Use of the longitudinal critical velocity in tunnel fires may destroy the stratification of smoke downstream, which is not conducive to the evacuation of downstream tunnel users. Reducing the longitudinal velocity can maintain stratification, but the appropriate longitudinal velocity range is unknown. Stratification of smoke under the longitudinal velocity is studied using Fire Dynamics Simulator (FDS). The aims of this paper are to explore the relationship between the longitudinal velocity and smoke stratification and to obtain a proper velocity range that can maintain clear downstream stratification. Based on Newman`s theory, the “stratification velocity” is proposed, which is the maximum velocity that maintains downstream smoke stratification. A good correspondence between the average Froude number within 200 m downstream and the longitudinal velocity and heat release rate (HRR) is found. The correlation between the temperature stratification and Froude number is obtained.Fr?=?0.49 can be used to distinguish whether the stratification is clear. The relationship between the critical velocity and the stratification velocity is also discussed.
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