Modelling respiratory droplet cloud transport: Group evaporation effect

Abstract

The transport of respiratory droplet cloud arising from human cough is investigated in the present work. A mathematical model based on particle mass, momentum and energy conservation in a Lagrangian framework is adopted to track individual respiratory droplets in a constant carrier flow. The objective here is to study the role of group evaporation of the droplet cloud on the suspension time and transport distance of the respiratory droplets. Two different models for the group effect are examined which are validated against in-house experiments using single-, dual- and triple-droplet suspension arrangements. The strength and limitations of the models are assessed. The results demonstrate that the Group evaporation number (G), which characterizes the collective vaporization of the droplet cloud, is of the order of ‘one’ which refers to reduced droplet evaporation rate within the cloud compared to an isolated droplet. In such cases, the droplet suspension time as well as the distance over which the cloud is transported are much longer in comparison to the case when an isolated droplet evaporation model is employed. Accordingly, even a gentle wind speed spreads the ejected droplets in the range of 10 m–50 m from the source (lower ambient temperature and higher humidity increase the spread). It is also found that the group evaporation is sensitive to the initial characteristic droplet size rather than the mass and duration of the injected liquid. Finally, the current results highlight the group evaporation effect as a potential contributor to enhancing the droplet suspension time in the air.