Aim: - To study the Reaction turbine
Apparatus used: - Model of Reaction turbine.
Theory: - Reaction Turbine: - In this type of turbine there is a gradual pressure drop and takes place continuously over the fixed and moving blades or over guide vanes and moving vanes. The function of the guides’ vanes is that they alter the direction of water as well as increases its velocity. As the water passes over the moving vanes its kinetic energy is absorbed by them. The inward flow reaction turbine having radial discharge at outlet is known as France’s 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.
Constructional details: - The main parts of the Francis turbine are: -
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.
Casing: - In case of 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 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. The casing is made of concrete or cast steel.
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 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.
Runner: - It is a circular wheel on which a series of radial curved vanes are fixed. The surface of the vanes is made 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.
Draft tube: - The pressure at the exit of the runner of a reaction turbine is generally less than atmosphere pressure. The water at exit cannot be directly discharged to the tail race. A tube or pipe of gradually increasing area is used for discharging water from the exit of the turbine to the tail race. This tube of increasing area is called draft tube. The draft tube, in addition to serve a passage for water discharge, has the following two purposes also.
· The turbine may be placed above the tail race and hence turbine may be inspected properly.
·
The kinetic energy rejected at
the outlet of the turbine is converted into useful pressure energy.
Specifications:
-
1. Type –Reaction Turbine
2.
Type of flow – Mixed
(Redial & Axial)
3.
Head –Medium 45 to 250m
4.
Specific speed – Medium 50
to 250
5.
Shaft position – Mainly
vertical (it may be horizontal also )
6.
Discharge – Medium
