Turbines – An IntroductionA turbine is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work, namely electricity.

Turbine Blade

Turbines have been used for centuries to convert freely available mechanical energy from rivers and wind into useful work, through a rotating shaft.

Classification of turbines based on working fluid:

When the working fluid is water turbines are called hydraulic turbines or hydroturbines.

When working fluid is air, and energy is extracted from the wind, the machine is called wind turbine.

When the working fluid is steam, turbines are called steam turbines.

A more generic name for turbines that employ a compressible gas as the working fluid is gas turbines.

Hydraulic Turbine

Steam Turbine

• Steam turbines are used for the generation of electricity in thermal power plants, such as plants using coal, fuel oil or nuclear power.

Steam Turbine (continued)

Gas Turbine

Working of a Gas Turbine to generate electricity

The working fluid in a gas turbine is a permanent gas, in contrast with a condensable vapour in the steam turbine, produced in a gas generator at high pressure by continuous combustion in a combustion chamber.

Gas Turbine used for electricity generation

Gas Turbine in Jet Engines

Utlizing Combination of Gas Turbine and Steam Turbine for Power Generation

Wind Turbine

For power generation For wind velocity measurement

Turbines can be further classified into two basic categories based on how they operate – Impulse Turbines and Reaction Turbines. Most hydro stations use either of these two turbines to produce electricity.

• In an Impulse turbine, the whole of the available energy of the fluid is converted to Kinetic Energy before the water acts on the moving parts of the turbine.

• Pelton Wheel is an example of such turbine.

Pelton Wheel (Impulse Turbine)

Pelton Wheels in a hydroplant Components in a Pelton Wheel

Water is blasted at these cups by one or more jets mounted in the surrounding casing. Momentum is transferred from water to cups, and a torque is created, causing the wheel to rotate.

This type of turbine is highly efficient.

In Reaction Turbines, the rotation is mainly achieved by the reaction forces created by the acceleration of the fluid in the runner (rotating blade). The basic principle is the same as a rotating lawn sprinkler in which water enters the arms of the sprinkler at low velocity and leaves through the jets at high velocity.

Newton's third law describes the transfer of energy for reaction turbines.

A Simple Reaction Turbine

• Reaction turbines consist of fixed guide vanes called stay vanes, adjustable guide vanes called wicket gates and rotating blades called runner blades.

• It also generally consists of a spiral casing or volute, as in hydraulic turbines. It surrounds the runner completely. The casing should be strong to withstand high pressure.

Top and Side View of a typical Reaction Turbine

• Flow enters tangentially at high pressure, is turned toward the runner by the stay vanes as it moves along the volute, and then passes through the wicket gates with a large tangential velocity component.

• Momentum is exchanged between the fluid and the runner, and the runner rotates.

• Unlike impulse turbine, the water completely fills the casing of a reaction turbine.

• Reaction turbine generally produces more power than an impulse turbine.

• Wicket gates control volume flow rate.

• There are two main types of Reaction Turbine – Francis and Kaplan Turbines.

Sectional and Top View of a Francis Reaction Turbine

Francis Turbine

Sectional View of a Kaplan Reaction Turbine

Both types of turbines are inward-flow reaction turbines.

Francis turbines utilize axial and/or radial flow concepts.

Kaplan turbines utilize axial flow of water.

Kaplan turbine is a propeller-type water turbine which has adjustable blades.

Kaplan Turbines

Various types of water turbine runners. From left to right: Pelton Wheel, two types of Francis Turbine and Kaplan Turbine

A Francis turbine runner, rated at nearly one million hp (750 MW), being installed at the Grand Coulee Dam, United States.

Euler Head and Efficiencies of Hydraulic Turbines

• Efficiency of turbines is a function of the available head.• Euler's Head: It is defined as energy transfer per unit weight.

• Hydraulic Efficiency - It is the ratio of power developed by the runner to the head of water (or energy) actually supplied to the turbine i.e.

• Mechanical Efficiency - It is the ratio of actual work available at the turbine shaft to energy imparted to the wheel.

• Overall Efficiency – The overall efficiency is based on the useful work output divided by the water power input.

Hydraulic Power plant showing Net Head

In impulse turbines, the total head available is first converted into the kinetic energy.

In the reaction turbines, the fluid passes first through a ring of stationary guide vanes in which only part of the available total head is converted into kinetic energy. The guide vanes discharge directly into the runner along the whole of its periphery, so that the fluid entering the runner has pressure energy as well as kinetic energy. The pressure energy is converted into kinetic energy in the runner.

Change of Pressure and Velocity in a Steam Impulse Turbine and a Steam Reaction Turbine

Questions and Answers:

1. What is a turbine? Classify turbines on the basis of the working fluid.

2. Discuss the various efficiencies associated with hydraulic turbines.

3. Name the two major reaction turbines used in hydroelectric power stations. State the major difference between impulse and reaction turbine in terms of operation.

4. Draw a Pelton Wheel. Discuss its various features and explain why it is an impulse turbine.

5. What is Euler’s head?