Numerical study of the flame geometry of pool fires in longitudinally ventilated tunnels: Fid-Bau Portal

Numerical study of the flame geometry of pool fires in longitudinally ventilated tunnels

Deng, Tao / Norris, Stuart / Sharma, Rajnish N.

Highlights • A flame geometry measuring method based on fuel consumption rate was proposed. • The flame geometry of a tunnel fire is different from that of an open fire. • Tunnel area and fire size are additional factors influencing the flame geometry. • Sub-critical ventilation rates have little influence on the flame base drag length. • The confined space concentrates the wind momentum and enhances the flame tilt.

Abstract In a windy environment, the flame tilt angle and base drag length are two parameters used to describe the flame geometry of liquid fuel fires. This work describes a numerical investigation using the FDS code to model three typical fire sizes in a longitudinally ventilated tunnel and a windy open space. The CFD modelling methodology was validated against experimental data from the literature. According to the findings, when the ventilation is weak, a backlayering flow occurs in the tunnel and causes a two-layer flow pattern upstream, which reduces the flow space and accelerates the ventilation. Consequently, the flame base drag length tends to remain constant, and the flame tilt angle becomes very difficult to predict. Unlike open fires, when the upstream is free of backlayering, the tunnel area and the heat release rate have been proven to be two additional factors that influence the flame geometry. The confined space causes more concentrated momentum of wind and speeds up the fuel consumption upstream of the dragging flame base. The former results in a more significant flame tilt, and the latter shortens the stay of fuel vapor on the floor. By introducing a dimensionless heat release rate and dimensionless tunnel cross-sectional area, a series of new prediction models covering all the governing parameters were derived, which have successfully estimated the longitudinal distance from the fire center to the maximum ceiling temperature in the tunnel.