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Fire Models

Fire Models 

Computer fire models are compilations of mathematical equations derived from basic physical principles or experimental data. The computer is used to solve systems of equations that would be difficult or tedious to solve by hand. The simplest computer models solve single equations, while more complex models are composed of hundreds or thousands of equations. A specific type of fire model, the room fire model, is capable of predicting the development of fire conditions in structures and is a helpful tool for fire investigators. Fire models are useful for predicting the development of a fire in a room or structure, for evaluating fire scenarios developed by an investigator, for comparing fire events to established time lines, and for conducting what-if analyses.

Due to the wide range of fire models available, varying levels of expertise are necessary to properly apply the models to investigations. Users of fire models must have an in-depth knowledge of the specific assumptions made by the models and the origins of the experimental correlations and data used as inputs. Each model will have specific limitations as a result of the assumptions made and the experimental methods used to derive correlations and input data.

There are two major categories of room fire models available to fire investigators: zone models and field models.

Zone Models

Zone models split rooms or enclosures up into one or more zones. The most commonly used models assume a room is made up of two zones; an upper layer consisting of heated combustion products, and a lower layer which is composed of cooler air relatively free of combustion products. In a two zone model, the fire forms the connection between the upper and lower layers. The layers are assumed to be well mixed, so that the conditions within each layer are constant. The predicted temperature within the hot layer, for example, is the same throughout. Many of the models include provisions for openings to the outside or to other rooms and for heat losses to the walls and ceiling. Model inputs typically include room dimensions and building materials, the sizes and locations of room openings, room furnishings characteristics, and the fire heat release rate. Outputs typically include prediction of sprinkler or fire alarm activation time, time to flashover, upper and lower layer temperatures, the height of the interface between the upper and lower layers, and combustion gas concentrations. Zone room fire models are available from several sources including the National Institute of Standards and Technology (NIST).

Field models

Field models, also known as computational fluid dynamics models (CFD), split a room or enclosure up into a large number of small three dimensional boxes called cells. The enclosure may contain hundreds of thousands of cells ranging in size from centimeters to meters. Field models are based on the basic physical principles of energy, mass, and momentum conservation. The computer calculates the movement of heat and smoke between the cells over time. At any point in time, it is possible to find the temperature, velocity, and gas concentrations within each of the cells. As in the case of the zone models, the properties within each cell are assumed to be constant. Due to the larger number of cells however, the conditions in the enclosure can be predicted in much greater detail. Field models are capable of predicting the conditions in very large and very small spaces, in spaces with complex shapes, and in complex multiple room configurations that are not possible with zone models. Due to the complexity of field models, they require a high level of expertise to operate and are currently run on expensive computer equipment. General purpose field models are available commercially from various sources. Advanced field models, specifically designed for fire safety analysis, are under development at the National Institute of Standards and Technology (NIST).