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

Abstract

In all kinds of liquid desiccant dehumidification systems, the temperature increase of the desiccant solution due to the effect of absorptive heating is one of the main reasons of performance deterioration. In this study, we look into the thermal effects during vapor absorption into single hygroscopic liquid desiccant droplets. Specifically, the effect of substrate conductivity on the transient heat and mass transfer process is analyzed in detail. The relative strength of the thermal effect and the solutal effect on the rate of vapor absorption is investigated and compared to the thermal effect by evaporative cooling taking place in pure water droplets. In the case of liquid desiccants, results indicate that the high thermal conductivity of copper substrates ensures more efficient heat removal, and the temperature at the droplet surface decreases more rapidly than that on Polytetrafluoroethylene (PTFE) substrates. As a result, the initial rate of vapor absorption on copper substrates slightly outweighs that on PTFE substrates. Further analysis by decomposing the vapor pressure difference indicates that the variation of vapor pressure caused by the temperature change during vapor absorption is much weaker than that induced by the concentration change. The conclusions demonstrate that a simplified isothermal model can be applied to capture the main mechanisms during vapor absorption into hygroscopic droplets even though it is evidenced to be unreliable for droplet evaporation.

Dynamics of hygroscopic aqueous solution droplets undergoing evaporation or vapour absorption

“…Depending on the ambient condition, the LiBr–H 2 O droplet either evaporates (at low ambient humidity) or absorbs water vapour (at high ambient humidity). Preferential evaporation/absorption induces concentration gradients, while the thermal effect along with water-vapour phase change induces temperature variation within the droplet (Wang et al 2020).…”

Role of volatility and thermal properties in droplet spreading: a generalisation to Tanner's law

“…2018; Wang et al. 2019, 2020, 2022). Specifically, the non-uniform mass flux tends to recede the liquid–air interface especially near the three phase contact line.…”

“…Compared with non-volatile liquids, the spreading of volatile liquids is more complex and remains elusive owing to the complexity added by interfacial phase change and non-equilibrium thermal transport (Berteloot et al 2008;Jambon-Puillet et al 2018;Wang et al 2019Wang et al , 2020Wang et al , 2022. Specifically, the non-uniform mass flux tends to recede the liquid-air interface especially near the three phase contact line.…”

On the Impact of Ambient Humidity and Evaporation on Demixing of Binary Mixtures

“…Wang et al, Hu et al, and Parimalanathan et al point out the role of moisture absorption (hygroscopicity) on spreading/contraction regimes of sessile droplets. Furthermore, the heat generated due to moisture condensation or absorption can also play a prominent role. , …”