Numerical flow simulation of pump stability for pumped storage units with analysis and visualization of the dynamic flow patterns

Abstract

With the increasing participation of alternative energy conversion sources in the energy market, the demand for energy storage and grid regulation services keep steadily growing. Pumped storage stations stand out currently as the most suitable and effective solution for these purposes. This leads to tougher requirements regarding the operation flexibility, regulation band and operating range of pumped storage units with reversible pump-turbines or ternary sets (with single or multi-stage pumps), operating at fixed or variable speed. Through the extension of the pump stable region, the pump regulation band can be extended (assuming the proper cavitation safety is provided) and in the case of reversible units the design of the hydraulic machine for turbine mode operation can be accordingly adjusted. The objective here is to model and examine details of the flow phenomena occurring as the pump goes through the unstable region at flows below minimum operating flow. A detailed understanding of the flow phenomena in this region can provide insights to improve the hydraulic design and extend the stable region of operation. Traditional model tests only provide data about the pump characteristic curve. The focus here will be transient computational fluid dynamic (CFD) simulations of a pump-turbine as it goes through the unstable low flow pumping region. The finite volume model includes the complete hydraulic machine from draft tube to spiral case and hybrid turbulence models were used, in this case scale adaptive simulation (SAS). For the analysis and assessment of the transient fluid flow at the different pump operating conditions, integral quantities such as the lift head were evaluated, the velocity field was visualized with flow streamlines, the pressure field with contour plots at the hydraulic surfaces and the vortical structures were identified, quantified and plotted using the criterion. The transient fluid flow calculation was started at an operating point in the pump stable operating range. The simulated pump flow was progressively reduced during the given numerical simulation until the set-in of the pump instability with the break-down of the lift head and pump flow continued to be reduced until the lift head started rising again. The numerical simulations were carried out for a low specific-speed reversible pump-turbine. Detailed model test results were available for the selected design and have been used to validate the numerical simulations.