Numerical Investigations on Vortical Structures in the Near Tongue Region of a Centrifugal Pump during Transient Operation

Centrifugal pumps are considered to be an integral part of process industries around the world. The flow structure within centrifugal pumps is very complex due to the interaction between the rotating impeller and the geometric features around it, such as tongue. Researchers have been analysing the effects of the interactions between impeller blades and the tongue, however, most of these studies are based on steady-state approximations where the impeller blades are modelled using frozen-rotor approach which leads to discrepancies in the predicted flow fields. In the present study, fully transient numerical investigations, on the generation and dissipation of vortical structures in the vicinity of the tongue region, have been carried out using a commercial Computational Fluid Dynamics (CFD) based solver. The instantaneous behaviour of a centrifugal pump is studied using the Sliding Mesh technique. Simulations have been carried out on both a constant rotating speed and under decelerating conditions. The second invariant of the velocity gradient tensor i.e. Q-criterion, has been employed to identify the generation and dissipation of vortical structures near the tongue region of the pump. The results indicate that the Q-criterion is fairly non-uniform downstream the tongue region due to the complex interaction between the impeller blades and the tongue. Furthermore, it has been observed that as the rotational speed of the centrifugal pump decreases, the Q-criterion in the near tongue region remains constant. The generation, expansion and subsequent mixing of two distinct vortical structures have been noticed downstream the tongue (within the volute), whereby the strength of these structures has been observed to be decreasing as the distance from the tongue increases.