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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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Impingement cooling is the most common technique for cooling <strong>of</strong> the<br />

nozzle support structure. Impingement jets are <strong>of</strong>ten directed toward the<br />

outer radius <strong>of</strong> the nozzle hooks that position nozzles in the support<br />

structure. This allows reduction <strong>of</strong> hoop stresses in the nozzle support<br />

structure, particularly when there is a concern for excessive radial<br />

temperature gradients through this structure. Another benefit <strong>of</strong> nozzle<br />

support structure cooling in addition to lowering metal temperature is a<br />

reduction <strong>of</strong> circumferential temperature gradients within the structure. This<br />

is a very important factor in achieving circumferentially uniform blade-tip<br />

clearances. Modulation <strong>of</strong> the cooling flow directed to the nozzle support<br />

structure can be used for active control <strong>of</strong> the tip clearances.<br />

The temperature <strong>of</strong> the turbine housing outer surface is <strong>of</strong>ten limited<br />

for reasons <strong>of</strong> safety and constraints from external engine hardware,<br />

including lube system and instrumentation. The U.S. standard for industrial<br />

gas turbines limits this outer shell temperature to about 450 8F. This is<br />

usually a trade<strong>of</strong>f between housing cooling and external insulation. Use <strong>of</strong> a<br />

highly effective external insulation is costly and also results in increased<br />

turbine housing temperatures that require application <strong>of</strong> more expensive,<br />

higher-grade alloys for the housing. Cooling <strong>of</strong> the housing results in<br />

additional heat losses with corresponding penalties for engine thermal<br />

efficiency and complications in design and engine maintenance. It is quite<br />

common to use ambient air for housing cooling utilizing energy <strong>of</strong> the spent<br />

cooling air from the turbine (or from the engine exhaust gas) to create<br />

suction (ejection) and flow <strong>of</strong> the ambient air along the housing.<br />

COMBUSTOR COOLING<br />

Effect <strong>of</strong> Combustor Design Evolution on Cooling<br />

Techniques<br />

This section will focus primarily on the liner, which usually consumes most<br />

<strong>of</strong> the combustor cooling flow budget. Other parts <strong>of</strong> the combustor<br />

including dome and transition to the nozzles are prone to design specifics<br />

and their cooling is not described in this section. However, some <strong>of</strong> the<br />

transition cooling issues were addressed earlier.<br />

Cooling requirements for the liner vary with a number <strong>of</strong> parameters,<br />

which have continually changed in the past 20 years <strong>of</strong> engine development.<br />

The major parameters are<br />

Hot gas temperature and type <strong>of</strong> fuel<br />

Cooling air temperature and allowable combustor pressure drop<br />

Allowable material temperatures, expected life, and durability<br />

Copyright © 2003 Marcel Dekker, Inc.

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