暂冲式环形射流诱导非定常激波聚焦的机理研究

Investigation on Mechanism of Unsteady Shock Wave Focus Induced by Blowdown Imploding Annular Jet

关键词:  环形射流  聚心碰撞  非定常激波  激波聚焦

Key words:  Annular jet  Imploding collision  Unsteady shock wave  Shock wave focusing

摘要:

利用暂冲式环形射流诱导非定常激波聚焦实验装置，在环形喷口宽度为9.4mm，喷口前后压比分别为6.1和17.9的条件下进行了环形射流聚心碰撞诱导非定常激波聚焦的实验研究，并以数值计算为补充，对流场的演化过程进行了详细的分析。结果表明，环形射流启动时形成的前导激波聚心碰撞是产生非定常激波的主要原因。在环形喷口宽度d=9.4mm，压比π=6.1的条件下，激波聚焦的峰值压力可达2.16MPa，为凹面腔内部初始压力的21.8倍。激波在凹面腔内聚焦后形成回传激波，凹面腔底部的气流经入射激波与回传激波两次压缩后，压力大大提升，向环形喷口回流，使下一循环中环形射流的欠膨胀度降低，前导激波强度也相应减弱，随之产生的非定常激波和激波聚焦强度也有所降低。此外，一个循环中输入凹面腔的能量由环形喷口宽度所决定，因此，当喷口前后压比大于临界压比1.89时，保持喷口宽度不变而仅改变喷口前后压比，所产生的非定常激波和激波聚焦强度基本不变。

Abstract:

Experimental investigation on the mechanism of shock wave focus induced by imploding annular jet was carried out with a shock wave focus apparatus. With the annular nozzle width d fixed to be 9.4mm，two conditions were tested under the pressure ratio (π) of 6.1 and 17.9，respectively. Meanwhile，Numerical simulation was performed as a supplement to visualize the flow field evolution in details. Results indicate that the implosion of start leading shock wave mainly contributes to the formation of the unsteady shock wave. Under the condition of d=9.4mm and π=6.1，peak pressure at the focus can attain 2.16MPa，21.8 times the value of the initial pressure. Echo shock wave forms after shock wave focuses. Then，the pressure inside the cavity is promoted substantially by incident and echo shock wave，so backflow set in at the exit of the annular nozzle. Accordingly，the under-expansion degree of the annular jet decreases in the next cycle，and so does the intensity of implosion and shock wave focus. In addition，the energy filled into the concave cavity during one cycle depends on the width of the annular nozzle. So，as long as the width of the nozzle is fixed when the pressure ratio exceeds 1.89，the intensity of the unsteady shock wave and the shock wave focus will remain unchanged.