Abstract
Under the circumstances of rapid expansion of diverse forms of volatile and intermittent renewable energy sources, hydropower stations have become increasingly indispensable for improving the quality of energy conversion processes. As a consequence, Francis turbines, one of the most popular options, need to operate under off-design conditions, particularly for partial load operation. In this paper, a prototype Francis turbine was used to investigate the pressure fluctuations and hydraulic axial thrust pulsation under four partial load conditions. The analyses of pressure fluctuations in the vaneless space, runner, and draft tube are discussed in detail. The observed precession frequency of the vortex rope is 0.24 times that of the runner rotational frequency, which is able to travel upstream (from the draft tube to the vaneless space). Frequencies of both 24.0 and 15.0 times that of the runner rotational frequency are detected in the recording points of the runner surface, while the main dominant frequency recorded in the vaneless zone is 15.0 times that of the runner rotational frequency. Apart from unsteady pressure fluctuations, the pulsating property of hydraulic axial thrust is discussed in depth. In conclusion, the pulsation of hydraulic axial thrust is derived from the pressure fluctuations of the runner surface and is more complicated than the pressure fluctuations.
Highlights
Renewable energy sources have increasingly contributed to global electricity generation [1]
To confirm the quality the numerical methodology of the of unsteady three‐dimensional flow simulations performed in thisofpaper and to assess the performance this Francis turbine, four partial simulations performed in this paper and to assess the performance of this
It should be noted that a frequency of 15.0 times that of the runner rotational frequency is found, which corresponds to the number of runner blades and signals that the pressure wave is able to travel from the runner domain to the draft tube wall in the whole hydraulic circuit
Summary
Renewable energy sources have increasingly contributed to global electricity generation [1]. Francis turbines are generally designed to operate at the best efficiency points (BEPs) and, theoretically, the flow leaving the runner is almost axial, with little swirl entering the draft tube. They found that pressure fluctuations resulted from the vortex precession within the range of 0.2–0.4 fn (runner rotational frequency), which were generated only in the conical part of the draft tube and disappeared in the straight diffuser. To investigate hydraulic axial thrust, the monitoring was conducted by corresponding points can be created by seeking the intersection of this intersecting line with a z‐axis. To investigate hydraulic axial thrust, the monitoring recording the time-resolved net lift force that acts on runner surfaces, including the runner blade, hub, was conducted by recording the time‐resolved net lift force that acts on runner surfaces, including and shroud.
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