Particle image velocimetry experiment of the inlet flow field in a mixed-flow pump during the startup period

The transient characteristics of the inlet flow field of mixed-flow pump during start-up process are hard to be measured experimentally. The elbow structure with viewport is designed before the mixed-flow pump for capturing the transient inlet flow fields of the mixed-flow pump. Meanwhile, the synchronous data acceptance system is also designed to collect the data from different sensors synchronously. Besides, uncertainty analysis is carried out after the experiment to ensure the accuracy and reliability of this test system. The transient external characteristics are obtained and the velocity distribution of a mixed-flow pump during the startup period is measured using particle image velocimetry technique. The results show that the overall uncertainty for the external characteristics was 0.2345%, which indicates the high accuracy of this test bench for capturing the transient flow condition in the mixed-flow pump during the startup period. At the beginning of the start-up process, the head of the mixed-flow pump increases and reaches the maximum once the rotation speed meets the maximum value and an instantaneous impulsive head appears at the end of the accelerating process. However, with the increase of the rotating speed, wall boundary layer gradually develops from laminar to turbulent flow, and the flow moves continuously from the hub center to the inlet. At the end stage of the accelerating process, more intensive entrainment effect of the impeller is found. When the rotation speed is steady, the influence of the elbow on the internal flow fields gradually decreases, compared with the startup process. The results can serve as a fundamental reference for the investigation of transient flow characteristics of the mixed-flow pump during the startup period.