Abstract: To investigate the unstable flow characteristics inside the centrifugal compressor with a vaneless diffuser, the proper orthogonal decomposition (POD) method was applied to the unsteady numerical simulation results to analyze the unsteady flow inside the vaneless diffuser under small flow conditions. The results indicate that the fluid field of vaneless diffuser is mainly influenced by the upstream blades and the downstream volute. In addition, POD method is capable of capturing the jet-wake flow structure, of which the characteristic frequency equals the blade passing frequency, as well as the unstable mode with low frequency. The characteristic frequency of the unstable mode is 22104Hz, and there exists several perturbations in different circumferential positions. These perturbations are mainly radial fluctuation but do not propagate in the circumferential direction. The reconstruction result of the unstable mode can visually show the development of the corresponding unstable flow. The sampling frequency of the flow field snapshots has an effect on the result of POD method.

Key words: centrifugal compressor; vaneless diffuser; proper orthogonal decomposition (POD); unstable condition