On the effects of walking speed, crowd density and human-to-source distance on pollutant dispersion in indoor spaces

• Published in: Building and environment 2024-07, Vol.259, p.111649, Article 111649
• Main Authors: Lim, H.D., Foat, Timothy G., Parker, Simon T., Vanderwel, Christina
• Format: Article
• Language: English
• Subjects: Crowd effects; Indoor air quality; Indoor airflow; Risk assessment; Scalar dispersion
• ISSN: 0360-1323; 1873-684X
• DOI: 10.1016/j.buildenv.2024.111649

The effects of walking speed, crowd density and human-to-source distance on pollutant dispersion in two scaled room models are investigated using simultaneous planar laser-induced fluorescence and particle-image velocimetry techniques. For a small 3 m high room, where the length-scales of the people and room are comparable, the walking motions significantly influenced the macro room mean flow patterns. This has a strong effect on the scalar dispersion properties as the magnitudes of the advective scalar fluxes are often comparable or larger than the turbulent scalar fluxes. As such, the scalar dispersion properties are case specific. For a large 9 m high room, the walking motion influenced only the local mean flow field. The increase in walking speed and crowd density improves the efficiency in which the scalar is transported and mixed with fresh ambient fluid out of the measurement plane (i.e. along the direction of the motion), leading to scalar-free zones observed on the opposite side of the room from the ventilation outlet. The area of the scalar-free zone increases with an increase in the walking speed and crowd density. The advective scalar fluxes are more sensitive to the human motion than the turbulent components, and as the mixing efficiency improves, the advective fluxes show a greater weakening with increased distance from source. The concentration PDFs in the near-source region can be described by the exponential function where the expected value at the 99% percentile can be derived as C99/crms′=4.61, which agreed well with the experimental measurements of 4.1 to 5.9. •Advective and turbulent scalar flux maps reveal underlying transport mechanism•Crowd effects can induce non-uniform and anisotropic scalar mixing•Scalar-free zone observed on the opposite side of the room from ventilation outlet•Dispersion patterns in small rooms are case specific due to non-linear interactions