Why is turbine blade cooling necessary and what are the basic methods?

Turbine blade cooling is necessary because turbine inlet temperatures (1700-2000K) exceed the melting point of blade materials (~1400K). Cooling allows operation 200-400K above uncooled limits, improving cycle efficiency. Basic methods include: Internal convection (cool air flows through passages inside the blade), Film cooling (air exits through holes creating a protective cool air layer), Impingement cooling (air jets directed at hot surfaces internally), and Transpiration (porous surface for uniform cooling). Cooling air is bled from the compressor, representing 5-15% of core flow. Advanced single-crystal superalloys and thermal barrier coatings further increase temperature capability.