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Turbine .lade .ooling.esign
The incorporation of blade cooling concepts into actual blade designs is very important. There are five different blade cooling designs.
.onvection and Impingement .ooling.Strut Insert .esign
The strut insert design shown in Figure 9-14 has a midchord section that isconvection-cooled through horizontalfins， and a leading edge that is impingement cooled. The coolant is discharged through a split trailing edge. The air flows up the central cavity formed by the strut insert and through holes at the leading edge of the insert to impingement cool the blade leading edge. The air then circulates through horizontal fins between the shell andstrut， and discharges through slots in the trailing edge. The temperature distribution for this design is shown in Figure 9-15.
The stresses in the strut insert are higher than those in the shell， and the stresses on the pressure side of the shell are higher than those on the suction side. Considerably more creep strain takes place toward the trailing edge than the leading edge. The creep strain distribution at the hub section is unbalanced. This unbalance can be improved by a more uniform wall temperature distribution.
Film and .onvection .ooling.esign
This type of blade design is shown in Figure 9-16. The midchord region isconvection-cooled， and the leading edges are both convection and film-cooled. The cooling air is injected through the blade base into two central

Figure 9-14. Strut insert blade Figure 9-1.. Film and convection-cooled blade

and one leading edge cavity. The air then circulates up and down a series ofvertical passages. At the leading edge， the air passes through a series of smallholes in the wall of the adjacent vertical passages， and then impinges on the inside surface of the leading edge and passes through film cooling holes. The trailing edge is convection-cooled by air discharging through slots. The temperature distribution for film and convection cooling design is shown inFigure 9-17. From the cooling distribution diagram， the hottest section canbe seen to be the trailing edge. Theweb， which is the most highly stressedbladepart， is also the coolest part of the blade.
A similar cooling scheme with some modifications is used in some of the latest gas turbine designs. The firing temperature of .E FA units is about 2350 0F (1288 0C)， which is the highest in the power generation industry. Toaccommodate this increased firing temperature， the FA employs advanced cooling techniques developed by .E Aircraft Engines. The first and second stage blades as well as all three-nozzle stages are air-cooled. The first stage blade is convectively cooled by means of an advanced aircraft-derived serpentine arrangement as shown in Figure 9-18.

Figure 9-17. Temperature distribution for film convection-cooled design oF (cooled)

Figure 9-18. .nternal of the frame FA blades showing cooling passages (.ourtesy GE Power Systems )
Cooling air exits through axial airways located on the bucket.s trailingedge andtip， and also through leading edge and sidewalls for film cooling.
Transpiration .ooling.esign
This design has a strut-supported porous shell (Figure 9-19). The shell attached to the strut is of wire from porous material. Cooling air flows upthe central plenum of thestrut， which is hollow with various-size metered holes on the strut surface. The metered air then passes through the porous shell. The shell material is cooled by a combination of convection and film cooling. This process is effective due to the infinite number of pores on the blade surface. The temperature distribution is shown in Figure 9-20.
The trailing edge of the strut develops the highest creep strain. This strain occurs despite the sharp stress relaxation at the trailing edge projection. The creep strain in the strut is well balanced. Transpiration cooling requires a material of porous mesh resistant to oxidation at a temperature of 1600 0F
(871.1 0C) or more. Otherwise， the superior creep properties of this design are insignificant. Since oxidation will close thepores， causing uneven coolingand high thermal stresses， the possibility of blade failure exists. The reason for superior creep property is a relatively low strut temperature 1400 0F average (760.0 0C)， which more than compensates for the high level of centrifugal stress required to support the porous shell.
　
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