Investigation of pollutants inter-unit dispersion and natural ventilation in buildings due to wind effect

Abstract

Inter-unit pollutant dispersion in naturally ventilated buildings through open windows has been received increasing attentions due to its short transportation route and the involvement of airborne infectious diseases. Previous studies regarding inter-unit dispersion are limited to isolated buildings. Given that almost all buildings in urban areas are surrounded by many other buildings, the effect of interference from surrounding buildings on the inter-unit dispersion is worthy of investigation. Among others, the upstream interfering building has the greatest effect when the downstream interfered building is located in its wake region. Moreover, there are few studies investigating the effect of building envelope features (such as, wing walls, bay windows and balconies) on natural ventilation performance and pollutant transportation in residential buildings. This study therefore intends to investigate the effect of surrounding buildings and building envelope features on natural ventilation and inter-unit dispersion. To achieve this research objective, three sub-works were carried out: (a) On-site measurement of outdoor microclimate in street canyons in urban environment, (b) Computational fluid dynamics (CFD) simulation of natural ventilation and inter-unit dispersion in multi-story buildings under the effect of surrounding buildings and (c) CFD simulation of inter-unit dispersion in multi-story buildings with the combined effect of surrounding buildings and envelope features. Compared with an isolated building, deep street canyons created by a cluster of buildings are known to be hampered by low wind speed and high pollutant concentrations, where the latter is especially evident in highly polluted urban areas. On-site measurements were conducted at two locations: a deep street canyon and an open space near an isolated building in Xi'an China in January 2015. During the winter, Xi'an is suffering from serious air pollution due to vehicle emissions, heating combustions and industrial emissions in surrounding areas. The local wind direction, wind speed, air temperature and relative humidity, as well as the three primary ambient air pollutants (PM10, PM2.5 and NO2) were measured. Compared with the scenario of an isolated building in an open space, the presence of surrounding buildings significantly modifies the air flow pattern and environmental quality around a building. The findings from the on-site measurements provided a basic background to the later CFD simulations. CFD simulations were conducted to evaluate the influence of an upstream building on the inter-unit dispersion and natural ventilation performance in the downstream interfered multistory buildings. A tracer gas was employed to simulate gaseous and fine particle pollutants. The presence of an upstream building greatly changes the airflow field and pollutant transportation routes in and around the downstream interfered building. The influence of the height of the upstream building was also examined. Under the normal wind incidence, the low upstream building greatly increases the average indoor air change rate per hour (ACH) values and the pollutant re-entry ratios (Rk) below the source unit on the windward side of the downstream interfered building. Under the oblique wind, a high upstream building greatly increases the average ACH values on the windward side and increases the Rk on the leeward side of the downstream building. CFD simulations were also conducted to investigate the combined effect of surrounding buildings and envelope features on the inter-unit dispersion and natural ventilation performance in multi-story buildings. The effect of three typical envelope features including balconies with upper and lower vents, wing walls, and bay windows were investigated. The results show that the influence of envelope features on ventilation performance and inter-unit dispersion is highly dependent on the surrounding environment including the approaching wind directions, the orientation of the target building and the height of the upstream building. For an isolated building, the presence of envelope features, particularly the vertical wing walls, improves largely the natural ventilation performance of the building but enhances pollutants inter-unit dispersion. However, when considering the effect of an upstream building, the presence of envelope features mostly lowers the natural ventilation performance but weakens the inter-unit dispersion, since the near-wall airflow pattern and pressure distribution are totally redefined.