Experimental study on ceiling smoke temperature distribution characteristics in bifurcated tunnel fires under branch longitudinal smoke extraction

Smoke control in underground traffic bifurcated tunnels is challenging because fire-induced smoke can spread through the main tunnel and branch tunnel, forming complex smoke transport paths. Branch longitudinal smoke extraction provides a potential approach for confining smoke near the bifurcation, but its ceiling smoke temperature characteristics and smoke confinement velocity remain unclear. In this study, 1:15 reduced-scale experiments were carried out in a bifurcated tunnel model to investigate the effects of ventilation velocity, distance between the fire and the bifurcation, and heat release rate on the ceiling smoke temperature distribution. Results indicate that when the fire is located at the bifurcation, increasing the ventilation velocity reduces the ceiling maximum smoke temperature rise in the main tunnel and accelerates the longitudinal temperature decay. When the fire is located in the upstream of the bifurcation, the ceiling maximum smoke temperature rise varies slightly with ventilation velocity and fire location within the tested range, whereas the temperature decay upstream of the fire and downstream of bifurcation are strongly affected by branch longitudinal smoke extraction. In contrast, the temperature decay between the fire and the bifurcation is weakly influenced. Predictive models were established for the ceiling smoke maximum temperature rise and the longitudinal temperature rise decay. Moreover, a dimensionless ventilation velocity of 𝑣 * 𝑒 ≥ 0 . 5 8 was proposed to confine hot smoke near the bifurcation or within the fire-bifurcation region. The results provide a quantitative basis for emergency ventilation design and smoke-confinement control in underground traffic bifurcated tunnels.