Human convection flow in spaces with and without ventilation: personal exposure to floor-released particles and cough-released droplets

Abstract

A human convective boundary layer plays an important role in pollution transport around the human body. It interacts with the surrounding airflows which modifies air movement around the human body and personal exposure. Understanding the influence of this interaction on the pollution spread around the human can be used to control and reduce personal exposure and improve HVAC design.

Experimental evaluation of an intermittent air supply system – Part 1: Thermal comfort and ventilation efficiency measurements

“…Another noteworthy point is that 0.4 m/s is the minimal employable supply velocity of AJS in order to completely destroy the thermal plume and avoid increased personal exposure to contaminants at the breathing zone [31,32]. This means there is no limit to what the maximum velocity can be as stated in ASHRAE standard 55 [24] if occupants are provided with individual control of the air movements.…”

“…The strategy was designed to operate with a minimum target velocity of 0.4 m/s at breathing height. At about this velocity, downward airflow is known to effectively blow away the thermal plume and reduce personal exposure to contaminants [31,32]. The velocity during low pulses should be less than 0.2 m/s to minimize draft risk and have perceptible airflow fluctuations [33].…”

Emission rates and the personal cloud effect associated with particle release from the perihuman environment

“…In a near vicinity of a person (up to 0.45 m distance from the body), the convective boundary layer dominates the transport of particles upwards to the breathing zone, thus elevating personal exposure for particles released in the immediate perihuman space. [29][30][31] Particles that escape this boundary layer mix with the surrounding room air. 32 In these experiments, when a subject performed seated moderate bodily movements, the contribution to cross-contamination at 1 m distance was 11% of self-inhaled PM 10 mass, and the value dropped further to 7% at 2 m distance.…”

Human body micro-environment: The benefits of controlling airflow interaction

“…In addition to the factors influencing the airflow interaction that were discussed above, the location of the pollution source is critical for exposure. It was reported by Licina et al [38] that in the case of particles generated at the feet of a seated thermal manikin the most favourable airflow patterns are from in front and from the side, because they minimize or eliminate the exposure at minimum velocity. Opposing flow from above interacts with the FCF and generates an unfavorable velocity field for pollution released at the feet and may even increase the exposure.…”