Lecture PPTs (24) - Lect 27 1 Lect-27 In this lecture...

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1 Lect- 27
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 2 Lect-27 In this lecture... Turbine Blade Cooling Blade cooling requirements Fundamentals of heat transfer
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 3 Lect-27 Turbine blade cooling For a given pressure ratio and adiabatic efficiency, the turbine work per unit mass is proportional to the inlet stagnation temperature. Therefore, typically a 1% increase in the turbine inlet temperature can cause 2-3% increase in the engine output. Therefore there are elaborate methods used for cooling the turbine nozzle and rotor blades. Turbine blades with cooling can withstand temperatures higher than that permissible by the blade materials.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 4 Lect-27 Turbine blade cooling Thrust of a jet engine is a direct function of the turbine inlet temperature. Brayton cycle analysis, effect of maximum cycle temperature on work output and efficiency. Materials that are presently available cannot withstand a temperature in excess of 1300 K. However, the turbine inlet temperature can be raised to temperatures higher than this by employing blade cooling techniques. Associated with the gain in performance is the mechanical, aerodynamic and thermodynamic complexities involved in design and analysis of these cooling techniques.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 5 Lect-27 Turbine blade cooling The environment in which the nozzles and rotors operate are very extreme. In addition to high temperatures, turbine stages are also subjected to significant variations in temperature. The flow is unsteady and highly turbulent resulting in random fluctuations in temperatures. The nozzle is subjected to the most severe operating conditions.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 6 Lect-27 Turbine blade cooling Because the relative Mach number that the rotor experiences, it perceives lower stagnation temperatures (about 200-300 K) than the nozzle. However the rotor experience far more stresses due to the high rotational speeds. The highest temperatures are felt primarily by the first stage. Cooling problems are less complicated in later stages of the turbine.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 7 Lect-27 Turbine blade cooling There are several modes of failure of a turbine blade. Oxidation/erosion/corrosion Occurs due to chemical and particulate attack from the hot gases. Creep Occurs as a result of prolonged exposure to high temperatures. Thermal fatigue As a result of repeated cycling through high thermal stresses.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 8 Lect-27 Turbine blade cooling Combustion products Stator Rotor Average radial temperature profile Average temperature profile entering a turbine stage
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay
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