Influence of Thermal and Moisture Capacities of Building Materials on Urban Microclimate by Multi-transport Modeling

Building materials that are commonly used in the built environment, e.g. concrete, asphalt, brick, play an important role in the absorption, transport and storage of heat and moisture. It is known that the temperature difference between air and building surfaces, responsible for buoyancy to occur, has a significant effect on the air-flow regime and, hence, the temperature in the urban environment. In addition, moisture redistribution within the built environment complicatedly influences the microclimatic conditions, as moisture is the key agent of evaporative cooling, as well as affecting the thermal capacity and conductivity of building materials. Local temperature distribution is also affected by parameters such as prevailing wind speed and direction, building and urban environment geometry and orientation with respect to the sun. Furthermore, these effects on the urban microclimate are inherently tridimensional. For example, convective heat and mass transfer coefficients, as well as the wind-driven rain intensity, have patterns on building facades with higher values near the top and side edges, around which an accelerated wind flow is observed. The present study makes use of a fully-integrated urban microclimate model, which takes into consideration wind flow, temperature and relative humidity in the air, and temperature and moisture content for building materials. Direct and diffuse solar radiation as well as thermal radiation with diffuse reflections are taken into account. The model calculates the distribution of wind-driven rain intensity using an Eulerian multiphase model. Conjugate heat and mass transport in air and building materials are coupled in such a way that the steady Reynolds-averaged Navier-Stokes (RANS) equations are solved together with unsteady equations of heat and moisture transfer in building materials for each timestep in a daily cycle of ambient temperature and relative humidity on a typical day. This approach is valid due to the fact that the time scale of transport in building materials is larger than the time scale of transport in air. The model allows for detailed spatial analysis of the cooling effect on the surfaces and air, as well as different contributions to cooling, such as convective cooling, sensible heat transfer due to rain and evaporation. Modeling results give information on the thermal storage within the street canyon surfaces, allow to compare heat-removal mechanisms and to analyze the universal thermal comfort index (UTCI) at different times of a day. The present study assesses the influence of heat storage in the built environment on local thermal conditions. To achieve this, an urban geometry composed of idealized street canyons is considered with a fully-developed approach flow. For simplicity, the conditions such as wind speed, wind direction and street-canyon orientation are kept constant. Heat and moisture transport within the street-canyon surfaces, i.e. building facades and ground, are coupled with the air flow. The results show, in general, larger spatial gradients in street-canyon surface temperatures at noon than at night time. For both night and day times, the highest surface temperatures are observed at the center of the street-canyon surfaces. Heat is stored in the street-canyon ground between sunrise and afternoon, period after which the heat flow switches direction. We found the largest difference between the ambient temperature and the average air temperature within the street canyon at pedestrian height to be, for this specific case, 1.5°C in the afternoon and the lowest 0.5°C just before the sunrise. In on-going work, the model is used to quantify the effect of different building materials as well as different surface albedos on the thermal comfort throughout the day.