Investigation of critical velocity in curved tunnel under the effects of different fire locations and turning radiuses

Toxic smoke is one of the main threats to people trapped in tunnel fires. Propagation characteristics of smoke flow differ between curved and straight tunnels, depending on the tunnel turning radius. In this study, experiments and numerical simulations were conducted to analyze the critical velocity of a curved tunnel under different fire scenarios, where different heat release rates, tunnel turning radiuses and fire locations were considered. Results showed that the critical velocity varied as the transverse fire location changed, reaching its minimum value when the fire was located at the transverse center of the tunnel, and increased gradually as the fire source approached the tunnel sidewall. For a given fire location, the critical velocity increased with the tunnel turning radius. It is speculated that a decrease in the tunnel turning radius may increase the resistance to smoke flow and has the advantage of preventing smoke propagation. A predicted correlation of the critical velocity considering the effects of different heat release rates, tunnel turning radiuses, and fire locations has been proposed. Predicted results of the improved correlation are consistent with results obtained from the numerical simulation. The outcomes of this study may provide a reference for tunnel ventilation system design.