1Lecture 21

2Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Axial Turbine

Characteristics Multi-staging Blade Cooling

3Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Turbine Characteristics

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Design Point

A typical aircraft gas turbine operating line would show choking after a pressure ratio A typical design point is where the flow is just choked Characteristics based on exit flow conditions would show higher pressure ratio operation possibilities

Choked flow

4Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Multi-staging of turbine is done extract more energy for mechanical power To restrict size and number of stages each stage does more work (aerothermodynamically loaded) Multi-spooling is done to make the spools rotate at different speeds

5Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

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A matched LP + HP turbine operation, HP turbine may be choked all the time , as the pressure ratio across the LP turbine change.

Multi-stage turbine characteristics

6Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

ParametersFront stages (HP)

Last stages (LP)

2 750 - 700 650 - 600

Rx0.20 0.25 0.35 0.45

M30.25 0.35

0.5 for turbojet and turbofan engines0.65 0.70 for turbo-prop engine

3 0 - 100

Typical Multi-stage turbine inlet and outlet parameters

7Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Turbine CoolingTurbine materials :Inconel, Monel

Additional Cooling Techniques for Life

8Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Gas Flow over a Turbine blade profile

Fig. shows the typical manner in which heat transfer co-efficient may vary locally around the surface of the turbine.

9Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

x x

Heat transfer coefficient = Quantity of heat transferred

surface area t T between hot gas & surface

01 02 mean0-bl

p-gas

2T +T UT = - (1 - 2.DR)2 2.c

Temperature on a blade surface as felt by it

10Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

11Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

12Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

13Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

14Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Coolant flow paths in a modern HP turbine stator

15Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Various blade cooling techniques provide various amounts of cooling

Maximum cooling is normally applied to first HP stage stator, which faces the highest temperature

Cooling is also applied to HP rotors. But the details of this technology is a little more complicated as the cooling has to be effected when the blades are rotating at high speeds

Modern LP stage stators are also cooled.

Last stage blades do not require cooling as the gas temperature is already substantially reduced.

16Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Over the last fifty years more effort has been given to turbine cooling rather than to turbine aerodynamics. As the flow in turbine is always in favourablepressure gradient, high turbine efficiency is comparatively easily achieved. Cooling actually reduces turbine efficiency slightly Cooling occurs differentially across the blade surface depending on the local temperature fields of gas and cooling available locally. Amount of local cooling may vary from 500 to nearly 5000 centigrade in modern blades Coatings are also applied on blade surfaces for saving the blades from high temperature

17Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

Next class

Radial Flow Turbines

18Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

19Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Lect 21

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