Effect of blockage ratio on critical velocity in tunnel fires

Tunnel blockage ratio, phi, is defined as the ratio of the cross-sectional area of the fire source to that of the tunnel. These fire sources in reality correspond to those that have considerable cross-sectional area such as trains or heavy goods vehicles. The effect of tunnel blockage ratio on critical velocity in tunnel fires is numerically studied. A proposed empirical consideration that accounts for the blockage ratio effect has been verified. A new formula for predicting the critical velocity in blocked tunnels has been presented based on the above consideration. Data from present numerical simulations and previous full-scale and reduced-scale tunnel fire experiments that have considerable tunnel blockage ratios are employed to examine this formula. The formula shows good agreements with these data, which can be used appropriately to calculate the critical velocity for tunnels with blockage.