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Turbine blade coolingTurbine blade cooling:Recent transport turbine engines such as PW 4000 and GE 90 have turbine inlet temperature of the order of 1380 deg CThis tem is high enough to soften the blade material High gas temperature accelerates oxidation and creep deformation of components Only air cooling is found practical in aircraft gas turbines.Air is bled from the compressor, carried aft and passed through the passages in turbine blades through their roots Air also cools the turbine discThe first stage turbine nozzles (guide vanes) and casing are also cooled.Cooling of turbine blades can be through i) Jet of air impinging on inside of leading edgeii) Convection cooling due to flow of air through blade internal passagesiii) Film cooling due to film of air directed chordwise over the blade surfaceTypical magnitude of cooling air required for a turbine stage:Designed to operate at 1500 deg Kfraction of entry gas mass flowannular wall = 0.016nozzle blades=0.025rotor blades=0.019rotor disc=0.005total=0.065Turbine blade cooling HP turbine may work at temperatures of about 2000 K, greater than the melting point of the leading nickel-based alloys from which they are cast. Therefore, the HP blades, non rotating guide vanes, and seal segments are cooled internally and externally using cooling air from the exit of the HP compressor at a temperature of about 1000 K (achieved through compression) at a pressure which is only about 2 bar greater than the inlet turbine pressure.

Turbine blade cooling air passages

Cooling performance parameter:

It is called blade relative temperature = (Tb Tcr ) / (Tg Tcr ) where Tb = mean blade temperature Tcr = coolant temperature at inlet = compressor delivery temperature Tg = mean effective gas temperature = static temperature + 0.85xstagnation temperature Turbine blade cooling: technology development

Internal and film coolingof HP turbine blades

HP turbine cooling air flow

Cooling of HP turbine blades

Rolls-Royce Spey turbine blade

Nozzle guide vane and turbine blade cooling arrangement

Rolls Royce system

Trent 500 hp turbine blade

Coolant feeding in Rolls Royce turbines:Although multi-passes is widely applied in current advanced turbine blade cooling, the main difference between Rolls-Royce and other gas turbine providers is the way of coolant feeding. Traditionally, Rolls-Royce prefers to apply both HP air and LP air to cool the same blade, which is seldom adopted by the other companies. The HP air is used as main cooling circuit and provides film cooling for the leading edge and pressure side, while LP air is used to cool the leading edge in convection cooling and discharged from the tip. Turbine blade cooling technology [Pratt & Whitney]