Thermal and flow characterization of supplementary airflow and ceiling temperature in tunnels with varying fire positions

To detect fire risks and ensure sufficient safety for the tunnel structure, it is essential to estimate the maximum ceiling gas temperature to which the structure is subjected. Nonetheless, research on the maximum ceiling gas temperature influenced by asymmetric supplemental airflow remains insufficient. This study conducts a series of numerical simulations to investigate the impact of longitudinal fire position on gas distribution and the maximum ceiling gas temperature in tunnels. The heat release rate increases from 3 to 10 MW, while the upstream-to-downstream length decreases from 1:1 to 1:8. Considering the velocity flow field characteristics in computational fluid dynamics simulations, the effects of the longitudinal fire position on the asymmetric airflow and the ceiling maximum temperature rise in the tunnel were studied. The results show that as the fire source gradually approaches the upstream end, the velocity difference between the upstream and downstream gradually increases, resulting in different airflow velocities on both sides of the near flame, which leads to the flame tilting to the downstream, thus affecting a series of fire thermal properties. Then, an ideal fire scenario with openings at both ends but only one end with supplementary air is proposed. In this scene, the influencing factor of fire thermal performance is only the velocity difference. It is verified in the longitudinal ventilation scenario, and the results are in good agreement to prove its effectiveness, thus making the temperature prediction formula in the longitudinal ventilation tunnel still have certain applicability in the natural ventilation tunnel.