Continuous Flow Diffusion Chambers (CFDC's) at Colorado State University
Contact: Dr. Paul J. DeMott, Research Scientist (pdemott@lamar.colostate.edu)
Collaborator: Dr. David C. Rogers, Scientist (dcrogers@ucar.edu)

Laboratory CFDC	Airborne CFDC in Wyoming King Air Cabin

(update 15 January 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.

Previous and Current Field and Laboratory Programs using the CFDC's

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