Impacts of ambient pressure on the stability of smoke layers and maximum smoke temperature under ceiling in ventilated tunnels

In tunnel fire, smoke is a great threat to successful emergency escapes, and its spreading patterns at high altitude tunnel may differ. Therefore, the impact of ambient pressure on the stability of smoke layers and maximum smoke temperature under ceiling in ventilated tunnels were investigated in this study. Results show that the temperature of smoke layer is negatively correlated with the longitudinal velocity and ambient pressure. Besides, the influence of longitudinal velocity at different ambient pressures on the shear velocity between the smoke and air layers was further studied. Using Richardson number ( Ri) and Froude number ( Fr), smoke flow can be classified into three patterns, namely stable and obvious smoke stratification ( Ri>3.2 or Fr < 0.38), a stable smoke layer but with blurred interface (2.3 < Ri < 3.2 or 0.38 < Fr < 0.53) and a completely unstable smoke stratification ( Ri < 2.3 or Fr > 0.53). Furthermore, a model has been developed to predict maximum smoke temperature under the tunnel ceiling, which agrees well with previous studies at standard atmospheric pressure. The results of the present work provide information for tunnel structure protection and safe evacuation in fire accidents at different ambient pressures.