Smoke bifurcation-induced low-temperature region in longitudinally ventilated tunnel fires: Phenomenon, modelling, and full-scale demonstration

Understanding smoke behavior in tunnel fires is essential for designing effective smoke control and evacuation strategies. This study investigates the behavior of smoke spread and bifurcation in longitudinally ventilated tunnel fires through a combination of full-scale fire experiments and numerical simulations. The findings show that intense ventilation disrupts the continuity of the downstream smoke layer and triggers the smoke layer bifurcation. This process is characterized by a three-dimensional spiral vortex structure, involving both radial and longitudinal diffusion, driven by the combined effects of thermal buoyancy, inertial forces, and sidewall confinement. The smoke bifurcation leads to a redistribution of mass and heat within the smoke layer, forming a central low-temperature region beneath the tunnel ceiling. The range of the smoke layer expands with increasing tunnel aspect ratio and ventilation velocity but decreases with higher heat release rates. Finally, a theoretical model to estimate the length of low-temperature region is proposed and validated, accounting for the complex interactions between the Coanda effect, anti-buoyancy sidewall jets, and plume behavior. This work challenges the traditional one-dimensional smoke flow assumption and offers a new understanding of the complex three-dimensional nature of smoke transport in tunnel fires, providing a more robust foundation for tunnel ventilation design and emergency response planning.