Unsteady flow characteristics of backflow vortices in an axial-flow pump at low flow rates

Abstract

Axial flow pumps are widely used in water conservancy, petrochemical and agricultural industries. Efficient operation is crucial for energy conservation and emission reduction. Improving efficiency under severe conditions requires studying the internal flow of axial-flow pumps, particularly at low flow rates where backflow vortices form near the impeller inlet. This study investigates the unsteady flow characteristics of backflow vortices at different flow rates in an axial-flow pump. Results show that backflow vortices form when the flow rate decreases to 0.59Q_d. As the flow rate further declines, the backflow vortex progresses upstream, contracts, and rebounds. The flow rate range is divided into three stages: Stage I with no backflow vortex, stage II with initial vortex development extending upstream and relatively fragmented, and stage III with vortex contraction and rebound forming a more coherent structure. Besides, backflow vortices induce significant pressure fluctuations and velocity oscillations with the primary frequency being 0.5 f_b. They exhibit a three-dimensional spiral motion involving changes in axial length, self-rotation, and revolution around the pump axis, with an angular velocity of approximately half the impeller’s rotational speed. This work enhances insights into backflow vortex behaviors, which is essential for optimizing pump design and improving operational stability in challenging environments.