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

Abstract

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

Effect of Substrate Conductivity on the Transient Thermal Transport of Hygroscopic Droplets during Vapor Absorption

“…At this ambient condition, the rate of vapor absorption is moderate while the vapor absorption phenomenon is apparent and the equilibrium state is easily reached. Due to vapor absorption, the volume of LiBr-H2O droplets increases rapidly at first and then slows down as the Based on mass conservation, the solute concentration inside the LiBr-H 2 O droplets (Figure 4a) can be calculated as established in the work of Wang et al [4]. In addition, the vapor pressure difference between the ambient and the droplet surface (Fig.…”

“…In addition, the vapor pressure difference between the ambient and the droplet surface (Fig. 4(b)) can be evaluated according to the fitting correlations derived by Patek et al [21] and further implemented by Wang et al for the absorption of LiBr-H 2 O liquid desiccant droplets [4]: ∆P = P vapor,ambient − P vapor,surface , P vapor,surface = P sat (Θ),…”

“…% LiBr-H2O solution from Sigma-Aldrich is used as the testing fluid for vapor absorption experiments, and the droplet volume is controlled as 2.5 ± 0. [4,18]. Before experiments, the substrate samples are cleaned using an ultrasonic bath and deionized water, and are further dried with filtered compressed air to remove any possible remaining dusts or contaminants.…”

“…Due to the existence of specific ions with strong adhesion to water molecules, the water vapor pressure at the droplet surface is reduced when compared to the partial vapor pressure of the surrounding air [3]. As a result, water vapor diffuses from the air side towards the liquid-air interface, and gets absorbed into the droplet [4]. Along with vapor-liquid phase change, the latent heat released will heat up the liquid solution, which is one of the main reasons of performance deterioration in all kinds of dehumidification devices [5].…”

“…In our previous research, we investigated the mechanisms of droplet growth and spreading [4], as well as the effects of ambient temperature, humidity, and surface wettability on the vapor absorption process [18]. In this study, we investigate the thermal effects and demonstrate its relation with the substrate properties during vapor absorption into hygroscopic liquid desiccant droplets.…”