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The Structure and Power Generation Principle of Francis Turbine

August 10, 2020

Francis turbine is a kind of counterattack turbine with a wide range of application heads, which can be used from 20~700m. It has higher efficiency and lower cavitation coefficient. The water inlet part of the turbine is a volute with steel plate lining. The seat ring pillar is also called a fixed guide vane. The guide vanes of the water guide mechanism are arranged around the runner. The seat ring pillar has strong upper and lower rings, and the volute and the upper and lower rings are welded together. The guide vane journal is supported by a bush (steel or nylon material) on the bottom ring and a sleeve fixed to the top cover. The bottom ring is fixed on the lower ring of the seat ring. The top cover is connected with the upper ring of the seat ring with screws. The transmission mechanism of the water guide is composed of a rotating arm arranged on the journal of the guide vane, a connecting rod and a control ring. The change of the opening of the guide vane from the outlet edge of the guide vane to the shortest distance from the back of the adjacent guide vane) is realized by the push-pull rod transmission control ring connected by the two servomotors of the water guide mechanism and the control ring.

 

Due to the high application head of the Francis turbine and the large bending load on the guide vane, the relative height of the guide vane is shorter than that of the axial flow turbine to reduce the span. In addition, as the water head increases, the overflow of the turbine decreases under the same power, which may reduce the overcurrent load surface of the flow channel, which generally decreases with the increase of the water head.

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The gap between the guide vane and the top cover and bottom ring of the turbine and the joint surface of the adjacent guide vane when shutting down will have water leakage. Generally, rubber or metal seals are used to minimize water leakage when the water guiding mechanism is closed. In high-head hydraulic turbines, a special tubular sealing device is sometimes used, and the inner cavity is filled with compressed air when shutting down, so that the end face can be completely sealed.

The runner is the core component of a water turbine that converts water flow energy into mechanical energy. The water flow enters the runner through the water guiding mechanism. The runner consists of an upper crown, a lower ring and blades. General Francis turbines have 14-19 blades. The blades, upper crown and lower ring form a solid overall rigid structure. The upper crown of the runner is connected with the lower flange of the main shaft. The drain cone is connected with the upper crown and is used to eliminate the water vortex. When the hydraulic turbine is working, part of the water flow is uselessly leaked through the gap between the rotating and non-rotating parts, thereby reducing the efficiency of the hydraulic turbine.

The sealing ring is to reduce flow leakage. When the water passes through the sealing ring space, it is blocked by the sudden expansion and contraction of the local hydraulic power, resulting in hydraulic loss, thereby reducing the flow rate and reducing the flow through the gap.


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