Influence of the water spray flow rate and angle on the critical velocity in tunnels with longitudinal ventilation

Abstract

Recently, the water spray system has been used in important tunnels to weaken the heat release rate of fires and cool the fire smoke, which has a significant impact on the critical velocity that is the minimum longitudinal ventilation required to prevent the fire smoke from spreading upstream of a tunnel. Therefore, a series of tunnel fires with eight heat release rates responding to from a car fire to a bus fire set as 5 MW, 10 MW, 15 MW, 20 MW, 30 MW, 40 MW, 50 MW and 60 MW were simulated to investigate the influence of the water spray flow rate (0 L/min, 200 L/min, 300 L/min, 400 L/min, 500 L/min and 600 L/min) and spray angle (30°, 60°, 90°, 120° and 150°) on the critical velocity by Fire Dynamics Simulator (FDS). The simulation data of the case without a water spray system were compared with previous studies and agreed well with previous model. However, previous model overestimates the critical velocity in the cases with a water spray system. The critical velocity decreases with the increase of water spray flow rate, while it increases firstly and then decreases with the increase of the spray angle. The change of the critical velocity is illustrated with the flow field and is caused by both the cooling effect of the water spray and inertial force of the longitudinal flow. Furthermore, a new correlation was proposed to predict the critical velocity in the tunnel with water spray system by introducing a modified coefficient β representing the water spray flow rate effect.