Turbine Section
Impulse/reaction turbines, nozzle guide vanes, fir-tree blade roots, creep and blade cooling.
Turbine section, summary notes
- 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.
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.
Turbine concept map
Turbine Section
Turbine section quiz
Turbine Section, quiz
1. Turbine blades are often internally air-cooled because:
Gas temperature exceeds the melting point of the blade materialIt makes the blades lighter, reducing the centrifugal load carried by the disc at high speedIt reduces noise, because the cooling flow smooths the pressure fluctuations behind each blade2. Turbine blade creep is:
Slow permanent stretching under sustained heat and centrifugal loadCracking of the blade from a single overload applied during a rapid accelerationCorrosion of the blade root, caused by sulphur compounds carried through with the fuel3. An engine takes in air at 15 °C and delivers it from the compressor at 450 °C. The temperature rise across the compressor is:
435 K465 K723 K4. A turbine entry temperature of 1200 °C expressed in kelvin is:
1473 K927 K1200 K5. The turbine's job is to:
Extract energy from the gas to drive the compressor and accessoriesCompress the incoming air before it reaches the combustion chamber and the fuel is addedIgnite the fuel and air mixture and hold the flame stable across the whole operating range6. Nozzle guide vanes (NGVs) ahead of a turbine rotor:
Accelerate and turn the gas onto the bladesSlow the gas downAdd fuel7. Turbine blades are attached to the disc by a:
Fir-tree root allowing for expansionWeldSingle rivet8. Creep is:
Slow permanent stretch under centrifugal load at high temperatureA sudden brittle fracture of the blade root under a single overload during accelerationCorrosion of the turbine disc, caused by sulphur compounds carried through in the fuel