10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Measurement of the Dynamics of Inertial, Rigid Nylon Fibers in Isotropic Air Turbulence
SOFIA KUPERMAN, Lilach Sabban, René van Hout, Technion – Israel Institute of Technology
Abstract Number: 399 Working Group: Remote/Regional Atmospheric Aerosol
Abstract Atmospheric dispersion of fibers is of importance in a variety of environmental and health related studies. For example, asbestos fibers may remain suspended in the air for a long time and when inhaled may get trapped in the lungs and cause cancer. Also, atmospheric ice crystals may resemble fibers and their orientation affects the earth’s albedo.
Very few experimental data exist regarding inertial fiber dispersion in isotropic turbulent flow. Most published experimental and numerical studies investigated neutrally buoyant (inertialess) particles. These studies have found that fibres preferentially align with the stretching direction of the fluid. Lately, numerical simulations have used Jeffery’s equation incorporating inertia and turbulence to study fiber dynamics. Here, rigid, inertial fibers in isotropic air turbulence were tracked using two-orthogonal digital inline holographic cinematography. The main goal of the present measurements is to gain insight into fiber length and inertial effects on their rotational and translational dynamics.
The present measurements were conducted in a turbulence chamber (40×40×40 cm3). Isotropic turbulence was created by air jets (random frequency and phase) generated by eight woofers located at the vertices of the chamber. The isotropic turbulence was characterized by a Taylor microscale Reynolds number of Reλ = 130, and Kolmogorov length and time scales of ηk = 177 μm and τk = 2.08 ms, respectively. The fibers were released from the top of the chamber using a specially designed dispenser. Care was taken to ensure that the fibers were not released in clumps. A two orthogonal view, holographic cinematography system was used to track fibers in the volume of interest (VOI, 17×17×17 mm3) at the intersection of the cameras’ fields of view, at the center of the chamber. The holographic cinematography system consisted of two high-speed, lensless cameras (Photron Ultima APX, 1024×1024 pixels at 2 kHz), a high-speed pulsed laser (Crystalaser, 527 nm, 10 μJ/pulse at 2 kHz), a spatial filter, collimating lens, beam splitter and mirrors. The collimated laser beams and the cameras were aligned, and to ensure time-resolved measurements, each camera acquired holograms at 2 kHz, i.e. a time separation of 0.5ms < 0.25τk. The holograms were subsequently digitally reconstructed in order to determine the time-resolved fiber tracks and their 3D orientation. Several different batches of nylon fibers with diameters and lengths ranging between 11 < D < 55 μm and 0.5 < L < 4 mm, respectively, were investigated. The corresponding Stokes number range, aspect ratios and length ratios were, 1.7 < St < 25, 4.3 < L/D < 20 and 4.3 < L/ηk < 23, respectively. These measurements provide insight into the effect of inertia and aspect ratio on the mean-squared rotation rate of the fibres as well as their translational velocities, important for validating fiber dispersion models.