Characteristics of human-related droplet dynamic patterns inside urban bus with numerical simulation approach: Impacts of solar radiation, ventilation modes and humidity

Nowadays, urban bus has become one of the closely human-related environments. The exhaled droplets when passengers breath, talk, cough and sneeze will undergo a series of significant dynamic process in the enclosed bus. The characteristics of aerosol dynamic process has been demonstrated to be great impact on aerosol transmission and the occupants' exposure levels, but has not been fully understood yet. This study analyzed the airflow organization and aerosol transmission during actual commuting period inside urban buses by employing the Eulerian-Lagrangian technique and realizable k-epsilon turbulence model. Three key factors including solar radiation, ventilation modes and humidity were added to reflect the impacts of the abovementioned parameters on aerosol dynamic and transmission patterns. The results showed that solar radiation will lead to a growth of up to 64.4% of bus interior surface temperature in winter. The air conditioning and ventilation inside the bus can meet the thermal comfort needs of passengers. The droplets start to spread in the entire compartment in 30-90 s and almost affects all the passengers, and those sprayed by standing passengers diffuse faster, affecting 8 passengers around the target passenger within 30 s. When relative humidity (RH) = 90%, the residential particle size after evaporation will be 27.9%-38.38% larger than that under RH = 30% with diameter over 15 mu m. The findings in the study should be a guidance when accurately forecasting the aerosol dynamic, transmission, exposure levels and smart control for ventilation system inside transport environment.