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

Abstract

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 experiments. As such, in the absence of thermocapillary convection, the evaporation is controlled by heat transfer to the interface, and the predicted behavior of the system is independent of choosing between the existing theoretical expressions for evaporation flux. Furthermore, we investigated the temperature discontinuity at the interface and confirmed that the discontinuity strongly depends on the heat flux from the vapor side, which depends on the geometrical shape of the interface.

Three-Dimensional Reconstruction of Evaporation-Induced Instabilities Using Volumetric Scanning Particle Image Velocimetry

“…Note the existence of the red layers below the green and blue layers below the interface. This phenomenon has also been observed in two dimensions by Song and Nobes [29] who performed planar PIV on the center plane of an evaporating water in the same cuvette used in this study and also by Li and Yoda [49] in the presence of buoyancy driven convection in a rectangular cavity and in a cylindrical tube in which 2D PIV was performed at the center of the tube during evaporation of water at low pressures [41]. However, the maximum velocity was detected to be exactly at the interface in the evaporation of methanol inside various size microtubes [33].…”

“…To prepare the suspension for PIV experiments, one drop of high-concentration particle solution, which contained 2% V/V of 2.0 μm solid fluorescent microspheres (Fluoro-Max R0200, Thermo Scientific, Waltham, MA, USA ), was distributed in 40 mL of deionized water and mixed gently to achieve a uniform dilute suspension. For similar experiments in a cylindrical tube (Reference [41]), we showed that using a 100 times more dilute suspension did not change the flow pattern and the behavior of particle motions immediately below the interface. Therefore, we conclude that the measured velocity field does not change with concentration within the range of particle concentration that is commonly used in PIV experiments.…”

“…It was found that the maximum velocities in the flow occurred close to but not exactly at the liquid-vapor interface as would be expected from a surface tension driven flow. The results presented in this paper provide a more detailed description of the flow compared to the previous 2D measurements in evaporating liquids and may be useful to help better understand the phenomena involved in this complicated process as they were in References [22,23,41]. Also, they promise the feasibility of using the continuity equation combined with planar PIV/PTV to be a powerful tool for 3D velocity measurements in applications where current techniques are difficult to be utilized or where a high resolution of the out-of-plane velocity is required to capture the small scales of the flow field.…”

“…Optics 2020, 1, FOR PEER REVIEW 14 used in this study and also by Li and Yoda [49] in the presence of buoyancy driven convection in a rectangular cavity and in a cylindrical tube in which 2D PIV was performed at the center of the tube during evaporation of water at low pressures [41]. However, the maximum velocity was detected to be exactly at the interface in the evaporation of methanol inside various size microtubes [33].…”

Temperature Discontinuity at an Evaporating Water Interface

“…0.24 to 15.6 K). Kazemi et al [20] measured temperature discontinuity of 0.24 K at vapor pressure of 435 Pa and mass flux of 2.41×10 −4 kg/(m 2 s). In the experiments, there was no heating element in the vapor phase.…”