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National Advisory Committee for Aeronautics, Report - Effect of Screens in Wide Angle Diffusers

An experimental investigation at low airspeeds was made of
the filling eject obserred when a screen or similar resistance is
placed across a difuser. The filling eject is found to be real in
that screens can prevent separation or restore separated flow in
difi'users eren of extreme dirergence and to depend principally on
screen location and pressure-drop coeficient of the screen.
Results are git-en for three diflerent difusers of circular cross
section with a variety of screen arrangements. Ejects of single
screens and multiple screens are shown. The mechanics of the
filling eject is explained, and possible eficiencies are discussed.
Results of arrangements of multiple screens in wide-angle
difiusers are given to show a possible application to damping
screens as used in wind tunnels to reduce turbulence.

An investigation of diffuser-screen combinations was under-
taken at- the National Bureau of Standards under the spon-
sorship and with the financial assistance of the National
Advisory Committee for Aeronautics in an effort to clarify
the so-called “filling effect” commonly observed when a
screen or similar resistance is placed at the mouth of a
wide-angle subsonic diffuser.

A wide-angle difi‘user is defined herein as one in which the
cross-sectional area increases so rapidly in the direction of
flow that separation is to be expected. Under ordinary condi-
tions this would include all conical .difi’users with walls
diverging with a total included angle greater than about 8°.

About the time that damping screens for reducing tur—
bulence were found to be of use in the larger wind tunnels, the
NACA adopted a rapidly expanding section just ahead of a
screen to reduce the loss through the screen. It appears to
have been this use of a wide-angle difi'user followed by a
screen that first aroused general curiosity and some skep-
ticism about the possibility of filling difi'users by this means.
Intuitively it could be seen that a screen would have a
tendency to spread the flow by its damming effect, but the
details of the effect were not clear. As far as is known, the
first quantitative study of the efl’ect was made by McLellan
and Nichols (reference 1), who were concerned with the
practical advantages of wide-angle diffusers just ahead of
heat exchangers.