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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 10 Evolution <strong>of</strong> nozzle geometry and cooling design.<br />

nozzle downstream. The flow field <strong>of</strong> the gas exiting the combustor and<br />

entering the stage 1 turbine nozzle is complicated including nonuniform<br />

radial and circumferential temperature pr<strong>of</strong>iles (hot streaks) and free-stream<br />

turbulence <strong>of</strong> some unknown intensity and scale. The parameter describing<br />

the degree <strong>of</strong> nonuniformity <strong>of</strong> combustor exit temperature is called pattern<br />

factor:<br />

Where<br />

PF ¼ðTg max TITÞ=ðTIT TcÞ<br />

Tg max is the highest gas temperature found at combustor exit plane.<br />

TIT (called sometimes CET) is the average turbine inlet (or combustor<br />

exit temperature).<br />

Tc is the compressor discharge temperature (recuperator exit<br />

temperature for an engine with recuperator).<br />

It is understandable that a lower PF at a given TIT requires a lower cooling<br />

effectiveness (lesser amount <strong>of</strong> cooling air for the same cooling technique).<br />

Typical PF values vary between 0.15 and 0.3.<br />

Copyright © 2003 Marcel Dekker, Inc.

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