Evolution of large-scale flow structures in an axial-flow pump during performance breakdown

Abstract

This paper presents a very large eddy simulation analysis of the unsteady flow in the pre-stall to stall transition process of an axial-flow pump, with the aim to elucidate the spatiotemporal evolution of large-scale flow structures during the performance breakdown of the pump. The transient flow is investigated utilizing a time-dependent flow rate computation scheme. The results demonstrate that, as the flow rate is dynamically reduced, the reduction in pump head is found lags behind the reduction in flow rate by approximately 15 impeller revolutions. The leading edge separation on the blade suction side (SS) evolves into a leading edge separation vortex (LSV) in conjunction with the dynamic reduction in flow rate. The attached flow on the SS in the vicinity of the hub and blade trailing edge squeezes the mainstream outwards, resulting in the formation of a cross passage vortex (CPV) on the tip side of the passage. The combined effect of the LSV, CPV, and tip-clearance flow induces a penetrating upstream flow in the tip region of the impeller, which gives rise to a swirling backflow within the inlet pipe. At stall, the CPV is stably attached to the SS and extends upstream of the leading edge of the neighboring blade. Furthermore, a trailing edge backflow is observed near the junction of the blade trailing edge and the hub, and it collides with the inflow near the hub, resulting in the formation of a hub-attached vortex.