Effect of inclined mainline on smoke backlayering length in a naturally branched tunnel fire

Abstract

In this study, the effect of the slope of the mainline tunnel on the characteristics of smoke movement and the distance of smoke backflow in a branched tunnel with an inclined downstream mainline was investigated. The downstream mainline tunnel slope varied from 0% to 7% at intervals of 1%. A virtual wind velocity was proposed as a means to correlate with the airflow velocity induced by the stack effect. The results showed that a significant airflow velocity was formed in the branched tunnel with an inclination of the mainline before shunting. When the tunnel slope and fire size were larger, the induced airflow velocity was enhanced due to the greater thermal pressure difference induced by the stack effect. The effect of the bifurcation angle on induced airflow velocity was limited, but could not be neglected under relatively large heat release rates. The smoke was well controlled into the horizontal mainline region due to the induced wind by the stack effect. The backlayering length was slightly reduced under stronger heat release rates but was more sensitive to the slope of the mainline tunnel. A prediction model for smoke backlayering length in a branched tunnel with a tilted downstream mainline was developed based on dimensionless velocity. The predicted value of the smoke backlayering length agreed well with the simulated results. This study contributes to the understanding of smoke movement in naturally branched tunnels with inclined downstream sections and guides extraction design.