Effects Of Arc Obstacles On Detonation Characteristics Of Hydrogen Air Premixed Gas

The transition process from deflagration to detonation is a physical phenomenon involving many complex processes,such as turbulence,shock-flame interaction,shock collision and reflection.Researches focused on this complex phenomenon have great significance for the application of pulse detonation engine.More disturbances can be generated by installing obstacles in semi-closed tubes,which can effectively reduce the DDT distance.In this paper,the self-designed arc obstacles are used to simulate and analyze the detonation characteristics of hydrogen-air premixed gas.The effects of the maximum blockage ratio and chord length of arc obstacles on the flame acceleration process and detonation initiation characteristics are studied.Also,the detonation characteristics in the tubes with arc and rectangle obstacles are compared.The main work and research contents of this paper are as follows:(1)The influence of maximum blockage ratio of arc obstacles on hydrogen-air detonation characteristics is simulated and analyzed.The results show that the arc obstacles can effectively reduce the deflagration-to-detonation transition distance and the optimal maximum blockage ratio is 0.7.As the maximum blockage ratio increases,the flame acceleration and initiation of detonation wave show 3 different regimes: For small maximum blockage ratio,the flame is affected by Squish flow,resulting the velocity differences between the flame near the wall and in the center of the tube,and the tulip flame formation.The tulip flame will generate two local explosion points and the explosion points develop and merge to initiate detonation.For intermediate maximum blockage ratio,under the interaction of the reflected shock wave and the obstacles,the flame is a narrow fingertip shape.The initiation of detonation wave is engendered by the development and fusion of multiple hot spots.For large maximum blockage ratio,the flame is also a narrow fingertip-shaped.The detonation wave is initiated by the coupling of the flame front with the leading shock wave at the throat of the obstacle.(2)Numerical simulation of hydrogen-air detonation characteristics in tubes equipped with arc obstacles of different chord lengths is carried out package.The results show the optimal chord length is half the diameter,in which the detonation wave propagates at a high velocity while maintaining small deflagration-to-detonation transition distance.Three different regimes were found in the flame acceleration and detonation initiation for different chord length: For the small chord length,the flame experienced the transition from fingertip to tulip flame,then recovered to fingertip flame again until initiation.The detonation is caused by the hot spot formed by the spontaneous combustion of unburned premixed gas.For the intermediate chord length,the tulip flame quenched by reverse flow will turn into fingertip flame under the jet effect of obstacles.Then,the fingertip flame is transformed into tulip flame again under the action of flow instability.The detonation is initiated by the fusion of two local hot spots produced by the coupling of tulip flame and diffraction wave.For the large chord length,the flame propagates as a fingertip flame.The detonation is initiated by the explosion of the flame surface compressed by reflected wave and shock wave.(3)The hydrogen-air detonation process in the tubes with arc obstacles and rectangle obstacles is analyzed and compared by the numerical simulation.The tubes with three different tube diameters(D = 30 mm,60 mm and 90 mm)and equipped with8 obstacles are studied.The results show that the performance of arc obstacles is better than that of rectangle obstacles,in which the intensity of the detonation is higher and the velocity of the detonation wave is about 6% higher than rectangle obstacles while arc obstacles can maintain small deflagration-to-detonation transition distance.In the process of the flame acceleration,the rectangle obstacles will produce higher turbulence intensity to accelerate the flame but also causes larger energy loss.For the initiation of detonation,in the arc obstacle tube,the initiation of the detonation is engendered by the development and fusion of multiple hot spots.However,for rectangular obstacles,the number of hot spots needed for detonation initiation is less,even a single hot spot can be initiated.In the process of the detonation propagation,the detonation in the arc obstacle tube has higher strength and velocity.For the rectangle obstacle,after the detonation front collides with the obstacle,part of the reflected wave will diffract at the entrance of the obstacle and interfere with the development of the transverse wave,which weakens the strength of the detonation front.