  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.
