Critical velocity for preventing thermal backlayering flow in tunnel fire using longitudinal ventilation system: Effect of floor-fire separation distance

Abstract

To design a reliable longitudinal ventilation system in a roadway tunnel, it is crucial to know the optimum value of the forced airflow velocity which must be applied to minimize hazards to users trapped in the building during a fire. This optimal value known as the critical longitudinal ventilation velocity, is a crucial criterion for tunnel safety when designing ventilation systems. Analysis of the influence of the fire development location from the tunnel floor on the critical velocity has rarely been studied, although this situation really exists due to the difference in vehicle sizes in practice. CFD simulations were performed in this paper to study this scenario and to estimate the critical velocity for a fire developing in different locations relative to the tunnel floor. The results indicate that as the elevation of the fire location increases from the tunnel floor, the critical velocity decreases. A new model involving the separation distance of the fire location on the tunnel floor and relating the critical velocity to the fire heat release rate is proposed. Furthermore, under critical conditions of ventilation velocity, the results show that the elevation of the fire source location from the floor has a significant effect on the maximum smoke temperature rise below the tunnel ceiling.