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