Evaporation from Air-Water Interfaces at Very Low Grashof Numbers
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Evaporation from Air-Water Interfaces at Very Low Grashof Numbers


The physics of air-water interface transport processes under very low mixing conditions is still poorly characterized due to the difficulty in balancing the combined heat and mass transfer mechanisms at play. Such processes are ubiquitous in lab facilities, biological systems and in a vast number of engineering applications. Correlations for mass transfer rates at very low Grashof numbers in weakly convective flows are not readily available, which often leads to incorrect estimations of evaporation rates derived from simplified 1-D models. This work investigates a number of fundamental free water-evaporation processes at very low Grashof (Gr) numbers.

First, the free evaporation from a small-scale circular pool at Gr numbers under dominant downward flow is examined. Experiments performed at normal conditions of temperature and pressure (NCTP) and for a wide range of relative humidity values are described and compared to numerical simulations. A downward thermally-induced flow originated at the rim of the pool overcomes the concentration-induced buoyancy, consequently forcing the far-stream dry air to descend into the lower temperature air-water interface.Experimental results show that a somewhat stable steady-state recirculation zone near the air-water interface develops for Gr numbers greater than or equal to 50. The Sherwood number (Sh) for this geometry scales with Gr^1/4 for drier free-stream boundary conditions, while it approaches a constant value for smaller mass transfer potentials (i.e., wetter environments). A Sh number correlation as a function of $Gr$ number is proposed, covering Gr numbers from 10 to 10^5.

The mass transport dynamics is also analyzed for free evaporation from open tubes. The mass transport results in complex patterns due to the interaction between the vertical walls and the buoyant flow. Evaporation undergoes a multi-regime process where, depending on the driving potential, stable convective cells develop inside the tube. These convective cells enhance the evaporation rate in most circumstances. The dependence of the diffusion-driven and convection-driven regimes as functions of the geometrical aspect ratio of the tube is studied. A new Sh number correlation valid for Gr numbers ranging from 50 to 4,000 is proposed. This correlation, which is nearly invariant with temperature, expresses the relationship between the Sh and Gr numbers for varying aspect ratios.

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