Francis Turbine

The Francis turbine was invented by an American inventor James B. Francis in the 1800s. The Francis turbine is an inward flow reaction turbine and is used in devices with a wide variety of head and flow. The Francis Turbine is the most commonly used turbine today to generate hydroelectricity. They are often used in large hydroelectric dam systems. The Francis turbine is a versatile turbine that can be built with either a horizontal or vertical axis and can be used in many different hydraulic head heights and water flow levels. This turbine also uses mobile guide vanes to direct the water into the turbine which can be set to allow the desired level of water flow into the turbine system.

Francis Turbine

Francis Turbine has a circular plate fixed to the rotating shaft perpendicular to its surface and passing through its center. This circular plate has curved channels on it; the plate with channels is collectively called as runner. The runner is encircled by a ring of stationary channels called as guide vanes. Guide vanes are housed in a spiral casing called as volute. The exit of the Francis turbine is at the center of the runner plate. There is a draft tube attached to the central exit of the runner. The design parameters such as, radius of the runner, curvature of channel, angle of vanes and the size of the turbine as whole depend on the available head and type of application altogether.

Components of Francis Turbine
a) Runner blades:  

Runner blades are the centers where the fluid strikes and the tangential force of the impact causes the shaft of the turbine to rotate, producing torque. Close attention in design of blade angles at inlet and outlet is necessary, as these are major parameters affecting power production. In Francis Turbine, water strikes the runner tangentially and leaves axially.

Runner of Francis Turbine

b) Spiral casing:

The spiral casing around the runner of the turbine is known as the volute casing or scroll case. Water passes through the spiral casing into guide vanes to pass into runner blades. Throughout its length, it has numerous openings at regular intervals to allow the working fluid to impinge on the blades of the runner. These openings convert the pressure energy of the fluid into momentum energy just before the fluid impinges on the blades. This maintains a constant flow rate despite the fact that numerous openings have been provided for the fluid to enter the blades, as the cross-sectional area of this casing decreases uniformly along the circumference.

Spiral Casing

c) Guide vane:

The primary functions of the guide or stay vanes is to convert the pressure energy of the fluid into the momentum energy. It also serves to direct the flow at design angles to the runner blades. Generally, in Francis Turbine, guide vane is movable and are adjusted as per the flow is required in the runner. Guide vanes are also used as controller for the flow of water in runner.

Guide Vane

d) Shaft:

Shaft is connected from the axis of runner to the generator. The rotation of runner rotates the shaft which rotates the generator and electricity is generated.

e) Draft tube:

The draft tube is a conduit which connects the runner exit to the tail race where the water is discharged from the turbine. Its primary function is to reduce the velocity of discharged water to minimize the loss of kinetic energy at the outlet. This permits the turbine to be set above the tail water without appreciable drop of available head. The types of draft tube are: (i) Simple elbow (ii) Moody Spreading tube (iii) Draft tube with circular inlet cone and rectangular disposal cone (iv) Draft tube with circular inlet cone and circular disposal cone.

Draft Tube with Circular Inlet Cone and Circular Disposal Cone

Do you know any other types of turbines? What did I miss to mention about Francis Turbines? Please let me know in the comment section below.