A Comparison of Propeller and Centrifugal Fans for Circulating the Air in a Wind Tunnel Page: 3 of 12
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N.A.C.A. Technical Note No. 281
to a variable speed compound wound, direct current motor. The
tunnel is of the open jet, single return passage type, having
guide vanes at the corners and a honeycomb at the large square
section of the entrance cone. It is shown diagrammatically in
Figure 1. The cross sections of the air jet and exit cone are
circular, but those of the return passage are rectangular.
The energy ratio, or the ratio of the kinetic energy of the
air passing through the throat per unit time to the electric in-
put power, was obtained for the tunnel with the centrifugal fan
at air speeds of 50, 75, and 100 M.P.H. The electric input to
the motor was measured by means of a standard voltmeter and am-
meter on the armature and series field line, and another pair of
smaller instruments on the shunt field line. The air speed at
the throat was obtained from the pressure difference between the
large square section of the entrance cone and the experiment cham-
ber, as measured by means of an N.A.C.A. micro-manometer and a
--
calibrated static pressure plate.
After the energy ratio had been determined, a record was
made of the dynamic pressure over a short period of time as ob-
tained from a Prandtl type Pitot-static tube and an N.A.C.A. pho-
tographically recording manometer, the fluctuations of the dy-
namic pressure being an indication of the relative smoothness of
flow.
After the above tests had been made the tunnel was modified
by replacing the centrifugal fan by a corner section: having guide2
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Weick, Fred E. A Comparison of Propeller and Centrifugal Fans for Circulating the Air in a Wind Tunnel, report, March 1928; (https://digital.library.unt.edu/ark:/67531/metadc53983/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.