Experimental Study on Mixed-convective Flows at the Building Scale and Analysis of the Local Convective Heat Transfer Coefficient at the Building Façades

The combined effects of urbanization and global warming give rise to temperature increases in urbanized areas. This phenomenon is known as the Urban Heat Island (UHI) effect. As a result of climate change, the frequency and intensity of heat waves will keep rising in the future. The combination of these effects has a negative impact on the health and thermal comfort of inhabitants, as well as on building energy consumption. Effective ventilation of urban outdoor areas is important for increasing urban air quality as it facilitates the removal of heat and pollutants. Urban flows are driven by the combined effects of wind and buoyancy, which are induced by increased building and ground surface temperatures. The understanding of mixed convective and buoyant flows governing urban microclimates is therefore meaningful for identifying strategies which can mitigate the impact of extreme weather, such as the use of green surfaces or reflective paints. It can also provide insights for the development of efficient control and management strategies of building installations taking the Urban Heat Island effect into account.

Wind tunnel measurements were conducted to study the influence of buoyancy on the flow in scaled urban street canyons with heated surfaces under different building configurations and roof geometries. The temperatures of each building surface and the ground were controlled independently in order to reproduce the characteristics of real-word urban environments, taking the effects of mutual shadowing and high thermal absorption surfaces into account. In contrast to field measurements, laboratory experiments allow for flow condition control and high spatial resolution measurements of entire flow fields. Particle Image Velocimetry (PIV) was used to analyze the main flow and turbulent properties in a section of the street canyon. Experiments were conducted using purposely designed buildings consisting of a heated aluminum core and a thin outer layer of Teflon. Infrared thermography was used to determine the temperature field on the building façades as well as the temperature distribution inside the canyon through the use of a non-conductive mesh. The distribution of the convective heat transfer coefficient was computed from the wall temperature fields acquired through infrared thermography.

The results were compared for different building configurations in order to assess various geometries in terms of air exchange rate and heat removal potential, thereby identifying promising layouts and setting the stage for  future research towards more detailed analyses including larger numbers of buildings and a wider range of flow conditions. Moreover, the results provide a data set of simultaneous flow, temperature and heat flux fields, which is necessary for the validation of numerical models.