Estimation of the effects of water mist system on the tunnel critical velocity due to smoke cooling

Abstract

A water mist system can reduce the tunnel critical velocity since it cools the smoke and reduces the buoyancy force. To study this, a theoretical model was derived using the theory of the critical Froude number and a global energy balance concept. The evaporation rate of the mists was found to be the dominating factor affecting the ratio between the critical velocities with and without the suppression. To test this, the CFD code FDS 6.7.3 was used to model a 60 m long tunnel with a water mist operating downstream to cool the smoke. Nine fire sizes in the range of 10 483 kW to 42 988 kW were modelled. With water mist operation, the theoretical model can accurately predict the reduction in critical velocity for small fire sizes. By the addition of a calibration factor to account for the change in the critical Froude number, the developed model can also provide reliable predictions for large fires whose critical velocities are independent of the heat release rate. The findings prove that the critical Froude number theory can be applied even in the presence of a water mist system and reveal a fact that only when the mist cools the rising hot plume and backlayering flow can the critical velocity be reduced.