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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 15g The effect <strong>of</strong> diffusers on pump efficiency.<br />

istics <strong>of</strong> typical centrifugal pump stages with and without vaned diffusers. It<br />

is clearly evident that stage efficiency is increased in the region <strong>of</strong> flow<br />

coefficient that is in proximity to the ‘‘design point.’’ This is the region<br />

where the impeller and diffuser are reasonably well matched. The<br />

performance increase indicated is the result <strong>of</strong> the recovery <strong>of</strong> kinetic<br />

energy in the diffuser flow passages. Also evident from the figure is the<br />

drop<strong>of</strong>f in efficiency compared with the nondiffused pump as we move<br />

farther from the design flow coefficient. This characteristic is the result <strong>of</strong><br />

the mismatch <strong>of</strong> the C vector (the absolute fluid outlet velocity from the<br />

impeller) with the vane angle <strong>of</strong> the diffuser’s leading edge. This leading edge<br />

becomes the source <strong>of</strong> incidence losses, which continue to increase as we<br />

move away from the design flow coefficient, and will eventually lead to<br />

separation (or ‘‘stall’’) at the leading edge. This phenomenon <strong>of</strong>ten leads to a<br />

discontinuity in the pump’s H–Q characteristic at this point. When this<br />

occurs, a certain hysteresis is normally exhibited, and an increase in flow<br />

coefficient beyond the discontinuity is required to recover ‘‘attached’’ flow.<br />

When a vaned diffuser is incorporated into a centrifugal stage, a small<br />

vaneless space is normally provided between the impeller exit and the leading<br />

edge <strong>of</strong> the diffuser. This vaneless space provides for mixing <strong>of</strong> the wakes<br />

generated by the impeller blades and allows the diffuser inlet flow to be<br />

somewhat more uniform. The clearance between impeller and diffuser<br />

generally attempts to balance minimum pressure losses (which are propor-<br />

Copyright © 2003 Marcel Dekker, Inc.

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