The water flow in the Francis turbine must be in the form of radial direction and exists through the turbine axially. When the turbine imparting then the water pressure must be decreases and it hits the blades when turbine rotate. It is the first turbine with a radial inflow. It is also known as reaction turbine. Compare to the impulse turbine reaction turbine has some features. We can see the major pressure drop in the turbine. Up to the entry point we can see the complete pressure drop in the impulsive turbine. During the operation in the turbine the water flow is completely filled.
Design of Francis Turbine:
Francis turbine consists of circular plates which are fixed to the rotating shafts and is perpendicular to the surface. The water flow takes place from the centre. We can notice the curved channels on the circular plates. The plate with the channels is known as runner. The runner is surrounded by a ring of motionless channels which is known as guide vanes. With the help of the spiral casing the guide vanes are mounted which is known as volute. We can see the exit of the flow from the centre of the runner plate. To the central exit of the runner the draft tube is attached. During the time of designing we can also notice the design parameters like angle of vane, radius of the runner, curvature of channel, size of the turbine must be depends upon the size of the head available.
Parts of Francis turbine:
Pen stock is a pipe and they are considered to have large size diameter. The source of water is coming from the reservoir and it takes the water to the turbine runner.
Scroll casing is also known as spiral casing. One of the ends is closed and the other end has the maximum cross sectional area. Along the direction of the flow the inlet velocity decreases continuously. At outlet we can see zero flow.
Guide vanes are also known as wicket gates. The main function or usages of the guide vanes are to guide the water towards the runner. The water flow must be an angle and that is appropriate for the design.
The main function of the governing mechanism is to change the angle of attack at runner by guide vane. In the turbine the flow rate of water must be changing.
Nearly 20 to 24 blades are attached to the runner. With the help of the runner blades and direction of water flow the runner is rotating.
It is one of the most important things in the turbines. To avoid leakage sealing is arranged to the shaft.
Draft tube is an expanding tube which is used to discharge the water through the runner and next to the tailrace. The water is lastly liquidated from the turbine. The main function of the draft tube is to reduce the water velocity at the time of discharge. At the outlet the water minimizes the kinematic losses. So it permits the turbine to set overhead the tail water deprived of any noticeable drop of existing head.
It provides drainage space.
Working of Francis Turbine:
In the Francis turbines the water must be enter into inlet nozzle and the velocity must be decreases. Then the water moves in the forward direction in the turbine but continuous decrease in the spiral casing area must be happened. With the constant speed the water strikes the turbine blades. On the runner the blades are mounted. The blades are designed like a thin air foil. So due to the design the water flow occurs at low pressure at one point and other side it creates a low pressure. So the motion takes place from the high pressure to the low pressure. Inside the blade it looks like a bucket it using the impulsive force for rotation of water. If the flow rate of water changes then by using the governing mechanism we can set the angle of attack of the water on the blade. At that case the turbine works properly. Initially the water passes from the blades and next later enters into the draft tube, where we can notice the continuous increment in the area and velocity of the water must be decreases.
Advantages of Francis Turbine:
• Changing of the head must be easily controlled
• Generator size must be small
• Runner size also small
• When the variation in the tail water level is relatively large then the operating head can be utilized compare with the total head.
Disadvantages of Francis Turbine:
• We can notice the wear when the water is un-cleaned.
• Inspection process is seen to difficult.
• Cavitation is the main danger
• Due to the water hammer effects it creates some problems.
• If head is below 50% then we can face the harmful effects mainly on the efficiency.
• Cavitation is also a big problem