Hydraulic loss distribution of pump-turbine operated in pump mode based on entropy production method
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Abstract:
Energy is irrevocably lost within the pump-turbine due to the activities of viscous forces near the wall. The conventional pressure drop method can not get exact details of the hydraulic loss within the machine's flow passageways. On the other hand, the entropy production method has obvious advantages in hydraulic loss assessment and it can accurately identify precise information on the position of irreversible losses.The composition and distribution of hydraulic loss under different flow rate operating points was explored for a prototype pump-turbine in pump mode using the entropy production theory. The entropy production method was verified to be reasonable and credible within a certain error range by comparison with the pressure drop method.The total entropy production and total hydraulic loss obtained by the method of differential pressure were consistent with the variation. With an increase in flow rate, the total entropy production decreased dramatically initially and then gradually increases. The entropy production rate caused by turbulence dissipation, direct dissipation, and wall shear stress exhibited the same variation pattern as the total entropy production. The major flow region's entropy production was predominantly induced by flow separation, backflow, and vortex creation. Entropy production was prominent in the main flow zone, with the entropy production rate caused by turbulence dissipation contributing the most to the total entropy production (50%-61%) and the entropy production rate caused by direct dissipation coming in second (37%-48%). The entropy production in the near-wall region primarily originated from the significant velocity gradient triggered by the wall shear stress, which could be roughly equivalent to friction loss and made a negligible 1%-2% contribution to total entropy production. Under various flow rate conditions, the hydraulic loss in the runner, guide vanes and stay vanes were dominant (67%-86%). Under low flow rate conditions, hydraulic loss in the draft tube was greater. However, under high flow rate conditions, hydraulic loss in spiral casing was greater. The distributions of the entropy production rate caused by turbulence dissipation and the entropy production rate caused by direct dissipation were highly consistent with the distribution of turbulent kinetic energy. But the entropy production rate caused by direct dissipation was mainly caused by strain rate, so it was closer to the main vortex regions, whereas the entropy production rate caused by turbulence dissipation was affected by turbulence intensity and had a wider distribution range in the flow field. High hydraulic loss under low flow conditions mainly came from the high-speed circulation in the vaneless region, vortices in the guide vane flow channels, and the flow separation within the elbow and the conical part of the draft tube. But the spiral casing’s hydraulic loss was much lesser. Hydraulic loss under the best efficiency operating point was small and mainly due to vortices in some stay vane flow channels and the blade wake. High hydraulic loss under high flow conditions mainly came from flow impact on the guide vanes, diffusion of unstable flow in stay vane flow channels, and the circumferentially spaced vortices and high-speed flows at the spiral casing inlet; Whereas the draft tube’s hydraulic loss was rarely small.The total entropy production and total hydraulic loss decreased significantly and then slowly increased with an increase in flow rate. The entropy production rate caused by turbulence dissipation contributed the most to total entropy production (50%-61%), with direct dissipation coming in second (37%-48%), and wall shear stress coming in last (1%-2%). Under various flow rate conditions, the hydraulic loss in the runner, guide vanes and stay vanes were dominant (67%-86%). Hydraulic loss in the draft tube was larger at low flow rate conditions. While the hydraulic loss in spiral casing was greater under high flow rate conditions. The entropy production distributions were highly consistent with the distribution of turbulent kinetic energy. The entropy production rate caused by direct dissipation was closer to the main vortex regions, whereas turbulence dissipation had a wider distribution range in the flow field. The detailed location of hydraulic loss within the pump-turbine’s flow domain strongly depended on flow conditions. Under low flow conditions, hydraulic loss mainly came from the high-speed circulation in the vaneless region, vortices in the guide vane flow channels, and the flow separation within the elbow and the conical part of the draft tube. Under the best efficiency operating point, the hydraulic loss was small and mainly due to vortices in some stay vane flow channels and the blade wake. Under high flow conditions, hydraulic loss mainly came from flow impact on the guide vanes, diffusion of unstable flow in stay vane flow channels, and the circumferentially spaced vortices and high-speed flows at the spiral casing inlet.
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