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### Power Plant Turbines

P M V Subbarao

Associate Professor

Mechanical Engineering Department

I I T Delhi

A Techno-economically feasible model for large Power Plants…………..

Using the steam to make the Power !

- Rotating the shaft is the ultimate goal of any power plant.
- As you have probably noticed, from the text and pictures on Steam Generators, there is no shaft.
- Which leads to the question:
"now that you have all this super energized steam or Gas, how do you get work from it ? "

A boilers / Combustor is only one part of a larger operation, granted, it's a large part but most important part of the operation is it's ability to apply all this steam power.

The Steam Turbine

- The more modern method of extracting mechanical energy from thermal energy is the steam turbine.
- Steam turbines have been the norm in various land based power plants for many years.
- Motive power in a steam turbine is obtained by the rate of change in momentum of a jet of steam impinging on a curved blade which is free to rotate.
- The steam is (partially or fully) expanded in a nozzle, resulting in the emission of a high/medium/low velocity jet.
- This jet of steam impinges on the moving vanes or blades, mounted on a shaft.
- Here it undergoes a change of direction and/or magnitude of motion which gives rise to a change in momentum and therefore a force.

Generation of Motive Power Through Newton’s Second Law

Vi

U

Freaction

Ve

Work is said to be done by a system iff

The sole effect external to the system

can be reduced to raising of weight

Ve

Vi

Analysis of Simple Stationary Impulse Blade- Consider a stationary 180 degree curved blade.
- A jet with a velocity Vi incidence on the blade.
- The blade deflects the jet along its surface and finally the jet leaves with a velocity Ve.
- The magnitude of velocity vector remains unchanged.
- However, the direction changes through 180 degrees.
- Ve = - Vi
- The change in velocity : - 2 Vi.
- A jet with a finite mass flow rate will experience a rate of change of momentum, FA:

FA

FR

The force acting on the blade:

However, this force cannot develop any motive power.

Kinetic power lost by the jet :

Power lost by jet = Power gained by the Blade

Initial Power of the jet :

Thermodynamic efficiency of an impulse blade :

An efficient impulse blade is bulky …… Suitable for Dense fluids…

Thermodynamic efficiency of a Reaction blade :

A compact Reaction blade is inefficient ……Suitable for Thin fluids…

Vre

Vae

U

Vri

Vai

Simple Impulse-Reaction Blade

Jet will lose power both by Impulse and Reaction.

One important and essential element in all these cases is a nozzle.

How To Provide A Mass Flow Rate

- Area for Flow of Fluid.
- Proportional to the Length of the Blade.
- More Number of Blade Spacings.

bi

ae

ai

be

Vai

Vae

Vri

Vre

Vai: Inlet Absolute Velocity

Vri: Inlet Relative Velocity

Vre: Exit Relative Velocity

Vae:Exit Absolute Velocity

ai: Inlet Nozzle Angle.

bi: Inlet Blade Angle.

be: Exit Blade Angle.

ai: Exit Nozzle Angle.

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