Evaluating and mitigating the individual-to-individual transmission of breathing-released aerosol in a densely occupied classroom with impinging jet supply

Classrooms are featured by prolonged and dense occupancy, while the transmission risk of respiratory aerosols is usually evaluated based on several sources, or the aerosols in the entire space, ignoring the effects of individual-to-individual transmission in the crowd. We define the average concentration of the tracer gas (i.e., Ci,av) released by the ith individual in the head regions of other students to assess the risk for cross-transmission from the infected to others, and the crowd-averaged Ci,av, (i.e., Cav‾) to evaluate the overall cross-transmission in the classroom. The impinging jet ventilation system is studied because it is promising for densely occupied spaces, while facing a drawback of recirculation airflow. Key findings reveal that increasing the supply velocity (vs) enhances the recirculation, disrupting thermal stratification at vs ≥ 1.5 m/s. While bulk indoor contaminant concentrations drop monotonically with the increase of vs. However, Cav‾ exhibits non-monotonic behavior: it declines from 6.56 ppm to 3.96 ppm as vs rises from 1.0 m/s to 2.0 m/s, but reaches a comparable low (4.20 ppm) at vs = 0.5 m/s. This demonstrates Cav‾’s superior sensitivity to interpersonal exposure risks compared to traditional spatially averaged metrics in crowded indoor spaces. Further analysis decouples the impacts of vs and supply airflow rate (Qs). As a result, Cav‾ increases with vs under a constant Qs, and drops as Qs rises under a constant vs. This study proposes decreasing the vs and increasing Qs to mitigate the cross-transmission between individuals in a crowd.