Francis and Kaplan Turbines | Fluid Mechanics for Mechanical Engineering PDF Download

Francis Turbine

The inward flow reaction turbine having radial discharge at the outlet is known as Francis turbine, after the name of J.B Francis an American engineer who in beginning designed inward radial flow reaction turbine.

In the modern Francis turbine, the water enters the runner of the turbine in the radial direction and leaves in the axial direction at the outlet of the runner. Thus the modern Francis turbine is a mixed flow type turbine.

1. Penstock: It is a long pipe at the outlet of which a nozzle is fitted. The water from reservoir flows through the penstock. The nozzle increases the kinetic energy of water flowing through the penstock.
2. Casing: In the case of a reaction turbine, casing and runner are always full of water. The water from the penstocks enters the casing which is of spiral shape in which area of cross-section of the casing goes on decreasing gradually. The casing completely surrounds the runner of the turbine. The casing is made of a spiral shape, so that the water may enter the runner at constant velocity throughout the circumference of the runner. The casing is made of concrete or cast steel.
3. Guide Mechanism: It consists of a stationary circular wheel all-round the runner of the turbine. The stationary guide vanes are fixed on the guide mechanism. The guide vanes allow the water to strike the vanes fixed on the runner without shake at the inlet. Also by a suitable arrangement, the width between two adjacent vanes of a guide’s mechanism can be altered so that the amount of water striking the runner can vary.
4. Runner: It is a circular wheel on which a series of radial curved vanes are fixed. The surface of the vanes is made very smooth. The radial curved vanes are so shaped that the water enters and leaves the runner without shock. The runners are made of cast steel, cast iron or stainless steel. They are keyed to the shaft.
5. Draft tube: The pressure at the exit of the runner of a reaction turbine is generally less than atmospheric pressure. The water at the exit cannot be directly discharged to the tailrace. A tube or pipe of a gradually increasing area is used for discharging water from the exit of the turbine to the tailrace. This tube of the increasing area is called the draft tube. The draft tube, in addition, to serve a passage for water discharge, has the following two purposes also.
   (i) The turbine may be placed above the tailrace and hence turbine may be inspected properly.
   (ii) The kinetic energy rejected at the outlet of the turbine is converted into useful pressure energy.

Kaplan Turbine

Kaplan Reaction turbines are axial flow turbines in which the flow is parallel to the axis of the shaft. They are the low head, high discharge turbine.
Kaplan is also known as a propeller turbine. Kaplan turbine is a propeller-type water turbine along with adjustable blades. Mainly it is designed for low head water applications.
In this water turn at right angles between the guide vanes, runner & then flow parallel to the shaft. It is an inward flow reaction turbine. The flow was along the radius from the periphery to the centre of the runner. (From outer dia to the inner dia of runner).
It is capable of giving high efficiency at overloads (up to 15-20%), at normal loads (up to 94%). The runner of this turbine is in the form of boss or hub which extends in a bigger dia. Casing with proper adjustment of blades during running. The blade angles should be properly adjusted so that water enters & flow through the runner blades without shock.

1. Penstock: It is the waterway used to carry the water from the reservoir to the turbine. At the inlet of the penstock, trash cracks are used to prevent the debris from going into the turbine.
2. Spiral or Scroll casing: In the case of reaction turbine casing and runner are always full of water. The water from the penstock enters the casing which is of spiral shape in which area of cross-section of the casing goes on decreasing gradually. The casing completely surrounds the runner of the turbine. The casing is made of spiral shape, so that the water may enter the runner at constant velocity throughout the circumference of the runner.
3. Guide Mechanism: It consists of a stationary circular wheel all-round the runner of the turbine. The stationary guide vanes are fixed on the guide mechanism. The guide vanes allow the water to strike the vanes fixed on the runner without shock at inlet. Also by a suitable arrangement, the width between two adjacent vanes of a guide mechanism can be altered so that the amount of water striking the runner can be varied. A space, called whirl Chamber, is provided between the guide vanes and the runner. In this chamber, the flow turns by 90° & move as a free vortex i.e without the aid of any external torque. The radial component changes into axial component due to the guidance from the fixed housing.
4. Runner: It is a circular wheel, also called ‘hub’ or ‘bass’ on which a series of radial curved vanes are fixed. The surface of the vanes is made very smooth. The radial curved vanes are so shaped that water enters and leaves the runner without shock. The runners are made of cast steel, cast iron or stainless steel. In the Kaplan turbine, the shaft is the extended part of the runner with a smaller diameter.
5. Draft tube: The pressure at the exit of an axial turbine is generally less than atmospheric pressure. The water at the exit cannot be directly discharged to the tailrace. A tube or pipe of a gradually increasing area is used for discharging water from the exit of the turbine to the tailrace. This tube of the increasing area is called the draft tube.

Working of the Kaplan turbine
Due to the low water heads, it allows the water flow at larger in the Kaplan turbine. With help of the guide vane the water enters. So the guide vanes are aligned to give the flow a suitable degree of swirl. The swirl is determined according to the rotor of the turbine. The water flow from the guide vanes is passed through the curved structure which forces the radial flow to the direction of the axial. The swirl is imparted by the inlet guide vanes and they are not in the form of a free vortex. With a component of the swirl in the form of axial flow are applies forces on the blades of the rotor. Due to the force, it loses both angular and linear momentum.

Advantages of Kaplan turbine

• Runner vanes are adjusted in the Kaplan
• Very low heads are required
• Very small no of blades are used nearly 3 to 8 blades
• Less resistance has to be overcome.

• Position of the shaft is only in the vertical direction
• Speed of the turbine is 250 to 850
• High speed generator is required
• Large Flow rate must be required.