title: Numerical Investigation of Sheath and Aerosol Flows in the Flow Combination Section of a Baron Fiber Classifier.
abstract: The Baron fiber classifier is an instrument used to separate fibers by length. The flow combination section (FCS) of this instrument is an upstream annular region, where an aerosol of uncharged fibers is introduced along with two sheath flows; length separation occurs by dielectrophoresis downstream in the flow classification section. In its current implementation at NIOSH, the instrument is capable of processing only very small quantities of fibers. In order to prepare large quantities of length-separated fibers for toxicological studies, the throughput of the instrument needs to be increased, and hence, higher aerosol flow rates need to be considered. However, higher aerosol flow rates may give rise to flow separation or vortex formation in the FCS, arising from the sudden expansion of the aerosol at the inlet nozzle. The goal of the present investigation is to understand the interaction of the sheath and aerosol flows inside the FCS, using computational fluid dynamics (CFD), and to identify possible limits to increasing aerosol flow rates. Numerical solutions are obtained using an axisymmetric model of the FCS, and solving the Navier-Stokes equations governing these flows; in this study, the aerosol flow is treated purely aerodynamically. Results of computations are presented for four different flow rates. The geometry of the converging outer cylinder, along with the two sheath flows, is effective in preventing vortex formation in the FCS for aerosol-to-sheath flow inlet velocity ratios below ~ 50. For higher aerosol flow rates, recirculation is observed in both inner and outer sheaths. Results for velocity, streamlines, and shear stress are presented.
authors.affiliation: Department of Mechanical and Materials Engineering, College of Engineering and Applied Science, University of Cincinnati, Cincinnati, Ohio 45221.
authors.forename: Prahit
authors.initials: P
authors.lastname: Dubey
authors.affiliation: Department of Mechanical and Materials Engineering, College of Engineering and Applied Science, University of Cincinnati, Cincinnati, Ohio 45221.
authors.forename: Urmila
authors.initials: U
authors.lastname: Ghia
authors.affiliation: Chemical Exposure and Monitoring Branch (CEMB), Division of Applied Research and Technology (DART), National Institute for Occupational Safety and Health (NIOSH), Centers for Disease Control and Prevention (CDC), 4676 Columbia Parkway, Cincinnati, Ohio 45226.
authors.forename: Leonid A
authors.initials: LA
authors.lastname: Turkevich
keywords: Aerosol; Baron Fiber Classifier; CFD; Fiber; Sheath Flow