Coupling effects of air conditioning and natural ventilation on the transmission and diffusion of bioaerosol in a classroom

The prevention of bioaerosol transmission has consistently been an issue that troubles people. The emergence of the bioaerosol has inflicted severe harm on society and humanity. Ventilation can effectively reduce the concentration of viral particles in indoor spaces. Nevertheless, few studies have been carried out on the distribution of particulate matter in indoor rooms under the coupling of ventilation and air conditioning. To study the effect of indoor airflow on bioaerosol particle distribution under the coupling of different window opening area ratios and indoor air conditioning operating modes, this study selected a university classroom as the research subject and simulated indoor airflow and temperature distribution using User-Defined Functions (UDFs) within Computational Fluid Dynamics (CFD) software. Building upon thermal environment evaluation indicators, the research investigated how air conditioning operation modes and window opening conditions affect the transmission dynamics of bioaerosols. The research findings demonstrate that: (1) The indoor airflow field generated by operating air conditioning or opening windows significantly enhances the diffusion rate of bioaerosol particles. Simulation results show that particles are transported from the emission source to the central zone of the classroom within 40 seconds and achieve full spatial distribution within 90 seconds. (2) When natural ventilation through open windows and mechanical air conditioning operate synergistically, an increase in the window opening area ratio ε from 16.7% to 40.1% leads to a reduction in aerosol relative concentration within the occupants' effective breathing zone from 0.56 to 0.06, resulting in a substantial decrease in exposure risk. (3) Adding natural ventilation to mechanical air conditioning can further reduce the average indoor aerosol concentration by 5%; however, the influx of hot outdoor air raises PMV values above 2.0, causing pronounced thermal discomfort and compromising overall indoor environmental quality. Indoor airflow critically influences aerosol transmission. Without air conditioning, respiratory virus particles diffuse slowly and remain spatially confined over short durations. When natural ventilation (open windows) and mechanical air conditioning operate together, they create a superimposed flow field that restricts aerosol dispersion—more so as window opening area increases. In summer, opening windows reduces bioaerosol exposure but introduces hot, humid outdoor air, raising indoor temperature and humidity, increasing PMV, and causing thermal discomfort. By jointly tuning air conditioning supply temperature and window opening area, a practical balance between pollutant control and thermal comfort can be achieved—supporting multi-objective indoor environmental optimization.