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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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tur—turbine<br />

w—wall<br />

?—mainstream (free-stream) condition<br />

TURBINE COOLING AS A KEY TECHNOLOGY FOR<br />

ADVANCEMENT OF GAS TURBINE ENGINES<br />

The performance <strong>of</strong> a gas turbine engine is strongly influenced by turbine<br />

inlet temperature, and substantial performance increases can be achieved<br />

with the maximum allowable turbine inlet temperature. From a performance<br />

standpoint, stoichiometric combustion with turbine inlet temperature<br />

around 2,000 8C (3,650 8F) would be a thermodynamic ideal because no<br />

work would be ‘‘wasted’’ to compress air needed to dilute combustion<br />

products. Therefore, the current industry trend pushes the turbine inlet<br />

temperature closer to the fuel stoichiometric temperature, especially for<br />

military engines. However, allowable component metal temperatures, even<br />

for the most advanced alloys and manufacturing processes, cannot exceed<br />

930–980 8C (1,700–1,800 8F). To operate at gas temperatures well above this<br />

limit, a highly efficient component cooling system is required. Advancement<br />

in cooling is a major means to achieve higher turbine inlet temperatures,<br />

leading to improved performance and life <strong>of</strong> the turbine. Heat transfer is<br />

thus a very important design element for all parts <strong>of</strong> a modern gas turbine,<br />

but especially the combustor and turbine sections. In discussing the state-<strong>of</strong>the-art<br />

cooling design <strong>of</strong> the hot section it is important to keep in mind that<br />

the turbine designer is constantly under extreme pressures <strong>of</strong> development<br />

schedule, affordability, durability, and various other interdisciplinary<br />

constraints, all these strongly affecting the selection <strong>of</strong> a cooling design.<br />

Cooling Challenges for Constantly Increasing Gas<br />

Temperatures and Compressor Pressure Ratio<br />

Progress in modern high-efficiency and high specific power gas turbine<br />

engines is typically measured by the increases in operating temperature and<br />

overall compressor pressure ratio. The most common simple cycle engines<br />

with higher pressure ratios and correspondingly increased gas temperatures<br />

can achieve greater power for the same size and weight and better overall<br />

engine fuel efficiency. Engines with recuperators do not benefit thermodynamically<br />

from high compressor pressure ratio. Advanced alloys for<br />

turbine airfoils can safely operate at metal temperatures below 980 8C<br />

(1,800 8F) and alloys for discs and stationary structures are limited to 700 8C<br />

Copyright © 2003 Marcel Dekker, Inc.

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