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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 47 Campbell diagram (frequency versus turbine speed) for a 60 Hz last-row<br />

turbine blade.<br />

Manufacturers generally tune through the first three families <strong>of</strong> modes<br />

(tangential, axial, torsional) to avoid resonance up through the seventh or<br />

eighth harmonic (420–480 Hz) <strong>of</strong> running speed. This is generally sufficient<br />

as the lower modes have higher associated energies. However, tenon fatigue<br />

failures have occurred in the 4th mode (second bending mode) excited by<br />

12th- or 13th-order harmonics.<br />

There is a limitation on the number <strong>of</strong> modes that can be tuned,<br />

because this is a function <strong>of</strong> <strong>of</strong>f-frequency requirements and manufacturing<br />

capability, illustrated schematically by example in Fig. 48. If, for example,<br />

the requirement is that the blade should be <strong>of</strong>f a resonant frequency by<br />

+1.5 Hz and the manufacturer can produce blades <strong>of</strong> a population that fits<br />

within a 40-Hz band, then it is only possible to tune modes with frequencies<br />

Copyright © 2003 Marcel Dekker, Inc.

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