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https://hdl.handle.net/2142/20292
Description
Title
Air-water transfer at wavy interfaces
Author(s)
Duke, Steve Richard
Issue Date
1996
Doctoral Committee Chair(s)
Hanratty, Thomas J.
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Engineering, Mechanical
Language
eng
Abstract
Gas-water exchange is usually controlled by a thin surface layer in the liquid just beneath the interface. The structure and dynamics of the interface profoundly influence the rate of mass transfer. Of particular importance to environmental and climatical cycles is the large (3 to 5 fold) enhancement of the rate of physical absorption of lightly soluble gases that accompanies wind-generated waves. This thesis relates oxygen absorption rates in stratified flow to the interfacial structure and to the near-interface hydrodynamics of the liquid.
Time-averaged mass transfer rates, two-dimensional liquid-side concentration fields, and interfacial windrow formations were measured during oxygen absorption in stratified wavy flow in a horizontal channel. The complication in making direct measurements is that the surface concentration layer is very thin (0.02 to 0.3 mm thick) and it is beneath a moving interface. A laser-induced fluorescence imaging method is described for capturing oxygen concentration variation and interfacial position for a two-dimensional field. A floating tea tracer method is described for visualizing interfacial streaks (or windrows) that form in convergence zones caused by large circulation flows.
Results are presented for flows that had depths of about 5 mm and interfacial friction velocities, $u\sp{\*},$ of 1.01 to 1.59 cm/s. The water surfaces had flat interfaces, two-dimensional ripple waves, broadcrested waves, and chaotic pebble waves.
The concentration fields show that increases in wave motions caused by increases in $u\sp{\*}$ are accompanied by a thinning of the average layer thickness, an increase in variations of the concentration layer within an image (along the dominant wave), and a decrease in the measured concentration at the interface. Layers were observed with thicknesses that varied from 0.11 to 0.53 mm. No layers extended beyond 0.6 mm, indicating that the bulk flow was well mixed, and, therefore, turbulent. A striking feature of the concentration fields for wavy flows is what appears to be layers of higher concentration that have detached from the surface and moved into the bulk flow.
Windrows were observed on the interface with spacings of 3 cm for broadcrested and 2 cm for chaotic pebble waves.
Results suggest that at least two mechanisms are operative. Langmuir-type circulations, with axes in the direction of mean wind-flow, are present. They exhibit themselves in the concentration fields as variations in the mean depth of the surface concentration-layer from image to image. This behavior is interpreted as streaks which meander in and out of the image sheet. A general thinning of the layer is observed when waves are present. This thinning is characteristic of what would be expected for an increase in turbulence diffusivity. It is not possible at this point to say which of these two distinct enhancements in transport accounts for the 300 to 500% enhancement with waves.
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