inlet is instrumented so that the suction mass flow, the sample mass flow,
the static pressure at the inlet tip, the pressure drop from front to rear
of the diffuser and the turbulence intensity at the exit of the porous
diffuser is measured. A pitot-static tube is deployed near the inlet so
that the local air speed is known. This value is used by the system that
controls the suction flow so that the inlet samples isokinetically.
LTI Flow and Control System Schematic
From TexAQS 2000 Campaign
Sample Flow Control:
The control system maintains isokinetic flow at the
inlet entrance without user supervision.
Computer controlled valves regulate both the suction and the sample
flows. The sample flow is
input by the user and the software and electronics adjust the suction flow
to compensate for variations in the sample flow and in the aircraft’s
true airspeed and altitude. The
sample flow may also be regulated by an aerosol characterizing instrument
that has an internal flow controller.
Material Blockage from Extended Use:
above are periodic flow calibrations of a diffuser. The porous material traps
ambient aerosol causing pore blockage. Thus,
the pressure drop as a function of flow rate through the material increases with
time. The PELTI
experiment comprised twelve flights predominately in the marine boundary
layer and produced an 8.5% increase in the porous material pressure drop.
The TexAQS2000 deployment comprised twelve flights predominately in
the boundary layer over Texas and produced another 11% increase. The porous
material was agitated in an ultrasonic cleaner with a mild detergent and
produced a 4% pressure drop reduction.
Improvements in cleaning techniques are presently being developed.