Numerical Analysis of Flow Phenomena Related to the Unstable Energy-Discharge Characteristic of a Pump-Turbine in Pump Mode

Abstract

Regions of positive slope in the pressure-discharge characteristics are one of the major concerns in design and operation of centrifugal pumps, as they are limiting the admissible operating range to values above the critical discharge. The industrial pump turbine of specific speed ν=0.42 (n q =66 min -1 ) proposed as QNET-CFD test case TA6-04 shows a marked saddle in the energy-discharge characteristic associated to a sudden drop of efficiency versus discharge at part load. The pump-turbine consists of a shrouded impeller with five blades, a diffuser with 22 guide and stay vanes and a spiral casing. CFD flow simulations on a reduced model were carried out with a finite volume Navier-Stokes code (CFX-5.7) using block-structured hexahedral meshes and the Menter-SST Turbulence model. Control of numerical quality has been performed. Reduced models with relatively low computational effort (mixing plane interface) already permit to capture the drop in efficiency and energy coefficient to analyze the flow phenomena inducing the drop of the energy coefficient Ψ that occurs at partial discharge.Analysis of local flow patterns and energy and velocity distributions at the rotor-stator interface that are related to the onset of recirculation are presented. INTRODUCTIONThe investigated industrial pump-turbine of specific speed ν=0.42 (n q =66 min -1 ) has been investigated experimentally by means of global performance measurements and local LDV measurements at guide vane inlet in pump mode and 2D-PIV measurements in the guide vane channels. They exposed the onset of a recirculating area at the hub side of the guide vanes that is related to the drop in the energy-discharge and the efficiency-discharge characteristics [1].

Hysteresis Characteristic in the Hump Region of a Pump-Turbine Model

“…As a consequence, more and more researchers have focused on studies of the hump instability characteristic using experimental and numerical methods. The hump characteristic is mainly caused by the prerotation, recirculation [1], and suction surface cavitation [2] at the runner inlet, backflow and vortex motion [3], rotating stall [4,5], and secondary flow pattern [6] in the diffuser channels. Lots of achievements have been obtained, which provide a deep understanding of the hump characteristic.…”

Hysteresis Characteristic in the Hump Region of a Pump-Turbine Model

“…As a consequence, more and more researchers have focused on studies of the hump instability characteristic using experimental and numerical methods. The hump characteristic is mainly caused by the prerotation, recirculation [1], and suction surface cavitation [2] at the runner inlet, backflow and vortex motion [3], rotating stall [4,5], and secondary flow pattern [6] in the diffuser channels. Lots of achievements have been obtained, which provide a deep understanding of the hump characteristic.…”

Fluid flow analysis of drooping phenomena in pump mode for a given guide vane setting of a pump-turbine model

“…The flow mechanism of a pump-turbine in pump mode is still not clear, especially for the drooping zone. Braun et al (2005) and Braun (2009) studied unstable energy-discharge characteristics of an industrial pump-turbine in pump mode and concluded that flow patterns, energy and velocity distributions at the rotor-stator interface are related to the onset of recirculation. However, the research just used one channel per component, which could not reflect interaction among the channels.…”