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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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nozzle vanes is limited by their oxidation. In some cases a low cycle fatigue<br />

can be detrimental for trailing edges <strong>of</strong> the nozzle vanes, particularly when<br />

they are very thin compared to the rest <strong>of</strong> the nozzle. Life <strong>of</strong> the turbine<br />

blades is usually limited by creep-rupture characteristics <strong>of</strong> a selected blade<br />

material, except for tips <strong>of</strong> unshrouded blades, which are typically exposed<br />

to low stresses from inertial forces and limited by the oxidation. Fifty<br />

percent reduction in the creep-rupture life <strong>of</strong> an advanced airfoil might<br />

result from the metal temperatures in stressed sections being only 20–30 8F<br />

(12–18 8C) above nominal design. For nozzles or oxidation-limited tip<br />

sections <strong>of</strong> the blade, the same life reduction results from metal temperatures<br />

at about 40–50 8F (25–32 8C) above nominal.<br />

The basic airfoil shape arises from the aerodynamic design, but<br />

modified by the knowledge that some, as yet unspecified, cooling system will<br />

have to be accommodated. Certain preliminary cooling assessments must be<br />

performed prior to a full specification <strong>of</strong> the airfoils. In order to keep metal<br />

temperatures within limits, the airfoil internal cooling must be arranged in a<br />

way so as to most effectively counterbalance external thermal loads. The<br />

designer can usually decide from past experience and design practices what<br />

type <strong>of</strong> cooling will be necessary (convective or film cooling) on each surface<br />

to be cooled.<br />

Depending on a specified life, the recommended metal temperature<br />

limits for advanced alloys are 1,800–1,900 8F (980–1,040 8C) for combustor<br />

liners and transitional parts, 1,900–2,000 8F (1,040–1,095 8C) for the nozzle<br />

vanes and end walls, 1,500–1,650 8F (815–900 8C) at blade highly stressed<br />

sections with allowance to rise to 1,800–1,900 8F (980–1,040 8C) in the<br />

oxidation limited tip section. Large turbine components like turbine discs<br />

and nozzle support structures, produced from forgings, are usually limited<br />

to temperatures below 1,400 8F (760 8C), and even at these relatively low<br />

temperatures might require oxidation protective coatings.<br />

Turbine discs and relatively high-temperature stationary structures<br />

that support turbine nozzles are susceptible to significant, mainly radial,<br />

transient temperature gradients inducing a low cycle fatigue. Effective<br />

protection (isolation) <strong>of</strong> these components from ingress <strong>of</strong> the hightemperature<br />

main-stream flow is essential for life <strong>of</strong> the components.<br />

Application <strong>of</strong> low-conductivity thermal barrier coatings can also provide<br />

significant assistance in confronting excessive transient temperature<br />

gradients in these components.<br />

Copyright © 2003 Marcel Dekker, Inc.

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