The influence of cross-sectional aspect ratio on ceiling temperature profile and mass flow rate of ceiling jet in tunnel fires

In this paper, the influence of tunnel cross-sectional aspect ratio on the ceiling temperature profile and mass flow rate (MFR) of ceiling jet is studied theoretically and numerically, and 13 tunnel cross sections with different aspect ratios ($\xi$) are considered. A total of 26 full-scale numerical simulation cases are conducted using Fire Dynamics Simulator, and small-scale experiments are used to verify the accuracy of the simulations. Results show that the maximum ceiling temperature is more sensitive to the tunnel height and decreases with increasing aspect ratio, which can be divided into two regions, $\xi$ <1 and $\xi$ ≥1. When $\xi$ ≥1, the maximum ceiling temperature varies more linearly. The initial locations of the one-dimensional spread for the tunnel with different tunnel cross-sectional aspect ratios are similar, which are concentrated at 15–20 m from the fire source when taking the MFR increase rate of 0.001 as the criterion. By introducing the sectional coefficient, the MFR model and temperature attenuation model of ceiling jet are developed for the tunnels with $\xi$ <1 and $\xi$ ≥1, respectively. The results of this paper could provide definite reference value for the smoke control in tunnel fires.