Brine-water experimental study on the propagation of stratified smoke flow in tunnel fires under subcritical longitudinal ventilation

In the early stage of tunnel fires, longitudinal ventilation velocity is often smaller than the critical velocity to preserve the smoke stratification and therefore facilitate the evacuation. A series of brine-water experiments are conducted to investigate the unsymmetrical propagation of the stratified smoke flow under subcritical ventilation. A light attenuation technique is used to measure the distribution of the reduced gravity in the tunnel. The ventilation velocity V and the source buoyancy flux per unit width B-0 are two dominant parameters for the flow. The reduced gravities at both sides of the source are quantitatively compared, and then a model is proposed to estimate the reduced gravities under different B-0 and V. In terms of the propagation velocity, two flow regimes are observed. When V/B-0(1/3) is smaller than 0.56, both the propagation velocity of the downstream smoke front, ud, and that of the backlayering smoke front, ub, are independent of time; moreover, ud is smaller than the propagation velocity of the smoke following the downstream smoke front. However, when V/B-0(1/3) is larger than 0.56, ud is still independent of time, but ub decreases with time; furthermore, ud is approximately equal to the propagation velocity of the smoke following the front. Prediction models regarding the propagation velocities at both sides of the fire are established. As the backlayering flow advances forward, its thickness approximately linearly decreases. The maximum thickness of the backlayering flow is correlated with V/B-0(1/3). The study might be useful for determining the ventilation and evacuation strategies in tunnel fires.