Numerical investigation on the effect of ambient pressure on mechanical smoke extraction efficiency in tunnel fires

Abstract This paper has analyzed the ambient pressure on the effect of the efficiency of the mechanical smoke extraction system in tunnel fires. These were simulated by Large Eddy Simulation (LES) with Fire Dynamics Simulator (FDS). The simulation results indicate that it is not the instance that the higher the exhaust rate, the better the smoke exhaust efficiency of the exhaust vent. There is a critical exhaust rate at which excessive fresh air entrainment, such as plug-holing of the smoke layer, can be avoided. The mechanical smoke exhausting process is affected by two competitive forces: horizontal inertial force and vertical suction force. The thickness and horizontal velocity of the thermal smoke layer in the tunnel increase as the ambient pressure decreases. At this time, the vertical suction force required for the occurrence of the plug-holing phenomenon increases, and thus the critical exhaust rate increases. A linear relationship between the dimensionless critical exhaust rate V c r i ∗ and the modified heat release rate Q ˙ ∗ was proposed to characterize the influence of ambient pressure on the mechanical smoke exhaust efficiency. When the exhaust rate is 0.5 times the critical exhaust rate, the smoke mass fraction exhausted from the exhaust vent (E) reaches the maximum value.