The Use of CFD Calculations to Evaluate Fire-Fighting Tactics in a Possible Backdraft Situation

Abstract

This paper is an attempt to integrate theoretical Computational Fluid Dynamics (CFD) calculations with practical fire-fighting tactics commonly used when arriving at the scene of an underventilated fire. The paper shows that CFD has a great potential in improving understanding and creating better effectiveness in the estimation of fire-fighting tactics. If burning has occurred in a lack of oxygen for a long time, excessive pyrolysis products may have accumulated in the fire compartment. If air is suddenly introduced in the compartment a backdraft may occur. The CFD code used for the simulations is fire dynamics simulator (FDS). In this paper, we focus on the conditions that can lead to backdraft, and not the deflagration or rapid combustion in itself. Therefore, the simulations focus on the gravity current and the mixing process between cold fresh air and hot smoke gases by considering a uniform temperature inside the building as initial condition. The different tactics studied include natural ventilation, positive pressure ventilation (PPV) and dilution by water mist. Their effectiveness is observed comparing them with a reference scenario, where no action is taken. The main objective of natural ventilation is to find the fire source, and the venting is more effective with several openings. Tactics involving PPV are very effective in evacuating the unburnt gases, but increases the mixing, and consequently the probability of backdraft during the early stage of operation. On the other hand, the addition of water mist can reduce the danger of backdraft by reducing the concentration of unreacted combustible gases below the critical fuel volume fraction (CFVF), where ignition cannot occur. If the dilution level is insufficient the danger of backdraft is increased, mainly because the process of gases evacuation is longer due to cooling, which reduces the density difference between hot and cold gases. During a fire-fighting operation, the choice of tactic depends mainly on whether there are people left in the building or not, but also on the fire-fightersÕ knowledge of the buildingÕs geometry and the fire conditions. If the situation shows signs of strongly underventilated conditions, the danger of backdraft has to be considered and the most appropriate mitigation tactics must be applied. 12 Nomenclature C airThe heat capacity of air (approximately 1 kJ/(kg K)) CFVF Critical fuel volume fraction CMC Critical mixture composition D(X) Dilution level, corresponds to a scenario with X % mass concentration GML Gas measurements line LFL Lower flammability limit M o Initial mass of hot gases M vap Mass of vapor added to obtain the desired dilution M CH4 Mass of unburnt gases (methane) Q fan Flow rate of the PPV fan tSimulation time T Temperature T final Temperature of hot gases after dilution T in Temperature of hot gases inside the compartment T out Temperature of cold gases outside the compartment (ambient) UFL Upper flammability limit v fan Velocity at the inflow boundary, due to PPV fan V gas Volume of hot gases in the en...

A CFD and Experimental Investigation of the Mixing Process in Compartment Fires

“…4, only the inside flow is seeded and it shows the formation of a small-scale pair of counter-rotating vortices (Obs 4.1) between cold entering air and hot inside air. The time resolved sequences of images shows successive vortex coalescence process where pairs of Van Karman vortices are ingested into a single large vortex [8]. Other visualizations have shown pulsating disturbances at the opening despite minimizing air disturbance in the room.…”

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