Abstract
In order to study the variation law of the flow field and pressure fluctuation in the hump section of the siphon outlet conduit, the flow field characteristics and frequency spectrum characteristics of the flow field were analyzed by combining a physical model test and numerical simulation under the conditions of the interaction between the axial flow pump and siphon outlet conduit, and the influence of the residual circulation at the outlet of the guide vane on the siphon outlet flow was investigated. Based on the influence of the flow field and hydraulic loss in the conduit, the equivalent surface method based on the Q criterion was used to analyze the vortex structure in the siphon outlet conduit and to analyze the internal vortex state. The results showed that with the increase of the flow rate, the intensity of the vortices in the cross-section of the hump section of the siphon outlet conduit decreased gradually, the average velocity circulation decreased gradually and the axial velocity distribution uniformity increased and tended to be stable; water flow stratification existed under three characteristic conditions with no circulation, and the hydraulic loss was greater with the circulation flow while it had a circulation under the small flow condition. Under the low flow rate conditions, the hydraulic loss was 6.6 times higher under the condition of circulation than without. Under a high flow condition, it was 1.3 times. Under the condition of a small flow rate, the vortex structure was distributed centrally at the inlet of the flow conduit, and under the other two characteristic conditions, the vortex structure mostly appeared as a strip; the pressure fluctuation in the hump section had obvious periodicity, and with the increase of the flow rate, the maximum pressure fluctuation amplitude in the hump section decreased gradually; with the decrease of the rotational speed, the pressure amplitude at the same measuring point in the hump section decreased gradually and at the optimum condition. Under the following conditions, the mean value of the pressure amplitude at the top of the hump section was reduced by 69.63%, and the mean value of the pressure amplitude at the bottom of the hump section was reduced by 63.5%. Under all the calculation conditions, the main frequency of pulsation at each measuring point of the hump section was twice the frequency of the rotation.
Highlights
The South-to-North Water Transfer Project is one of the most important water conservancy projects in China in the 21st century
The physical model test and CFD method were used to analyze the flow field and pulsation characteristics in the hump section of a siphon outlet conduit, and the vortex structure in the siphon outlet conduit was analyzed by the isosurface method based on the Q criterion
(1) With the increase of the flow rate, the intensity of the vortices in the cross-section of the hump section of the siphon outlet conduit decreased gradually, the average velocity circulation decreased from 0.056 m2/s to 0.008 m2/s, and the uniformity of the axial velocity distribution increased from 55.4% to 75.4% and tended to be stable
Summary
The South-to-North Water Transfer Project is one of the most important water conservancy projects in China in the 21st century. It is the largest inter-basin water transfer project in the world. A total of 35 pumping stations have been constructed in Phase I of the east route of the South-to-North Water Transfer Project. The siphon outlet conduit is one of the common outlet structures for large-scale low-head pump units. It is suitable for situations where the water level of the outlet pool varies little. Its cut-off mode has the advantages of simplicity, reliability and economy
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