,such as CH₄+2O₂,C₃H₈+5O₂,C₂H₄+3O₂,CH₄+2O₂+nAr.The formation distance of stable detonation is subdivided into the deflagration accelerated propagation distance and the overdrive detonation propagation distance,which is based on the C-J velocity positions in the acceleration stage and the attenuation stage as the boundary nodes.?3?The effects of initial pressure and argon dilution concentration on the deflagration acceleration propagation distance in a free accelerating tube were investigated experimentally.Taking typical combustible mixture CH₄+2O₂ as an example,the variation rule of deflagrating acceleration distance decreases with the increase of initial pressure in a"step-type"manner,and increases nonlinearly with the increase of argon dilution concentration.?4?The effects of initial pressure and argon dilution concentration on the propagation distance of overdrive detonation and the formation distance of overall steady detonation in a free accelerating tube were experimentally studied.Taking CH₄+2O₂ as an example,the overdrive detonation propagation distance also increases with the increase of initial pressure,and the stable detonation formation distance shows a similar"step-type"decreasing trend due to the large influence of the accelerated detonation propagation stage.With the increase of the argon gas dilution concentration,the propagation distance of overdrive detonation decreases gradually,while the steady detonation formation distance increases accordingly,which is similar to the variation of the propagation distance of accelerated detonation.Results show that the propagation law of DDT is mainly affected by the propagation of deflagration flame at the acceleration stage.?5?The effects of spiral tube length and blocking rate on the DDT were experimentally studied in premix gas C₂H₄-Air.The results show that the steady detonation growth process under the accelerating development of spiral tube obstacle is consistent with the free acceleration condition,and the addition of spiral tube obstacle can obviously accelerate the transition from deflagration to detonation.The increase of the spiral tube obstacle length can accelerate the whole process of DDT.Under this experimental condition,the DDT process is accelerated obviously with the increase of blocking rate.As long as the initial pressure is higher,the effects of blocking rate of spiral tube is more obvious.