自由活塞激波风洞压缩管非等熵流动分析

在自由活塞驱动的高超声速地面试验设备中，自由活塞压缩器的运行状态对于试验气流状态参数、试验时间及设备安全性起到决定性的作用.研究基于FD-21自由活塞激波风洞结构参数，针对典型的活塞压缩器运行状态展开数值模拟和等熵理论预测，分析压缩管中的波系结构和非等熵效应引起的流动参数变化.进一步地，将压缩管中的总熵变来源分解为激波和黏性两部分；改变驱动压力、活塞质量、压缩管初始压力和压缩管长度进行数值模拟，分析熵变变化规律，并进行参数影响的归一化分析，结果表明归一化后的熵变仅与压缩比有关；最后，对等熵理论进行修正，修正后的压缩管压力与实验和数值结果更为吻合. 

Analysis of the Non-Isentropic Flow in the Compression Tube of a Free Piston Shock Tunnel

In the hypersonic test facilities driven by free piston compressors, the operation condition of the free piston compressor is conclusive to the status of the test gas, the effective test time and the facility safety. Based on the structural parameters of FD-21 free piston shock tunnel and the typical operation condition of the free piston compressor, quasi-one-dimensional simulation and isentropic theoretical prediction of the flow in the compression tube were conducted, and the complex wave structures as well as the changes of the flow parameters induced by the non-isentropic effect were analyzed. The source of the entropy change in the compression tube was attributed to two mechanisms: the shock wave and the viscosity. Through the transverse comparison of simulation results with the variation of driving pressure, piston mass, initial pressure in the compression tube and compression tube length, the entropy change attributed to each mechanism was normalized with piston compressor operation parameters, and a unified entropy change formula was obtained. Finally, the isentropic model of piston compressor operation was modified with the entropy change profile, and better agreement with experimental and numerical results was acquired.