CFAST Model

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.

2. 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.

3. 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.

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

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

6. 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.

7. 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 in FRI3D

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.

FRI3D uses the location of the sources as modeled by the user and its dimensions to output the appropriate CFAST directives needed.

CFAST Compartments in FRI3D

Boundary objects as modeled in FRI3D specify the outer compartments. How a zone is turned into CFAST compartments is set by the zone's Use Boundaries property in the inspector.

One compartment per zone (default)

With Use Boundaries off, the zone becomes a single CFAST compartment the size of its bounding box. Every fire, target and vent in the zone is placed in that compartment. A vent can still connect the zone to another zone or to the outside, so a scenario that spans several zones is simulated as several connected compartments.

Multiple compartments in a zone

With Use Boundaries on, each boundary object in the zone becomes its own CFAST compartment, so a zone can be split into several rooms, for example two rooms sharing a wall, or a room with an adjoining corridor. CFAST then calculates a separate hot gas layer temperature and layer height for each room, and smoke and heat move between them only through the vents that connect them.

FRI3D assigns everything to the right compartment from its position:

• Fires, targets, detection and suppression devices go into the boundary that contains them. An item in the small gap left by the wall thickness between two boundaries goes into the nearest one. A target is placed by its point closest to the fire, so a cable tray running through more than one room is measured in the room where it is closest to the fire.
• Vents connect the boundaries they overlap. A vent in a wall shared by two boundaries connects those two rooms; a vent that overlaps only one boundary connects that room to the outside. A mechanical vent's flow direction comes from its orientation in the 3D view, not from its From/To compartment fields, so check the vents after switching Use Boundaries on for an existing zone.

To model a zone as several rooms, model each room as its own boundary object, and place the vents (doors, openings, ducts) that connect them in the shared walls, floors and ceilings.

Things to be aware of:

• A vent can only connect boundaries of the same zone, or a boundary and the outside. To connect to a room in a different zone, leave Use Boundaries off.
• Heat conduction through the walls shared by two compartments is not modeled; the rooms exchange heat only through their vents.
• The CFAST visualizations show the layer height and temperatures of each compartment separately, and also color the ceiling and floor surfaces.

CFAST Targets in FRI3D

The CFAST Device directives generated by FRI3D are placed at the closest location to the fire on the surface of the equipment or plant item.

CFAST Results

The results of the CFAST simulation and their associated device temperature along with the zone temperature is used for equipment failures.

CFAST Visualizations in FRI3D

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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