Unsteady constant value calculation and particle image velocimetry experiment in full passages of centrifugal pump impeller

Abstract: The centrifugal pump is one of the most important energy conversion devices and is widely used in almost all industry and agriculture, which consumes more than 21% of the total power consumption in China. Small changes in impeller geometry can lead to significant changes in hydraulic performance, such as the total head, efficiency, and cavitation characteristics. Even in the same impeller, the impeller passages show asymmetrical and unsteady flow characteristics under design and off-design conditions, mainly due to the complex three-dimensional shape of pump. Therefore, it is necessary to know more detailed knowledge of the local and instantaneous features of the impeller flow for study on the more flexible pumps that maintain high efficiencies at a broader range of operating conditions. A variety of measuring techniques have been applied to centrifugal pumps in striving for accurate quantitative flow descriptions. The particle image velocimetry (PIV) technique is a powerful tool, which offers both more information on the instantaneous spatial flow structures and, at the same time, considerably reduces acquisition time. In the existing research, only one passage was taken as the research goal, mainly due to the limitation of the test conditions. So, there was improving space in flow measurement for all flow passages of a centrifugal pump impeller. On the other hand, most of numerical results were usually validated by performance test, which was worth further verification through the PIV test. In this study, a special PIV system and a shrouded centrifugal pump impeller were designed and then the flow performance inside six rotating passages of the pump was detailedly measured using the PIV. The absolute velocity and relative velocity fields of six passages in an impeller were successfully measured under different working conditions. Relative velocity fields were also computed with the standard k-ε turbulence model of three-dimensional Reynolds-averaged Navier-Stokes equations and a commercial solver fluent software was used to divide grids. The unsteady constant value calculation results were confirmed by the performance experiment, the maximum error of head was only 4.62% with allowable acceptance. From the velocity and relative velocity measurements, the different flow patterns in the two passages near the tongue and the other passages were revealed. At the flow passage close to the tongue, the absolute velocity was reduced with the increasing of the flow rate since vortex, or backflow appeared at the zone. At larger flow rate, the relative velocity at the passage close to tongue was significantly larger than that at the other passages. The flow field distribution showed more obvious differences apart from design conditions. And there existed dead zone whose relative velocity was small at the two passages near the tongue, and its area was increased with the increase of flow rate. The results showed that relative velocity distribution trend of numerical calculation and experimental results under design conditions was in agreement, and relative velocity value of that was different. The study demonstrates that the PIV technique is efficient method to obtain reliable and detailed velocity data over a full impeller passage and it provides a reference for internal flow characteristic study in centrifugal pumps.