Multi-Scale Computational Fluid Dynamics Investigation of Ventilation-Controlled and Gravity-Controlled Respiratory Aerosol Transport in an Office Environment: Implications for Heating, Ventilation, and Air Conditioning Design and Airborne Infection

This study presents a comprehensive transient Computational Fluid Dynamics–Discrete Phase Model (CFD-DPM) investigation of aerosol and droplet transport in an air-conditioned office containing one infected occupant and three susceptible occupants. The transition from ventilation-controlled to gravity-dominated transport of respiratory aerosols in office settings remains poorly characterized. Continuous emission over 300 s was simulated for particle diameters of 1, 5, 10, and 50 µm. The continuous airflow field was resolved using the pressure-based solver, energy equation, and Shear Stress Transport k-ω turbulence model, validated against experimental temperature and relative humidity data with mean absolute errors of 0.59 °C and 1.26% relative humidity, respectively. The airflow model was validated against temperature and relative humidity data only; the discrete-phase predictions were not validated directly against experimental aerosol data. A critical finding is the identification of three distinct transport regimes: Regime I (1–5 µm), characterized by ventilation-dominated behavior with strong airflow coupling, approximately 50% outlet removal, and 38–39% airborne fraction at 300 s; Regime II (10 µm), representing a mixed-transition zone where gravitational effects begin to compete with aerodynamic forces; and Regime III (50 µm), dominated by gravitational settling with 93% deposition and only 1.6% outlet removal at 300 s. The critical transition diameter between ventilation-controlled and gravity-dominated transport was estimated to be approximately 20–30 µm based on interpolation between the 10 µm and 50 µm cases and dimensionless analysis; this estimate requires further validation with intermediate particle sizes. Dimensionless analysis revealed that particles with Stokes numbers below 0.1 remain strongly coupled to the airflow, while those exceeding 1.0 exhibit ballistic trajectories dominated by inertia and gravity. Outlet-2 consistently served as the dominant removal path for all particle sizes, confirming an asymmetric ventilation transport route attributable to the supply jet trajectory and recirculation patterns. Human 2 received the highest body-deposition fraction for aerosol-sized particles (0.287% for 1 µm, 0.123% for 5 and 10 µm at 300 s), identifying this location as a critical exposure zone. For 50 µm droplets, susceptible occupant deposition was negligible (<0.0005%) because gravitational settling deposited most mass on the floor (64.1%) and the infected occupant (25.5%) before far-field transport occurred. These findings establish a quantitative framework for designing ventilation systems that optimize removal of aerosol-sized particles while recognizing the inherent limitations of air extraction for droplet control. The proposed transport-regime classification provides a framework for HVAC engineers, with recommendations for return grille placement, air change rates, and supplementary source-zone controls for large droplets. These recommendations are derived from a single office geometry and ventilation configuration and should be validated across different room layouts, ventilation strategies, and operating conditions before being generalized. The methodology and findings contribute to the growing body of knowledge on airborne infection risk mitigation in occupied spaces, with direct applicability to post-pandemic building design and operation.