Flow Evolution and Energy Loss Mechanism in Accidental Shutdown Process of a Large Submersible Mixed-Flow Pump System

Abstract

To investigate the accidental shutdown process for a large vertical and submersible mixed-flow pump unit in a drainage pumping station, a three-dimensional numerical method for this transition process was proposed according to the angular momentum balance theorem and rigid body rotation technology. We obtained the transient performance curves of the unit and the internal flow characteristics of the full flow channel. In addition, we also analyzed the energy loss distribution of the through-flow components during this progress based on the entropy production theory. The results show that the whole runaway process needs to go through four stages: the pump mode, the pump braking mode, the turbine mode, and the stable runaway mode. The pressure amplitude changes greatly in impeller and guide vanes, and the main fluctuation frequency is the blade frequency. There are higher harmonic frequencies in the dynamic rotor-stator interface. As the rotating speed increases in turbine mode, the negative pressure area near the impeller blade's trailing edge gradually increases. The entropy production method can be used to determine the location, intensity of energy loss during the transient process. If the rotating speed exceeds the allowed value in the runaway turbine mode, the interaction between blade tip vortex and hub vortex rope may cause loss of stability.

A Numerical Study on the Performance of a Pumping Station with Bell-Mouth-Based Vertical Pumps during an Accidental Shutdown

“…Also, the range of utilization of the 4-quadrant characteristics is very broad in PAT (Pump as Turbine) applications, from the PAT turbine startup [27][28][29], through the pump-turbine transition [30,31], up to the "S" zone close to the turbine runaway conditions, where the so-called S-instability occurs [32][33][34][35][36][37]. The simulation of pump performance during the accidental shutdown can be found, e.g., in [38], where a large vertical mixed-flow pump with a draft tube is modeled inside the commercial CFD software ANSYS Fluent with the angular momentum acting on the impeller updated in each time step by a User-Defined Function (UDF), or in [39], where the runaway and runaway shutdown processes of the tubular turbine inside the circulating cooling water system are simulated based on the combination of CFD and the method of characteristics (MOC) and also using the Fluent UDF. The process of power failure in the case of the submersible tubular pump is also solved in [40] using the direct combination of the Fluent code and the 6DOF model based on the fourth-order multi-point Adams-Moulton formula.…”