Impeller-volute interaction around tongue region in centrifugal pump under rotating stall condition

Abstract: The impeller-volute interaction around the tongue region in centrifugal pump is always very strong, which usually causes vibration and noise. The tongue region is one of the most critical regions for pressure fluctuation, and in order to reveal the impeller-volute interaction around this region under rotating stall condition, a volute-type centrifugal pump was chosen as the research object to investigate by numerical simulation. A number of reference locations were arranged in the near-tongue region for recording the pressure fluctuations. The entire computational domain including impeller and volute was divided into 4.2 million grid cells, and the time step was set as 2.3×10-4 s, totally 360 time steps per impeller revolution.. Corresponding to a Courant number, which was estimated to be smaller than 1.0. Large eddy simulation was applied to simulate the three-dimensional unsteady viscous incompressible flow in the centrifugal pump. The predictions of the numerical model were compared with experimental results of flow-head curve. Good agreement between the simulated and experiment results was obtained. The largest head deviation under different flow rates was 8%, due to the smooth wall assumption during the simulations. Several flow rates were chosen ranging from 100% to 25% of the nominal flow rate for determining the stall point. The internal flow field and pressure fluctuation characteristics at different operating points were obtained. Spectra of pressure pulsation signals were analyzed, and the frequency was normalized by the rotation frequency. The regions with lower values in the pressure field were referred as rotating cells in that the stall was always accompanied by pressure reduction. It has been found that the rotating stall phenomenon occurred as the flow rate was decreased to 0.70 of nominal flow rate. Three stall cells near the entrance of passages could be observed in the pressure distribution. As the flow rate was further decreased, the area of stall cells was larger. Under the rotating stall condition, 50% of the blade passing frequency (i.e. 3 times of rotation frequency) was presented due to the alternate stalled and unstalled passages. When the rotating stall occurred, the amplitude of pressure fluctuation was much higher than that at unstalled points. Stall cells had significant effect on the pressure fluctuations in the volute. The maximum amplitude of dominant frequency was located at the tongue whether it was under rotating stall or unstalled condition. The amplitudes at the other locations were reduced gradually along the flow direction. Furthermore, a vortex always appeared in the fixed position (near the tongue) when the blade passed the tongue. It could be called "fixed stall" phenomenon, which was caused by the non-uniform pressure distribution on the volute and strong adverse pressure gradient in the tongue region. At 0.50 of nominal flow rate, the rotating stall cells played the leading role. The dominant frequency was 3 times of rotation frequency, which was 50% of the blade passing frequency. However, when the flow rate was decreased to 0.25 of nominal flow rate, the "fixed stall" cells played the leading role. The dominant frequency was 6 times of rotation frequency, which was the blade passing frequency. This research can provide useful reference for the secure and stable operation of centrifugal pumps.