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by Stephen Ricci and James Saunders, Battelle Science & Technology International, Columbus, OH
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The U.S. Army is building a
large test facility to evaluate
instruments for the optical
detection of chemical and biological
agents from 5 km away. The
facility will disperse chemicals that
optically simulate agents in a
chamber to calibrate and test the
instruments. The facility is large:
the test chamber is 15 x 18 m2 in
cross-section and 135 m long. An
optical port 3 m in diameter is
located at each end of the chamber.
Unfortunately, the optical
ports cannot be conventional windows
because unlimited optical
access is required over a wide
range of wavelengths. The simulants,
while benign compared to
the chemical and biological agents,
must still be contained within the
chamber and only allowed to exit
through filters. Consequently, large
air curtains have been put to use
instead of windows.
Schematic of experimental containment facility (top view), 19 m long and 4 m wide;
the curtains and filtered blower outlets are oriented vertically (into the page) and
are 3 m high; air is drawn in through the dampered opening at left because the
blowers draw more air than the curtains emit
Battelle used CFD and experiments
to design the air curtain containment
system. To represent one
end of the test section, two large
rooms are connected by a 9m long
air curtain chamber, with a large
port between each room and the air
curtain chamber. One room serves
as a dissemination chamber and is
completely enclosed except for the
port to the air curtain chamber. The
opposite room has an opening to
the ambient, simulating the optical
port. The air curtain chamber has
three air curtains. Opposite each
curtain is a large suction blower,
which is designed to pull in the air
curtain flow and surrounding air, filter
it, and discharge it.
In the original arrangement, curtains #1 and #2 oscillate during operation; curtain
#1 (right) flaps to the left at one time (top) and to the right at a later time (bottom)
The experimental objectives
were to demonstrate a five-orderof-
magnitude concentration drop
from the dissemination chamber
to the ambient. The first experiments
were promising because
the required containment was
achieved. However, smoke visualization
showed curtain oscillations
when more than one curtain was
operating, a result that FLUENT
predicted accurately.
Physically, the oscillations are
caused by the instability that develops
when the curtains interact with
different surrounding pressure
regions and with each other. When
curtain #1 fluctuates toward curtain
#2, the pressure rises between
the curtains, and forces them
apart. This is complicated by the
colliding secondary flows from the
air curtains as they fluctuate
toward each other. Curtain #2 is
prevented from swinging away
from curtain #1 because ambient
air is flowing hard into the #2
blower outlet. The pressure rise
then pushes curtain #1 away.
When it does, the pressure drops
between the curtains, drawing the
curtains together once again, and
the oscillation continues.
Based upon the CFD predictions,
Battelle modified the curtain
placement to generate a stable
vortex pattern between the air curtains.
Both model predictions and
experiments in the test facility verified
the required containment and
stable operation. By using the
unsteady modeling capabilities of
FLUENT to offer insight into the
physics of air curtain oscillations, a
design that met the performance
targets could be found in a cost
effective manner.
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