Working Principle of Impulse Turbines and Reaction Turbines.

Working Principles of a Steam Turbine :

High pressure steam is fed to the turbine and passes along the machine axis through multiple rows of alternately fixed and moving blades. From the steam inlet port of the turbine towards the exhaust point, the blades and the turbine cavity are progressively larger to allow for the expansion of the steam.
The stationary blades act as nozzles in which the steam expands and emerges at an increased speed but lower pressure. (Bernoulli's conservation of energy principle - Kinetic energy increases as pressure energy falls). As the steam impacts on the moving blades it imparts some of its kinetic energy to the moving blades.

There are two basic steam turbine types, impulse turbines and reaction turbines, whose blades are designed control the speed, direction and pressure of the steam as is passes through the turbine.

Impulse Turbines :

The steam jets are directed at the turbine's bucket shaped rotor blades where the pressure exerted by the jets causes the rotor to rotate and the velocity of the steam to reduce as it imparts its kinetic energy to the blades. The blades in turn change change the direction of flow of the steam however its pressure remains constant as it passes through the rotor blades since the cross section of the chamber between the blades is constant. Impulse turbines are therefore also known as constant pressure turbines.
The next series of fixed blades reverses the direction of the steam before it passes to the second row of moving blades.

Reaction Turbines :

The rotor blades of the reaction turbine are shaped more like aerofoils, arranged such that the cross section of the chambers formed between the fixed blades diminishes from the inlet side towards the exhaust side of the blades. The chambers between the rotor blades essentially form nozzles so that as the steam progresses through the chambers its velocity increases while at the same time its pressure decreases, just as in the nozzles formed by the fixed blades. Thus the pressure decreases in both the fixed and moving blades. As the steam emerges in a jet from between the rotor blades, it creates a reactive force on the blades which in turn creates the turning moment on the turbine rotor, just as in steam engine. (Newton's Third Law - For every action there is an equal and opposite reaction)
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