Infectious diseases, such as COVID-19, can be transmitted by the exhaled virus-laden respiratory droplets. In indoor environments, ventilation systems are widely used to dilute or remove such droplets. To improve their performance and lower the infection risk, investigations on droplet transport under ventilation systems are of great importance.
Respiratory droplets in indoor environments constitute a complex multiphase system. The sizes of droplets, affecting the balance between aerodynamic and gravitational forces acting on them, play an important role in their trajectories. In addition, the respiratory droplets are multicomponent, containing volatile and non-volatile substances. They can evaporate, which causes changes in size and significantly affects droplet transport. Computational fluid dynamics (CFD) provides a powerful tool to investigate the droplet transport. To consider the effect of evaporation, a zero-dimensional (0D) evaporation model for the salt-water droplets, which assumes salt is homogeneously distributed inside a droplet, is widely used to resemble the respiratory droplets. However, the salt is found to be not homogeneously distributed inside the droplets [1]. A salt shell may form due to the enrichment of salt in the droplet surface, resulting in the droplet residue larger than the prediction of 0D model. Therefore, the one-dimensional (1D) evaporation model, considering the salt diffusion inside the droplets, was adopted by some researchers [1, 2]. This, however, increases the computational costs. Therefore, in this study, the factors affecting salt distribution inside the droplets are analyzed using the 1D model. Based on the results, a simple 0D model with corrections considering the non-homogeneous salt distribution is developed. The corrected model is validated with experiment [1]. Then it is coupled with the CFD-population balance modelling (CFD-PBM) approach to trace the transport of respiratory droplets in ventilated indoor environments.
An example of the obtained information is shown in Figure 1, where the evolutions of salt concentration profiles inside the NaCl-water droplets are presented. The setups are from the work of Gregson et al. [1], and the results agree well with their simulations. It can be seen that increasing ambient temperature or decreasing the initial salt concentration leads to a more non-homogeneous salt distribution inside the droplets. This is because such measures cause a higher evaporation rate so that the salt enriched in the droplet surface does not have enough time to diffuse to the center of the droplet.

References:
[1] Gregson FKA, Robinson JF, Miles REH, et al. Drying kinetics of salt solution droplets: Water evaporation rates and crystallization. The Journal of Physical Chemistry B, 2018, 123(1): 266-276.
[2] Ozler G, Grosshans H. Airborne virus transmission: Increased spreading due to formation of hollow particles. Environmental Research, 2023, 237: 116953.