Experimental investigation on the impact of thermal stratification on aerosol behavior in indoor environments

Ventilation modes significantly impact the dispersion and deposition of pathogen-laden aerosols in indoor environments, thereby affecting both direct and indirect disease transmission. This study investigates the influence of thermal stratification as a result of the ventilation mode. Several HVAC parameters are examined in the experiments, including vent location, air exchange rate, heating or cooling mode, and the resulting conditions, which can be either stratified or mixed. Test aerosols are fluorescein particles in the 0.3–5 μm size range, characteristic of those reported in the literature for human expiratory activities, and releases are complemented by co-injection of CO₂ to allow for a broader measurement of dispersion. A body heat simulator and a heated injection system are used to account for the buoyant plume rise of human exhalation and body heat. Particle deposition on horizontal and vertical surfaces is quantified through deposition plates located throughout the test chamber. Dispersion and deposition are as expected from a lumped box model when the ventilation mode promotes mixed conditions (air exchange rates of 0.5–5 h⁻¹). When conditions were thermally stratified, the location of the return vent had a substantial impact on the measured concentrations; locating the return at the floor creates a dead-end volume at the top half (breathing zone) of the room where aerosols accumulate, whereas positioning the return on the ceiling offer the most efficient mode for removing contaminants. The deposition was an important sink for airborne particulates, and deposition observed on the walls and ceiling was higher than anticipated. There are novel comparisons between the deposition rates and measured friction velocities in the room to attempt to qualify the relative roles of turbulence, gravity, and Brownian deposition mechanisms; however, most of the deposition could be attributed to electrostatic effects. The findings in this study can have serious ramifications for developing HVAC designs that aim to minimize the risk of indoor disease transmission.