Runaway characteristics of bidirectional horizontal axial flow pump with super low head based on entropy production theory

A low-head pump station is often required to deliver water in two directions, resulting in the risk of runaway accidents under the bidirectional operation. This study aims to investigate the runaway transition of the pump under the Forward Runaway Condition (FRC) and Backward Runaway Condition (BRC), thereby establishing the full flow system of the low-head horizontal axial flow pump. The volume of fluid (VOF) was applied to determine the position of the free surface, and then to simulate the volume fraction of water and air in the upstream and downstream domains. The Shear Stress Transport (SST) k-ω turbulence model was selected to close the governing equations, where the eddy viscosity was modified to account for the transport of the principal turbulent shear stress. The Entropy Production Rate (EPR) mainly included the Entropy Production By Direct Dissipation (EPDD), Turbulence Dissipation (EPTD), and Wall Shear Stress (EPWS). The User-Defined Function (UDF) in the Fluent software was applied to control the real-time speed of the impeller using the torque balance equation. The Grid Convergence Index (GCI) was also calculated to verify the grid independence. A model test was conducted to verify the accuracy of three-dimensional simulation and entropy generation. The results show that the flow rate and rotation speed of the pump decreased first, and then increased, whereas, the torque generally presented a downward trend, while fluctuated around 0 value under FRC and BRC. Furthermore, the torque fluctuation amplitude under FRC was significantly higher than that under BRC in the runaway state, due to the strong Rotor-Stator Interaction (RSI) under FRC. The EPDD was dominant in the total simulation domain, followed by the EPTD and EPWS. Additionally, the total entropy production in the impeller was the highest in each simulation domain, due mainly to the larger velocity gradient and the stronger rotor-stator interaction. Additionally, the guide vane was located in the inflow direction of the impeller under FRC in the turbine or runaway state, where the smoother flow state and the lower EPDD and EPTD under FRC, compared with the BRC. As for the inlet conduit during the runaway state, the EPDD was slightly higher than the EPTD under FRC and BRC. However, the EPDD in the outlet conduit was much higher than the EPTD. More importantly, the upstream transformed from the outflow to inflow domain, and then the EPDD, EPTD, and EDWS were gradually close to zero, whereas, the downstream transformed from inflow to outflow domain, and then the EPDD, EPTD, and EDWS gradually increased during the runaway. There was a seriously unstable flow pattern in the inlet and outlet channel, leading to the strong vortices and reflux areas, particularly when the flow rate was zero (tQ=0). Correspondingly, the velocity gradient and turbulent kinetic energy were small at the low velocity, leading to the smaller total entropy production at the inlet and outlet conduit under FRC and BRC. Additionally, the span of velocity gradient in the downstream was larger under BRC than that under FRC, so did the EPDD in the runaway state (42.5 s). The vortex core gathered at the inlet side of the impeller blade under pump condition at tQ=0. Consequently, the distribution of vortex and EPR was similar at different blade-to-blade surfaces. The reason was that there was a large velocity gradient near the vortex core, particularly leading to a larger entropy yield, indicating that the vortex was the cause of energy loss.