28.02.2013 Views

Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

(4) ellipiticity, such as caused by thermal distortion <strong>of</strong> stationary parts such<br />

as end walls, seals, and nozzle diaphragms, (5) nonuniform spacing,<br />

gauging, or thickness <strong>of</strong> stationary blades.<br />

Frequency Response and the Campbell Diagram<br />

Natural modes <strong>of</strong> vibration for turbine blades can be divided into three<br />

basic categories: (1) tangential modes where blades vibrate perpendicular to<br />

the axis <strong>of</strong> the turbine in the plane <strong>of</strong> maximum blade flexibility; (2) axial<br />

modes; and (3) torsional (twisting) vibration. Figure 46 shows aschematic<br />

<strong>of</strong> the typical first three modes <strong>of</strong> vibration <strong>of</strong> a single blade. These are<br />

tangential (first mode), axial (second mode), and torsional (third mode). In<br />

practice, actual blade vibration modes will usually be a combination <strong>of</strong> these<br />

simple modes, and thus actual measured vibration will be considerably more<br />

complex even for single blades and for grouped blades. Because <strong>of</strong> the<br />

complexity in the shape <strong>of</strong> blade vibrations, a number <strong>of</strong> physical terms for<br />

the higher modes have been developed and can lead to significant confusion<br />

unless specifically defined for the particular problem being analyzed. The use<br />

<strong>of</strong> computer-generated mode shape diagrams, visually identifying the<br />

displacements and shapes, can be <strong>of</strong> significant benefit in understanding<br />

how the actual deformations are occurring.<br />

High cycle fatigue <strong>of</strong> LP turbine blades usually occurs because <strong>of</strong><br />

structural resonances at frequencies synchronous with harmonics <strong>of</strong> the<br />

running speed. The tuning <strong>of</strong> longer blades so that no structural resonances<br />

occur with the first several harmonics <strong>of</strong> rotational frequency has been<br />

standard design practice ever since the principle was introduced by W.<br />

Campbell in 1924. Parallel work was done by D. Smith. Subsequent<br />

refinement was introduced by M. Prohl [96]. Blades are also tuned to avoid<br />

frequencies close to harmonics to allow a margin to account for slight<br />

differences in manufacturing and assembly tolerances. If these goals are<br />

achieved, only aerodynamic flow excitations should remain as forcing<br />

functions for dynamic stresses.<br />

The Campbell diagram provides a representation <strong>of</strong> blade natural<br />

frequencies against rotational speed and machine harmonics as shown in<br />

Fig. 47. Note that at the rated speed, NR, the blades are tuned such that<br />

none <strong>of</strong> the blade natural frequencies (horizontal bands in the figure)<br />

intersects a machine harmonic (or multiple <strong>of</strong> operating speed), H. The<br />

width <strong>of</strong> the horizontal bands indicate expected scatter from manufacturing<br />

and assembly tolerances. The rise in blade frequencies from zero to rated<br />

speed <strong>of</strong> the turbine is caused by ‘‘speed’’ or ‘‘spin’’ stiffening.<br />

If the machine speed is above or below the rated level, there can be an<br />

intersection <strong>of</strong> the two lines leading to a condition <strong>of</strong> resonance. Turbines<br />

Copyright © 2003 Marcel Dekker, Inc.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!