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

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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-fighters’ knowledge of the building’s 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.

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Abbreviations

Cair :

The 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

\(M_{{{\text{CH}}_{4} }} \) :

Mass of unburnt gases (methane)

Q fan :

Flow rate of the PPV fan

t :

Simulation 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 enclosure

V w :

Volume of water to cool the gases from T in to T final

X :

Mass concentration of unburnt gases (methane)

y :

Dilution factor (%)

Z low :

Lower vertical limit of the flammability region

Z up :

Lower vertical limit of the flammability region

ΔH w :

Heat of vaporization for water (2260 kJ/kg)

Δz :

Vertical width of the flammability region. Δz = Z up − Z low

ρ gas :

Density of gases

ρ in :

Density of hot gases inside the compartment (approximately 353/T in)

ρ out :

Density of cold gases outside the compartment

ρ w :

Water density in kg/L (1 kg/L)

χ :

Dilution ratio

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Acknowledgements

The present work was performed as a part of the project “Under-Ventilated Compartments Fires (FIRENET)” (Co. No. HPRN-CT-2002-00197). The project is supported by the EU Research Training Network FP5, which is gratefully acknowledged.

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Authors and Affiliations

  1. Civil and Environmental Department, Faculty of Engineering, University of Iceland, Reykjavik, Iceland

    Georges Guigay & Jónas Elíasson

  2. Øresund Safety Advisers Ltd, Malmo, Sweden

    Daniel Gojkovic

  3. Helsingborg Fire Department, Helsingborg, Sweden

    Lars-Göran Bengtsson

  4. Iceland Fire Authority, Reykjavik, Iceland

    Björn Karlsson

Authors
  1. Georges Guigay
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  2. Daniel Gojkovic
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  3. Lars-Göran Bengtsson
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  4. Björn Karlsson
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  5. Jónas Elíasson
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Correspondence to Georges Guigay.

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Guigay, G., Gojkovic, D., Bengtsson, LG. et al. The Use of CFD Calculations to Evaluate Fire-Fighting Tactics in a Possible Backdraft Situation. Fire Technol 45, 287–311 (2009). https://doi.org/10.1007/s10694-008-0058-4

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