Film Cooling of Turbine Blades in Gas Turbines
- Draw Cooling Air From Compressor
- Injection of Coolant Onto Blade Surface
- Creation of an Insulating Sublayer
- Lower the Effective Gas Temperature in the Boundary Layer
- High Cooling Effectiveness
- Little Rise of Heat Transfer Due to Jet-Mainstream Interaction
- Avoid Loss of Coolant into Main Stream Due to Jet Lift-Off
- Minimize Necessary Coolant Mass Flow Rate
- Reliably limit the operating temperature for the blading to tolerable values
(prevent thermal damage, longer service intervals etc.)
Problem Areas in Film Cooling
- Jet Detachment - Coolant is Directly Injected into Main Fluid --> Poor
Coverage by Cooling Film
- Unfavorable Increase of Local Heat Transfer due to Strong Mixing
- Aerodynamic Losses Induced by Cooling Film
Prediction of film cooling performance
Prediction of film cooling performance is a very tough issue.
First of all, the heat transfer coefficient is directly linked to the boundary
layer state, which is extremely difficult to predicted on a film cooled blade.
Then, the film cooling effectiveness on a surface depends on many geometrical
parameters like shape, orientation and arrangement of the holes, number of rows
or surface curvature. The flat plate is the only case for which an empirical
correlation concerning film cooling effectiveness is available.