Evaporative heat flux at the air-water interface is a quantity of importance that is required in the design of engineering equipment such as a solar distillation system. The heat flux at the water surface depends on the temperature gradient set-up in water and in air above, along with evaporation rates of water, and hence the humidity gradient developed in the air-gap. In the present study, a rectangular cavity partially-filled with initially hot water is allowed to cool, both by heat transfer to a cooler surface and by evaporation. The cooling rate of the hot water body is studied in time to determine the resulting interfacial heat flux. In experiments, a Mach-Zehnder interferometer is utilized to record isotherms as well as temperature profiles across the cross-section of water and as a function of time. Convection patterns formed in water by surface cooling could arise from buoyancy as well as Marangoni convection. These patterns are simulated numerically in a two-layer air-water system and compared against experiments. The typical values of Rayleigh number and Marangoni number are of order 10⁶ and 10³ respectively. Results show that the evaporation process initially generates a diffusion layer, followed by buoyancy-driven convection in the body of water and is diffusion-dominated once again at later times. The value of estimated evaporative heat flux is small during thermal diffusion in water and higher for convection, when compared with the spatiotemporally averaged correlations reported in the solar still literature.
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