Study of smoke back-layering length with different longitudinal fire locations in inclined tunnels under natural ventilation

Abstract To investigate the vehicle fire hazard at different locations in inclined tunnels, the effect of longitudinal fire locations on smoke behavior characteristics induced by inclined tunnel fires was proposed by simulation and theoretical analysis. A series of tunnel models with slope of 4% was conducted by Fire Dynamics Simulator (FDS) software for fire simulation, in which three fire scenarios were considered corresponding to the variation of fire location, upstream length and downstream length respectively. The results show that smoke back-layering length drops progressively with increasing of downstream length, and upstream inlet airflow velocity is linearly positively correlated with downstream length, while the upstream length has a limited impact on smoke movement. A new prediction equation of the dimensionless smoke back-layering length account for downstream length and tunnel slope was established, through the addition of height difference between fire source and downstream exit. The established correlation shows that the dimensionless smoke back-layering length is logarithmically related to cubic power of downstream length, and the predicted results agreed well with simulated data as tunnel slope from 3.5% to 7.5%. The results can provide beneficial suggestion in ventilation system design and emergency evacuation for inclined tunnels.