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National Advisory Committee for Aeronautics, Technical Notes - A Comparison of Propeller and Centrifugal Fans for Circulating the Air in a Wind Tunnel

The tests described in this paper afford a direct comparison
of the efficiency and smoothness of flow obtained with propeller
fan and multiblade centrifugal fan drives in the same wind-tunnel
The propeller fan was found to be superior to the centrifugal fan
in that the efficiency was about twice as great, and the flow is
much smoother.

While aircraft propeller type fans are used in most wind
tunnels for circulating the air, in some cases multiblade cen—
trifugal fans are employed. The tests herein described give a
direct comparison, as regards efficiency and smoothness of flpwm.
produced, between a propeller and a centrifugal type fan in the
same wind tunnel.

The N.A.C.A. six—inch wind tunnel at the Langley Memorial
Aeronautical Laboratory Was used for the tests. As originally
built, the air in this tunnel was circulated by means of a Sturte—
vant multivane centrifugal fan, design 5, size 8, direct connected
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 l. 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.