Hydrodynamic characteristics and mass transfermechanism near free interface

Abstract

Mass transfer across the free interface was characterized by using a single eddy with random surface renewal. The velocity fluctuations in the immediate vicinity of free interface were measured with a hot film anemometer. And mass transfer rates and eddy exposure times were analyzed by using the method of a simple deterministic approach. The velocity scale, the length scale, and the eddy exposure time of each single eddy passing a fixed location close to the interface were directly extracted from the fluctuating raw data and compared with those obtained by the statistical approach. The analysis of bulk concentration measurements demonstrated that even though the diffusion coefficient was strongly dependent on the concentration of solute the interfacial mass transfer rate remained almost constant. Therefore, the diffusion coefficient that should be used in correlating mass transfer data should correspond to the interfacial composition. The very small eddies were predicted too high to compare with mass transfer rates obtained by concentration measurements. The mass transfer rates were mainly contributed by the Pandtl sized eddies and even larger eddies, i.e., the small x-directional movements in the surface were less important in mass transfer than were the larger movements which relate to surface renewal. This conclusion was also found in the energy spectra. The turbulence energy spectra close to the surface showed no extended region of slope -5/3. Both the length scale and the velocity scale of eddites near the interface took part in the mass transfer processes. Through the eddy length scale distribution, it was shown that eddy length scales having the major portion were not changed although the turbulence intensities were increased. The portion of large velocity scale was, however, increased with Reynolds numbers. The contribution of eddies with small exposure time was increased as the liquid became more turbulent. It was shown that the eddy exposure time dist...