The role of radial secondary flow in the process of rotating stall for a 1.5-stage axial compressor

Rotating stall is one of the most important sources of performance deterioration and system instability in axial flow compressors and, therefore, it is of great value for compressor aerodynamic design and the development of active control strategies. It has been widely demonstrated that peculiar flow mechanisms, such as tip clearance leakage vortices, play a decisive role in the genesis of rotating instabilities (RI) and stall inception, however it is still very hard to predict and interpret such mechanisms accurately. This paper presents the results of both experimental and numerical researches dealing with forming process of rotating stall in a 1.5-stage low-speed industrial axial compressor at Harbin Engineering University. A new physical mechanism for explaining the birth of rotating instabilities based on the detection of spike waves is introduced as a result of a deep analysis recognising the role due to low-energy flow migration of along the rotor spanwise direction. To this purpose, transient pressure signals obtained from both experiments and simulations were adopted to describe flow details during onset and scanning pressure contours on the compressor casing were obtained both experimentally and numerically during the process of approaching stall point so that the transition process from RI to the appearance of spike waves could be well captured. The experimental results showed that the frequencies of RI dropped nearly linearly against the flow coefficient and the strength of radial secondary flows increased distinctly, which induces the boundary layer separation near leading edge in the tip region at last. Full-annulus URANS numerical simulations using SST turbulence model were performed and validated adequately by test data, and then employed to capture more flow details and support better understanding of the flow physics. In fact, a lateral tornado-type vortex occurring before the trailing edge backflow and featuring the same frequency of early pressure disturbances was observed numerically. Under the combined effect of the lateral tornado-type vortex and the tip secondary vortex, a trailing edge reverse flow was observed which produced a huge increase of blockage. A radial tornado vortex was formed connecting the suction side and casing wall at the same time finally leading to the spike-wave rotating stall.