Stall Mechanism Of High-Pressure Ratio Centrifugal Compressors

The compressor stability is the key issue for the success of aero-engine development.Owning the advantages of compact structure,fewer parts,high-pressure ratio,and high reliability,centrifugal compressors have been widely used in small and medium aero-engines.The continuously increasing pressure ratio for centrifugal compressors puts a higher challenge to the aerodynamic stability of centrifugal compressors.Further understanding the stall mechanism in centrifugal compressors,and broadening the stable working range are of great significance for improving the performance and reliability of small and medium aero-engines.Based on the current research status and the development trend of high-pressure ratio centrifugal compressors,this thesis focuses on the stall mode and stall mechanism of high-pressure ratio centrifugal compressors.Numerical simulations and experiments have been carried out in the following three aspects:(1)Research on the stall mode and stall mechanism in the diffuser.By the whole-annual unsteady simulations,the inception and development of stall in the centrifugal compressor are captured under 60%-100%design speed.As a result,the existence of three different types of diffuser stall,the forward-travelling rotating stall,the backward-travelling rotating stall and the rotating instability,are proved in the high-pressure ratio centrifugal compressors.The forward-travelling stall cell in the diffuser is only affected by the incidence,which confirms the classic stall propagation model proposed by Emmons;the backward-travelling stall cell in the diffuser,owning to the combined effects of incidence,shock and the number of stall cells,propagates in the opposite direction of the centrifugal impeller rotation direction,which is different from the classic stall propagation model proposed by Emmons;for the rotating instability in the diffuser,the number of stall cells is changing by time,and the duration of propagation process for stall cell in the circumferential direction is relatively short.It is worth pointing out that the flow instability is limited to the diffuser passage for the high-pressure ratio centrifugal compressors.(2)Research on the stall mode and stall mechanism in centrifugal compressor stage.At off-design speeds,the compressor stage stall will occur inside the centrifugal compressor,that is,the centrifugal impeller and diffuser will stall at the same time.However,the stall inceptions vary from the compressor to compressor.At 60%to 90%design speed for the high-pressure ratio centrifugal compressor,the impeller stalls first,and then the diffuser is affected by the impeller and stalls;At 60%to 80%design speed for the ultra-high-pressure ratio centrifugal compressor,the diffuser stalls first,and the impeller stalls only after being affected by the diffuser.The main reason for the impeller stall is the expansion of the reversed flow at the impeller inlet near the blade tip,while the diffuser stall is because of the increase of positive incidence near the vane tip.(3)Research on the related mechanism of stall and flow parameters in centrifugal compressors.The flow structures before stall in a centrifugal impeller at off-design speed is mainly divided into two types:the radial migration of flow on the suction side near the leading edge of the impeller main blade and the tornado-like vortex attached between the impeller blade suction surface and the casing.These two flow phenomena are associated with the radial pressure gradient distributions;besides,the stability boundary of the diffuser is fitted by extracting the flow parameters.Through the research in this thesis,the stall mode and stall mechanism in the high-pressure ratio centrifugal compressors is explored and revealed,and the related mechanism among the stall mode,the stall mechanism and the flow parameters are established,which provides the basic support for designing of a high-stability centrifugal compressor.