(532e) Drag Correlation of Drop Motion on Fibers

The objective of this paper is to correlate a drag coefficient to the Reynolds number for axial motion of barrel drops on fibers. This work includes effects of vibration induced motion of droplets and coalescence. The study of motion of drops is important to understand the drainage behavior of droplets. Drainage of liquid helps to eliminate moisture from media samples before applying thermal energy and hence reducing the drying cost.

A significant amount of literature describes the mechanisms of droplet capture, coalescence, and drainage from filter media and models are developed at a scale that accounts for the liquid held in the filter through averaged parameters such as saturation. The study of drop motion on fibers is of scientific and economic interest for many possible applications like printing, coatings, drug delivery and release, and filters to remove or neutralize harmful chemicals or particulates from air streams.

Gas convection induced drop motion in fibrous materials occurs in coalescing filters, clothes dryers, textile manufacturing, convection ovens, and dewatering of filter cakes. Droplet removal can significantly reduce drying costs by reducing the free moisture contained in fibrous materials prior to applying thermal drying techniques. An experiment is designed to produce couette flow of air enabling the drop motion. Couette flow is produced with a rotating surface for air flow past the fiber and drops to produce linear laminar air velocity field.

The couette flow velocity profile is produced for Reynolds number less than 2000 to ensure laminar flow. The channel is 1 mm deep with the fiber located at the centerline of the channel. Drops are attached to the fibers by exposing the fibers to liquid aerosols. FLUENT software is used to numerically model the couette flow device to evaluate the air velocity profile to ensure the velocity field is in laminar flow.

Different liquids and fiber materials are used having different wetting properties. The drops are observed to attach and move on the fibers as prolate spheroids (barrel drops) attached symmetrically around the fiber. The drops are observed through a microscope to move or not move in a stochastic random behavior under identical conditions. In the presence of low frequency vibrations the drops are observed to move more consistently. The vibrations are induced into the fiber and drops through a speaker in contact with the fiber sample holder.

The experimental drag coefficient verses Reynolds number data are compared with a simple model from transport theory and they are compared with results from a computational model solved by FLUENT software. The results show the simple model is inadequate to predict the drag coefficient though it does show the same general trend. The theoretical 1-D model and the FLUENT 3-D model show reasonable agreement.