The Critical Velocity and the Fire Development
Published: 19 April, 2010
The “critical velocity” is strongly influenced by the heat release rate and the tunnel geometry. The critical velocity generally increases with increasing fire heat output for small to medium sized fires, however there is a ceiling for the critical velocity – called “super” critical velocity when the critical velocity becomes insensitive to the fire power and remains constant once the fire reaches certain level.
The current techniques for prediction of the values of the critical velocity for various tunnels were predominately based on semi-empirical equations obtained from the Froude Number preservation combining with some experimental data. Although the methods provided simple and effective semi-empirical solution to determine the value of the critical velocity, the Froude Number modeling failed to explain the existence of the super critical velocity. The fire plume theory explains that the super critical velocity is caused by the intermittent flame occupying the ceiling region when the heat release rate reaching the critical level and fire zone dominates the full height of the tunnel. The plume theory has observed the changes in fire and plume sizes and linked the fire development with the critical velocity. However it still couldn’t describe the physical conditions which could be used to predict the super critical velocity. This paper examines the combustion characteristics of the fire zone under different ventilation conditions and establishes some fundamental changes in fire burning status when heat release rate reaches the critical level for the super critical velocity. The fire development under the ventilation is analyzed using combustion theory. Both CFD simulations and experimental measurements are used to identify the flame status under ventilation.
