EFFECT OF PRE-WHIRL ON UNSTABLE FLOW INCEPTION IN A CENTRIFUGAL IMPELLER

摘要

A casing treatment is one of the efficient devices for suppression of unstable flow at small flow rates in axial compressors and blowers, but not always common in centrifugal compressors. It was verified experimentally as well as numerically by the authors that the implementation of an inlet ring groove arrangement was fairly efficient for suppression of unstable flow occurring at small flow rates in a centrifugal impeller integrated with inducer. According to the authors previous study based on the numerical simulation, the recirculation flow is formed from the ring groove located near the inducer tip throat to the blower inlet through the annular bypass, and both the tip leakage flow and the reverse flow near the inducer tip are sucked into the ring groove, resulting in the unstable flow suppression. In this case, a large-positive flow incidence is reduced successfully due to not only the increased incoming flow rate but also the pre-whirl induced by the recirculation flow. In the present study, the effect of the pre-whirl induced by the recirculation flow on the flow incidence and the impeller characteristics were analyzed numerically and compared with the experimental results. In order to change the swirl intensity of the recirculation flow, twenty guide vanes were installed circumferentially in the flow passage of the ring groove arrangement, and the setting angle of the guide vane was changed stepwise. It is clearly shown that the flow incidence distribution between hub and tip and the recirculation flow rate vary remarkably by changing the setting angle of the guide vane. The concluding remarks obtained are as follows. (1) The formation of inlet recirculation is very effective for suppression of the unstable flow at small flow rates. (2) The better impeller characteristic at large flow rates can be achieved by both implementations of the rear groove with the rounded edge and the front groove. (3) The better impeller characteristic at small flow rates can be achieved by suppression of the pre-whirl intensity. (4) By means of the advanced type of the ring groove arrangement with the guide vane, the unstable flow rate range of the tested impeller was reduced by about 53% without deterioration of the impeller characteristics in the whole flow rate range even at small and large flow rates.