Coupled CFD, Radiation and Building Energy Model for Studying the Impact of Building Height Topology and Buoyancy on Local Heat Island Formation in Urban Environments

In past decades urban areas grew continuously. Due to the urban heat island (UHI) effect the temperatures in urban areas are increased. The local microclimate strongly influences the human comfort and health in urban areas as well as space cooling and heating demands of buildings. The mechanisms causing the UHI effect have to be understood in detail to be able to improve the human comfort and decrease the energy demand in urban areas. One of the most efficient ways of removing heat from urban areas is wind driven ventilation. Knowing the detailed heat fluxes caused by forced (wind) and free (buoyancy) convection is important to be able to design urban areas such to improve the removal of heat. In this study flow fields and heat fluxes are analysed for a generic urban area with 23 buildings. Six different urban topologies are studied, where the footprints of the buildings and the total volume of the 23 buildings is kept constant while the building height of the individual buildings is changed (figure). The impact of the different building height topologies on local air temperatures is studied with coupled CFD (computational fluid dynamics) and building energy simulations. The coupling approach is presented in Allegrini et al. 2015a. Convective and turbulent heat fluxes as defined in Allegrini et al. 2015b are considered. The simulations are conducted for different wind speeds to investigate the impact of buoyancy on the local heat island formation. The results show that the average temperatures are similar for cases with different building height topologies and the same wind speeds, but significant differences are observed regarding local air temperatures. The average temperatures in the urban environments change depending on the wind speeds and if buoyancy is considered in the simulations or not. A detailed analysis based on a comparison of simulations where buoyancy is neglected or considered, shows a strong impact of buoyancy on the averaged and local air temperatures. Buoyancy increases the ventilation of the urban environment and lowers the air temperatures. This leads to lower local air temperatures for cases with the lower wind speeds used in this study. Finally, a correlation is proposed showing that the normalised increase in local air temperature is linked to the air volume fluxes (convective exchange) and local thermal diffusivity (turbulent exchange). This means that with higher air volume fluxes and higher thermal diffusivities more heat can be removed from the urban environment leading to lower local heat island intensities. If the air volume fluxes and the thermal diffusivities are low, the local air temperatures have to increase, because the heat from the buildings has to be removed in smaller air volumes. The correlations show that the heat removal potential from an urban environment can be estimated directly from the flow field, when the velocities and momentum eddy diffusivity are known (using the turbulent Prandtl number).

References:

• Allegrini J, Dorer V, Carmeliet J: Influence of morphologies on the microclimate in urban neighbourhoods. Journal of Wind Engineering and Industrial Aerodynamics 144: 108-117, 2015a
• Allegrini J, Dorer V, Carmeliet J: Coupled CFD, radiation and building energy model for studying heat fluxes in an urban environment with generic building configurations. Sustainable Cities and Society 19: 385-394, 2015b