Modelling of evaporation of cough droplets in inhomogeneous humidity fields using the multi-component Eulerian-Lagrangian approach

Abstract

A B S T R A C TThis study employed a multi-component Eulerian-Lagrangian approach to model the evaporation and dispersion of cough droplets in quiescent air. The approach is featured with a continuity equation being explicitly solved for water vapor, which allows comprehensively considering the effects of inhomogeneous humidity field on droplets evaporation and movement. The computational fluid dynamics (CFD) computations based on the approach achieved a satisfactory agreement with the theoretical models reported in the literature. The results demonstrated that the evaporation-generated vapor and super-saturated wet air exhaled from the respiratory tracks forms a "vapor plume" in front of the respiratory track opening, which, despite the short life time, significantly impedes the evaporation of the droplets captured in it. The study also revealed that due to the droplet size reduction induced by evaporation, both the number density of airborne droplets and mass concentration of inhalable pathogens remarkably increased, which can result in a higher risk of infection. Parametric studies were finally conducted to evaluate the factors affecting droplet evaporation. Summary: The study demonstrated the importance of considering inhomogeneous humidity field when modelling the evaporation and dispersion of cough droplets. The multi-component Eulerian-Lagrangian model presented in this study provides a comprehensive approach to address different influential factors in a wide parametric range, which will enhance the assessment of the health risks associated with droplet exposure.

Thermal effect of human body on cough droplets evaporation and dispersion in an enclosed space

“…The composition of saliva droplets was assumed as 98.2% water and 1.8% non-volatile solid compounds based on the existing literature [13,16]. The evaporation process of the heavily weighed water component was controlled by the equilibrium vapour pressure relative to the ambient pressure at the surface of droplets.…”

“…Since the inhomogeneous vapour concentration would significantly affect the droplet trajectories by changing the function of time and droplets size, it is important to fully address the inhomogeneous water vapour concentration on droplet evaporation. With that in need, a multi-component Eulerian-Lagrangian approach that explicitly solves the continuity equation of the water vapour was proposed in the previous study [16] to enhance the mathematical models for modelling evaporation process of coughing.…”

“…The same process could take more than 100s for droplets with initial diameter of 100 μm under an opposite ideal condition of 100% RH [14]. To further test the impact of droplet size and RH change on evaporation process, our previous research [16] conducted several case studies with combinations of various ambient RH (10%-90%), exhaled humidity and droplets amount. The outcomes demonstrated that the dynamic behaviours of droplets were significantly affected by evaporation and RH, including the change of droplet dispersion, promote increase of pathogen mass carried by find and ultrafine droplets.…”

“…However, this essential factor: the thermal flow in indoor environment from human body heat, was generally overlooked when integrating the droplets evaporation into indoor environment. Since investigating the cough droplets evaporation process would inevitably increase the computational cost, the computational domains were very restricted in most existing studies [12,14,16]. Cough was generally released through a narrow surface imitating the mouth only, whereas human model was either completely removed or overly simplified (i.e.…”

Influence of wind and relative humidity on the social distancing effectiveness to prevent COVID-19 airborne transmission: A numerical study

“…Supported by benchmark experimental data from cough aerosol characterizations (Bourouiba, Dehandschoewercker, & Bush, 2014;Johnson & Morawska, 2009;Xie, Li, Sun, & Liu, 2009;Yang, Lee, Chen, Wu, & Yu, 2007), numerical studies (Chen & Zhao, 2010;Gupta, Lin, & Chen, 2011;He, Niu, Gao, Zhu, & Wu, 2011;Li, Shang, Yan, Yang, & Tu, 2018;Y. K. Yang, Kang, Hwang, & Park, 2017;Zhang & Li, 2012;Zhu, Kato, & Yang, 2006) have been done to either characterize cough aerosol clouds or simulate aerosol transport phenomena in both indoor and outdoor environments.…”