What is it about?

In this paper we investigate how differences in temperature within a room affect the dispersion of the tiny particles that we exhale while we speak. Temperature indoors is rarely completely uniform, particularly when ventilation is not optimal. Thermal stratification occurs when warmer air - which is less dense - travels upwards and the colder air remains at a lower height. We explored a wide range of exhaled particles, going from slightly less that half a millimetre to 1 μm. We found that in all cases the smallest aerosols (< 20 μm) can travel up to 4 m from the source, while larger droplets are unaffected by stratification and do not follow a ballistic trajectory, falling out within 0.5 m of the source in all cases. Strong thermal stratification was also shown to increase aerosol concentration at the breathing height by up to 27%. We also identified a change in the behavior of aerosols within the size range of 12-20 μm in the presence of stratification. Aerosols smaller than 12 μm will remain locked up and linger in the air currents; whereas aerosols over this threshold detach from the exhalation cloud gradually and fall slowly towards the ground.

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Why is it important?

Being able to predict the fluid dynamics of expiratory events is crucial to design effective mitigation strategies to make indoor spaces infection resilient. We reproduce a realistic size distribution of the particles we expire, using a methodology that allows us to track every one of them individually in space and time without making any pre-assumption on its behaviour. The study of the effect of thermal stratification showcases and quantifies how these particles have distinct behaviours depending on their size and the environmental conditions. Our work shows that temperature gradients that are acceptable from a thermal comfort perspective may be unacceptable from a cross-infection perspective, highlighting the critical role that the selection of a ventilation strategy may play in mitigating airborne pathogen spread indoors. Visualizing these phenomena is in itself important to provide the general public and decisions makers with tools to understand better phenomena that are not always intuitive.


I believe that the fundamental value of this work is to introduce a tool that allows us to consider the fate of expiratory particles individually, not as a uniform cloud. We started investigating a phenomenon, thermal stratification, where the impact of the environmental conditions on the different size fractions is of essence. There are many more situations in which the discrete nature of the expiratory particles is critical to the understanding of the underlying phenomena and the design of mitigation strategies and we will pursue this line of research. We are particularly interested in investigating in detail ventilation effects. This is also the first journal paper by my student Aleksandra Monka and is well-deserved recognition to her outstanding work.

Bruño Fraga
University of Birmingham

Read the Original

This page is a summary of: Influence of thermal stratification on the transport of polydispersed expiratory particles, Physics of Fluids, October 2023, American Institute of Physics, DOI: 10.1063/5.0163971.
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