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

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

where Rth is the Reynolds number based on the hydraulic diameter at the<br />

blade passage minimum area (referred to as the throat) defined as<br />

Rth ¼ r oVo<br />

m o<br />

2hs cosðaoÞ<br />

h þ s cosðaoÞ<br />

ð53Þ<br />

where h is the blade height and s is the spacing between the blades at the<br />

mean radius. The blade axial chord is identified by cx. In both Eqs. (52) and<br />

(53), the subscript ‘‘o’’ refers to blade-row outlet conditions, either stator or<br />

rotor (for the rotor, the absolute velocity V is replaced by the relative<br />

velocity W, standard practice for all ‘‘blade-row’’ relations). The reference<br />

loss coefficient x ref is a function <strong>of</strong> blade turning and thickness and can be<br />

found in Fig. 13. Compared to Smith’s chart, this correlation requires more<br />

knowledge <strong>of</strong> the turbine geometry, but no more than would be required in<br />

a conceptual turbine design. The losses predicted by this method are only<br />

valid for the optimum blade chord-to-spacing ratio and for zero incidence.<br />

Tip clearance losses must also be added in the final determination <strong>of</strong> turbine<br />

efficiency. Like Smith’s chart, this correlation results in a total-to-total<br />

efficiency for the turbine.<br />

The optimum value <strong>of</strong> blade chord-to-spacing ratio can be found using<br />

the definition <strong>of</strong> the Zweifel coefficient [17]:<br />

z ¼ 2 cos ao<br />

sinðai aoÞ ð54Þ<br />

cx=s cos ai<br />

where the subscript ‘‘i’’ refers to blade-row inlet. Zweifel [17] states that<br />

optimum solidity ðcx=sÞ occurs when z ¼ 0:8.<br />

Tip clearance losses are caused by flow leakage through the gap<br />

between the turbine blade and the stationary shroud. This flow does not get<br />

turned by the turbine blade; so it does not result in work extraction. In<br />

addition, the flow through the clearance region causes a reduction <strong>of</strong> the<br />

pressure loading across the blade tip, further reducing the turbine efficiency.<br />

The leakage flow is primarily controlled by the radial clearance, but is also<br />

affected by the geometry <strong>of</strong> the shroud and the blade reaction. Leakage<br />

effects can be reduced by attaching a shroud to the turbine blade tips, which<br />

eliminates the tip unloading phenomenon. For preliminary design purposes,<br />

the tip clearance loss for unshrouded turbine wheels can be approximated by<br />

Z<br />

Z zc<br />

¼ 1 Kc<br />

rtip<br />

rmean<br />

Copyright © 2003 Marcel Dekker, Inc.<br />

cr<br />

h<br />

ð55Þ

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

Saved successfully!

Ooh no, something went wrong!