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Handbook of Turbomachinery Second Edition Revised - Ventech!

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The classical approach to satisfying radial equilibrium is to use a free<br />

vortex variation in the vector diagram from the hub to the tip <strong>of</strong> the rotor<br />

blade. A free vortex variation is obtained by holding the product <strong>of</strong> the<br />

radius and tangential velocity constant ðrVy ¼ constantÞ. When this is done,<br />

the axial velocity Vx is invariant with radius. Until the widespread use <strong>of</strong><br />

computers in turbine design, almost all turbines employed free vortex<br />

diagrams due to their simplicity. For preliminary design purposes, the free<br />

vortex diagram is more than satisfactory.<br />

Aside from its simplicity, the free vortex diagram has other<br />

advantages. Holding rVy constant implies that the work extraction is<br />

constant with radius. With Vx constant, the mass flow varies little with<br />

radius. This implies that the mean section vector diagram is an excellent<br />

representation <strong>of</strong> the entire turbine from both a work and mass flow<br />

standpoint.<br />

When using a free vortex distribution, there are two key items to<br />

examine in addition to the mean vector diagram. The hub diagram suffers<br />

from low reaction due to the increase in Vy and should be checked to ensure<br />

at least a zero value <strong>of</strong> reaction. From hub to tip, the reduction in Vy and<br />

increase in U cause a large change in the rotor inlet relative flow angle, with<br />

the rotor tip section tending to overhang the hub section. By choosing a<br />

moderate hub-to-tip radius ratio (if possible), both low hub reaction and<br />

excessive rotor blade twist can be avoided.<br />

For a zero exit swirl vector diagram, some simple relations can be<br />

developed for the allowable mean radius work coefficient and the hub-to-tip<br />

twist <strong>of</strong> the rotor blade. For a zero exit swirl diagram, zero reaction occurs<br />

for a work coefficient <strong>of</strong> 2.0. Using this as an upper limit at the hub, the<br />

work coefficient at mean radius is found from<br />

lm ¼ 2 rh<br />

rm<br />

2<br />

ð58Þ<br />

For a turbine with a hub-to-tip radius ratio <strong>of</strong> 0.7, the maximum work<br />

coefficient at mean radius for impulse conditions at the hub is 1.356. The<br />

deviation in inlet flow angle to the rotor from hub to tip for a free vortex<br />

distribution is given by<br />

Db 1 ¼ b 1h<br />

¼ tan<br />

b 1t<br />

" #<br />

1<br />

fmðrm=rhÞ 1 lmðrm=rhÞ 2<br />

tan<br />

" #<br />

1<br />

fmðrm=rtÞ 1 lmðrm=rtÞ 2<br />

ð59Þ<br />

For a vector diagram with lm ¼ 1:356; rh=rt ¼ 0:7, and<br />

Copyright © 2003 Marcel Dekker, Inc.

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