Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Process Engineering, Biotechnology, Nutrition Technology
The turbulence intersities and spectra in water very close to an air interface have been measured with a hot wire anemometer. The turbulence in the water has been induced in three ways: with a stirrer, by a submerged jet, and by flow of a thin film of water over inclined plates, both rough and smooth.We find that the root-mean-square fluctuation velocity Ṽx′ shows for the first system no regular trend with the distance y below the surface, while for the second system Ṽx′ increases markedly as the surface is approached. In water films over the rough plates, Ṽx′ can be as high as four times the shear stress velocity.The turbulence energy spectra close to the surfaces in the first and second systems show (Figure 6) no extended region of slope -5/3, whereas for flow over the plates, there is an appreciable subrange of this slope (Figures 8 and 9).Intensities of turbulence in the surface region are as high as 30 to 50% in the first system, up to 100% in the second system, and 17% over smooth plates and up to 60% over rough plates.These turbulence characteristics can be related to the rates of mass transfer for oxygen absorbing into water by comparing plots of k/D1/2 with the square roots of the different eddy frequencies. The Levich treatment [interpreted by Equation (15)] gives good agreement with the stirred cell and jet results. For the smooth and rough plates, mass transfer depends on eddies intermediate in size between the x directional and the y directional large eddies. In all cases, the Kolmogoroff eddy frequencies are much too high to correlate with mass transfer rates.Further, the energy spectra show that 60 to 80% of the total eddy energy lies in the larger eddies, with only 1% (or less) in the Kolmogoroff range.We conclude that the Prandtl sized eddies, and even larger eddies, determine mass transfer rates at a free surface.Direct measurement of concentration fluctuation frequencies, using platinum microelectrodes, was found to be unsatisfactory because of the poor frequency response of the amplified signals.
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