Numerical Analysis of a Model Pump-turbine Internal Flow Behavior in Pump Hump District

Purpose Numerically analyzed the flow characteristic and explored the hydrodynamic mechanism of the S-shaped region formation of a Francis pump-turbine. Design/methodology/approach Three-dimensional steady and unsteady simulations were performed for a number of operating conditions at the optimal guide vanes opening. The steady Reynolds averaged Navier-Stokes equations with the SST turbulence model were solved to model the internal flow within the entire flow passage. The predicted discharge-speed curve agrees well with the model test at generating mode. This paper compared the hydrodynamic characteristics of for off-design cases in S-shaped region with the optimal operating case, and more analysis focuses particularly on very low positive and negative discharge cases with the same unit speed. Findings At runaway case towards smaller discharge, the relative circumferential velocity becomes stronger in the vaneless, which generates the “water ring” and blocks the flow between guide vane and runner. The runner inlet attack angle becomes larger, and the runner blade passages nearly filled with flow separation and vortexes. The deterioration of runner blade flow leads to the dramatic decrease of runner torque, which tends to reduce the runner rotation speed. In this situation, the internal flow can’t maintain the larger rotating speed at very low positive discharge cases, so the unit discharge-speed curves bend to S-shaped near runaway case. Originality/value The analysis method of four off-design cases on S-shaped region with the comparison of optimal operation case and the calculated attack angles are adopted to explore the mechanism of S characteristic. The flow characteristic and quantitative analysis all explain the bending of the unit discharge-speed curves.

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Numerical Analysis of the Effect of Misaligned Guide Vanes on Improving S-Shaped Characteristics for a Pump-Turbine

Journal of Fluids Engineering ◽

10.1115/1.4038077 ◽

2017 ◽

Vol 140 (3) ◽

Cited By ~ 4

Author(s):

Xiao Yexiang ◽

Zhu Wei ◽

Wang Zhengwei ◽

Zhang Jin ◽

Ahn Soo-Hwang ◽

...

Keyword(s):

Numerical Analysis ◽

High Speed ◽

Flow Behavior ◽

Internal Flow ◽

Operating Conditions ◽

Guide Vanes ◽

Electrical Grid ◽

Runner Blade ◽

Discharge Rates ◽

Unit Speed

The S-shaped characteristic curves in pump-turbines complicate synchronization with the electrical grid and affect system safety. Misaligned guide vanes (MGVs) are one of the most effective solutions to avoid S-shaped characteristics. The internal flow mechanism with the MGV for improving S-shaped characteristics was studied by numerical analysis. Six operating conditions were modeled in the S-shaped region. Four guide vanes were arranged as the MGVs to qualitatively and quantitatively analyze the flow behavior. The internal flow was quite complex at the four operating points without the MGV; here, the attack angle and the flow behavior had no obvious difference at each vane. For the similar conditions with MGVs, attack angles and internal flow fields varied clearly at each vane, especially in the vaneless region and in the runner blade passages. For the same discharge rates, total openings, and rotating speeds, the internal flows were quite different between with and without the MGVs. The MGVs disrupt the high-speed circumferential water ring (appreciably faster compared to the main flow) in the vaneless region and maintain operation with higher unit speeds. Consequently, the unit speed is larger at the same unit discharge in the S-shaped region. Therefore, the MGV method can reduce S-shaped characteristics.

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