Multiscale Coupling of Phase Transition and Urban Flow in Two‐Phase Chlorine Dispersion in Street Canyons

ABSTRACT

Accidental releases of liquefied hazardous gases in urban environments pose severe risks to public safety, yet existing models typically resolve either near‐field two‐phase jets or urban‐scale single‐phase dispersion, neglecting their coupled interactions. In this study, a unified computational framework is developed to explicitly resolve multiscale coupling among source dynamics, droplet phase transition, and turbulent urban flow during two‐phase dispersion of liquefied chlorine in street canyons. Comparative simulations with ammonia are performed to highlight the role of thermophysical properties, and the results reveal that chlorine evaporates more rapidly due to the combined effects of lower latent heat and smaller initial droplet size, resulting in fundamentally different cloud evolutions. The concentration field exhibits strong asymmetry, with enhanced accumulation on the windward side driven by preferential droplet transport and localized evaporation. Hazardous zones at the pedestrian height are quantified using threshold concentrations of 14, 35, and 900 ppm. Among the representative cases considered, wind velocity shows a stronger influence on the extent and evolution of hazardous regions, whereas ambient temperature exhibits a comparatively smaller effect on the final hazard footprint within the examined temperature range. A reduced street aspect ratio enhances end‐region ventilation and suppresses early‐stage hazard development. These results demonstrate that two‐phase effects fundamentally alter dispersion behavior in street canyons and cannot be captured by conventional single‐phase models. The proposed framework provides mechanistic insights and supports improved hazard assessment and urban safety design.