Enhancing Fire Safety in Urban Bifurcated Tunnels An Investigation of Slope and Curvature Effects on Fire Dynamics and Smoke Temperature Distribution

Urban underground tunnels, particularly bifurcated roads, are essential to modern transportation systems but face significant fire safety challenges. This study investigates the fire resilience of urban bifurcated tunnels under natural ventilation, focusing on how the ramp slope and curvature influence the fire thermal environment and the associated safety implications—a topic with limited quantified research. By combining model-scale fire experiments with numerical simulations, the research quantifies the effects of tunnel geometry—specifically, slope and curvature—on smoke temperature distribution, flame morphology, and longitudinal temperature attenuation. Key findings show that increasing the tunnel slope accelerates smoke flow downstream, resulting in significant temperature increases, while curvature has a comparatively subtler influence on the transverse distribution of high-temperature zones. The maximum observed temperature difference across curvature variations was limited to 20 K (less than 5% variability), whereas slope variations induced temperature changes of up to 70 K (approximately 17% variability). A predictive model for maximum temperature rise and longitudinal temperature attenuation was developed based on these findings, offering essential insights for fire-safe underground infrastructure design. This work advances fire safety standards and informs emergency response strategies in complex urban tunnel systems.