Airborne pollutant dilution inside the deep street canyons subjecting to thermal buoyancy driven flows: Effects of representative urban skylines

The air flow and pollutant dispersion within a group of street canyons ventilated merely by thermal buoyancy force induced by heated building surfaces are examined by CFD model using SST k-omega turbulence model for different urban skyline configurations. Pollutants emitted from the bottom of street canyon roughly mimic the traffic exhaust releasing. Numerical results are validated well with former theoretical results on thermal boundary flow adjacent to a heated vertical wall. The air exchange rate per hour (ACH) and pollutant retention time are adopted to evaluate the canyon ventilation performance. An exponential relationship could be established between the pollutant retention time and the thermal boundary flow rate. A semi-empirical formula is proposed by using the theoretical results of thermal boundary layer and two empirical constants derived from the present simulation results, which could be used to evaluate the ventilation performance at the urban design. As the convergence flow at the street canyon roof decays from urban rim to urban center, the pollutant retention time differs from canyon to canyon. The protuberant skyline configuration is found more effectively in purifying the street canyons at urban edge, in contrast, the concave skyline configuration shows higher purification efficiency at urban center. Present research could benefit for design purpose and environmental impact assessment.