Human convective boundary layer and its impact on personal exposure

Abstract

People spend most of their time indoors and they are constantly exposed to pollution that affects their health, comfort and productivity. Due to strong economic and environmental pressures to reduce building energy consumption, low air velocity design is gaining popularity; hence buoyancy flows generated by heat sources are gaining more prominent influence in space airflow formation and on the indoor environment overall. In such spaces with low air supply velocity, air mixing is minimized and the pollution emitted from localized indoor sources is non-uniformly distributed. The large spatial differences in pollution concentration mean that personal exposure, rather than average space concentration, determines the risk of elevated exposure. Current room air distribution design practice does not take into account the air movement induced by the thermal flows from occupants, which often results in inaccurate exposure prediction. This highlights the importance of a detailed understanding of the complex air movements that take place in the vicinity of the human body and their impact on personal exposure. The two objectives of the present work are: (i) to examine the extent to which the room air temperature, ventilation flow, body posture, clothing insulation/design, table positioning and chair design affect the airflow characteristics (velocity, turbulence and temperature) around the human body; and (ii) to examine the pollution distribution within the human convective boundary layer (CBL) and personal exposure to gaseous and particulate pollutants as a function of the factors that influence the human CBL, and of different locations of the pollution sources. In this work, the empirical results were obtained primarily by using a thermal manikin to simulate a human in the indoor environment. In spaces with low air mixing, an increase of the ambient temperature from 20 to 26 ˚C widened the CBL flow in front of a seated manikin, but did not influence the shape of the CBL in front of a standing manikin. The same temperature increase caused a reduction of the peak velocity from 0.24 to 0.16 m/s in front of the seated manikin. Dressing the nude manikin in a thin-tight clothing ensemble reduced the peak velocity in the breathing zone by 17%, and by 40% for a thick-loose ensemble. A lack of hair on the head increased the peak velocity from 0.17 to 0.187 m/s. Apart from their thermal insulation, clothing and chair design had a minor influence on the velocity profile beyond 5 cm distance from the body. Closing the gap between the table and the manikin reduced the peak velocity from 0.17 to 0.111 m/s. At a room air temperature of 23 ˚C, with the manikin leaning backwards the peak velocity was 0.185 m/s, which is 45% above the case with the manikin leaning forward. The direction and magnitude of the surrounding airflows considerably influence the airflow distribution around the human body. Downward flow with a velocity of 0.175 m/s at a room air temperature of 23 ˚C did not influence the convective flow in the breathing zone, while the flow at 0.30 m/s affected the CBL at the nose level, reducing the peak velocity from 0.185 to 0.10 m/s. In order to completely break away the human CBL, downward flow had to be supplied with a velocity of 0.425 m/s. Transverse horizontal flow disturbed the CBL at the breathing zone even at 0.175 m/s. With a seated manikin exposed to airflow from below with a velocity of 0.30 and 0.425 m/s assisting the CBL, the peak velocity in the breathing zone was reduced and the flow pattern around the body was affected, compared to the assisting flow of 0.175 m/s or quiescent conditions. In this case, the airflow interaction was strongly affected by the presence of the chair. The results also show that Particle Image Velocimetry (PIV) and Pseudo Color Visualization (PCV) techniques can be adequately employed for the human CBL investigation. The results show that reducing the room air temperature from 23 to 20 ˚C increased the fluctuations of air temperature close to the surface of the body. Large standard deviation of air temperature fluctuations, up to 1.2 ˚C, was easured in the region of the chest, and up to 2.9 ˚C when the exhalation was applied. Leaning the manikin backwards increased the air temperature and standard deviation of air temperature fluctuations in the breathing zone, while a forward body inclination had the opposite effect. Exhalation through the mouth created a steady temperature drop with increasing distance from the mouth, without disturbing conditions in the region of the chest. Exhalation through the nose did not affect the air temperature in front of the chest due to the physics of the jets flow from the nose. Only very small discrepancies between the results obtained with the breathing thermal manikin and a real human subject were found. This suggests that the thermal manikin can be used for accurate measurements of an occupant’s thermal microenvironment. The results also suggest that a detailed understanding of the distribution of pollutants in the vicinity of a human body is essential for understanding exposure in spaces with low air mixing. The pollution source location had a considerable influence on the pollution concentrations measured in the breathing zone and on the extent to which the pollution spread to the surroundings. The highest breathing zone concentrations were measured when the pollution source was located at the chest, while there was negligible exposure to any the pollution emitted from the upper back or behind the chair. Based on the results obtained in a single plane, it was shown that a decrease in personal exposure to pollutants released from or around the human body increased the extent to which the pollution spread to the surroundings. Reduced room air temperature and backward body inclination both intensified the transport of pollution to the breathing zone and increased personal exposure. The front edge of a table positioned at zero distance from the human body reduced pollution/clean air transport to the breathing zone, but when it was positioned 10 cm from the body it increased the transport of pollution/clean air from beneath. For accurate predictions of personal exposure, the characteristics of the CBL must be considered, as it can transport pollution around the human body. The best way to control and reduce personal exposure when the pollution originates at the feet is to employ transverse flow from in front and from the side, relative to the exposed occupant. Airflows from above opposing the CBL and from behind transverse to the CBL, create the most unfavourable velocity field that exhibits a non-linear dependence between the supply airflow rate and personal exposure. Without a better understanding of the airflow patterns in a room the ventilation rate may therefore be increased in vain.