Study On The Initiation And Propagation Characteristics Of Detonation Waves In Variable Cross-section Tubes

With the development of aerospace and space exploration,high-performance aircraft urgently need advanced propulsion devices.Pulse detonation engines are based on detonation combustion,which has the advantages of high thermal efficiency,large thrust-to-weight ratio,and wide applicability.It is one of the ideal power devices in the future aerospace field.Deflagration to Detonation Transition(DDT)is a key research content in the research field of pulse detonation engines.Effectively reducing the detonation distance and shortening the detonation time is an important measure to improve the performance of pulse detonation engines.In this paper,on the basis of fully investigating the dynamics of DDT research at home and abroad,the self-designed variable cross-section detonation pipeline device is adopted for the impact of the change of pipeline cross section on the DDT process.The pressure sensor and ion probe system are used to research the DDT in the cross section shrinkage pipeline.Experiments were carried out to obtain the influence of the cross-sectional deflation structure of the pipeline on the detonation wave initiation and propagation characteristics;the two-dimensional numerical simulation of the DDT process in the cross-section deflation and sudden expansion pipeline was carried out to analyze the flame and the pressure wave propagation behavior in the detonation wave initiation process,and the influencing factors of the DDT process are also analyzed,the main research contents and conclusions are as follows:(1)Three types of cross-sectional shrinkage pipe devices with different detonator lengths were designed.The experimental research on the initiation and propagation characteristics of methane / oxygen detonation waves in the cross-section shrinkage pipe and straight pipe showed that when the cross-section shrinkage ratio was 5 : 3.When the length of the detonating tube is 30 cm,the cross-sectional shrinkage has a significant acceleration effect on the DDT process.At an initial pressure of 20 and15 kPa,the initiation distance of the cross-sectional shrinking pipe is 22% and 19%shorter than that in the straight pipe,but the cross-sectional shrinkage has no effect onthe state of stable detonation waves.The effect of initial conditions on the detonation and propagation characteristics of detonation waves was studied using a combination of experiments and numerical simulations.The results show that with the increase of the initial pressure,the DDT process is gradually accelerated and the peak pressure in the pipeline is gradually increased.However,the initial pressure has little effect on the stable detonation speed;when the length of the detonator is from 30 to 50 cm,the DDT distance decreases first and then increases.The peak pressure of the stable detonation wave increases with the length of the detonator.(2)We studied the flame and pressure wave development of the methane /oxygen DDT process in the cross-sectional shrinkage and abrupt expansion pipeline through numerical simulation,and analyzed the influence of the abrupt structural parameters of the pipeline on the DDT process.The results showed that the pressure wave propagates to the variable cross-section in the cross-sectional shrinkage pipe.Due to the obstruction of the cross section,a return pressure wave is formed.Due to the influence of the return pressure wave,the flame propagation speed temporarily decreases,but the flame enters the detonation tube from the detonator After that,the speed quickly picked up and gradually propagated in the shape of a tulip flame until detonation was formed;when the inner diameter of the detonator was 40-60 mm,that is,the cross-sectional expansion ratio was 4: 3-2: 1,the detonation distance was shortened and then increased;When the size of the bombs is the same,the initiation distance and flame delay time decrease with the increase of the contraction angle.In the cross-sectional expansion pipe,the flame will not be hindered by the return pressure wave at the cross-sectional change,but will be temporarily accelerated by suddenly entering a large space.When the inner diameter of the detonator is suitable,the flame has formed a tulip flame in the detonator.After the flame enters the detonation tube,the tulip flame is destroyed,and when the degree of cross-sectional expansion(section expansion ratio,expansion angle)is small,the tulip flame can be reformed in the detonation tube,and eventually detonation is formed;The cross-sectional expansion ratio decreases with the increase;when the detonator and detonator have the same size,the larger the expansion angle,the shorter thedetonation distance and time.(3)Through the comparison of the flame propagation speed of DDT process in different variable cross-section pipes,it can be concluded that the appropriate detonator size in the cross-sectional shrinkage pipe can greatly shorten the DDT distance,otherwise it will hinder the formation of detonation waves;The accelerated detonation process of the flame in the abrupt expansion pipeline is mainly affected by the size of the detonating tube and less affected by the change in cross section.For the propagation characteristics of the detonation wave passing through the abrupt section of the cross section,when the cross section shrinkage is relatively small,a part of the detonation wave can be directly introduced into the small-diameter pipeline to maintain its original state and continue to propagate.When the cross-sectional sudden expansion is relatively small,the detonation wave will not be decoupled after passing through the cross-sectional sudden expansion section,but the shape of the detonation wave will change from a plane wave to a spherical wave,and eventually form a plane wave propagation again.