The stall and surge directly impact on the safety and reliability of compressors. The spike-type and modal-type stall inception exist in compressors. At present, few studies pay attention to the stall inception of centrifugal compressor, such as the formation reason for the stall inception and the action by the volute tongue on the stall precursor. This paper investigated the stall characteristics of a high-speed small-flow centrifugal compressor and illustrated the relationship between the volute tongue and the location of stall inception. In addition, the mechanism of stall inception was also clarified. Both the analysis of initial flow structures and the comparison of the frequency spectrum characteristics at different monitoring points show that the spike-type stall occurs at about 115° circumferential position in this centrifugal compressor. The nonaxisymmetric geometry structure of the volute leads to the uneven circumferential pressure distribution. The blockade effect of the volute tongue results in high static pressure area near the volute tongue. The disturbance caused by high static pressure adversely propagates into the diffuser, resulting in the static pressure peak value at different radii. As the pressure peaks adversely migrate to the impeller inlet and induce the leading edge spillover near the corresponding blade, the spike-type stall occurs. Therefore, the volute tongue both induces the stall inception and determines the circumferential position of the stall inception at the centrifugal compressor inlet.

The present paper numerically and experimentally investigates the stall inception mechanisms in a centrifugal compressor with volute. Current studies about stall inception pay more attention on the axial compressors than the centrifugal compressors; especially, the circumferential position of stall inception onset and the stall process in the centrifugal compressor with asymmetric volute structure have not been studied sufficiently yet. In this work, the compressor performance experiment was conducted and the casing wall static pressure distributions were obtained by 72 static pressure sensors first. Then, the full annular unsteady simulations were carried out at different stable operating points, and the time-averaged static pressure distributions were compared with the experimental results. Finally, the stall process of the compressor was investigated by unsteady simulations in detail. Results show that the stall inception onset is determined by the impeller leading edge (LE) spillage flow, and the occurrence time of trailing edge (TE) backflow is prior to the LE spillage. The nonuniform static pressure circumferential distribution at impeller outlet induced by volute tongue causes the two stall inception regions locating at certain circumferential positions, which are 120 deg and 300 deg circumferential positions at impeller LE, corresponding to the circumferential static pressure peak (PP) and bulge regions at impeller outlet, respectively. In detail, at rotor revolution 2.86, a small disturbance that the incoming/tip clearance flow interface is perpendicular to axial direction occurs at 120 deg position, but this disturbance did not cause the compressor stall. Then at revolution 7, the first stall inception zone (spillage region) occurs at 120 deg position, causing the compressor stall with positive pressure ratio performance. At approximately revolution 23, the second stall inception zone occurs at about 300 deg position; however, both the intensity and size of this stall inception zone are smaller than those of the first stall inception zone. These two stall inception zones are not moving along circumferential direction because the stall inception circumferential position is dominated by the impeller outlet static pressure distribution. Even then, the obvious low frequency signals appear after the spillage crossing two blade LEs, because at this moment, the spillage vortex caused by the tip leakage flow begins to shed. However, due to the asymmetric structure limitation, this vortex cannot move across full annular. Furthermore, the spillage vortexes cause the local low static pressure zone ahead of blade LE in the centrifugal compressor with volute, suggesting that the spillage can be predicted by the steady casing wall static pressure measuring. The development of blockage zones at impeller LE is also investigated quantitatively by analyzing the stall blockage effect.

Stall and surge behaviors deteriorate the compressor performance and stability of the operating range. To avoid the effect of stall and surge behaviors, the characteristics of the pre-stall disturbance have been considered in recent years. However, investigations of the pre-stall and stall disturbances in centrifugal compressors remain insufficient owing to the various geometric structures in centrifugal compressors. In this study, a centrifugal compressor with a volute and vaneless diffuser was studied using high-response static pressure measurements and numerical simulations. The pressure disturbance evolution from near-choke to near-surge conditions was obtained, and the characteristics of the pre-stall and stall disturbances were analyzed. The results showed that the compressor instability route could be divided into pre-stall and stall stages, which could be accurately predicted based on the periodic static pressure irregularity and skewness. The pre-stall stage was dominated by the rotating instability (RI) phenomenon, and the frequency of the RI was 43% of the blade passing frequency at 70% N_max. The experimental results demonstrated that the first onset of RI occurred at the 120 deg–180 deg circumferential position and maintained a local pattern; then, RI occurred over the whole annulus when the flowrate was decreased. RI was induced by the vortex structure attached to the casing wall, leading to a sharp pressure trough in the static pressure trace. The RI exhibited an evolution pattern of initially increasing and then fading, which can be explained by the enlargement and movement of the impeller inlet vortex. The large blockage effect of RI triggered a stall with disturbances at 20% and 80% of the impeller shaft frequency, and these disturbance frequencies had a trend of shifting to one dominant frequency of 22% the impeller shaft frequency. Furthermore, stall cell merging and the coexistence of RI/stall phenomena occurred during the stall stage.