Impulse Turbine vs Reaction Turbine
Turbines are a class of turbo machinery used to convert the energy in a flowing fluid into mechanical energy by the use of rotor mechanisms. Turbines, in general, convert either thermal or kinetic energy of the fluid into work. Gas turbines and steam turbines are thermal turbo machinery, where the work is generated from the enthalpy change of the working fluid; i.e. the potential energy of the fluid in the form of pressure is converted into mechanical energy.
The basic structure of an axial flow turbine is designed to allow a continuous flow of fluid while extracting the energy. In thermal turbines, the working fluid at a high temperature and a pressure is directed through a series of rotors consisting of angled blades mounted on a rotating disk attached to the shaft. In between each rotor disks, stationary blades are mounted, which act as nozzles and guides the fluid flow.
Turbines are classified using many parameters, and the impulse and reaction division is based on the method of converting the energy of a fluid into mechanical energy. An impulse turbine generates mechanical energy completely from the impulse of the fluid when impact on the rotor blades. A reaction turbine uses the fluid from the nozzle to create momentum on the stator wheel.
More about Impulse Turbine
Impulse turbines convert the energy of the fluid in the form of pressure by changing the direction of the fluid flow when impacted on the rotor blades. The change in the momentum results in an impulse on the turbine blades and the rotor moves. The process is explained using the newtons second law.
In an impulse turbine, the velocity of the fluid is increased by passing through a series of nozzles before being directed to the rotor blades. The stator blades act as the nozzles and increase the velocity by reducing the pressure. Fluid stream with higher velocity (momentum) then impacts with the rotor blades, to transfer the momentum to the rotor blades. During these stages, the fluid properties undergo changes which are characteristic to the impulse turbines. The pressure drop occurs completely in the nozzles (i.e the stators), and the velocity increases significantly in the stators and drops in the rotors. In essence, the impulse turbines only convert the kinetic energy of fluid, not the pressure.
Pelton wheels and de Laval turbines are examples of the impulse turbines.
More about Reaction Turbine
Reaction turbines convert the energy of the fluid by the reaction on the rotor blades, when the fluid undergoes a change in momentum. This process can be compared to the reaction on a rocket by the exhaust gas of the rocket. The process of the reaction turbines are best explained using Newton’s second law.
A series of nozzles increases the velocity of the fluid stream in the stator stage. This creates a pressure drop and an increase in velocity. Then the fluid stream is directed to the rotor blades, which are also acting as nozzles. This further reduces the pressure, but the velocity also drops as a result of transfer of kinetic energy to rotor blades. In reaction turbines, not only the kinetic energy of the fluid, but also the energy in the fluid in the form of pressure is converted into mechanical energy of the rotor shaft.
Francis turbine, Kaplan turbine, and many of the modern steam turbines belong to this category.
In modern turbine design, operation principles are used to generate optimal energy output and the nature of the turbine is expressed by the degree of reaction (Λ) of the turbine. The parameter is basically the ratio between the pressure drop in the rotor stage and the stator stage.
Λ = (enthalpy change in the rotor stage) / (enthalpy change in the stator stage)
What is the difference between Impulse Turbine and the Reaction Turbine?
In an impulse turbine, pressure (enthalpy) drop occurs completely in the stator stage, and in reaction turbine pressure (enthalpy) drops in both rotor and stator stages. {If the fluid is compressible, (usually) the gas expands in both rotor and stator stages in reaction turbines.}
The reaction turbines have two sets of nozzles (in the stator and rotor) while impulse turbines have nozzles only in the stator.
In reaction turbines, both pressure and kinetic energy are converted to shaft energy while, in impulse turbines, only the kinetic energy is used to generate shaft energy.
The operation of impulse turbine is explained using Newton’s third law, and the reaction turbines are explained using Newton’s second law.