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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