Affiliations: Department of Chemical Engineering, University of
California Santa Barbara, Santa Barbara, CA 93106, USA
Abstract: Mechanisms governing heat and mass transfer at air-water interfaces
may be studied experimentally and by mean of Direct Numerical Simulations
(DNS). Flow visualizations play a central role in unraveling the mechanisms
that govern these transfer rates. In particular visualizations show that the
flow is organized in large structures. These are sweeps, high-speed (relative
to the interface velocity) fluid traveling toward the interface, and ejections,
low speed fluid moving away from the interface region. It is the frequency with
which these large flow structures refresh the interface that controls mass
transfer. On the liquid side, flow fluctuations in the near-interface region
are relatively unimpeded, so fluid poor in solute can be transported by sweeps
to the interface, and then pick up solute through diffusion before being
carried away again. On the gas side, flow fluctuations in the near-interface
region are strongly impeded, so mass transfer is controlled by sweeps and
ejections, i.e., any events with significant interface-normal velocity. The
frequency, with which these large flow structures are generated, can be
computed from the DNS. Simple parameterizations, based on the mechanisms
discussed above, can be developed and appear to predict mass transfer
velocities in excellent agreement with experimental and numerical results. The
parameterizations capture the effect of capillary waves.
Keywords: mass transfer, capillary waves, DNS, turbulent flow