The counterattack turbine is a kind of hydraulic machinery that uses the pressure of the water flow to convert water energy into mechanical energy.
(1) Structure. The main structural components of the counterattack turbine are the runner, the water diversion chamber, the water guiding mechanism and the draft tube.
1) Runner. The runner is a part of the water turbine that converts the energy of the water flow into rotating mechanical energy. Depending on the direction of water energy conversion, the runner structures of various counterattack turbines are also different. Francis turbine runner is composed of streamlined twisted blades, crown and lower ring and other main vertical components; axial flow turbine runner is composed of blades, runner body and drain cone and other main components: diagonal flow turbine runner structure is more complex. The blade placement angle can be changed with the working conditions and matched with the guide vane opening. The blade rotation center line is at an oblique angle (45°-60°) to the axis of the turbine.
2) Water diversion chamber. Its function is to make the water flow evenly into the water guiding mechanism, reduce energy loss, and improve the efficiency of the turbine. Large and medium-sized turbines often use circular cross-section metal volutes with heads above 50m, and trapezoidal cross-section concrete volutes for those below 50m.
3) Water guiding mechanism. It is generally composed of a certain number of streamlined guide vanes and their rotating mechanisms evenly arranged on the periphery of the runner. Its function is to guide the water flow evenly into the runner, and by adjusting the opening of the guide vane, to change the flow rate of the turbine to meet the load requirements of the generator set, and it also plays a role of sealing water when it is fully closed.
4) Draft tube. The water flow at the outlet of the runner still has a part of the surplus energy that has not been used. The role of the draft tube is to recover this part of energy and discharge the water downstream. Draft tube is divided into two types, straight cone and curved. The former has a large energy coefficient and is generally suitable for small horizontal and tubular turbines; the latter has lower hydraulic performance than straight cones, but has a smaller digging depth, and is widely used in large and medium-sized counterattack turbines.
(2) Classification. According to the axial direction of the water flow through the runner, the impact turbine is divided into a Francis turbine, a diagonal flow turbine, an axial flow turbine and a tubular turbine.
1) Francis turbine. Francis (radial axial flow or Francis) turbine is a counter-attack turbine in which water flows radially from the circumference of the runner to the axial direction. This kind of turbine has a wide range of applicable heads (30-700m), simple structure, small volume and low cost. The largest Francis turbine that has been put into operation in China is the Ertan Hydropower Plant, with a rated output power of 582 MW and a maximum output power of 621 MW.
2) Axial flow turbine. The axial flow turbine is a counterattack turbine in which water flows in from the axial direction and flows out of the runner in the axial direction. This type of turbine is divided into two types: fixed-blade type (screw type) and rotary type (Kaplan type). The blades of the former are fixed, and the blades of the latter can be rotated. The water passing capacity of the axial flow turbine is greater than that of the Francis turbine. Because the blades of the paddle turbine can change position with changes in load, they have higher efficiency in a wide range of load changes. The anti-cavitation performance and mechanical strength of the axial flow turbine are worse than that of the Francis turbine, and the structure is also more complicated. At present, the applicable head of this kind of turbine has reached 80m or more.
3) Tubular turbine. The water flow of this kind of water turbine flows axially out of the runner, and there is no rotation before and after the runner. The utilization head range is 3-20. . The fuselage has the advantages of small height, good water flow conditions, high efficiency, less civil engineering, low cost, no need for volutes and curved draft tubes, and the lower the head, the more obvious the advantages.
Tubular turbines are divided into two types: full-through-flow and semi-through-flow according to generator connection and transmission mode. Semi-through-flow turbines are further divided into bulb type, shaft type and shaft extension type. Among them, the shaft extension type is also divided into two types. There are oblique axis and horizontal axis. At present, the most widely used bulb tubular type, shaft extension type and vertical shaft type are mostly used in small units. In recent years, the shaft type has also been used in large and medium-sized units.
The generator of the shaft extension tubular unit is installed outside the waterway, and the generator is connected to the turbine with a longer inclined shaft or horizontal shaft. This shaft extension type structure is simpler than the bulb type.
4) Diagonal flow turbine. The structure and size of the diagonal flow (also called diagonal) turbine are between the mixed flow and the axial flow. The main difference is that the centerline of the runner blades is at a certain angle to the centerline of the turbine. Due to the structural characteristics, the unit is not allowed to sink during operation, so an axial displacement signal protection device is installed in the second structure to prevent accidents that the blades and the runner chamber collide. The utilization head range of diagonal flow turbine is 25～200m.
Post time: Oct-19-2021
