This study has been conducted to analyze the effect of vehicular blockage on critical velocity and backlayering length experimentally through a 3 m model tunnel (1:50 of a standard tunnel length). Three vehicle sizes in two or three arrays, occupying 3-15% of tunnel cross-section in six scenarios, were used. Based on this study, critical velocity decreases in the case that ventilation velocity reaches the fire source directly. However, critical velocity increases when the path of ventilation flows are blocked by blockages due to an increase in heat release rate through vehicular obstructions. In addition, the influence of blockages on backlayering length and its dimensionless analysis by the ratio of longitudinal ventilation velocity to critical velocity is also presented. It is found that the backlayering length follows an exponential relation with and without blockages. Moreover, the analysis of backlayering length with respect to the blockage section length shows that as backflow occurs along the ceiling and flows over blockage section, its heat transfer to surrounding increases leading to shorter backlayering length due to the enhanced inertial force and heat loss.

backlayering length, critical velocity, heat release rate.