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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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TURBINE NOZZLE COOLING<br />

In the design <strong>of</strong> air-cooled turbine nozzles, a number <strong>of</strong> factors have to be<br />

considered in order to achieve an optimized design (more details for cooling<br />

design optimization can be found in the section on advanced experimental<br />

heat transfer and cooling validation). In addition to the specified engine<br />

thermodynamic performance, these should include<br />

Life requirements<br />

Field operating conditions<br />

Aerodynamic design<br />

Combustor characteristics<br />

Emission constraints<br />

Coolant supply system<br />

Material properties<br />

Geometric constraints<br />

Manufacturability and cost<br />

Selection <strong>of</strong> the nozzle cooling design concept is strongly affected by the<br />

component life target and environment where the engine is operated. The<br />

presence <strong>of</strong> foreign particles in the compressed air and corrosive chemicals<br />

in the fuel as well as the number <strong>of</strong> thermal cycles over the life <strong>of</strong> the<br />

component, and <strong>of</strong>f-design operation result in certain decisions regarding<br />

the selected design concept. Examples <strong>of</strong> design constraints associated with<br />

these factors are the smallest allowable size <strong>of</strong> the cooling holes, application<br />

<strong>of</strong> thermal barrier coatings, expected erosion-induced surface roughness and<br />

corresponding changes in the external heat-transfer coefficients, cooling air<br />

supply pressure margins over downstream pressures at the air discharge<br />

locations, etc.<br />

The shape <strong>of</strong> a turbine airfoil that is selected for the best aerodynamic<br />

performance might be compromised to be acceptable from a cooling design<br />

standpoint. Constraints associated with the cooling might require redistribution<br />

<strong>of</strong> the external heat-transfer coefficients, resulting in a larger<br />

diameter <strong>of</strong> the leading edge, changes in velocity distribution and airfoil<br />

shape, and even the number and size <strong>of</strong> the airfoils. Figure 10 illustrates the<br />

evolution <strong>of</strong> these changes using Rolls Royce nozzles as an example.<br />

Interaction with Combustor<br />

Combustors are responsible for generating the upstream turbulence <strong>of</strong><br />

various intensity and scale, temperature pr<strong>of</strong>ile (pattern factor), and velocity<br />

boundary conditions for turbine nozzles. Emissivity characteristics <strong>of</strong> the<br />

combustor also <strong>of</strong>ten play an important part in the thermal loading <strong>of</strong> the<br />

Copyright © 2003 Marcel Dekker, Inc.

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