Flow instabilities such as Rotating Stall and Surge limit the operating range of centrifugal compressors at low mass-flow rates. Employing compressible Large Eddy Simulations (LES), their generation mechanisms are exposed. Toward low mass-flow rate operating conditions, flow reversal over the blade tips (generated by the back pressure) causes an inflection point of the inlet flow profile. There, a shear-layer induces vortical structures circulating at the compressor inlet. Traces of these flow structures are observed until far downstream in the radial diffuser. The tip leakage flow exhibits angular momentum imparted by the impeller, which deteriorates the incidence angles at the blade tips through an over imposed swirling component to the incoming flow. We show that the impeller is incapable to maintain constant efficiency at surge operating conditions due to the extreme alteration of the incidence angle. This induces unsteady flow momentum transfer downstream, which is reflected as compression wave at the compressor outlet traveling toward the impeller. There, the pressure oscillations govern the tip leakage flow and hence, the incidence angles at the impeller. When these individual self-exited processes occurs in-phase, a surge limit-cycle establishes.

中文翻译：

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离心式压缩机旋转失速和喘振的产生机制

旋转失速和喘振等流动不稳定性限制了离心式压缩机在低质量流量下的运行范围。采用可压缩的大涡模拟 (LES)，它们的生成机制被暴露出来。在低质量流量运行条件下，叶尖上的流动反向（由背压产生）会导致入口流动剖面的拐点。在那里，剪切层引起在压缩机入口处循环的涡流结构。这些流动结构的痕迹一直观察到径向扩散器的远下游。叶尖泄漏流表现出由叶轮赋予的角动量，这会通过对进入流施加过度的涡流分量而使叶尖处的入射角恶化。我们表明，由于入射角的极端变化，叶轮无法在喘振操作条件下保持恒定效率。这会引起下游不稳定的流动动量传递，这在压缩机出口处被反射为压缩波，向叶轮传播。在那里，压力振荡控制着叶尖泄漏流，因此也控制着叶轮的入射角。当这些单独的自退出过程同相发生时，就会建立一个浪涌极限循环。