Steam turbines are heart of power plant, they are the devices which transform thermal energy in fluid to mechanical energy.
Energy Absorption from fluid - Role of Rotor Blades
When 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.
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| 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.
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| 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 Fluid
A 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 Rotors
When 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 Transfer
Total 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 Turbines
When 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.
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| 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 Turbines
When 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.
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| 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.
