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Continuous Flow Diffusion
Chambers (CFDC's) at Colorado State University
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Contact: Dr. Paul J. DeMott, Research Scientist (pdemott@lamar.colostate.edu)
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Collaborator: Dr. David C. Rogers, Scientist (dcrogers@raf.atd.ucar.edu)
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Laboratory CFDC
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Airborne CFDC in Wyoming King Air Cabin
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(update 4 February 2002)
Two continuous flow diffusion chambers (CFDC's) were developed at Colorado
State University's Department
of Atmospheric Science during the late 1990's through research grants
sponsored by the National Science Foundation Division
of Atmospheric Sciences. Modifications and improvements have occurred
with additional funding from the National Aeronautics and Space Administration
(Atmospheric Effects
of Aviation Project
, FIRE
Arctic Cloud Experiment, CRYSTAL-FACE
experiment). Many instrument developments were done in collaboration with
Dr. David C. Rogers, formerly of CSU and now at the National Center for
Atmopspheric Research. We continue to collaborate with Dr. Rogers on instrument
developments. Additional descriptions of the aircraft CFDC and measurement
campaigns with this instrument may be found on
Dave's
CFDC page. This page repeats some of the information contained
on Dave's page, but includes some other material, including details on
upgrades performed during Summer 2001.
A continuous flow diffusion chamber is intended to expose aerosol particles
to well-defined conditions of temperature and water vapor content in order
to effect and study phase changes. We developed a continuous flow diffusion
chamber technique for studying ice nucleating aerosol particles in laboratory
experiments and from airborne platforms. This page gives some information
about the airborne and laboratory CFDC's, with plots of the sampling conditions,
illustrations of experimental configurations, and photos of the CFDC's.
A link is provided to summaries of recent experimental campaigns.
Physical Construction, Operating Principles, Computations and Modeling
The continuous flow diffusion chambers (schematic)
are oriented for vertical flow through an annular space. They are constructed
of two cylindrical, thin, ebonized copper walls that are separated by approximately
1.1 cm. The walls of the CFDC are force-cooled
either by circulating coolant through copper tubing coils surrounding the
outer wall and inside the inner wall (laboratory CFDC) or by using these
same coolant coils as evaporators for refrigeration compressor units (aircraft
CFDC). In operation, the walls are coated with ice, achieved by flooding
the chamber with water. An inlet manifold
directs sample air containing aerosol particles into the center of a laminar
flow field where the sample is surrounded on either side by particle-free
sheath air (or N2). By varying the set temperatures of
the two walls, the warm wall provides a vapor source to the cold wall
so that water vapor and temperature fields are created. These fields and
airflow determine the conditions of exposure for the aerosols during their
typical 5 to 20 s residence time in the CFDC. Ice
particles grow to relatively large sizes compared to aerosol particles
and are distinguished from them using an optical particle counter (0.4
to 20 mm) at the base of the CFDC. The
aircraft CFDC transitions to a hydrphobic warm wall surface in the lower
third of the device so that liquid water drops formed at RH>100% will evaporate,
leaving only ice crystals as large particles. The only other physical differences
between the two devices is the fact that the laboratory CFDC is approximately
50% longer, providing additional ice crystal growth time at ambient lab
pressures and the laboratory device has associated equipment
for aerosol generation and preconditioning. An impactor is sometimes
used following the optical counter to collect ice crystals onto specialized
transmission electron microscope (TEM) grids for analysis of the residual
particles. Calculations of air flow, temperature,
and humidity are made assuming steady-state conditions (Rogers,
1988). The
temperature and supersaturation range
are
determined by wall temperatures and air flow. The laboratory CFDC was designed
from the onset (circa 1998) to use a colder temperature
range. During 2001, the refrigeration system for the aircraft
instrument was upgraded to use a two-stage compressor configuration capable
of achieving -65C sample conditions. The profile
of air velocity in the CFDC is based on analytical solutions for
the steady state.We are also looking at using computational
fluid dynamicsto study the air flow, thermodynamics and particle
evolution in the chamber. The data system for the CFDC chambers is based
on a Pentium PC. Acquisition, control and display
software utilizes National Instrument's(NI) LabVIEW graphical development
environment interfaced with an NI Fieldpoint modular distributed I/O system.
Air Flow and Inlet Impactors
Air flow schematic
Previous Installations
Weights
and moments of CFD racks in Wyoming King Air
Airflow
Distribution in NCAR Electra (25k gif) during Lake-ICE/SnowBand project.
CFD
in NCAR Electra during Lake-ICE/SnowBand project.
CFD
in NASA DC-8 for NASA-SUCCESS project.
CFD
in DC-8 high rack, front view;
Rear view
CSU
air sample inlet/outlet on NASA DC-8;
Many air probes during SUCCESS
Miscellaneous Images
Aircraft CFDC with new data and refrigeration systems
Climet optical particle counter and TEM grid
impactor
Instrument-related Publications
Rogers, D.C., 1988: Development of a continuous flow thermal gradient diffusion
chamber for ice nucleation studies. Atmos. Res., 22, 149-181.
Rogers, D.C., P.J. DeMott, S.M. Kreidenweis and Y. Chen, 2001: A continuous
flow diffusion chamber for airborne measurements of ice nuclei, J. Atmos.
Oceanic Technol., 18, 725-741.
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