PhD Defence Fahmi Yigit | Bio-Aerosols: Understanding & modelling the impact of ventilation design and environmental changes

Bio-Aerosols: Understanding & modelling the impact of ventilation design and environmental changes

Fahmi Yigit is a PhD student in the Department of Physics of Fluids. (Co)Promotors are prof.dr. D. Lohse and prof.dr. A. Marin from the Faculty of Science & Technology.

Bio-aerosols, including airborne bacteria, fungi and viruses, move through indoor environments through interactions between respiratory emissions, ventilation flows, thermal plumes and environmental conditions. Ventilation and air-cleaning systems are commonly assessed using airflow rates, air changes per hour, filtration efficiencies and laboratory removal rates. However, these indicators do not necessarily reflect the actual protection of occupants or products.

This thesis develops an exposure-oriented framework for evaluating the real-world hygienic effectiveness of ventilation and air-treatment systems. It introduces the Hygienic Air Delivery Rate (HADR), a metric that considers both the amount of air treated and whether this hygienically treated air effectively reaches the occupied zone. Classroom experiments show that identical nominal airflow rates can produce substantially different hygienic outcomes because of airflow distribution, short-circuiting, thermal convection and occupant interaction.

The thesis also examines the relationship between infection-risk reduction and energy efficiency. It demonstrates how intelligent airflow design and validated technologies, including UV-C disinfection, can improve hygienic performance without relying solely on increased outdoor-air supply.

Finally, temperature and relative humidity are incorporated into airborne infection-risk assessment. By extending Wells–Riley concepts with thermodynamic principles and visualising them in a Mollier diagram, the thesis explains how environmental conditions affect aerosol behaviour, pathogen viability and infection risk.

Overall, this research shifts ventilation assessment from equipment-centred performance towards occupant-centred exposure reduction. A system should be judged not only by how much air it moves or cleans, but by how effectively it protects the people and products that depend on it.