Backlayering Distance of Thermal Fumes in Tunnel Fire Experiments Using a Large-Scale Model

In this study, the backlayering distance of thermal fumes in a tunnel fire was examined by using a large-scale model tunnel. A 1/5-scale model was constructed taking into consideration both the similarity of thermal characteristics in the wall and Froude similarity. Experimental parameters were the heat release rate of the fire source and the longitudinal ventilation velocity. The following conclusions were obtained. A new expression was proposed to calculate the dimensionless backlayering distance based on the dimensionless heat release rate and Froude number. Constants included in this expression were obtained from experimental data by using the least-squares method. The backlayering distance calculated by the new expression was compared with that calculated by previous expressions developed by other researchers. The new expression has the ability to calculate not only the backlayering distance but also the critical velocity