Simple model for control of fire gases in a ventilated tunnel

A model of a tunnel fire is proposed to describe the behavior of the fire gases spreading in opposite direction to a fresh air current induced by tunnel ventilation. In particular, the model yields an analytic formula for estimating the critical ventilation velocity, which is required to prevent backlayering, i.e. to arrest the fire gases close to the fire source. It can be shown that the critical ventilation velocity mainly depends on the heat-release rate of the fire and the height of the tunnel. An important finding, already suggested by other theories, is that for large heat-release rates the critical velocity does not depend on the heat-release rate. The model has been validated by comparison with the experimental data obtained from fire tests in an instrumented tunnel. The formula of the critical ventilation velocity, which is also given in dimensionless form, is generally applicable for ventilation rates and heat-release rates occurring in real tunnel fires.