Experiments were performed to measure the mass transfer coefficients for natural convection evaporation from a horizontal water layer. Owing to the latent heat requirements of the evaporation process, the temperature of the water layer is depressed relative to the ambient temperature. This gives rise to a buoyancy which overpowers the oppositely directed buoyancy associated with the concentration gradient of the water vapor. The experiments encompassed two orders of magnitude in the Rayleigh number and, in addition, the effects of two geometrical parameters were investigated. One of these is the size of a horizontal annular frame which surrounds the water containment pan, while the other is the step caused by the difference between the water level and the height of the pan sidewall. Both the presence of the frame and of the step decrease the mass transfer coefficient. Narrow frames give rise to a sharp decrease, but further enlargements of frame size have little additional effect. The step-height effect was found to be independent of both the Rayleigh number and the frame size, enabling a universal correlation. Furthermore, for all of the investigated steps and frames, Sh ∼Ra0.205.

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