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Handbook of Turbomachinery Second Edition Revised - Ventech!

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circumference <strong>of</strong> the disk, although a variety <strong>of</strong> other shapes are possible.<br />

Figure 49 illustrates these dominant sinusoidal shapes. The one nodal<br />

diameter figure shows the simplest <strong>of</strong> these modes. In this mode there are<br />

two zero points or nodes (indicated by the solid points on the circle) and a<br />

displacement around the disk equal to one sine wave. The displacement<br />

reaches a maximum positive displacement on the right side <strong>of</strong> the drawing<br />

and a maximum negative displacement 1808 around the disk from that<br />

point.<br />

The interference diagram has been developed to calculate the effect <strong>of</strong><br />

nodal disc vibrations on blade vibrations. The interference diagram plots the<br />

disk–blade modal shapes (using nodal diameters) against frequency. An<br />

example <strong>of</strong> the interference diagram is shown in Fig. 50 for an analysis <strong>of</strong> a<br />

six-blade group. The basic premise <strong>of</strong> the interference diagram is that<br />

resonance cannot occur unless the nodal diameter <strong>of</strong> the mode in question<br />

coincides with the per-rev excitation, i.e., four per-rev interfering with the<br />

fourth nodal diameter. The nodal diameter concept is used to describe the<br />

mode shapes in terms <strong>of</strong> the number <strong>of</strong> nodes (locations <strong>of</strong> zero motion)<br />

observed in a given mode. Thus, Fig. 50 shows the relationship between the<br />

blade group natural frequency, the blade–disk mode shape (expressed in<br />

terms <strong>of</strong> nodal diameters), and per-rev excitation. The closed symbols<br />

represent single modes. The data used on the interference diagram are from<br />

a finite-element analysis [80].<br />

A constant-speed line (1,800 rpm in Fig. 50) is drawn on the<br />

interference diagram. Wherever the speed line crosses a set <strong>of</strong> nodal<br />

diameter modes, a resonant condition is possible. In this example, the speed<br />

line passes near the fourth nodal diameter <strong>of</strong> the first tangential mode set,<br />

indicating a possible resonance. The advantage <strong>of</strong> the interference diagram<br />

over the Campbell diagram is that the latter would not have identified this<br />

potential resonance since it does not include the disk modal effects.<br />

Aerodynamic Analysis and Flow Analysis <strong>of</strong> Blades<br />

Aerodynamic and flow analysis has always been important in the design and<br />

analysis <strong>of</strong> turbines. However, recent developments in computational fluid<br />

dynamic (CFD) codes have allowed significantly better analysis than was<br />

available even in the recent past. Such tools were not available to the turbine<br />

manufacturers when most units were originally designed.<br />

Turbine heat rate and output improvements <strong>of</strong> 2–6% have been<br />

reportedly achieved in turbines from several manufacturers as a result <strong>of</strong><br />

optimizing the aerodynamics <strong>of</strong> the original turbine flow paths [103, 104].<br />

Complex three-dimensional CFD codes have been increasingly used to<br />

evaluate the redesign <strong>of</strong> turbine components such as exhaust hoods and/or<br />

Copyright © 2003 Marcel Dekker, Inc.

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