CFD characterization of street canyon heating by solar radiation on building walls

Abstract: Heat exchange processes between building walls and external air within street canyons is an important topic in air quality modelling considering that the thermal fields largely affect local flow dynamics and pollutant concentration distribution. Despite the abundance of numerical studies, many questions still remain unanswered, ultimately limiting the inclusion of heat effects in mesocale atmospheric models. Of interest in this study is the assessment of radiative processes at building walls in terms of emissivity (i.e. is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature), shape factors (i.e. the proportion of the radiation which leaves the surface of a building and strikes toward the surface of another building) and their relation to the Richardson number. The study is approached via the computational fluid dynamics code OpenFOAM, with large eddy simulations (LES) extension to model turbulence within the canyon and Boussinesq approach to model heat transfer. Consequences on pollutant concentrations are also analysed. A model for setting the thermal boundary conditions on the building surfaces is developed, based on a database that gives the correlations between the emissivity of a surface and the heat flux emitted by this surface, under certain weather conditions, as a function of the latitude and longitude of the surface, the orientation, the time of the day. As a novel result an exponential law between the transfer coefficient and Ri (as well as with the emissivity) is proposed as a function of the canyon aspect ratio.