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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 9 Typical turbine hot section finite-element model.<br />

hollow components. One <strong>of</strong> the most challenging engineering objectives in<br />

turbomachinery design and a crucial factor for engine performance is to<br />

minimize and maintain tight turbine blade-tip clearances over the life <strong>of</strong> the<br />

engine. This requires a full range <strong>of</strong> comprehensive design and analysis,<br />

including concept development combined with intricate transient fluid<br />

mechanics, heat transfer, and stress-deflection analysis <strong>of</strong> the entire<br />

multicomponent structure, followed by engine test verification. Turbine<br />

blade-tip leakages represent one <strong>of</strong> the largest sources <strong>of</strong> turbine efficiency<br />

loss. Maintaining tight operating tip clearances without tip rubs is a major<br />

task for a cross-disciplinary design. Proper thermal management <strong>of</strong> the<br />

entire engine rotor-to-stator transient behavior, particularly for the<br />

nonshrouded blades in the diverging gas path, creates an extraordinary<br />

challenge. Both thermal and inertial transient displacements as well as some<br />

aerodynamic loading effects should be considered. Passive tip clearance<br />

control, which is based on close matching between transient growth <strong>of</strong> rotor<br />

and stator, typically requires a bulky stationary structure, unacceptable for<br />

aero engines. Active and semiactive [17–20] tip clearance controlling<br />

techniques, based on modulation <strong>of</strong> the cooling air supply to the stationary<br />

structure during transient operation, present an alternative solution when<br />

lighter structures are required.<br />

Several factors must be considered to insure minimal tip clearance at<br />

steady state without rubs, particularly in the case <strong>of</strong> unshrouded blades:<br />

Differences in thermal radial growth between the turbine rotor<br />

assembly and nozzle tip shroud support structure during transients.<br />

This is usually the most severe during hot restarts, when tip<br />

Copyright © 2003 Marcel Dekker, Inc.

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