Abstract
Horizontal axis double-runner Francis turbines have great advantages in the development of small hydropower plants, but the arrangement of double runners aggravates the complexity of the water flow between runners, and the mutual influence of the two runners cannot be ignored. In order to explore the relationship between the performance and the internal flow field and investigate the pressure pulsation characteristics of the double-runner Francis turbine, the steady and unsteady numerical analysis of the full flow channel of a prototype turbine was carried out based on the Realizable k-epsilon model and the polyhedral mesh method. The results show that the relationship between the average efficiency of the two runners and the flow difference between the runners is negatively correlated. As the flow rate difference between the runners on both sides increases, the average efficiency of the runners decreases. The draft tube flow of a horizontal-axis turbine has a profound effect on the flow field characteristics in the runner. When the working conditions change, the turning and converging timing of the mainstream at the outlet of the two runners will change. The movement of the mainstream promotes the change in location of the dead water zone. The existence of the vortex zone makes the pressure distribution at the outlet of the runner uneven, which is an important reason for the asymmetry of the flow in the runner. The analysis of pressure pulsation and its frequency spectrum shows that when the working conditions change, the low-frequency, strong pressure pulsation area on the surface of the guide vane will regularly migrate between the two runners, while the high-frequency pressure pulsation that occurs in the bladeless zone will dissipate in the runner. The doubling of the blade frequency on the pressure surface and back surface of the blades gradually attenuates with the increase of frequency. The pressure pulsation attenuation on the surface of the high-position blade conforms to the linear law, and the attenuation of the pressure pulsation on the surface of the low-position blade conforms to the exponential law. The research in this paper provides a certain reference value for revealing the flow field mechanism and pressure pulsation characteristics of the double-runner Francis turbine.
Highlights
Small hydropower units, which are widely used in rural and remote hilly areas [1], often face complex operating conditions in which the head and flow rate vary over a range, and they have a greater magnitude of pulsations in the velocity and pressure in the flow field of the turbine when the operating point is further away from the design point [2]
When the flow rate of a single runner is less than 40% of the rated flow rate, the efficiency of the turbine will be seriously reduced due to the deterioration of flow regime and hydraulic vibration, while a double runner allows the hydraulic turbine to operate in the efficient zone by closing the guide vane or barrel valve of one runner [9]
To compare the difference in pressure pulsation characteristics between horizontal and vertical axis turbines, Chirag [19] researched both the horizontal and vertical Francis turbines, where the results showed that the non-synchronous pressure pulsation had less influence on the internal flow field of the horizontal turbine than the
Summary
Favrel [20] investigated the influence of the geometry of draft tubes of the Francis turbine on the pressure pulsation and energy loss under off-design conditions, and the results fully showed that the draft tube shape significantly affects the pressure pulsation distribution in the runner channel of the turbine To some extent, both experimental and numerical methods reveal that the channel structure can highly affect the spatial-temporal distribution and frequency characteristics of the pressure pulsation of the turbine. On the basis of the research work carried out by the above scholars, based on the horizontal axis double-runner hydraulic unit which has been put into operation in a power plant in northern Europe, the three-dimensional solid model of the whole flow channel is established, and the three-dimensional steady and unsteady turbulent flow is calculated and analyzed by the CFD numerical calculation method. A spectrum analysis of the pressure pulsation monitoring data is carried out to quantitatively analyze the hydraulic stability of such units
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