According to the working principle, hydraulic turbines can be divided into two categories: impulse turbines and reaction turbines.
The performance of the runner turbine of the impact turbine is relatively high, and it rotates due to the impact of water flow, and the pressure of water flow is constant during the working process, mainly due to the conversion of kinetic energy; The runner of the impact turbine rotates under the reaction force of the water flow, and the pressure energy and kinetic energy of the water flow change during the working process, but mainly the conversion of pressure energy.
I. impact turbine
according to the flow direction of water, impact turbines can be divided into two types: cutting type (also known as bucket type) and oblique type. The structure of oblique impact water turbine is basically the same as that of bucket turbine, except that the jet direction has an inclination angle, which is only used for small units.
theoretical analysis proves that the efficiency is the highest when the circumferential velocity at the pitch circle of the bucket is about half of the jet velocity. When the load of this turbine changes, the inlet speed direction of the runner remains unchanged. In addition, all these turbines are used in high-head power stations, and the head change is relatively small, so the efficiency is less affected by the load change, and the efficiency curve is relatively flat, with the highest efficiency exceeding 91%.
second, the impact turbine
the impact turbine can be divided into mixed flow, axial flow, oblique flow and tubular flow.
1. Axial flow turbine
is suitable for power stations with lower head. Under the same water head, its specific speed is higher than that of Francis turbine.
the blades of the axial-flow fixed-blade turbine are fixed on the runner body. Generally, the installation height is 3-5m. The blade placement angle cannot be changed during operation, with simple structure and low efficiency. It is suitable for power stations with small load change or the number of units can be adjusted to adapt to the load change.
Kaplan turbine was invented by Austrian engineer Kaplan in 192, so it is also called Kaplan turbine. Generally, the installation height is 3-8m. The runner blades are generally operated by hydraulic relays installed in the runner body, which can rotate according to the change of water head and load, so as to maintain the optimal cooperation between the rotating angle of movable guide vanes and the rotating angle of blades, thus improving the average efficiency. Some of the highest efficiencies of this kind of turbines have exceeded 94%. A typical example is Gezhouba.
2. The water flow between the guide vanes and the runner of the tubular turbine
basically does not change direction. In addition, the straight conical draft tube is adopted, so the discharge flow does not need to turn in the draft tube, so it has high efficiency, large flow capacity and high specific speed, and is especially suitable for small riverbed power stations with low water head of 3-2m. Installation diagram of hydraulic turbine This kind of hydraulic turbine can also realize bidirectional power generation when installed in tidal power station. This kind of turbine has many structures, among which the bulb turbine is the most widely used.
the generator of the bulb unit is installed in the watertight bulb body. Its runner can be designed as fixed paddle or rotary paddle. Among them, it can be subdivided into tubular type and semi-tubular type. The world's largest bulb turbine (paddle semi-tubular flow) was installed in the second power station of Rockisland, USA, with a head of 12.1m, a rotating speed of 85.7 rpm, a runner diameter of 7.4m, and a stand-alone power of 54 MW. It was put into operation in 1978.
3. Francis turbine
is the most widely used turbine in the world. It was invented by American engineer Francis in 1849, so it is also called Francis turbine. Compared with axial-flow propeller, its structure is simpler, its operation is stable, and its highest efficiency is higher than that of axial flow. However, when the head and load change greatly, its average efficiency is lower than that of axial-flow propeller, and some of its highest efficiency has exceeded 95%. Francis turbine is suitable for a wide range of water heads, ranging from 5 to 7 meters, but the most widely used is 4 to 3 meters.
Francis runner is generally made of low-carbon steel or low-alloy steel castings or cast-welded structure. In order to improve the performance of cavitation resistance and sand wear resistance, stainless steel can be surfacing on the parts prone to cavitation, or stainless steel blades can be used, and sometimes the whole runner can be made of stainless steel. The casting and welding structure can reduce the cost, make the size of the runner more accurate and the surface of the runner smoother, which is beneficial to improve the efficiency of the turbine. The blades, the upper crown and the lower ring can also be made of different materials respectively. A typical example is Liujiaxia in China.
4. Inclined flow turbine
was invented by Swiss engineer Derria in 1956, so it is also called Derria turbine. Its blades are obliquely installed on the runner turbine. With the change of water head and load, the hydraulic relay in the runner operates the blades to rotate around its axis accordingly. Its highest efficiency is slightly lower than that of Francis turbine, but its average efficiency is much higher than that of Francis turbine. Compared with axial-flow propeller turbine, it has better cavitation resistance and lower runaway speed, and is suitable for 4~12 meters head.
Because of the complicated structure and high cost, diagonal flow turbines are generally only used when mixed flow or axial flow turbines are not suitable or not ideal. This kind of turbine can also be used as a reversible pump turbine. When it is started under the condition of water pump, the runner blades can be closed into a nearly closed cone, thus reducing the starting load of the motor.