Study of critical velocity and backlayering length in longitudinally ventilated tunnel fires

Experimental tests and theoretical analyses were conducted to investigate the critical velocity together with the backlayering length in tunnel fires. The experiments were performed in two longitudinally ventilated model tunnels. The proposed correlations for critical velocity are found to comply well with experimental data in both tunnels. The critical Froude number and the critical Richardson number were analyzed using the experimental data. The backlayering length was related to the ratio of longitudinal ventilation velocity to critical velocity. Experimental data show that the relation between the ratio of ventilation velocity to critical velocity and the dimensionless backlayering length follows an exponential relation. A correlation based on experimental data to predict the backlayering length is proposed. Further, comparison of experimental data of critical velocity and backlayering length with results from large-scale tests shows that there is a good agreement in both scales. The effect of accident vehicle obstruction on critical velocity and backlayering length was also analyzed. Experimental data show that the decrease in rate of critical velocity due to obstruction is slightly greater than the ratio of cross-sectional area of the model vehicle to tunnel cross-sectional area, and the backlayering length with an accident vehicle set inside the tunnel gets smaller.