Thermal plume diffusion on building facades: Mechanisms and model development under low wind speeds

Understanding the diffusion behavior of thermal plumes released from building façades is critical for mitigating localized heat accumulation and enhancing urban thermal comfort, especially under low wind conditions commonly found in dense cities. This study aims to investigate the formation, diffusion, and rise characteristics of façade-induced thermal plumes under calm and low-wind environments. To achieve this, a combination of wind tunnel experiments using a static wind platform, hot-wire anemometry, and Particle Image Velocimetry (PIV) was employed, alongside computational fluid dynamics (CFD) simulations. The results show that thermal plumes primarily exhibit strong vertical rise with limited lateral diffusion, resulting in significant local heat buildup near façades. Wind direction and façade heat flux density notably influence plume behavior, compressing plumes under windward conditions and enhancing vertical rise under leeward scenarios. The rise angle decreases with increasing Richardson number (Ri), and this relationship is well described by the functions y=52.6×Ri/(4.8+Ri) and y=45.3×Ri/(4.1+Ri). Additionally, semi-empirical formulas for plume diffusion were proposed for static wind scenarios, with correction factors introduced to adapt to varying wind speeds. This research provides a novel coupling of experimental and numerical approaches to quantify façade thermal plume dynamics and offers practical insights for optimizing building design, improving natural ventilation strategies, and informing sustainable urban microclimate planning.