Coupled CFD, radiation and porous media model for evaluating the micro-climate in an urban environment

• A coupled model to study effect of evaporative cooling in urban area is proposed.
• Drying of a wet porous windward wall for two different wind speeds is studied.
• Lower surface and air temperatures caused by evaporation improve the comfort.
• Less comfortable conditions were observed for low wind speed case.
• Cooling efficiency was higher during low wind speed case.

This paper introduces a coupled model to predict the micro-climatic conditions in an urban street canyon. The model couples three sub-models: (i) a Computational Fluid Dynamics (CFD) model, which solves the convective heat, air and vapor transport, (ii) a Building Envelope Heat and Moisture (BE-HAM) model which solves heat and moisture transfer and storage within porous materials, and (iii) a radiation model (RAD) which accounts for the radiative heat exchange between the urban surfaces and the sky. The applicability of the model is demonstrated by two case studies. In both, the effect of evaporative cooling on the surface and air temperatures in a street canyon as well as on the thermal comfort is evaluated. The case studies differ in the free-stream wind conditions. We found that the reduced surface and air temperatures during evaporative cooling have a positive influence on the comfort sensation. Nevertheless, this effect is partly canceled out by the increase in vapor pressure. Comparing both cases, we observed less comfortable conditions for the case with lower free-stream wind speed due to the more pronounced buoyancy effects and the reduced mixing of the air in the street canyon. The case studies illustrate the complex interplay among temperatures, vapor pressures, and wind speeds, as well as the spatial and temporal variation of these quantities, which stresses the need for detailed modeling approaches, accounting for the full complexity of the urban micro-climate.