Study on the influence of branch slope on smoke movement mechanism in bifurcated tunnel fire under natural ventilation

In order to investigate the impact mechanism of branch slope on fire smoke in underground bifurcation spaces, a series of small-scale fire experiments were conducted. This study measured relevant data under natural ventilation conditions for four different branch slopes and six fire power levels, focusing on the highest ceiling temperature in the tunnel and the attenuation pattern of ceiling temperatures along the longitudinal direction in both the main and branch tunnels. It also analyzed the temperature field, velocity field, and pressure field at the bifurcation. The results indicate that a decrease in branch slope leads to an increase in the highest temperature in the main tunnel, primarily due to the enhanced chimney effect that intensifies the entrainment of smoke and accelerates the flow of fire smoke. A modified model for the highest temperature, considering the influence of branch slope, was proposed. Branch slope has a certain impact on the longitudinal temperature attenuation of the tunnel ceiling, with a relatively weaker effect on the main tunnel. Based on empirical formulas, a formula for longitudinal temperature attenuation involving slope, including both the main and branch tunnels, was developed. Fire smoke accelerates at the bifurcation point, a phenomenon that intensifies with an increase in the slope of the branch tunnel. The research in this paper provides a reference for fire safety issues in bifurcated tunnel structures.