Suppression of the unstable head curve in mixed flow pump with adjustable vane by use of shallow grooves

Abstract

In order to operate turbo machinery widely, a mixed flow pump with adjustable vane is often used. However, the head-capacity curve of a mixed flow pump tends to be unstable with a positive slope characteristic in the range of low capacity. The unstable head curve is one of a serious problem for the stable pumping operation. This paper studied about suppressions of the unstable head curves in a mixed flow pump with adjustable vane by use of the shallow grooves. The specific speed of the tested mixed flow pump with adjustable vane was about 700 [m3/min, min−1, m]. In order to clarify the main reason of the unstable head curves, Computational Fluid Dynamics (CFD) simulation was carried out. It was confirmed that the positive slope head curve was mainly caused by the suddenly increase of the hydraulic loss due to the tip leakage flow at the middle of the impeller tip. In addition, it was also suggested that the rapidly hydraulic loss at the low capacity range was caused by a peculiar meridional shape for a mechanism of adjustable vane. Therefore, in order to improve the tip leakage flow at the low capacity range, the shallow radial grooves which were mounted on the middle of the impeller tip was tested experimentally and CFD simulation was also carried out. As a result, it was clearly confirmed that the shallow radial grooves were very effective to suppress the unstable head curve. In addition, it was also confirmed that the improvement of the tip leakage flow at the range of low capacity by the result of CFD simulation. On the other hand, it was also showed that the best efficiency becomes low compared with the original pump. It was suggested that the internal flow characteristics at the best efficiency was strongly affected by the radial groove. Therefore, in order to recovery the best efficiency, the shallow angled grooves which have a same angle of the impeller tip blade were also proposed and tested experimentally. As a result, it was confirmed that the decrease of best efficiency improves compared with the radial grooves and almost the same effect of the suppression of unstable head curve.