The blade runs hotter than its own melting point
In a modern jet engine the gas hitting the turbine is hotter than the melting point of the nickel alloy the blades are made of. The metal survives because it is cooled from the inside.
Compressor air, still hundreds of degrees but far cooler than the gas path, is fed through channels cast inside the blade and out through small holes. It lays a film over the surface. A ceramic coating stands between the gas and the alloy. Together they let the turbine inlet temperature sit above the melting point of the metal while the metal itself stays solid.
The reason to bother is efficiency. A hotter peak, for the same fuel, is a larger share of the fuel’s energy turned into work. The cooling air is not free — it was compressed, and bleeding it costs thrust — which is why the holes and the coatings are engineered as tightly as the alloy.
Caveats
- The gas temperature and the metal temperature are different numbers. The claim is that the gas is hotter than the alloy can melt at, not that the blade is molten.
- Not every engine runs this far above the limit. The hottest military and large civil turbines are the case in point.
Sources
- 1Cumpsty N.. Jet Propulsion. Cambridge University Press, 2003.