Determination of evaporation rate of warm water placed inside a partially-filled top cooled enclosure

Abstract

Evaporation is an important step in clean water production technologies such as solar desalination. Here, a warm body of impure water evaporates in a differentially heated cavity, the vapour condenses over a cooler surface and the condensate is collected as salt-free water. While buoyancy-driven convection is set-up in air, the water body is subjected to evaporative cooling and buoyancy-driven flow is setup in the water volume as well. Evaporation-induced buoyant flow in water is imaged in the present study using a Mach-Zehnder interferometer in the infinite and wedge fringe settings. The corresponding interfacial heat flux, that includes the contribution from evaporation, is evaluated by analyzing the interferograms. From a modeling perspective, the evaporation rate is the central quantity of importance. It is determined in the present study using an ab initio model for an air-water system that solves mass, momentum, energy, and moisture transport equations with convective flow and jump conditions in the heat flux at the interface. The instantaneous mass and energy fluxes thus determined have also been compared with the kinetic theory model and Dunkle's correlation that is frequently adopted in the design of solar stills. Among the three models, the ab initio approach shows close agreement with experiments. The match with Dunkle's correlation is less satisfactory. The ab initio model closely follows the flow transitions in the air-phase and provides a justification for a good match with the experimentally derived instantaneous evaporative heat and mass fluxes. The ab initio approach is further adopted to parametrically study the effect of initial water temperature, top surface temperature and water depth on the evaporation rate.