Critical velocity in point extraction for dual longitudinally ventilated tunnel fire

The critical velocity used to control the smoke flow can effectively prevent the back-layering of fire in the tunnel. Therefore, understanding the critical velocity can help increase evacuation time for better tunnel safety. This paper establishes a model to predict the critical velocity in a smoke point extraction system through theoretical analysis. A formula for calculating the critical velocity was obtained by using small-scale model tests. The results indicated that the critical velocity was exponentially related to the distance between the smoke vent and the fire source. It was also related to the volume flow rate of the smoke when the dimensionless distance was less than 2.7. The critical velocity remained almost constant as the dimensionless heat release rate was greater than 0.12 and the dimensionless distance was greater than 2.7. There was a good agreement in the variation tendency of the critical velocity between the testing result of the two-point extraction system and that of the longitudinal ventilation system. The critical velocity in the two-point extraction system varied from 0.22 to 0.59 m/s, which was less than that given by the longitudinal ventilation system.