Theoretical model for predicting smoke back-layering length downstream of centralised ceiling exhaust vent in tunnel fires

Abstract

Passing a certain amount of smoke through a centralised ceiling exhaust vent with an acceptable back-layering length can considerably reduce the smoke exhaust rate of a centralised exhaust system. Accordingly, this study conducted a theoretical analysis of the back-layering length of smoke downstream of the centralised ceiling exhaust vent and proposed a theoretical prediction model considering the smoke exhaust efficiency, exhaust vent layout, and the distance between the fire source and vent. This model can be applied across multiple exhaust vent layouts with a wide range of smoke exhaust rates. In addition, the theoretical prediction model was verified with a series of reduced-scale experiments. Thereafter, we compared the proposed theoretical model with existing relevant models. The results revealed that the existing models are applicable only for a specific range of the smoke exhaust rate because these models were either empirically derived for a particular range of smoke exhaust rate or developed assuming pure smoke flow in the vent. Furthermore, the proposed model was employed to quantitatively analyse the influence of the exhaust rate, exhaust vent layout, and the distance between the exhaust vent and fire source on the smoke back-layering length. Finally, we discussed the contribution of the proposed model for optimising the exhaust vent layout and reducing the smoke exhaust rate of the system.