Upper room UVGI effectiveness with dispersed pathogens at different droplet sizes in spaces conditioned by chilled ceiling and mixed displacement ventilation system

• Airborne and droplet pathogen transport is modeled in mixed CC/DV spaces.
• The effectiveness of UVGI is of large pathogen-carrying particles is studied.
• The model for airborne pathogens transport was experimentally validated.
• The transport of large pathogen-carrying particles is validated with CFD.
• UVGI is less effective with mixed DV for large pathogens droplet sizes.

This paper investigates by modeling and experimentation the dispersion of bacteria and performance of Upper Room Ultraviolet Germicidal Irradiation (UVGI) in spaces conditioned by combined chilled ceiling (CC) and mixed displacement ventilation (DV) system. The effects of gravitational settling and deposition were taken into account in mathematical modeling to reasonably simulate the transport of pathogen via expiratory droplets of all size ranges. The model was experimentally validated with and without the use of UVGI for different ventilation rates. Experiments were conducted in a CC/DV test room with a constant generation of Serratia marcescens, an extremely UV-susceptible microorganism to measure the airborne bacteria concentration in the room air and quantify the bacteria deposition on horizontal surfaces. The experimental bacteria colony forming units (CFU) count was used to validate a CFD model developed to predict the deposition of pathogen carriers. The mathematical model was then substantiated using the CFD predictions of pathogen concentration in the space for large carrying particles. The validated mathematical model was then used to study the effect of pathogen-carrying particle sizes on the microbiological indoor air quality. Results showed that the droplet size affects the value of maximal bacteria concentration and the height at which it occurs. Moreover, the UV disinfection rate achieved in the upper zone decreases from 88% to 78% when the size of pathogen-carrying particles increases from 2.5 μm to 20 μm meaning that greater percentage of bacteria are removed by deposition when bacteria is carried by large particles.