CFAST is a two-zone fire model used to calculate the evolving distribution of smoke, fire gases and temperature throughout compartments of a constructed facility during a fire. In CFAST, each compartment is divided into two gas layers.

The modeling equations used in CFAST take the mathematical form of an initial value problem for a system of ordinary differential equations (ODEs). These equations are derived using the conservation of mass, the conservation of energy (equivalently the first law of thermodynamics), the ideal gas law and relations for density and internal energy. These equations predict as functions of time quantities such as pressure, layer height and temperatures given the accumulation of mass and enthalpy in the two layers.

CFAST Features

Zone Model Features The following is a list of generic features and limitations of CFAST as a representative zone model. Most of the aspects are common to all zone models.

1. Model calculates a single hot gas layer temperature, layer height and layer composition for each room. Model has multiple room capability.

1. Model does not predict fire growth rates, heat release rates or generation of smoke and other products of combustion. These data are either required as input or are generated from engineering correlations. No effect of temperature or radiation on the fire growth is directly calculated.

2. The effects of hot gas layer temperature on radiation, fire growth and heat release rates are not calculated. The effect of oxygen depletion on energy release rate is estimated based on a correlation.

3. The model can treat natural and forced/mechanical ventilation.

4. Heat losses through walls are calculated via a simple transient heat conduction approximation with convective and radiative heating boundary conditions.

5. There is no coupling between the thermal or radiation conditions in the compartment and the burning fuel. Such effects must be accounted for in the specification of the source fire. This often involves an iterative process.

6. The effects of oxygen depletion are accounted for by reducing the heat release rate as a function of oxygen concentration. The energy release rate is reduced from the ambient value to zero when a limiting oxygen concentration (an input variable) is reached.

CFAST Sources

A fire in CFAST is implemented as a source of fuel which is released at a specified rate.

This fuel is converted into enthalpy (the conversion factor is the heat of combustion) and mass (the conversion factor is the yield of a particular species) as it burns.

A time-dependent heat release rate (HRR) is used from the fire material. The specified heat of combustion is used to calculate the mass loss rate of fuel, from which the production rate of combustion products can be calculated using specified product yields. The heat release and the corresponding product generation rates go to zero when the lower oxygen limit is reached, and are replaced by the appropriate production rate of unburned fuel gas which is transported from zone to zone until there is sufficient oxygen and a high enough temperature to support combustion.

CFAST Properties

CFAST Visualizations

Since CFAST is a zone model the visualizations are restricted to visualizating at compartment boundaries, flame, upper and lower zone quantities, target temperatures and other quantities of interest enforced with the limiation of a zone model for fire.

Visualizations and options are described in a following section

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