Clearance flow patterns and pressure distribution of a pump-turbine: Measurement and simulation of a rotating disk flow

Abstract

The clearance flow patterns and pressure distribution determine the clearance axial hydraulic thrust of a pump turbine, which has a substantial impact on the unit axial imbalance. However, due to the tiny size and complex shape of the clearance flow channel, there is no detailed description of the flow patterns and pressure characteristics. In this study, we conducted a model test with particle image velocimetry (PIV) measurements and CFD simulation of a rotating disk flow that was a simplified model of the pump-turbine clearance flow. It is shown that a typical Batchelor flow is formed in the clearance region, demonstrating a “core region + double-boundary layers” distribution for the circumferential velocity along the clearance height direction; the core region rotates at a speed of only around 41–42% of the rotating disk speed and is independent of the clearance inlet pressure and clearance height. Driven by centrifugal force, the flow is radially outward around the rotating disk, while inward around the stationary disk in the meridian section, showing secondary flow vortices. The pressure in the clearance region has a circumferentially symmetric and radially quadratic distribution. Based on the liquid differential equilibrium equation and core region circumferential velocity, the pressure and clearance axial hydraulic thrust in the clearance region can be expressed as a function of the clearance inlet pressure and the square of the runner rotating speed. These findings can be used to investigate axial force imbalance issues of the pump-turbine unit.

Numerical Study on the Effect of Crown Clearance Thickness on High-Head Pump Turbines

“…In Equation (10), Dh is the runner diameter. In the analysis, the gap thickness coefficient λ increases or decreases by 0.006, 0.011 or 0.023.…”

“…Unstructured grids are used in geometrically irregular areas to improve geometric adaptability, improve grid quality and enhance calculation convergence. Among them, the sealing cavity of the upper crown cavity, the sealing cavity of the lower ring cavity, the stay vanes, the guide vanes and the In Equation (10), D h is the runner diameter. In the analysis, the gap thickness coefficient λ increases or decreases by 0.006, 0.011 or 0.023.…”

“…Studies have shown that, under load shedding conditions, the gap flow between the runner and the fixed part determines the precise simulation of the pump turbine pulsation characteristics and unsteady vortex [9]. Hou XX et al [10] used the method of combining PIV measurements and a numerical simulation to determine the reasons that affect the gap flow pattern and axial water thrust, and they quantitatively expressed the axial water thrust as a function of the square of the rotational speed and the gap inlet pressure. Liu YS et al [11] studied the influence of different runner sinking axial deviation values on the axial water thrust of Francis turbines under rated conditions and found that, when the runner sinks, the axial water thrust on the upper ring increases, whereas the axial water thrust on the lower ring decreases.…”