Background:
Sampling of aerosols from aircraft is difficult primarily because of the
large difference between the velocity of the aircraft and the velocity of
the air that enters a typical instrument or sample collection device. The
range of aircraft velocities is from about 80 m/s to over 200 m/s. Aerosol
instruments and filter collectors typically accept velocities on the order
of 10 m/s. It is often necessary to locate instruments inside the aircraft
forcing the inclusion of bends in the sample lines. Bends can place limits
on the air velocities in the lines, since excessive velocities can lead to
large losses of particles in bends. Thus it is often necessary to slow the
flow of the air by factors of 20 or more before subjecting the sample
stream to a bend or entering an instrument. Done incorrectly, this slowing
can lead to significant aerosol losses due to turbulent deposition (Huebert
et al., 1990).
The University of Denver Low Turbulence Inlet permits the slowing of
aerosol samples from approximately 100 m/s, which is the true air speed of
the aircraft, to a few meters per second without subjecting the sample to
turbulent deposition in the diffuser. This turbulence reduction is
achieved through boundary layer suction in the diffuser. The slowed sample
can then be transported to various instruments and samplers in the
aircraft with minimal losses. The LTI enhances the population of particles
with aerodynamic diameters larger than a few microns by a known amount
depending on aerodynamic diameter. The LTI has been shown in tests (Huebert
et al., 2000) to deliver more supermicron particles than other inlet
designs because it avoids turbulent deposition.
The fundamental concepts underlying the DU Low Turbulence Inlet can be see
in the figure below. Air enters the inlet which is aligned as
closely as possible with the local wind vector. The leading edge of
the inlet is elliptical in shape which permits the inlet to function when
the alignment between the local wind vector and the inlet diffuser axis
vary in normal research flight. The flows are adjusted so that the
mass flow velocity in the throat of the inlet nearly equals that of the
free stream velocity. This sampling condition is referred to as
isokinetic sampling. Deviations from isokinetic sampling can be
compensated for in data reduction by correcting for enhancement or
reduction of concentration resulting from mismatched flows. Most of
the entering flow is pulled through the porous diffuser cone. This suction
suppresses the formation of the boundary layer and the generation of
turbulence. Roughly 20% of the entering flow exits the rear of the
diffuser. This laminar flow has not experienced the turbulence that
accompanies slowing in solid diffusers and causes particles to be
deposited on the walls. The mass mixing ratio of supermicron
particles is probably enhanced in the sample flow as compared with the
ambient. This inertial enhancement results from the sharp bending of
streamlines due to the expansion and the suction flow in the diffuser and
the inability of the large particles to follow the streamlines. The
resulting enhancements can be predicted since the flow is laminar and the
particles do not come in contact with the wall. The resulting
corrections do not depend upon assumptions concerning particle transport
in turbulent flow nor upon assumptions concerning particle bounce.
More
Information:
LTI
FUNCTION LTI
DESIGN
LTI MODELING
LTI
RESULTS
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