Session: 17-01-01: Research Posters

Paper Number: 150242

150242 - Water Droplet Evaporation in Air: Effects of Interfacial Thermal and Mass Diffusion Resistance 

Evaporation of micro/nanoscale water droplets in air is a process highly relevant to a variety of environmental, industrial, and biomedical applications such as development of effective ventilation methods for indoor droplet control, superfine inkjet printing, spray cooling, delivery of medical aerosols to the lungs, and prevention of the airborne spreading of viruses. The ability to predict the evaporation characteristics such as evaporation time, droplet temperature, and evaporation rate of water microdroplets in air at different temperature, pressure and humidity is of great importance to aforementioned applications.

In conventional hydrodynamic modeling of water droplet evaporation in air, it is usually assumed that thermodynamic equilibrium exists at the water-air interface, i.e., the water temperature is the same as the air temperature at the interface and the water vapor density at the interface equals to the saturated vapor density. The saturated water vapor near the water droplet surface is transported to the ambient air through mass diffusion or convection. When the convection effects on droplet evaporation are negligible, the conventional hydrodynamic model predicts that the diameter square (D²) of a water droplet evaporating in air will decrease at a constant rate, i.e., the D² law. The experimental studies showed that the D² law gives a reasonable prediction of the evaporation rate of mm-sized water droplets. However, for water droplets with a diameter of a few μm, it was found that the droplet evaporation time (i.e., the time for complete evaporation of the droplet) measured by experiment is about twice the value of the prediction from the conventional hydrodynamic model. When the droplet size is further reduced to nanoscale, the molecular dynamics (MD) simulation results showed that the droplet evaporation time at low ambient pressure condition could be several times longer than the prediction from the D² law. In our recent MD studies of Ar nanodroplet evaporation in a Ne gas, we explicitly show that, due to the thermal resistance and mass diffusion resistance at the liquid-gas interface, there is an evident temperature jump at the interface and the vapor density near the evaporating droplet surface is well below the saturated vapor density, indicating the invalidity of the thermodynamic equilibrium assumption. Therefore, we infer that the thermodynamic equilibrium at the water-air interface is also an invalid assumption for evaporation of micro/nanoscale water droplets in air, and the interfacial thermal and mass diffusion resistance have stronger effects on the evaporation of smaller water droplets in air, which results in a longer droplet evaporation time than the prediction from the conventional hydrodynamic model.

To understand the effects of interfacial resistance on water droplet evaporation in air, it is essential to develop a theoretical model that can predict the thermal resistance and mass diffusion resistance at the water-air interface. In this work, we will first use the kinetic theory-based model to understand heat and mass transfer at the water-air interface. The interfacial thermal and mass diffusion resistance predicted from the kinetic theory-based model will then be incorporated into the hydrodynamic model to study how the water droplet size and the surrounding air temperature, pressure and humidity will affect the interfacial resistance and the evaporation rate of water droplet in air. The evaporation characteristics of water droplets in air predicted from our kinetic and hydrodynamic combined model are corroborated by MD simulation results as well as the recent experimental data. Our modeling results show that, at normal temperature and pressure (i.e., 25^oC and 1 atm by NIST), the equivalent length of thermal resistance and mass diffusion resistance at water-air interface is ~ 200 nm and 100 nm, respectively. This indicates that, at normal temperature and pressure, the effects of interfacial thermal and mass diffusion resistance on evaporation of water droplets in air cannot be ignored if the droplet diameter is less than 40 μm.

Presenting Author: Wazih Tausif Missouri University of Science and Technology

Presenting Author Biography: Wazih Tausif is a Ph.D. student in the department of Mechanical and Aerospace Engineering at Missouri University of Science and Technology. His research focuses on computational micro/nanoscale heat and mass transfer.

Authors:

Wazih Tausif Missouri University of Science and Technology
Zhi Liang Missouri University of Science and Technology