Simulating respiratory droplet dispersion in ventilated office spaces: Effects of particle size and ventilation rate

The COVID-19 (coronavirus disease 2019, an infectious disease caused by the coronavirus SARS-CoV-2) outbreak has brought attention to the critical role of airborne respiratory droplets and aerosols in the transmission of viruses. This study analyzed the transport and dispersion of respiratory droplets in a ventilated office space housing two mannequins separated by a partition. It was assumed that one of the mannequins emits respiratory droplets while speaking. The airflow patterns and droplet dispersion in the presence of two different-height partitions, 1.372 m and 1.626 m, were investigated using the transition k–kl–ω turbulence model. The ANSYS-Fluent code was employed to simulate the airflow in the room. The Lagrangian particle tracking method was used to evaluate the droplet velocity and trajectories. The discrete random walk (DRW) model was employed to account for turbulence fluctuation effects on particle dispersion. The influence of various ventilation flow rates, from 1.975 to 5.6, on the dispersion of droplets of different sizes 1, 10, and 20 μm, emitted by the speaking mannequin was evaluated. Furthermore, the concentration and distribution of droplets of varying sizes near the receiving mannequin were analyzed. A velocity inlet of 1.1 m/s was applied at the emitter mannequin’s mouth, directing flow toward the receptor, and a total of 12,125 water-density droplets were released. Droplet concentrations were evaluated using the two-way coupling approach of the ANSYS-Fluent code and particle plane-crossing estimate using MATLAB software for dilute droplet concentration. The predicted concentrations of droplets of different sizes in the breathing zone of the receptor mannequin were compared with the earlier enhanced diffusion model estimates, and general agreement was found. This study revealed that increasing the ventilation flow rate (air changes per hour, ACH) leads to a decrease in the concentration of smaller-sized particles in the breathing zone of the receptor mannequin. The larger droplets had higher gravitational sedimentation velocity, resulting in lower concentrations in the breathing zone of the receptor mannequin. As expected, the highest concentration was observed for 1-μm particles at ACH = 1.975 (the lowest ACH studied). This trend was observed by both the diffusion model and Lagrangian particle tracking models. The simulation results also showed that the concentration of respiratory droplets was highly nonuniform and was significantly affected by the ventilation flow pattern in the room.