FRANCIS TURBINE

TURBINE

A Machine for producing continuous power in which a wheel or rotor typically fitted with vanes is made to revolve by a fast moving flow of water steam,gas ,air or other fluid .
The kinetic energy of the moving fluid is converted to Mechanical energy by the impulse or reaction of the fluid .
Key parts of a turbine are set of blades that catches the moving fluid ,the shaft that rotates as the blades moves and generator that is driven by the shaft.

TYPES OF TURBINE

• PELTON TURBINE
• KAPLAN TURBINE
• FRANCIS TURBINE

FRANCIS TURBINE

It is the most commonly used turbine used under medium head about 45-400 meter.It is a reaction turbine and is used for the electricity generation and is preferred more in comparison to kalpan and pelton turbines.
The important features of francis turbine is described below.

• Used under medium head 45-400m.
• Used under flow rate between 10-700cubic meter.
• Water enters into the runner Radially and leaves axially and so it is also called Mixed flow turbines
• Curved vanes are used and when water flows
• through the vanes a low pressure is created at oneside and high pressure is created at the other side due to which a lift force is created.
• Vanes have a bucket kind of shape at the outlet so water will hit and produce an impulse force before leaving the runner.
• Both impulse force and lift force will make the runner rotate.So Francis turbine is not a pure

Reaction turbine.

• Runner is connected to a generator with a help of a shaft for electricity generation.
• Arrangement of turbine shaft is fitted inside a casing .
• Flow of water takes place via a nozzle .
• Stay vanes and guide vanes are fitted and the purpose of them is to convert one part of potential energy to kinetic energy .
• Stay vanes directs the water flow into the runner section towards the blades.

WORKING OF FRANCIS TURBINE

As water enter the turbine through the inlet nozzle its velocity will decrease as it moves forward in the turbine but the continuous decrease in the area of spiral casing will not let it happen. Design of spiral casing makes shore that the water strike the each blade of the turbine as a constant speed. Blades are mounted on the runner. Design of the blade is like a thin air foil so when water flow over it low pressure is created at one point and high pressure is created on other side and motion take place from high pressure to low pressure. Lower side of the blade is like a bucket which use the impulse forces of water in the rotation.






If water flow rate change then governing mechanism came into use and change the angle of attack of water on to the blade so the turbine can work properly. After the water passes the blades it enter the draft tube whose continuous increase in area decrease the velocity of the water.

Figure shows the different parts associated with the francis turbine.Francis turbine is placed inside a casing and guide vanes are used as shown in the figure .Water from the penstock enters into the casing and reaches to the turbine blades where the lift force and impulse force rotates the blades .

APPLICATION OF FRANCIS TURBINE

• Electricity production can be estimated by the help of flowrate and head.
• Francis turbine may be designed for wide range of head and flow.
• It has high efficiency.
• They may be used as Pump.

ADVANTAGES

• Easy control on changing head.
• Size of runner and generator is small.
• Small change in efficiency over time.
• Operating head can be utilized even when the variation in tail water level is relatively largewhen compared to the total head.

DISADVANTAGES

• Unclean water can create problem 
• Cavitation is and ever present danger.
• Head 50 percent lower can be harmful effect on the effieciency as well as cavitation danger becomes more serious.