Working of Steam Turbine ~ Learn Engineering

3/2/2014 Working of Steam Turbine ~ Learn Engineering

http://www.learnengineering.org/2013/02/working-of-steam-turbine.html 1/4

Working of Steam Turbine| |

Steam turbines are heart of power plant, they are the devices which transform thermal energy in fluid to mechanical

energy. In this video lecture working of steam turbine is explained in a logical manner.

Summary of above lecture is given below.

Energy Absorption from fluid - Role of Rotor BladesWhen high energy fluid (high pressure and high temperature) passes through series of rotor blades, it absorbs energy from

fluid and starts rotating, thus it transforms thermal energy in fluid to mechanical energy.

Fig.1 Rotating blades of turbine helps in transforming thermal in fluid to mechanical energy

So series of such blade which eventually transform thermal energy are the most vital part of a steam turbine. One of such

rotor set is shown in figure below.

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Fig.2 A typical steam turbine rotor

If you take a close look at one of the blade, it would be clear that a blade is a collection of airfoil cross sections from

bottom to top. When flow passes through such airfoils it induces a low pressure on bottom surface and high pressure on

top surface of airfoil as shown in figure below.

Fig.3 Fluid flow around airfoil cross sectioned blade induces a high pressure (P) and low pressure(P) on blade surfaces

This pressure difference will induce a resultant force in upward direction, thus making the blade rotate. So some part of

fluid energy will get transformed to mechanical energy of blade. Before analyzing energy transfer from fluid to blade, we

will have a close look at energy associated with a fluid.

Energy Associated with a FluidA flowing fluid can have 3 components of energy components

Kinetic energy - Virtue of its velocity

Pressure Energy - Virtue of its pressure

Internal Energy - Virtue of its temperature

Last 2 components of energy together known as enthalpy. So total energy in a fluid can be represented as sum of kinetic

energy and enthalpy.

Energy Transfer to RotorsWhen fluid passes through rotor blades it loses some amount of energy to the rotor blades. Due to this both kinetic and

enthalpy energy of fluid come down for a typical rotor. As kinetic energy comes down velocity of flow decreases. If we

directly pass this stream to next stage of rotor blades it will not transfer much energy because of low velocity of flow

stream. So before passing the stream to next rotor stage we have to increase the velocity first. This is achieved with use

of a set of stationary nozzle blades, also known as stator. When fluid passes through stator blades velocity of fluid

increase due to its special shape thus one part of enthalpy energy will get converted into kinetic energy. Thus enthalpy of

stream reduces and kinetic energy of stream increase. It is to be noted that here there is no energy addition or removal

from flow, what happens here is conversion of enthaply energy into kinetic energy. Now this steam of fluid can be passed

to next rotor blades and process can be repeated. Velocity and enthalpy variation of flow is shown in following figure.

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Fig.4 Velocity and enthalpy variations across rotor and stator stages of a typical steam turbine

Degree of Energy TransferTotal energy transfer to the rotor blade is sum of decrease in kinetic energy and decrease in enthalpy. Degree of

contribution of each term is an important parameter in axial flow machines. This is represented by a term called of degree

of reaction, which is defined as

Where both enthalpy change and kinetic energy changes are defined across the rotor blade.

0 % Reaction - Impulse TurbinesWhen D.O.R = 0 there will not be any enthalpy change across the rotor, such a turbine is known as impulse turbine. Blades

of such a turbine would like as shown below.

Fig.5 A typical impulse turbine rotor cross section and flow pattern

Here incoming flow stream hits the blade and produces and impulse force on it. Since enthalpy across the blade does not

change temperature will also remain same. There will be minor pressure drop across the rotor, but this is almost negligible.

Here energy transfer to the blade is purely due to decrease in kinetic energy of fluid.

100 % Reaction TurbinesWhen D.O.R = 1 kinetic energy change across the rotor will be zero, energy transfer will be purely due to decrease in

enthalpy. Since kinetic energy is same across the rotor absolute value of velocities remain same. This is shown in figure

below.

Fig.6 A typical reaction turbine rotor cross section and flow pattern

Usually people use compromise of above two discussed cases,that is 50% D.O.R . Such turbines are known as Parson

turbines, where both kinetic and enthalpy energy transfer contribute equally to power transfer to rotor.

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Working of Steam Turbine ~ Learn Engineering