Study of ventilation and virus propagation in an urban bus induced by the HVAC and by opening of windows

Abstract

The external and internal airflow and air renewal inside urban buses have taken especial relevance since the COVID-2 pandemic. Computational fluid dynamics (CFD) simulations, which focus on the estimation of indoor airflow are not conclusive about the impact of using Heat, Ventilation and Air Conditioning (HVAC) systems on diseases’ transmission risk while travelling with open windows has shown to be a good strategy to renew the indoor air. In order to estimate the COVID-2 airborne transmission by aerosols, a real urban bus was simulated by CFD. Twenty passengers (containing the driver) were included in the model with a typical inhalation–exhalation breathing cycle. The concentrations of air exhaled by ten of them were tracked during 30 min using Eulerian scalar tracer, and the concentrations inhaled by the twenty passengers were monitored. Then, the well-known Wells & Riley risk model was applied in order to estimate the cumulative inhaled viruses and the subsequent transmission risk. Four scenarios were considered: HVAC off with closed windows (Case 1), HVAC on with closed windows and 100% of air recirculation (Case 2), HVAC on with closed windows and 75% of air recirculation (Case 3), and HVAC off and the bus moving at 20 km/h with some windows opened (Case 4). Results clearly showed that the motionless condition (Case 1) caused the highest transmission risk around the emitters with negligible risk far from them. On the contrary, the HVAC on reduced the maximum risk to only 6% (Case 2) and 3% (Case 3) of the risk estimated for Case 1. Finally, travelling with some open windows promotes a large air renewal, reducing almost completely the transmission risk.