Numerical modeling on mechanical smoke extraction efficiency of multiple lateral smoke extraction vents system in an immersed tunnel

In the case of immersed tunnel fires, lateral smoke extraction is a method employed to control the smoke movement with vents set in tunnel sidewalls due to limited space in height. This paper has investigated the smoke extraction efficiency of multiple lateral vents system using large eddy simulations (LES) in a fire dynamics simulator (FDS). The analysis is done on the impact of the lateral vent aspect ratio AR (n), lateral extraction velocity (v), and vent spacing distance (d) at various heat release rates HRRs (Q) on smoke extraction efficiency (E). The results show that the vent spacing distance (d) has a negligible impact on the smoke extraction efficiency. The smoke mass flow rate and smoke layer thickness in the tunnel changed significantly because of lateral extraction vents. The smoke extraction efficiency increases with dimensionless HRR Q* and vent aspect ratio n at constant lateral extraction velocity v, but decreases drastically with increasing dimensionless lateral extraction velocity v* due to an increase in fresh air drawn into the lateral vents. The smoke extraction efficiency reached to 100% at 1 m/s extraction velocity for vents of 3.25 aspect ratio. Finally, a correlation for smoke extraction efficiency in one-dimensional smoke spread stage was established based on dimensional analysis, which can provide a reference guide for designing a ventilation system in tunnels with lateral vents.