Fire-induced temperature distribution beneath ceiling and air entrainment coefficient characteristics in a tunnel with point extraction system

This study investigates the effect of a point extraction system on air entrainment characteristics of a one-dimensional smoke movement stage in a ventilation tunnel under various ceiling extraction velocities. Reduced scale model tunnel experiments are conducted. The vertical smoke layer temperature is recorded, and the variation characteristics of smoke layer thickness are calculated by an integral ratio method. Results show that the air entrainment coefficient increases with fire heat release rates for a given ceiling extraction velocity, but decreases with increasing ceiling extraction velocities for a given heat release rate. A dimensionless factor f(V) is introduced and the revised model of air entrainment coefficient under various ceiling extraction velocities is proposed. The entrainment coefficient for smoke flows in tunnel fires is found to be a function of the Richardson number. All the experimental data are well correlated by the new proposed model for the air entrainment coefficient with a single-point extraction system. This study proposes new empirical entrainment design formulae of tunnel fires under the effect of point ceiling extraction system.