Numerical Investigation of Stratification Parameter Effect on Natural Ventilation Performance

This paper analyses numerically (by Large Eddy Simulation) the natural ventilation effect by a duct exhaust system on smoke thermal stratification with increasing duct section and duct height in a tunnel fire. The flow pattern behavior in the tunnel with duct is described first. Then an application method for the stratification parameter « S» is presented. For the larger duct section, results show that exist a strong disturbance within the smoke layer under the ceiling with severe instability occurrence between the hot and the cold layers. However, in case of the higher duct, the stack effect appears and increases to a critical condition in which the smoke area thickness beneath vent decreases to 0, leading to the occurrence of the plug-holing phenomenon. In these cases, the stratification was purely destroyed. Result show also that the stratification was obtained where the stratification parameter seems to be greater than 1 and a correlation criterion to characterize the smoke flow stratification is predicted.