A turbine blade turns hotter gas than the metal could ever endure, and it survives because it is hollow. Follow cold air from the blade root, through a winding internal channel, out of tiny surface holes and across the blade.
A turbine blade is the curved vane that hot gas flows over to spin the turbine disc. In a modern engine the gas is hotter than the metal's melting point, so the blade is built hollow and cooled from the inside. The trick is a set of internal channels, rows of tiny surface holes and a heat-resistant coating.
From the outside a blade looks like a solid metal vane. The reason it can survive, though, is that it is not solid at all â the cooling passages are hidden inside, so cutting the blade open is the only way to see how the air is routed.
Hot combustion gas sweeps past the airfoil from its leading edge to its trailing edge. The blade extracts energy from that gas, but the gas itself is far hotter than the metal can stand.
Cooler compressor air is tapped off and fed up through a duct in the blade root. The root platform also locks the blade into the disc and carries its huge spinning load.
Inside the blade the air follows a winding, zig-zag serpentine channel. The long path forces it to pick up heat from the surrounding metal as it travels, so the metal is cooled all the way along the blade.
Some of the air is not kept inside. It escapes through rows of tiny film cooling holes and spreads across the surface, laying down a thin, moving film of cool air between the blade and the hot gas.
Finally, a ceramic thermal barrier coating on the outer surface slows heat from entering the metal, and a tip shroud seals the end of the blade. Cooling and insulation together keep the blade alive.
By being cooled from the inside. Cold air flows through internal channels and along the surface so the metal itself is kept far below the temperature of the gas around it.
It is a zig-zag channel cast inside the blade. Routing the air up and down lengthens its path, so it absorbs more heat from the metal before it leaves.
Air inside the blade escapes through rows of tiny holes and spreads over the surface. That thin layer of cool air shields the metal from the hot gas, like a moving blanket.
Because it is already available and much cooler than the combustion gas. Tapping a little compressor air costs some efficiency but protects blades that could not otherwise survive.
It is a ceramic layer on the surface that slows heat flowing into the metal. It works together with the internal cooling, not instead of it.