Abstract
The positive slope on the pump performance curve of pump-turbines suggests potential operational instabilities in pump mode. Previous research has indicated that the increase of the hydraulic loss caused by sudden changes of flow patterns in pump-turbines is responsible for the positive slope, however its detailed flow mechanism is still unclear. A low specific speed model pump-turbine was numerically investigated against experiments in the present study, by applying unsteady RANS (Reynolds-Averaged Navier–Stokes equations) simulations with a v2-f turbulence model. The mechanism of occurrence of the positive slope on the pump performance curve was discussed regarding the energy balance, as this region appears when the value of ∂ P u ∂ Q is larger than the critical value P u Q . An unsteady local loss analysis, derived from the energy equation, was conducted to illustrate the contribution of local flow patterns to the loss in corresponding hydraulic components. The variation of the kinetic energy of the mean flow was taken into account for the first time so that this method can be applied to highly time dependent flow patterns, e.g., a rotating stall in the present study. The investigations on the flow patterns revealed that some guide vane channels stalled with a larger discharge coefficient than the positive slope region. Several guide vane channels near the stalled channels were stalling with minor decrease of the discharge coefficient, leading to sudden increases of the input power and the loss. When the discharge coefficient slightly decreased in further, the pump-turbine operated into the positive slope region, and the rotating stall with 3 stall cells appeared, proven by the FFT (Fast Fourier Transform) and cross-phase analysis on the pressure fluctuations.
Highlights
Pumped hydroelectric storage (PHS) is widely established for utility-scale electricity storage, utilizing relatively cheap electricity from the power grid during off-peak hours to move water from a lower reservoir to an upper one to store energy, and releasing the water from the upper reservoir to generate power at a higher price during periods of high electricity demand
When reducing the of the positive slope is that thepower value and of the rate, the decrease of the input increase of the torque can result in the decrease of the useful power, which may lead to the positive slope in the pump performance curve
Flow rate, the decrease of the input power and the increase of the torque can result in the decrease of the useful power, which may lead to the positive slope in the pump performance curve
Summary
Pumped hydroelectric storage (PHS) is widely established for utility-scale electricity storage, utilizing relatively cheap electricity from the power grid during off-peak hours to move water from a lower reservoir to an upper one to store energy, and releasing the water from the upper reservoir to generate power at a higher price during periods of high electricity demand. Unsteady simulation is necessary to predict flow patterns in the pump-turbine operating in the positive slope region, considering that these flow patterns are highly time-dependent (e.g., rotating stall). It is generally accepted that the sudden change of flow patterns leading to a sharp increase of the hydraulic loss is responsible to the positive slope [1], its detailed flow mechanism is still unclear, since it is difficult to quantitatively measure the difference described above, and to discuss the relationship between this difference and the hydraulic loss. The unsteady local loss analysis based on the energy equation was occurrence positive slope. The unsteady local loss analysis based energy equation was occurrence of of thethe positive slope.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
