Reducing indoor particle exposure using mobile air purifiers - experimental and numerical analysis

Aerosol particles are one of the main routes of transmission of COVID-19. Mobile air purifiers are used to reduce the risk of infection indoors. We focus on an air purifier which generates a defined volumetric air flow through a highly efficient filter material. We investigate the transport of aerosol particles from an infected dummy equipped with an aerosol generator to receiving thermal dummies. For analysis, we use up to 12 optical particle counters to monitor the particle concentration with high spatial resolution. Based on the measurement data, a computational fluid dynamics (CFD) model is set up and validated. The experimental and numerical methods are used to investigate how the risk of infection suggested by the particle exposure in an exemplary lecture hall can be reduced by a clever choice of orientation of the air purifier. The particle concentration at head height deviates by 13 % for variations of location and orientation. Finally, CFD simulation was used to monitor the particle fates. The steady simulation results fit quite well to the experimental findings and provide additional information about particle path and for assessing comfort level due to air flow. Practical implicationsDifferent installation locations and operating conditions of the air purifier are evaluated and the use of thermal dummies mimics the conditions of practical use cases. The measurement results show the integral particle mass over time in the "faces of the dummies", representing the potentially inhaled particle load of persons present in the room. At an air change per hour of 5, the cumulated PM1 mass at head level was reduced by 75 %, independently of the location of the infected dummy, compared to the "natural decay" case showing that filtration is an effective means of reducing aerosol particle concentrations. It turns out that obstructing the outlet stream of the air purifier may be particularly advantageous.