Numerical investigation on the unstable flow at off-design condition in a mixed-flow pump

Abstract

The mixed-flow pump is wildly used in industrial and agricultural fields, and the requirements for its stability become more and more restricted with the improvement of the capacities. When the pump is operated under part-loading conditions, unstable characteristic like rotating stall happens and brings intensive vibration and noise. To reveal the internal unstable flow characteristics in mixed-flow pump, unsteady numerical simulation is conducted under different flow rates in the current research. The secondary flow and the vortex induced by the unsteady flow in the pump, which vary with the change of the flow rate, are analyzed according to the simulation results. Pressure fluctuation in different parts of the impeller is numerically predicted and correlated to the rotating stall propagation process. With the reduction of the flow discharge, a critical flow rate (0.6 Qdes in the current model pump) is found related to the rotating stall inception. Unstable vortex structure is found in part of the flow passages of the impeller under the stall inception stage; 0.56 Qdes is the flow rate related to the deep rotating stall under which stable stall vortexes are found in each flow passage of the impeller. Three types of pressure fluctuation characteristics representing different flow field pattern in the impeller are found. When the pump is working at designed flow rate, the pressure fluctuation is regular with the dominant frequency being the rotation frequency fs of the rotor. At the stall inception condition, when the stall core appears in the flow passages of the impeller in turn, the amplitude of the pressure fluctuation is increased and the dominant frequency is changed to a low-frequency signal. When the flow rate is reduced below deep stall condition, owning to the reflux from the guide vane and the intensive rotor–stator interaction, the blade passing frequency of the guide vane fb2 becomes the dominant frequency. Finally, unsteady characteristics of the flow field under stall inception condition are analyzed to demonstrate the propagation process of the rotating stall. The stall vortex is found to propagate from the stalled flow passage to the next, through the tip area of the blade leading edge. During the propagation process, the first vortex shrinks and disappears gradually, while the second vortex grows continuously, and the third vortex shows up at the vanishment of the first stall vortex. This research provides detailed information for the unstable flow especially related to rotating stall evolution with the variation of the operating flow rate of the mixed-flow pump with guide vane.