Abstract
Power production from intermittent renewable energy resources, such as solar and wind, has increased in the past few decades, leading researchers and engineers to establish techniques to preserve a stable electrical grid. Consequently, hydraulic turbines are being used more frequently in transient operating modes to regulate the grid. The present work provides a comprehensive numerical study on the transient flow field of a high-head Francis turbine model throughout the shutdown sequence. The computations were performed using OpenFOAM, utilizing the SST-SAS turbulence model. A Laplacian smoothing scheme is employed to conduct the mesh deformation of the guide vane domain. The time-averaged draft tube velocity field at the steady Best Efficiency Point (BEP) is validated against experimental data. Then different aspects of the transient flow field in the shutdown sequence are carefully assessed and explained for the first time. Short-Time Fourier Transform (STFT) analysis is carried out on the fluctuating part of the static pressure and force signals. High-amplitude low-frequency oscillations, due to the formation of a Rotating Vortex Rope (RVR) were observed during a specific period of the shutdown sequence. Thereafter, at deep part load conditions, the RVR vanishes and, a wide range of stochastic frequencies are identified at minimum load. A signal coherence analysis was accomplished to distinguish the deterministic and stochastic frequencies. The variation of the velocity field in the draft tube is described in detail with the help of velocity triangles. An in-depth explanation of the formation and variation of vortical structures during the whole sequence is presented. The physical mechanism of formation and destruction of the RVR is thoroughly explained.
Highlights
In recent years, global energy consumption has been increasing annually by 1.5% [1]
The purpose of the present paper is to provide an in-depth analysis of the transient turbulent flow field of a high-head Francis turbine model during a shutdown sequence
A Particle Image Velocimetry (PIV) plane was established in the conical part of the draft tube, in which two inplane velocity components were measured
Summary
Global energy consumption has been increasing annually by 1.5% [1]. Electrical energy production from renewable energy resources, such as hydro, solar, wind, and biomass, has been growing fast to fulfill the current energy demand, owing to their availability and environmental friendliness. An experimental investigation of a Francis turbine during shutdown and startup indicated that a faster transient procedure increases the pressure fluctuations notably [18]. To the best of the authors’ knowledge, no comprehensive CFD study on details of the transient flow field of the Francis turbines during the shutdown sequence has been reported previously. The purpose of the present paper is to provide an in-depth analysis of the transient turbulent flow field of a high-head Francis turbine model during a shutdown sequence. Many aspects and features of a Francis turbine flow field during the shutdown are examined and described for the first time in this study, such as the time-variation of different high and low frequencies related to pressure and force pulsations, coherence analysis of fluctuations, as well as temporal variation of the velocity field.
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