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Handbook of Turbomachinery Second Edition Revised - Ventech!

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deterministic unsteady part arises from rotor–stator interactions and the<br />

rotor motion through the stator field (or vice versa, depending on the<br />

location <strong>of</strong> the reference axis). If this is a multistage turbine, there will also<br />

be unsteady effects from wakes <strong>of</strong> upstream stages, but these will not be<br />

repeating in the same time cycle as the influences from the immediate<br />

upstream airfoil row. To remove the unsteadiness due to upstream stages,<br />

the last <strong>of</strong> the three averaging processes is applied.<br />

Because each new averaging creates new correlation terms, by the time<br />

we have completed the last averaging there are 25 correlation terms in just<br />

the r-momentum equation alone. As in the Reynolds averaging <strong>of</strong> the<br />

Navier–Stokes equations, a model must be provided for each <strong>of</strong> these<br />

correlation terms.<br />

Despite the effort needed to apply models for all the correlation terms,<br />

the finished (fully derived) model can be implemented as a reasonably<br />

efficient computer code. Whereas the analysis <strong>of</strong> a given design may require<br />

several hundred hours with a massively parallel computer for a fully<br />

unsteady calculation, in the same time several hundred runs <strong>of</strong> a code using<br />

the APM could be made. This should certainly be enough to firm up the<br />

design <strong>of</strong> a given machine.<br />

Figure 20 shows some comparisons <strong>of</strong> using average passage<br />

calculations to predict the total pressure and total temperature pr<strong>of</strong>iles<br />

after a LPT stage. (No heat-transfer results seem to be available as yet from<br />

Average Passage Methods.) Looking at the figure, especially when noting<br />

the increment <strong>of</strong> the scale shown, the prediction <strong>of</strong> the shape and magnitude<br />

<strong>of</strong> the pr<strong>of</strong>iles is close to—if not actually superior to—most steady 3D<br />

methods.<br />

How the APM approach will perform in heat-flux calculations is yet to<br />

be tested.<br />

Figure 20 Results <strong>of</strong> APNASA calculations <strong>of</strong> flow through a low pressure<br />

turbine. A high pressure stage is located upstream.<br />

Copyright © 2003 Marcel Dekker, Inc.

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