Gas turbine blades, combustor liners and exhaust components operate in gas hotter than the material can withstand. Film cooling places a thin protective layer of cool air between the surface and the hot gas.
Blowing ratio: too little is bad, too much is worse
Cooling effectiveness rises with the blowing ratio (the ratio of coolant to mainstream mass flux) — up to a point.
If the ratio becomes too high, the jet penetrates into the mainstream instead of staying attached, and lifts off. The surface is then unprotected; worse, the lifted jet generates a counter-rotating vortex pair that draws hot gas towards the surface. The result is spending more air and getting worse cooling.
The optimum lies in a narrow, geometry-dependent range, and that is the real subject of the design.
Hole geometry
Inclination angle. A shallow angle to the surface (typically around 30°) helps the jet stay attached; a normal hole separates immediately.
Fan-shaped exit. Widening the hole exit lowers the jet velocity and increases lateral spreading. It gives markedly better coverage for the same air flow, which is why it is standard in modern designs.
Compound angle. Adding a lateral angle relative to the flow improves lateral coverage.
Modelling warnings
Resolving the holes is expensive. Solving every hole explodes the mesh. A common approach is to represent the holes as source terms — but that approach cannot see the jet lift-off or the vortex pair, which is exactly the critical phenomenon. In the critical region the holes must be resolved.
A conjugate solution is needed. The metal temperature is set both by the external hot gas and by the internal cooling passage. See the conjugate heat transfer note.
Turbulent mixing governs. How quickly the jet is diluted is set by the turbulence model, and models differ systematically in this configuration. The result should be used comparatively.
References
- Menter, F. R. (1994). Two-equation eddy-viscosity turbulence models for engineering applications. AIAA Journal, 32(8), 1598–1605. DOI: 10.2514/3.12149
- Modest, M. F. & Mazumder, S. (2021). Radiative Heat Transfer, 4th ed. Academic Press. ISBN 978-0-323-98406-5.