Evaluation of outdoor thermal comfort under different building external-wall-surface with different reflective directional properties using CFD analysis and model experiment

Abstract

Recently, urban heat island (UHI) has become more intense due to the increase in artificial constructions in cities. In addition, in many office buildings in big cities in Japan, except for indoor workplaces, some outdoor workplaces that surround a building are provided to improve people productivity. Therefore, a comfortable outdoor thermal environment is considered as an important topic. As one of the countermeasures for mitigating the UHI phenomenon, highly reflective (HR) materials are applied to external-wall surfaces of buildings to increase the solar reflectivity of the external-wall surface to finally reduce the heat emitted from the building external walls. These applied HR materials mainly possess diffuse HR (DHR) directional properties. In recent years, retro-reflective (RR) materials have been studied to replace the traditional DHR materials for possible application in building facades. The present study aims to use the computational-fluid-dynamics (CFD) analysis to predict the outdoor thermal environment, including air temperature (Ta), wet-bulb globe temperature (WBGT), and standard effective temperature (SET*) under two different building-facade types (DHR and RR properties). To further analyze the effect of DHR and RR building facades on the thermal environment, in addition to evaluating the three thermal-environment indicators ( Ta , WBGT, and SET*), this study evaluates two outdoor radiation-environment indicators, including sol-air temperature (SAT) and change operative temperature (COT) using an actual facade-model experiment. The results show that the RR building facade are more effective in reducing WBGT, SET*, SAT, and COT. • Determined the reflective directional property of RR materials using our developed optical apparatus. • Simulated urban blocks with different reflective directional building facades (DHR and RR). • Used computational-fluid-dynamics analysis to evaluate the outdoor thermal environmental factors. • Implemented a facade experiment to investigate the effect of DHR and RR facades on outdoor environment. • Demonstrated that RR facade was more effective in reducing the environmental factors, WBGT, SET*, SAT, and COT.