Turbine Section (blades, nozzles, cooling, clearances)
What this domain tests
Turbine Section questions focus on extracting work from hot gas without exceeding metal limits: nozzle guide vanes versus rotor blades, impulse and reaction staging, cooling schemes, tip clearance, and creep.
Core rules
- Nozzle guide vanes accelerate and turn the gas; rotor blades extract work. A reaction stage drops pressure in both stator and rotor; an impulse stage drops pressure mainly in the nozzle.
- Turbine inlet temperature is limited by blade material and cooling. Cooling methods include internal convection, impingement, film cooling, and thermal barrier coatings.
- Tip clearance is a direct efficiency loss. Active clearance control uses cooling air on the casing to shrink clearance at cruise.
- Creep is time-at-temperature dependent; exceeding turbine temperature limits consumes blade life even if the engine still runs.
Worked example
A turbine extracts work from 50 kg/s of gas with cp = 1.15 kJ/(kg.K) and a temperature drop of 400 K. Power = mdot x cp x delta T = 50 x 1.15 x 400 = 23,000 kW = 23 MW. If cooling air equal to 8% of the main flow is taken from the compressor, that air does little turbine work and penalizes cycle efficiency; the reason is that it bypasses the combustor and enters the turbine at a lower temperature.
Traps
Calling vanes blades; assuming cooling air is free; treating tip clearance as fixed; confusing creep with fatigue; ignoring the cooling penalty in performance calculations.