Smoke movement in tilted tunnel fires with longitudinal ventilation

• A tunnel model tilted at angles 0°, 3°, 6° and 9° to the horizontal was used.
• Critical velocity for preventing backlayering was studied by modeling experiments.
• Non-dimensional critical longitudinal velocity was related to heat release rate.
• Higher critical velocities for preventing backlayering required in tilted tunnels.
• A corrected empirical formula for critical velocity in a tilted tunnel is proposed.

Studies on smoke movement in a tilted tunnel fire with longitudinal ventilation will be reported in this paper. Analytical equations on fires acting at the smoke layer were studied first. Numerical simulations with Computational Fluid Dynamics (CFD) were carried out. Physical scale modeling experiments were performed to study the critical velocity for preventing back-layering in a tunnel model with tilted angles at 0°, 3°, 6° and 9° to the horizontal. Froude number modeling was used to deduce empirical formula on the location of smoke stagnant point under longitudinal ventilation. The normalized heat release rate Q̇″ is a key factor and the critical value of Q̇″ is 0.12. This study was focused on Q̇″ less than 0.1 while deriving the relations between critical velocity and heat release rate. The results are similar to some findings reported earlier in the literature. Non-dimensional critical longitudinal velocity for preventing back-layering effect was found to be varied with heat release rate to the power 1/3. The critical velocities for preventing back-layering for tilted tunnels are higher than the values required for horizontal tunnels. However, a corrected empirical formula for critical velocity in a tilted tunnel is proposed based on the experimental and numerical results.