Experimental And Numerical Studies On The Internal And External Flow Field In Multi-cycle Pulse Detonation Engine

Pulsed Detonation Engine (PDE) is a new-concept power device, which utilizes pulsed detonation waves to produce force. Compared with Conventional propulsion turbine engine, PDEs are more compact and reliable because of no rotational component. Since 1980s, a series of important achievements were achieved in the studies of the experimental and numerical simulation. However, these researches seldom concerned the dynamics structure of the external flow filed. While PDE is working, the dynamics structure of the internal flow filed affects the development of the external flow field, the external flow field is counteractive in the internal flow field which led to the instability of thrust and performance, and there are greater differences of dynamics mechanism of the internal and external flow field among multi cycles. So that researches in these aspects are both academically and practically significant.In this paper, the experiments on the process of PDE were performed and the pressure-time profiles of the external flow field were obtained. Moreover, some visualization tests of the external flow field were implemented by using YA-16 high-speed shadowgraph imaging system, and the clear sequential shadowgraph images were also obtained in tests. In addition, by solving axisymmetnc Euler equations in conjunction with the chemistry, numerical simulation of two cycles of pulse detonation engine was performed, and a detailed chemical mechanism of H₂-O₂-N₂ system, which included 19 element reactions and 9 species, was involved. An in situ adaptive tabulation (ISAT) method was improved to enhance the computational performance of combustion chemistry. Furthermore, the computational shadowgraph images were plotted based on the numerical results and relevant computational flow imaging methods.Based on the experimental results, numerical results and computational shadowgraph images, the dynamic characteristics of the internal and external flow fields were elucidated systematically. In this paper, after the donation wave vented external flow field in the first cycle, it decayed into a non-reactive spherical shock wave followed by hot combustion products. A vortex ring was induced by Baroclinic effects and Helmholtz instabilities in the vicinity of the thruster exit which functioned as a convergent/divergent fluid nozzle. The combustion products behind the shock wave formed the Prandtl-Meyer flow in the exit of the tube due to the under expansions, and the suspended shock occurred which was connected with a low-pressure core of the vortex ring in order to adjust the pressure difference between expansion flow field and compression flow field behind the leading shock wave. Due to decreasing mass flow rate, the angle of the fluid nozzle varied which resulted in the modification of intensity, shape and standing location of the suspended shock. After the donation wave vented external flow field in the second cycle, under the directional propagating of the gas in the front of the leading shock wave and the very high mass flow rate behind the leading shock, the leading shock front in the vicinity of the axis protruded gradually and a cucurbit-like formed. The characteristics of the vortex ring, suspended shock wave and leading shock wave in the two cycles, were showed obviously from the clear sequential experimental shadowgraph images and computational shadowgraph images.In addition, the characteristic in the external flow field of the exited reactive shock waves with different velocities was discussed in detail according to the experimental shadowgraph images. Namely, when the other venting conditions were fixed, with velocity of the exited shock wave increasing, the bending degree of the suspended shock wave increased, the thickness of the crest of the mushroom-like combustion products increased and the shape of the secondary upward-facing shock wave became more round. Effects of nozzles on PDE cycle efficiency were also analyzed. Namely, the duration of thrust platform of the straight nozzle was the longest, then followed the divergent nozzle, finally was the convergent nozzle.