CFD Modeling of Flammable Gas Concentration Levels and Empirical Validation

Hubert Biteau, Ph.D., P.E., CFEI
Exponent, Failure Analysis Associates; Atlanta, GA

Nicholas Nava, P.E., CFEI, CVFI
Exponent, Failure Analysis Associates; Bowie, MD

Presented at the International Symposium on Fire Investigation Science and Technology, 2018


A tool which is of particular interest in fire and explosion investigations is the modeling of flammable gas concentrations. The use of computational fluid dynamics (CFD) modeling, particularly NIST’s Fire Dynamics Simulator (FDS) is well known in the fire protection engineering community. Considerable research has been performed by engineers and scientists from NIST and within the fire protection engineering community to validate this model.

The use of FDS in modeling gas dispersion for natural gas and propane gas leaks has been the focus of past work by others. Recent interest has surrounded the use of flammable and combustible liquids during cleaning and construction type activities. Flammable gas is produced due to evaporation of the liquid phase. The dispersion of this flammable gas throughout compartments above the Lower Explosive Limit (LEL) can result in a fire or explosion when in contact with a competent ignition source.

The scope of this paper is to demonstrate FDS’s ability to accurately model flammable gas concentration and dispersion. Experimental testing was conducted to determine the evaporation rate of frequently used flammable and combustible liquids. Combustible gas monitors were used to spatially measure the flammable gas concentrations in proximity of a flammable and combustible liquid spill in a controlled diked area. The experiment was then modeled using FDS. Experimental data and modeling results were compared.

This work is of particular interest for those individuals involved in fire investigations who consider using FDS to model flammable gas concentrations. First, results of this experimental work will be presented. Secondly, a comparison of the experimental and model results will demonstrate FDS’s ability to accurately depict flammable gas evaporation and dispersion. Finally, recommendations will be made for the critical input parameters needed to allow for the use of the model for fire origin and cause determinations.

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