Effect of blockage on critical ventilation velocity in longitudinally ventilated tunnel fires

Effect of blockage on critical ventilation velocity in longitudinally ventilated tunnel fires was investigated based on small-scale experiments and numerical simulations. Blockages were located right upstream of the fire or 3.5 m upstream of the fire. At each location, blockages have five blockage ratios and two orientations. Experimental data shows that the factors as blockage ratio, blockage-fire distance and blockage orientation could have coupling effect on the critical ventilation velocity. For blockage right upstream of the fire, the critical ventilation velocity decreases with the increasing blockage ratio when blockages are horizontally positioned, while there is first decrease and then increase in the critical ventilation velocity for blockage vertically positioned. A correlation was proposed to predict the critical ventilation velocity for blockages right upstream of the fire and horizontally positioned. For blockage 3.5 m upstream of the fire, the change in the critical ventilation velocity is very slight with the increasing blockage ratio for blockages vertically positioned, however, when blockages are horizontally positioned, there is an increase in the critical ventilation velocity for large blockage ratios. The change in the critical ventilation velocity with increasing blockage ratio is more obvious for blockages right upstream of the fire than that for blockages 3.5 m upstream of the fire. Numerical simulations were also conducted in the tunnel without fire. The altered tunnel flow field due to the obstruction of blockage was observed and its effect on the critical ventilation velocity was addressed. Comparison of the critical ventilation velocity between the experimental data and the results from previous studies showed that the critical ventilation velocity changed differently with the existence of blockage. The reason for this difference might be that the relative position between the blockage and the fire, and the fuel type could have coupling effect on the critical ventilation velocity.