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National Advisory Committee for Aeronautics, Technical Notes - Effect of the Reversal of Air Flow Upon the Discharge Coefficient of Durley Orifices

In laying out apparatus for the laboratory testing of
superchargers for aircraft engines at the Langley Memorial
Aeronautical Laboratory, it was found most convenient to
measure the air on the inlet side by drawing the air from
the atmosphere through thin plate orifices at low veloci—
ties into a box and thence through a throttle into a large
reservoir which in turn was in free communication with the
supercharger. The orifice coefficients for this condition
were unknown, as the desired type of orifice had been cal—
brated with the flow from the box into the atmosphere, in—
stead of the reverse direction which would Obtain with this
apparatus. The present experiments were therefore under—
taken in order to obtain information regarding the rela—
tionship between the coefficients for flow in the two di—
rections.

The apparatus used for these experiments is shown di—
agrammatically in Fig. 1. When the flow was from 4 to 1
(see Fig. l), the blower maintained an air pressure of about
5" Hg. in the large tank, which was throttled into the small-
er tank and led into the orifice box through a restricted
passage formed by a short length of 1/2" pipe. The flow of
air through the box was thus freed from the pressure pulsa-
tions that existed at the blower. Runs with the flow from
1 to 4 (see Fig. 1) were made by reversing the blower, and
consequently the air direction, by reversing the direction
of rotation of the driving motor.

The method followed was to take a series-of observa—
tions with the flow from 4 to l, and then to take another
similar series with the direction of air flow reversed and
the orifice plates inverted in their places. With each size
orifice runs were made with the flow in both directions such
that the pressure drops over the reference orifice, and con—
sequently the ratios of pressures on the two sides of this
orifice were very nearly identical.

By inverting the orifices When the direction of flow
was reversed the effects of inaccurate orifices were elimi-
nated, since the air always flowed through each orifice in
the same relative direction. As the orifices were very care—
fully made, this precaution was almost superfluous, but the
change was easily made and it removed all possible suspicion
of error due to any irregularities in the orifices.