9. Flame Length

In a large tunnel fire, the flame impinges on the ceiling and then extends along the tunnel ceiling. When the longitudinal ventilation rate is high, flames only exist on the downstream side of the fire source, while under low ventilation rate, flames exist both upstream and downstream of the fire source. In a large tunnel fire, the horizontal distance between the fire source and the flame tip is defined as a flame length. A theoretical model of flame length in a large tunnel fire is proposed in this chapter. A large amount of data relevant to the flame length from model and large-scale tunnel fire tests is used to verify the model. The results show that the downstream flame length increases approximately linearly with the heat release rate (HRR) but is insensitive to the longitudinal ventilation velocity. The flame length under high ventilation rate approximately equals the downstream flame length under low ventilation rate. As the longitudinal ventilation velocity decreases, the upstream flame length increases, and so does the total flame length. Dimensionless equations that correlate well with test data are presented. Calculations of the flame lengths of jet flames arisen from pressurized fuel tanks are presented and discussed.