Thermal stratification characterisation of large tunnel fires under forced ventilation

Large tunnel fires exhibit thermal stratification dynamics that are distinct from small fires, yet most existing correlations for smoke stratification are derived from small-scale or reduced-scale experiments. This computational study investigates thermal stratification in tunnels using two high-intensity fires (30 MW and 80 MW) under varying ventilation velocities. Unlike previous research that assumes ambient floor temperatures, the results show that large fires generate floor temperatures significantly above ambient due to radiative heat transfer from the extended ceiling flame. The critical temperature ratio for stable stratification was found to be $\Delta T_{cf}/\Delta T_{avg} = 1.9$, compared to the traditional value of 1.7, with a corresponding critical Froude number of 0.858. A new empirical correlation relating the temperature ratio to the Froude number is proposed and compared against existing correlations from the literature. The discrepancy in critical Froude numbers reported across previous studies is discussed, and the limitations of the Froude number as a sole descriptor of stratification stability in large fires are highlighted.