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Handbook of Turbomachinery Second Edition Revised - Ventech!

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fluctuations seen by a rotating blade as it passes each stationary blade or<br />

traverses each stationary blade pitch They are among the most pervasive<br />

sources <strong>of</strong> synchronous excitation in steam turbines. They occur at the<br />

nozzle passing frequency and its harmonics. Key contributors to nozzlewake<br />

excitations occur from [46]<br />

Viscous wake caused by the presence <strong>of</strong> a boundary layer.<br />

Trailing-edge thickness. A turbulent wake is formed by the passage <strong>of</strong><br />

flow past a stationary blade. There is a design trade<strong>of</strong>f pertaining to<br />

the thickness <strong>of</strong> the blade’s trailing edge. Too thick leads to<br />

increased wake, while too thin is difficult to manufacture and has<br />

unacceptable strength levels.<br />

Separation wake caused by premature separation <strong>of</strong> the flow before<br />

reaching the trailing edge. These wakes can be caused by inadequate<br />

design or by operating at <strong>of</strong>f-design conditions and can lead to a<br />

larger wake with potentially large flow fluctuations.<br />

Nonuniform deposits on blade airfoil surfaces can trip boundary<br />

layers.<br />

Shock waves and expansion waves. Shock waves can be formed when<br />

supersonic velocities are reached in the stationary blade passages.<br />

Another flow condition that can result in an unsteady flow is the<br />

formation <strong>of</strong> a choked, underexpanded jet. This latter condition can<br />

cause excitation in the passing rotating blade as well as disturb the<br />

flow through the rotating blade rows.<br />

Wave reinforcement. It has been proposed that reinforcement <strong>of</strong> waves<br />

bouncing between rotating and stationary blades in the same stage<br />

may occur leading to increased wave amplitudes and corresponding<br />

blade vibrations [66].<br />

Moisture impingement. The impingement <strong>of</strong> moisture coming <strong>of</strong>f<br />

stationary blades and hitting the rotating blades can result in forces<br />

on LP blades (as well as erosion) and thus induce vibration.<br />

Of these sources the first three (viscous wakes, trailing-edge thickness, and<br />

separation wakes) affect HP, IP, and LP turbine blades. The last three<br />

(shock waves, wave reinforcement, and moisture impingement) occur in LP<br />

turbines.<br />

Figure 40 shows one example <strong>of</strong> these types <strong>of</strong> nonuniform flows. The<br />

figure illustrates the variation <strong>of</strong> the steam force across the discharge from a<br />

fixed blade row that will result in nonuniform loading on the passing<br />

rotating blades.<br />

2. Spatially nonuniform flows caused by geometric asymmetries.<br />

This category includes nonuniform pressures, velocities, or angles <strong>of</strong> flow<br />

Copyright © 2003 Marcel Dekker, Inc.

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