Inter-Blade Vortex and Vortex Rope Characteristics of a Pump-Turbine in Turbine Mode under Low Flow Rate Conditions

Abstract

Pump-turbines are often used to provide a stable power supply with a constant frequency in response to intermittent renewable energy resources. However, existing pumped-storage power stations often operate under off-design conditions because of the increasing amounts of inconsistent renewable resources that have been added to the grid. Under off-design low flow rate conditions, inter-blade vortex and vortex rope phenomena usually develop in the runner and draft tube passages, respectively, in turbine mode. These vortices cause complicated flow patterns and pressure fluctuations that destabilize the operation of the pump-turbine system. Therefore, this study investigates the influence of correlation between the inter-blade vortex and vortex rope phenomena under low flow rate conditions. Three-dimensional steady- and unsteady-state Reynolds-averaged Navier–Stokes equations were calculated with a two-phase flow analysis using a shear stress transport as the turbulence model. The inter-blade vortices in the runner passages were captured well at the low flow rate conditions, and the vortex rope was found to develop within a specific range of low flow rates. These vortex regions showed a blockage effect and complicated flow characteristics with backflow in the passages. Moreover, higher unsteady pressure characteristics occurred at locations where the vortices were especially pronounced.

Effect of Air Injection on the Internal Flow Characteristics in the Draft Tube of a Francis Turbine Model

“…In the location of 0.1D 2 , all of the observed conditions exhibited the highest normalized frequency (0.25f n ) in the low-frequency region, corresponding to a vortex rope in the DT. The frequency of a vortex rope with precession commonly corresponds to 0.2 to 0.4 times the rotational frequency [2,32,33]. The magnitude observed for the condition without air injection and with an anti-swirl fin was larger than those observed for other conditions, but this magnitude decreased at an injection level of 0.5%Q.…”

“…In the location of 0.1D2, all of the observed conditions exhibited the highest normalized frequency (0.25fn) in the low-frequency region, corresponding to a vortex rope in the DT. The frequency of a vortex rope with precession commonly corresponds to 0.2 to 0.4 times the rotational frequency [2,32,33]. The magnitude observed for the condition The unsteady pressure characteristics based on the flow rate of air injection in the DT (see Figure 12) were investigated via fast Fourier transformation (FFT) analysis of the pressures during a total of eight revolutions of the runner.…”

Vortex structure and pressure fluctuation of a pump-turbine in pump mode using large Eddy simulation

“…This study reveals the evolution mechanism of vortex rope in the draft tube. Kim et al 20 investigated the internal flow and unsteady pressure characteristics in the turbine mode of a pump-turbine under low flow rate. It was found that the position of the inter-blade vortex changes with the flow rate.…”

Numerical Investigation of Inner Flow Characteristics of a Prototype Pump Turbine with a Single Pier in Draft Tube at Part Load Conditions

“…Compared with Francis and Kaplan turbines, pump turbines are influenced by the S-region characteristic, and the pressure fluctuation characteristics are affected by both the rotor-stator interaction and the draft tube vortex rope under some conditions [12]. Kim et al [13] studied the internal flow in the draft tube of a pump turbine under partial flow rate conditions. By comparing the swirling number and the shape of the vortex rope, it is indicated that the vortex rope phenomenon is more obvious at medium flow rates, but at lower flow rates the swirling intensity is stronger.…”