Isothermal and Near-Isothermal Free Evaporation of Water from Open Tubes in Air

The physics of air-water interface transport processes under very low mixing conditions is still poorly characterized. Consequently, correlations for mass transfer rates at very low Grashof numbers in weakly convective flows are not readily available, which often leads incorrect estimations of evaporation rates that are derived from simplified 1-D models. Here we investigate the evaporation from open tubes that results in complex massline patterns due to the interaction of the vertical walls with the buoyant flow in near isothermal conditions. Using numerical simulations validated by experimental data for typical Normal Conditions of Temperature and Pressure (NCTP), we analyze the water vapor transport dynamics for open tubes with aspect ratios ranging from 2 to 11 at the temperature of 290 K and 310 K, and relative humidity values from 0 to 99% , all at 1 atm. We show the dependence of the diffusion-driven and convection-driven processes as functions of the geometrical aspect ratio of the tubes. We propose a new Sherwood number correlations valid for both isothermal and near-isothermal processes in the range of Grashof numbers from 50 to 4,0 0 0. This correlation, which is optimized for air-water interfaces and is nearly invariant with temperatures near NCTP, expresses the Sherwood and Grashof numbers for varying aspect ratios. The proposed correlation estimates evaporation rates within 5% of values obtained from 3-D numerical simulations for the entire range of Grashof and Sherwood numbers under study.(c) 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )