Introduction



The Francis turbine is a type of mixed flow reaction turbine that was developed by James B. Francis. This turbine is used for medium heads ranging from 40 m to 600 m with medium discharge. Water enters the runner and flows towards the center of the wheel in the radial direction and leaves parallel to the axis of the turbine. Francis turbines are the most common water turbine in use today and are primarily used for electrical power production. The power output which generally for Francis turbine ranges from just a few kilowatts up to 800 MW.
When Francis turbine is not properly designed, a detrimental phenomenon called Cavitation can occur which can be catastrophic in nature. The cavitation problem is produced by bubbles of vapor when the pressure of the liquid is lower than the vapor pressure. This produces erosion in the moving part of the turbine’s runner and shaft.

Main parts of a Francis Turbine



Penstock

It is a large size conduit which conveys water from the upstream of dam/reservoir to the turbine blade

Spiral/ Scroll Casing

The complete turbine can be covered with a casing with a gradually decreasing cross sectional area along the flow direction. The maximum area is at the junction where penstock delivers the water into the volute/scroll

Essentially, the casing of the turbine avoids the critical parts like impeller blades, guide vanes, a runner from damages because of external load. The main advantage of such a design is as follows:

1. Even distribution of liquid along the runner
   
   
2. Formation of eddies is avoided which in turn avoids loss of head
   
   
   
   

Guide Blades/ Wicket Gates
It is the only controlling part of the whole turbine, which opens and closes depending upon the demand of power requirement. In case of more power output requirements, it opens wider to allow more water to hit the blades of the rotor and when low power output requires it closes itself to cease the flow of water. If guide vanes are absent then the turbine cannot work efficiently henceforth its efficiency would decrease.

Governing Mechanism Governing mechanism is used to change the position of the wicket gates to affect in change in water flow rate, depending on the load conditions requirement. It consists of a stationary circular wheel all around the runner of the turbine. The stationary guide vanes are fixed on the guide mechanism where pivoting the guide vanes, the area of the blade passage is increased or decreased to vary the flow rate of water.



Runner and Runner Blade The heart of a Francis turbine is the runner blade, it is the rotating part which help in generating electricity. The runner blade shaft is connected to the shaft of the generator which rotates the generator rotor to produce electricity. Runner is a circular wheel on which a series of radial curved vanes are fixed. The curve of the blades is so designed that the water enters and leaves the runner without shock. The flow in the runner of a Francis turbine is partly axial and partly radial.

Draft Tube The pressure at the exit of the runner of Francis Turbine is generally less than atmospheric pressure. The water at exit of runner cannot be discharged directly to the tail race. A pipe of gradually increasing cross section area is used for discharging water from the exit of turbine to the tail race. This tube of increasing area along its length is called Draft Tube. Please note that one end of the draft tube is connected to the outlet of runner while the other end is sub-merged below the level of water in the tail-race. The main function of draft tube is to decrease the pressure at the runner exit to a value less than standard atmospheric pressure and henceforth increase the effective working head.



Working of Francis Turbine • With help of the penstock the water is brought to the runner through wicket gates from the reservoir. According to the loading conditions the opening between the vanes can be adjusted to vary the quantity of water entering the turbine through governing mechanism.
• The water enters the runner at a high pressure but with a low velocity. As the water flows over the vanes the pressure head is gradually converted into velocity head. The kinetic energy generated is the cause of rotation of the turbine. In other words, the hydraulic energy is converted into mechanical energy.
• The water leaving the runner enters the tailrace after passing through the draft tube which enlarges gradually and the enlarged end is submerged deeply in the tailrace water. Due to this arrangement a suction head is created at the exit of the runner.

Advantages of Francis Turbine
• The advantages of the Francis turbine include the following:
• Wear and tear had delayed effect on efficiency of Francis turbine compared to Pelton turbine
• Operating head variation can be simply controlled in Francis turbine
• Even at the low discharge of water, head failure does not occur • Runner and generator size is small.
• Only little changes in efficiency over the time
• Operating head can be utilized even when the variation in tailwater level is relatively large when compared to the total head.

Disadvantages of Francis Turbine
The disadvantages of the Francis turbine include the following.
• Dirty water can cause extremely rapid wear in high head Francis turbine
• The repair and inspection are much harder reasonably
• Cavitation is an ever-present hazard
• Cavitation can reduce the efficiency and can even cause failure if not dealt with