A study of spike and modal-deep stall phenomena of centrifugal compressor at small mass-flow rate

In this paper, the spatiotemporal evolution characteristics of spike and modal-deep stall cells in a centrifugal compressor are studied at a small mass-flow rate. Three operating points are selected at different stall stages, and the internal unstable flow mechanism is deeply explored based on the downtrend of pressure ratio and isentropic efficiency on the external characteristic curve. The dimensionless mass flow rate (M*) is proposed to reflect the centrifugal compression rate of the whole machine under various stall conditions. A hump-like pulse is captured at deep stall conditions, and its fluctuation amplitude is an order of magnitude different from the near-stall to stall conditions. The fluctuation characteristics at different operating conditions are quantitatively analyzed from the perspective of the time domain. The fluctuation and spectral characteristics of flow field parameters at different stall conditions are obtained by collecting dynamic data of the flow field at different positions and performing a fast Fourier transform. The characteristic frequency of the flow field is closely correlated with the spatiotemporal flow structure. The fitting curve of static pressure distribution along the blade profile at deep stall conditions is increasingly divergent. The stall signals of two different modes are captured by the frequency-domain analysis of the pressure fluctuation on the inner flow surface of the impeller. The spatiotemporal correlation between the evolution of stall clusters in the impeller domain and the existence of vaneless diffusers and volutes is explored, from the axial flow of the impeller domain to circumferential flow. The stall mechanism of circumferential flow in the vaneless zone of the centrifugal compressor is deeply explored from the time domain and frequency domain based on the pressure fluctuation and its root mean square error value.