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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Table 2 Materials Selection Criteria<br />

Failure modes Cost<br />

Strength at<br />

high<br />

temperature<br />

Low-cycle<br />

fatigue<br />

strength<br />

High-cycle<br />

fatigue<br />

strength<br />

Manufacturing<br />

process or<br />

mode <strong>of</strong><br />

fabrication Weight Others<br />

Material cost Casting Density Containment<br />

(ductility,<br />

Fabrication<br />

cost<br />

Vendor<br />

availability<br />

Hardness and<br />

ease <strong>of</strong><br />

machining<br />

Strength/<br />

weight<br />

strength)<br />

Thermal<br />

expansion<br />

Forging — Thermal<br />

conductivity<br />

Creep strength — Repairability — Specific heat<br />

Fast fracture,<br />

cyclic crack<br />

growth rate<br />

— Availability — —<br />

Corrosion — Consistency, — —<br />

resistance<br />

cleanliness <strong>of</strong><br />

product<br />

<strong>of</strong> the turbine are made <strong>of</strong> alloys <strong>of</strong> nickel, those used commonly being<br />

Hastalloy, Inconel, and Waspalloy for compressor/turbine casings, shafts,<br />

and other areas. Blades and vanes are routinely made <strong>of</strong> single-crystal,<br />

directionally solidified, or equi-axed alloys <strong>of</strong> various compositions.<br />

Waspalloy or Inconel may be used also to make seal plates for turbine<br />

discs. In some experimental engines, turbine discs and some seal plates are<br />

made <strong>of</strong> sintered alloys <strong>of</strong> nickel. Some <strong>of</strong> them are capable <strong>of</strong> withstanding<br />

extremely high temperatures, running as high as 1,450 8F.<br />

2. Aerodynamic design: This involves the definition <strong>of</strong> the airfoil<br />

contours, both <strong>of</strong> the compressor and <strong>of</strong> the turbine stages. The design starts<br />

with the velocity triangles laid out during preliminary design. The blade<br />

angles should match the inlet and exit angles specified by the velocity<br />

triangles. The contour <strong>of</strong> the blade is then designed to provide a smooth<br />

flow and to ensure that there is little separation at all points on the blade.<br />

Proper turning <strong>of</strong> flow over the blade and the contour design ensure that<br />

blade loading is as desired. Usually, HPT blades turn out to be somewhat<br />

thicker at the leading edge than LPT blades. In addition, they do not exhibit<br />

large radial twists. The design <strong>of</strong> compressor airfoils (refer to Chapter 6) is<br />

different from that <strong>of</strong> turbine airfoils (refer to Chapter 7). This is due to the<br />

Copyright © 2003 Marcel Dekker, Inc.

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