As part of the technology utilization program at the NASA Lewis
Research Center, efforts are underway to transfer aerospace technologies
to new areas of practical application. One such effort involves
using advanced computational fluid dynamics (CFD) codes for turbomachinery
to analyze the internal fluid dynamics of low-speed fans and blowers.
This year, the Kirby Company in Cleveland, Ohio, approached NASA
with a request for technologies that could help them improve their
vacuum cleaners. Of particular interest to Kirby is the high-frequency
blade-passing noise generation of their vacuum cleaner fan at
low airflow rates.
To assess the current capability, Lewis researchers studied two
Kirby centrifugal fan impellers, a standard (baseline) impeller
and a modified (tapered) impeller, analyzing them aerodynamically
with a three-dimensional viscous turbomachinery CFD code (RVC3D)
that was developed by R.V. Chima at Lewis. Both impellers were
simulated at the same airflow (102 ft3/min)
and at slightly different rotational speeds corresponding to the
measured differences in Kirby tests performed on the two impellers.
The Kirby test data indicated that the tapered impeller generated
significantly less noise at all flow rates, but especially at
very low flow rates. The main goals of the CFD simulations were
to investigate whether substantial differences could be seen in
the computed results for a relatively high airflow rate and to
determine if those differences suggested the observed trend in
noise reduction.
Contour plots of the computed (absolute) total-pressure fields
just downstream of the impeller exit for the baseline and tapered
impellers are shown in the figures. A complete inversion of a
high total-pressure region is apparent from the results. In particular,
the baseline impeller produces higher total-pressures near the
shroud (the top of the passage in the top figure), whereas the
tapered impeller produces higher total-pressures near the hub
(bottom of the passage in the bottom figure). These differences
are much larger than expected since there is only a modest change
in the impeller geometry between the baseline and tapered configurations.
In view of the spanwise extent and general character of the circumferential
(left-to-right) variations in total-pressure, we concluded that
the CFD results do allow a reasonable inference about relative
noise generation. That is, compared with the baseline impeller,
the tapered impeller should generate less noise.
Previous articleLast updated May 9, 1997
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