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National Advisory Committee for Aeronautics, Technical Notes - Experimental Study of Effect of Vaneless Diffuser Diameter on Diffuser Performance

For vaneless-diffuser diameters of 54, 27, 24, and 20 inches
were studied. by successively cutting down a 54-inch vaneless
diffuser am]. running it in a variable-component rig with a commer-
cial mixed-flow impeller to determine the variations in both com-
pressor and diffuser efficiency with diffuser diameter. Runs were
made at actual impeller tip speeds of 500 to 1300 feet per second
through a range of air flows from open throttle to surge to inves-
tigate the relation between diffuser efficiency and diffuser
diameter under widely varying operating conditions. Total-pressure
surveys were made at several radii throughout the diffuser and these,
, together with the standard. measurements, provided information con-
cerning the magnitude and the location of the important losses in
efficiency through the diffuser.

Substantial variations in over-all compressor performance were
produced by changes in diffuser diameter; over-all compressor effi-
ciency increased with increasing diffuser diameter. In all the
diffusers, losses in efficiency were limited to the entrance and
exit regions, losses in the interior of the diffusers being negli-
gible. Diffuser-exit losses increased from 5 to 15 points as diffuser
dimeter decreased, the losses being approximately inversely propor- '
tional to the square of the diffuser diameter. Diffuser-entrance
losses amounted to about Q.06 'and showed no systematic variation
with diffuser diameter. In general, mean entrance and exit losses
varied only slightly with load coefficient and tip speed.
No important effects of diffuser diameter on impeller effi-
ciency were observed.

The vaneless diffuser, as compared with the vaned diffuser, has
the desirable characteristic of a relatively flat efficiency curve
over a wider operating range but has two serious disadvantages, com-
paratively low peak efficiency and excessive size. Reference 1
describes a method of designing vaneless diffusers of wide operating
range and presents experimental evidence that vaneless diffusers can
be designed to give peak compressor efficiencies that compare favor-
ably with those obtained by using the same impeller and the manu-
facturer's waned diffuser. However, because the diameter of the
vaneless diffuser is 34 inches and that of the vaned diffuser is 17,
a study of the relation between vaneless-diffuser diameter and effi-
ciency was made at the NAGA Cleveland laboratory to determine
whether the size could be reduced to dimensions comparable with
those of the vaned diffuser without incurring prohibitive penalties
in efficiency.