Experimental investigation of relative velocity field based on image rotation method in pump impeller

Abstract

The continuous instantaneous flow field in pump impeller is vital for studying the mechanism of some complex flow phenomenon. In this paper, a high frequency PIV system with maximum sampling frequency 10 k Hz is built and used to measure the continuous flow field in impeller. In order to reduce the minimum sampling frequency required for PIV while ensuring the accuracy of flow field measurement, an image rotation method was proposed for calculating the relative velocity flow field in impeller. This method removed the particle displacement caused by impeller rotation in circumferential direction before the FFT calculation was performed. The relative velocity value calculated by velocity triangle method and image rotation method are compared and analyzed at different sampling line positions for different experimental conditions. The results show the image rotation method can not only obtain a more accurate velocity field near the blade area, but also has a lower requirement for the sampling frequency of the PIV system. The relative velocity error between two methods shows a trend of decreasing and then increasing when the sampling frequency decreases from 10 k Hz to 1 k Hz. Meanwhile, the relationship between the impeller speed and optimal sampling frequency range is established, which provides guidance for the selection of the appropriate PIV sampling frequency used for the measurement of the flow field in impeller.