Experimental and simulation study of stack emission and contaminant dispersion around typical building configurations and arrays in Hong Kong

Abstract

Wind velocity and outdoor ventilation play an essential role in diluting air pollutants and improving air quality in urban localities; however, the benefits of sufficient airflow could be significantly offset by the blockage effects from building structures. Ample research has been performed on wind flow around cubical buildings, but little has been conducted on the airflow in the wakes of buildings with configurations and arrays commonly found but considerably unique in Hong Kong. This thesis assessed the health impacts of exhaust stack emissions on the local community, and investigated the effects of wind direction, building configuration, and array design on airflow patterns and pollutant dispersion in the areas surrounding the point source. To this end, three individual studies were conducted, assessing: (1) The impacts of stack emissions on air quality in a small urban setting, (2) The effects of incident wind angles and building configurations on airflow patterns of building wakes and leeward walls, and (3) The effects of building arrays on airflow and contaminant distributions in the central space of buildings. Based on the release concentration and the potential hazards to human health, fifteen chemicals that were possibly emitted from a research building were selected for one year-long air monitoring. A tracer gas study was also performed to identify the dilution factor of the environment, and validate two turbulence models, renormalized group (RNG) and realizable (RLZ) k-ε. Statistical tests demonstrated that RNG outperformed RLZ k-ε for the prediction of pollutant dispersion and concentration distribution in the emissions study. In the assessment of wind direction and building configurations, it was found that when the wind approached lateral movement (90°), the downwind length and maximum bilateral width of the low-wind-velocity (LWV) zone in the wake of "T"-shaped buildings decreased. When the incident wind was oblique (45°), the length and width of the LWV zone in the wake of "+"-shaped buildings also decreased. Furthermore, it was found that air pressure on the leeward walls of the "T"- and "+"-shaped buildings gradually decreased with building height. Two common building arrays, i.e., 'L'- and 'U'-shaped, in Hong Kong were studied, revealing that the former maintained a stronger performance by forming a smaller LWV zone in the central space between buildings. The L-shaped array performed best at an incident wind angle of 225°; whereas a 90° incident angle produced the largest LWV zone for the U-shaped array. Although generally, the L-shaped array better distributed pollutants, the U-shaped array with a 180° wind angle had a smaller high pollutant concentration area than the L-array with a wind angle of 225°. Further, the worst vertical dispersion corresponded to a 135° wind angle for the 'L'-shaped array. To conclude, appropriate selection of building configurations and arrays, as well as their orientations, will allow for the most effective use of wind flow to enhance ventilation and pollutant dispersion.