Section 15.6

Turbine Section

Impulse/reaction turbines, nozzle guide vanes, fir-tree blade roots, creep and blade cooling.

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The turbine extracts energy from the hot, high-pressure gas to drive the compressor and accessories (and, in turboprop/shaft engines, the propeller/rotor). Nozzle guide vanes (NGVs) ahead of each rotor accelerate and turn the gas onto the blades.
Turbines are impulse, reaction or (usually) impulse-reaction. In an impulse stage the gas is turned by fixed nozzles and the pressure drop is across the vanes; in a reaction stage the blades themselves act as nozzles and the pressure drops across the moving blades.
Blades are attached to the disc by a 'fir-tree' root that allows for expansion and is retained axially; they run in the hottest gas stream, so they suffer creep (slow permanent stretch under load at temperature), thermal fatigue and oxidation.
Cooling lets turbine entry temperature exceed the metal's melting point: cool compressor air is fed through internal passages and out of film-cooling holes to blanket the blade surface; single-crystal blades and ceramic thermal-barrier coatings raise the limit further.
The NGVs are the first components to see combustion gas and set the mass flow through the engine; the turbine's condition is watched through EGT/ITT limits.
⚠ Exam trap: turbine blades are held by a loose-fitting 'fir-tree' root to allow thermal expansion; creep is the life-limiting slow stretch under centrifugal load at high temperature, and film cooling lets gas temperature exceed the blade's melting point.

Solved examples

How can a turbine run in gas hotter than the melting point of its blade material without the blades melting?
The blades are internally air-cooled: relatively cool air bled from the compressor passes through passages inside each blade and exits through tiny film-cooling holes, forming a thin insulating film of cooler air over the surface. Combined with thermal-barrier ceramic coatings and single-crystal alloys, this keeps the metal below its limit even though the gas is hotter than the melting point.

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