Ventilation effectiveness measurements and CFD simulations in classrooms for infection risk control

Ventilation is one of the main engineering measures to control the airborne respiratory infection transmission in shared indoor spaces. Virus removal rate by ventilation depends not only on ventilation rate but also on ventilation effectiveness. In this paper we study ventilation effectiveness and local air quality index. We conducted experiments in different classroom layouts, in the case of different point source locations corresponding to infector positions. Tracer gas measurements with CO2 were conducted at each room with thermal dummies to simulate occupancy. These field measurement results were used to validate computational fluid dynamics (CFD) simulation models which were applied for larger number of possible source positions. Ventilation effectiveness was calculated both from measurement and CFD results. CFD provided more detailed picture about room airflows and concentration distribution. The findings demonstrated reasonable agreement between CFD simulation results and field measurements, but some discrepancies were observed in classroom measurements with one source location. The main finding of the study is that common mixing ventilation air distribution systems, which provide fully mixed conditions in the case of normal occupancy/distributed source, do not provide fully mixing in the case of point contaminant source. Ventilation effectiveness values were reduced to 0.73-0.82 with a point source. Only in smallest classroom, the values higher than 1 were achieved. Thus, in infection risk-based ventilation design, ventilation effectiveness may have remarkable effect on required ventilation rates and therefore needs to be carefully designed and taken into account.