Smoke ventilation in operational fire fighting.

Ventilating a fire compartment during operational fire fighting procedures may have unpredictable consequences. In some cases the ventilation is advantageous: the hot gases are removed from the fire enclosure, the visibility improves and the enclosure cools down. In some cases the opposite happens: with the accelerated burning rate, more smoke is spread around, and the temperatures rise. The most dramatic consequence is the initiation of a backdraft, where the pyrolyzed gases ignite instantaneously, in the worst case causing a severe explosion. The effect of ventilating the fire compartment was studied systematically by quarter scale laboratory tests. The fire was initiated in a one-storey three-room compartment subject to different horizontal ventilation conditions. Both natural and positive pressure ventilation (PPV) were applied. The tests revealed many critical factors affecting the success of the attack. When properly used, PPV clearly improves the survival probability in the compartment: the visibility dramatically improves, and the temperatures are low everywhere outside the fire room. A fire spread zone model code (BRI2T) was applied to a few principal test scenarios. The model simulates well scenarios with no vigorous turbulent mixing of the gas layers, but predicting dependences between different parameters is tedious because the model (like all zone models) does not contain a feedback between varying ventilation conditions and the heat release rate. Due to these limitations, the available zone models are not suitable for PPV applications.