Turbines have been used for centuries to convert freely availablemechanical energy from rivers and wind into useful work, through arotating shaft.Classification of turbines based on working fluid:When the working fluid is water turbines are called hydraulic turbinesor hydroturbines.When working fluid is air, and energy is extracted from the wind, themachine 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 asthe working fluid is gas turbines.
Gas TurbineThe working fluid in a gas turbine is a permanent gas, in contrast with acondensable vapour in the steam turbine, produced in a gas generator athigh pressure by continuous combustion in a combustion chamber. Gas Turbine used for electricity generation Working of a Gas Turbine to generate electricity
Utlizing Combination of Gas Turbine and Steam Turbine for Power Generation
Wind TurbineFor power generation For wind velocity measurement
Turbines can be further classified into two basic categories based onhow they operate – Impulse Turbines and Reaction Turbines. Mosthydro 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 WheelWater is blasted at these cups by one or more jets mounted in thesurrounding 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 reactionforces created by the acceleration of the fluid in the runner (rotatingblade). The basic principle is the same as a rotating lawn sprinkler inwhich water enters the arms of the sprinkler at low velocity and leavesthrough the jets at high velocity.Newtons third law describes the transfer of energy for reactionturbines. 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
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 adjustableblades. Kaplan Turbines
Various types of water turbine runners. From left to right: Pelton Wheel, two types ofFrancis Turbine and Kaplan Turbine
A Francis turbine runner, rated at nearly one million hp (750MW), 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.• Eulers 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.
In impulse turbines, the total head available is first converted into thekinetic energy.In the reaction turbines, the fluid passes first through a ring of stationaryguide vanes in which only part of the available total head is convertedinto kinetic energy. The guide vanes discharge directly into the runneralong the whole of its periphery, so that the fluid entering the runnerhas pressure energy as well as kinetic energy. The pressure energy isconverted 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?