A simulation study on the effect of bifurcation angles on the temperature characteristics in bifurcated tunnel fires

The fire safety design of bifurcated tunnels poses unique challenges, mainly due to complex smoke dynamics. These tunnels are common in modern urban underground traffic networks. This study presents a comprehensive numerical investigation into how the bifurcation angle (θ) influences the maximum gas temperature rise ( ΔT max ) beneath the ceiling and its longitudinal decay in naturally ventilated bifurcated tunnel fires. We used the Fire Dynamics Simulator (FDS) to conduct high-fidelity simulations. The bifurcation angle (30°, 45°, 60°, 90°) and the heat release rate (5–30 MW) were systematically varied. Dimensional analysis and simulation data reveal that the classic dimensionless relationship ΔT max /T 0 ∝ kQ * 2/3 remains valid, but the proportionality constant k is strongly geometry-dependent. We established a generalized empirical model: ΔT max /T 0 = (5.855 + 1.921sinθ) Q * 2/3 . This model shows that ΔT max increases with the bifurcation angle for a given fire size. Furthermore, we analyze the longitudinal temperature distribution. In the main tunnel upstream and downstream of the fire source, temperature decay follows a double-exponential model. In the branch tunnel, the decay characteristics are significantly modulated by θ. We provide predictive correlations for all key decay parameters as functions of sinθ. This work provides fundamental insights and practical, quantitative tools for performance-based fire-safety design.