The determination of eddy mass diffusivities for the air‐water system in a wetted‐wall column

Abstract

AbstractEddy mass diffusivities as a function of radial position for different Reynolds numbers have been determined for air flowing countercurrently to water in a wetted‐wall column. The ratios of the eddy mass diffusivity to the eddy viscosity also were determined. The wetted‐wall column was constructed from Pyrex glass with an I.D. of 2.75 in. and a wetted section 39 in. in length. An approach section was constructed from the same material and of adequate length to insure full development of the velocity profile. The gas phase Reynolds numbers varied from 3,200 to 54,300. The liquid phase flow was laminar. The concentration and velocity profiles of the air stream were measured experimentally and bulk temperatures of all streams were obtained. A probe was developed to measure the concentration profiles. The probe consisted of platinum wires wrapped in a helix around a glass capillary tube on which a thin layer of lithium chloride in polyvinyl alcohol had been deposited. The resistance of the probe was a reliable indication of the water vapor concentration in the air stream. The probe was calibrated and used for all concentration measurements. Eddy mass diffusivities were evaluated from a solution of the steady state mass diffusion equation by means of a digital computer with experimental velocity and concentration profiles. Qualitatively, the eddy diffusivity profiles are similar to those which have been obtained for heat and momentum transfer. When these profiles are used with the diffusion expression, they are capable of accurate evaluation of the concentration profile. The ratio of the eddy mass diffusivity to the eddy viscosity was determined and was found to be less than 1.0 over most of the cross section, but the ratio increased to values greater than 1.0 near the center and near the walls of the column.