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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