Ke Zhong

Numerical Analysis of Solar Radiation Effects on Flow Patterns in Street Canyons

Abstract A three dimensional computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier-Stokes equations with the standard k-ɛ turbulence scheme is applied to simulate the influence of thermal effects on air flow pattern, pressure distribution and air exchange in a regular street canyon with different thermal conditions and different background wind speeds. The results show that the thermal conditions can greatly affect the flow patterns inside the street canyon, and an impact factor w is defined to discuss the influence of thermal conditions on the static pressure distribution around the buildings. The value of w decreases with the increase of the wind speed for both upwind and downwind buildings. When the wind speed reaches a critical value (i.e., Vc), the influence of thermal effects on the pressure distributions near the buildings can be ignored. The air exchange rate in the canyon remains unchanged when the wind speed is above Vc.

Indoor environmental quality of high occupancy dormitory buildings in winter in Shanghai, China

Maintaining acceptable air quality and providing adequate thermal comfort are conflicting goals for ventilation requirements in crowded, unheated rooms in winter. The former requires a maximal ventilation rate for indoor pollutant removal, but the latter prefers a minimal rate. To understand the indoor thermal environment in winter in the climatic zone that features hot summer and cold winter, the indoor environmental quality in three selected rooms of university dormitory buildings with high occupancy density in Shanghai, China was monitored simultaneously and continuously from December 2010 to January 2011. The results show that, without space heating equipment in winter, the quality of indoor thermal environment in the student dormitory was poor, and that the indoor temperature ranges of the sampling rooms were completely out of the thermal comfort zone as defined by ASHRAE. To maintain an endurable thermal environment, the occupants had to close the windows to reduce the heat loss, which resulted in elevated indoor CO2 concentrations during sleeping hours due to a lack of ventilation. The indoor CO2 levels exceeding 2000 ppm (4000 mg/m3) were observed in several occasions. To improve the indoor air environment, central heating systems or individual heating units should be provided for dormitory buildings in the Yangtze River Delta region.

Air Environment and Energy Performance of a Building Courtyard in the Heating Season

Courtyards are now used in many new buildings in China for taking advantage of better natural ventilation and thus improving the microclimate. However, when the building is equipped with a central air-conditioning (AC) system for space heating in winter, the warm air infiltration from the heated rooms enters into the courtyard and leads to changes in the natural ventilation characteristics in the courtyard. In the present study, the air environment and energy performance of a courtyard style office building in Shanghai were monitored in winter. The internal and external environmental parameters such as air temperature, CO2 and particle concentrations were measured simultaneously and continuously when the building was heated or was not in use. The results show that when there is no apparent pollutant source in the courtyard, a better microenvironment can be established and the air quality in the space is almost the same as the ambient air, because its natural ventilation is enhanced by the warm air infiltration from the heated rooms. Compared to a building without a courtyard, the building envelope heat loss can be reduced by 20% or 11% in winter for buildings with or without a corridor facing the courtyard enclosed by glass windows, respectively, due to the warm air infiltration which increases the air temperature in the courtyard.

Numerical Modeling of the Flow and Pollutant Dispersion in Street Canyons with Ground Thermal Effect

Thermal stratification affects the flow in and above urban street canyons. Such thermal effect is often not noticed, and can lead to pessimistic or optimistic results of the air quality in urban street canyons under calm conditions and low wind speeds. A three-dimensional CFD model is applied to simulate the flow patterns and particle concentrations in a street canyon under different aspect ratios and ground thermal conditions. The model is validated by the experimental data found in the literature. The simulation results are used to evaluate the flow and pollutant dispersion properties in the canyon. The results show that the ground thermal conditions can significantly affect the ventilation performance of the street canyon, which improves with the increased temperature difference (ΔT) between the ambient air and the ground of the canyon. The increased ΔT enhances the buoyancy induced secondary flow in the street canyon and hence reduce the particle concentrations in the canyon, with this influence more pronounced for small street widths.