Vehicle Exhaust Distribution In Street Canyons And Its Effect On Built Environment

With the rapid increase of vehicle population, deterioration of air quality caused by the emission of traffic-related pollutant becomes the main environment problem in large cities. The great deal of pollutant emitted from vehicles must influence indoor environment of buildings around streets. Hence, it is necessary to investigate the distribution of traffic-related pollutants and its transfer process into i space of buildings.In this paper, the street canyon, a typical urban microscale environment, was chosen as the research object. A series of works had been carried out around the dispersion of traffic-related pollutant in street canyons and its effect on built environment.Firstly, four commonly used turbulent models based on Reynolds averaged Navier–Stokes equation (RANS) approach, including standard k-εmodel, RNG k-ε, Realizable k-εand RSM model, was evaluated using two numerical examples. One numerical example focused on the pollutant dispersion in a two-dimensional street canyon and the other focused on air flow around a single cube building. The calculation precision of each model was validated using two extensive experimental databases obtained from the atmospheric boundary layer wind tunnel. And among the studied turbulence models, the RNG k-εturbulence model was found to be the most optimum turbulence model to investigate problems in this paper.Secondly, the effects of aerodynamic and thermal factors on air flow structure and pollutant dispersion in street canyons were investigated using CFD technology.The research about street canyon configuration and wind direction showed that following types of street canyon layout were propitious to reduce residence time of traffic-related pollutant in people stay zone: (1) street canyon aspect rate was lower than 1.6; (2) street canyon length to width ratio was lower than 3; (3) the street canyon was belonged to step-up canyons (the down-wind building was higher than the up-wind building). Moreover, axis of street canyons should be avoided to be vertical to the local main wind direction. The optimum location of air intakes was mainly affected by canyon length to width ratio. If the canyon length to width ratio was higher than 5, air intakes should be located on the back fa?ade of buildings. While the canyon length to width ratio was lower than 5, air intakes should be located on the rooftop.When directly using CFD model to simulation the airflow affected by thermal buoyancy, it is too time-consuming to be viable. Hence, a simplified method was used in this paper. A time-saving model was used to calculate canyon surface temperature in advance. The calculated surface temperature was used as thermal boundary condition in the CFD simulation. And special distribution of temperature, airflow velocity and pollutant concentration were calculated through CFD model. Through this method, the surface temperature was decoupled in CFD simulation from air temperature and airflow in the street canyon. Hence, it greatly reduced the calculation amount.The model built in this paper to predict thermal environment in street canyons was based on the theory of thermal balance and atmospheric boundary layer dynamics. This model considered the shade effect of buildings on solar radiation, the multi-reflection of radiation between building surfaces and the road, the effect of vegetation and anthropogenic heat on local thermal environment. Comparing with other models in references, this model used three-dimensional wind model rather than commonly used one-dimensional model to calculate wind speed near canyon surface. As a result, the precise of heating transfer calculation related to convection was improved. Furthermore, heat exchange between the canyon space and the overlaying atmosphere was also modeled based on the classical theory of dynamics of atmospheric boundary layer.The model was validated through a field measurement which is carried out in a street canyon located near the center of Harbin, China. The filed measurement result showed that the model can well predict temperature of canyon surface. Caparing with CFD model, the model built in this paper only needed small calculation source. So it could be also used for long time dynamic analysis of urban thermal enviroment.Using CFD model and the model mentioned above, thermal buoyancy effect on airflow and pollutant disperion in street cnayon was studied. The day simulated was a typical summary day, and the wind speed at the top of street canyon was appointed as 2m/s and 1m/s respectively. The calculation results showed that thermal buoyancy effect could greatly affect street canyon air exchange rate, airflow and pollutant distribution when the ambient wind speed was low. It was also found that the simplified method commonly used in many studies, which only considered a simple surface (such as road surface of windward surface) heating, could not reflect the effect of thermal buoyancy.Finally, a simulation method based on CFD model, model for predicting thermal enviroment of street canyon and the indoor multi-zone network model, was presented in this paper to investigate the effect of traffic-related pollutant on indoor enviroment. This method considered the thermal buoyancy effect, spacial distribution of wind pressure and pollutant at building surface and air exchange induced by thermal pressure. Through this method, the transfer rule of traffic-related pollutant from outdoor to indoor enviroment was investigated under different street aspect ratio, ambient wind speed and wind direction.