Coupled thermal transport and mass diffusion during vapor absorption into hygroscopic liquid desiccant droplets

摘要

Phase change phenomena at a droplet scale have gained extensive attention in recent years due to the unique aspects that can be exploited in a wide range of domestic and industrial applications. Different from existing studies on droplet evaporation and/or dropwise condensation, this work focuses on the mechanisms of vapor absorption into hygroscopic liquid desiccant droplets, which are of interest to many dehumidification and absorption processes. In particular we investigate the coupled heat and mass transport during vapor absorption into single droplets using optical imaging and infrared thermography. Driven by the vapor pressure difference between the ambient and the droplet surface, desiccant droplets grow due to water uptake. The droplet growth rate and final expansion ratio depend on the ambient temperature and relative humidity. After liquid desiccant droplet deposition onto the substrate, and as a consequence of the initial fast vapor absorption, droplets experience an increase in temperature. They then gradually cool down as a result of heat dissipation into the substrate and into the ambient combined with the decrease in the vapor absorption rate, i.e., heat of absorption. The temperature rise measured by infrared thermography is confirmed by the calculation of the heat of absorption for six representative environmental conditions. Furthermore the vapor pressure at the droplet surface is estimated taking account of the changes of interfacial temperature and salt concentration within the droplet bulk. As water absorbs into the droplets, the salt concentration decreases and so does the driving force for vapor diffusion and hence the heat of absorption. As a contrast, experiments with evaporating water droplets show different evolution of droplet profile, different dynamics of the triple contact line, as well as the occurrence of evaporative cooling. We conclude on the importance of taking account of the coupling mechanisms of absorptive heating and volume growth during vapor absorption into liquid desiccant droplets. Findings presented here provide a valuable extension to existing literature of phase change at the droplet scale, and contribute to a more complete understanding of the behaviors of liquid desiccant droplets in dehumidification processes. (C) 2019 Elsevier Ltd. All rights reserved.