Computer fire modeling is now prevalent in fire litigation and subrogation (Information about fire modeling for building design can be found here). Put simply, fire models are computer programs that solve coupled sets of mathematical equations that encode the physical laws of nature governing fire dynamics. For example, fire models calculate compartment fire temperatures by applying the principle of energy conservation. Fire modeling makes it possible for scientists and engineers to analyze and reconstruct fires, but a high degree of training, education, and experience is necessary to correctly apply fire models.

The two main types of fire models are zone models and field models. Zone fire models capitalize on the empirical observation that smoke tends to collect near the ceiling of a compartment due to buoyancy, but the air near the floor remains relatively cool and clear (this is why we are taught to crawl on the floor during a fire!). In zone fire modeling, each room (or compartment) is divided into two distinct layers: a hot upper layer (containing most of the fire's heat and smoke), and a cool lower layer (which remains relatively uncontaminated by smoke). The temperature of each layer is calculated from conservation of energy, the amount of toxic combustion products in each layer is calculated from conservation of chemical species, and the height of each zone is calculated from conservation of mass. In the US, most zone fire modeling is conducted using CFAST, a model developed by the US National Institute of Standards and Technology (NIST).

By design, zone fire modeling provides the user with limited spatial information. That is, if a grease fire occurs in a kitchen, a zone fire model can only provide information about the temperature near the ceiling, averaged over the entire kitchen. In comparison, field fire modeling provides a much more detailed description of the fire environment, and a field fire model would show that temperatures near the burning grease are higher than temperatures near the ceiling.

Field fire modeling is based on Computational Fluid Dynamics (CFD) principles. In a CFD-based fire modeling, each compartment is broken into thousands (sometimes hundreds of thousands or even millions) of separate cells. Mathematical descriptions of mass, energy, species, and momentum conservation are enforced at each cell, and the fire model solves these coupled equations to give a detailed description of the fire flow field. The most widely-used field fire model is Fire Dynamics Simulator (FDS), a public domain fire model developed primarily by NIST. FDS fire modeling has found widespread applications in the fire litigation community.

Fire modeling produces graphical representations of fire conditions (smoke concentration, temperature, flame position) that can effectively convey technical information to a judge or jury. Some of the more common fire modeling applications encountered in fire litigation include:

• Fire development modeling (how fast did a fire spread and how intense did it become)
• Smoke detector and heat detector activation modeling (how long did it take for smoke or heat detectors to activate)
• Tenability analysis (how much smoke, heat, and toxic gases were present at various locations and would the conditions cause occupants to become incapacitated)
• Establishment of a fire time line (to determine major events such as ignition, detector activation, onset of flashover, window breakage, spread to adjacent rooms or areas, etc.)
• Egress modeling (how long would it take occupants to evacuate a given space)
• Wildland fire modeling

A word of caution: Fire modeling can be a useful tool to support a fire expert's opinion, but it has a high potential for misuse. Unfortunately, fire models are often mis-applied -- sometimes deliberately to obtain a desired result, and other times inadvertently due to inexperience, lack of training/education, inability to recognize model limitations, or unjustified assumptions. It is for this reason that in addition to conducting fire modeling in support of fire litigation, Reax Engineering Inc. is often requested to analyze fire modeling conducted by other expert witnesses to bring to light any technical deficiencies. For any inquiries related to fire modeling or fire model applications, please contact Chris Lautenberger

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