Although evaporation is considered to be a surface phenomenon, the rate of molecular transport across a liquid–vapor boundary is strongly dependent on the coupled fluid dynamics and heat transfer in the bulk fluids. Recent experimental thermocouple measurements of the temperature field near the interface of evaporating water into its vapor have begun to show the role of heat transfer in evaporation. However, the role of fluid dynamics has not been explored sufficiently. Here, we have developed a mathematical model to describe the coupling of the heat, mass, and momentum transfer in the fluids with the transport phenomena at the interface. The model was used to understand the experimentally obtained velocity field in the liquid and temperature profiles in the liquid and vapor, in evaporation from a concave meniscus for various vacuum pressures. By using the model, we have shown that an opposing buoyancy flow suppressed the thermocapillary flow in the liquid during evaporation at low pressures in our experi...

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Experimental and Numerical Study of the Evaporation of Water at Low Pressures.

This work presents a study on the evaporation of a volatile liquid droplet placed on a substrate and surrounded by air. We consider in this work the case of a pure liquid droplet (water, ethanol and butanol), and the case of a binary mixture droplet (water-ethanol and water-butanol). The aim of this work is to study the effect of the volatility of each component within a pure or binary droplet on the evaporation kinetics. The volatility of a liquid is considered as the ability of a substance to evaporate. One liquid is more volatile than another is the one with the lowest boiling temperature. We noticed that for pure liquid droplets, the volatility of the liquid considerably influences its life and cooling effect. After evaporation, the more volatile the liquid, the shorter the life of the droplet and the greater its cooling. The results of the simulation were compared and validated by the literature. Experimental results are used for validation and interpretation of the evaporation of water-butanol binary droplet. The behavior of the butanol in the mixture is different to that when it is pure. Despite its low volatility compared to water, strongly diluted in a water-based droplet, butanol evaporates in a preferential way. In fact, in the diluted mixture, the fraction of butanol evaporates first and is exhausted before that of water. The binary droplet tends to become a pure water droplet. We also show that, the same as in the case of the water-ethanol, the addition of butanol in water accelerates evaporation and decreases the droplet’s life.

Study of the evaporation kinetics of pure and binary droplets: volatility effect

Recently interest has arisen in the use of so-call ed self-rewetting mixtures for micro-scale heat transfer systems. Such fluids, in which the surface tension can increase with increasing temperature, are expected to offer superior evaporative cooling perf ormance by extending the region of operation before dryout of the heated surface sets in. Whilst improved p rformance has been shown in some practical situat ions using these fluids, it is not entirely clear as to the mechanism of such improvements. We have studied the flow within evaporating sessile drops of 1-pentanol-water mixtures using micro-PIV and have observed three stages in the evaporation p rocess. During the first stage there appears to be a single toroidal vortex with flow inwards along the base of the drop. The vortex only occupies the central reg ion of the drop and appears to pulsate, reducing in size d uring evaporation. This is followed by a second tra nsition stage to a third stage in which the flow is directe d radially outward, as observed by us for pure wate r droplet evaporation and in the latter stages of ethanol/wat er drop evaporation. Temperature measurements, usin g IR thermography suggest that the initial stage of evap oration may be controlled by thermal Marangoni effe cts as opposed to the concentration driven Marangoni flows po tulated for ethanol-water mixtures.

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Flow measurement using micro-PIV and related temperature distributions within evaporating sessile drops of self-rewetting mixtures of 1-pentanol and water

Johan Ebeling

Papers

Flow measurement using micro-PIV and related temperature distributions within evaporating sessile drops of self-rewetting mixtures of 1-pentanol and water