A hydraulic turbine is a machine which converts the hydraulic energy into mechanical energy. The hydraulic turbines are also known as water turbines. Following two types of hydraulic turbines are important.
1. Impulse turbine
In an impulse turbine, the total energy at the inlet of a turbine is only kinetic energy. The pressure of water both at entering and leaving the vanes is atmospheric. It is used for high head of water. A Pelton wheel is a tangential flow impulse turbine.
2. Reaction turbine
In a reaction turbine, the total energy at the inlet of a turbine is kinetic energy as well as pressure energy. It is used for low head of water. The Francis and Kaplan turbines are inward flow and axial flow reaction turbines respectively.
What is a reaction turbine? How to calculate hydraulic and overall efficiency of reaction turbine? What is difference between a Kaplan turbine and Francis turbine?
The following important points may be noted for reaction turbines :
(a) In a reaction turbine, the water enters the wheel under pressure and flows over the vanes.
(b) The hydraulic efficiency of a reaction turbine is the ratio of the workdone on the wheel to the energy (or head of water) actually supplied to the turbine.
(c) The overall efficiency of a reaction turbine is the ratio of the power produced by the turbine to the energy actually supplied by the turbine.
(d) A Kaplan turbine is an axial flow reaction turbine. The number of blades are generally 4 to 8 in a Kaplan turbine runner.
(e) A Francis turbine is an outward flow reaction turbine. The number of blades are generally 16 to 24 in a Francis turbine runner.