Gas Turbine Blade Cooling

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Of the various means of producing mechanical power, the turbine is in many respects the most satisfactory. The absence of reciprocating and rubbing members means that balancing problems are few, that the lubricating oil consumption is exceptionally low, and that reliability can be high. The inherent advantages of the turbine were first realized using water as the working fluid, and then it changed to steam for electricity generation. Since the cost of setting up an installation for producing steam is very high hot gases themselves started to be used for running the turbine. Then the turbines were called Gas Turbines

In a Gas Turbine the working fluid, which is compressed to a very high pressure is allowed to expand. The power developed by the turbine can be increased by the addition of energy or increasing the temperature of the working fluid prior to expansion. The expansion of the hot working fluid then produces a greater power output.

The compressor takes atmospheric air and compresses to a very high pressure. Then in the combustion chamber it is imparted high energy. Then it is allowed to expand through the turbine. We will get the power O/P at the shaft. Depending on how the working fluid is guided to expand and the shape of the turbine blades, they are classified into Radial flow and Tangential flow turbines. Even if the increase in inlet temperature of the working fluid increases the work O/P and hence the efficiency of the turbine there are certain limitations for that, so we must cool the turbine using appropriate cooling methods.

Gas turbine blade is that part of the turbine which will guide the gas through the required angle to expand with minimum loss. It is usually small in size and the largest one will be only of the size of the palm of the hand. The gas enters the row of nozzle blades called stator blades or nozzle guide vanes with a static pressure and temperature P1&T1 and a velocity C1. It is expanded to P2&T2 and leaves with an increased velocity C2 at an angle α2. The rotor blade inlet angle will be chosen to suit the direction β2 of the gas velocity V2 relative to the blade at the inlet.β2 and V2 are found by vectorial subtraction of the blade speed from the absolute velocity C2. After being deflected the gas get expanded in the rotor blade passages and leaves at P3&T3 with relative velocity V3 at angle β3. Vectorial addition of U yields the magnitude and direction of the gas velocity at exit from the stage C3 and α3. α3 is known as swirl angle.

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