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### Fire Modelling

 Fire model Norminal fires Time equivalences Compartment fires Zone Models CFD / field models Parametric Localised One-zone Two-zone Complexity Simple Intermediate Advanced Fire Behaviour Post-flashover fires Pre-flashover fires Post-flashover fires Pre-flashover / localised fires Complete temperature-time relationships Temperature distribution Uniform in whole compartment Non-uniform along plume Uniform Uniform in each layer Time and space dependent Input parameters Fire type No physical parameters Fire load Ventilation conditions Thermal properties of boundary Compartment size Fire load & size Height of ceiling Fire load Ventilation conditions Thermal properties of boundary Compartment size Detailed input for heat & mass balance of the system Detailed input for solving the fundamental equations of the fluid flow Design tools BSEN1991-1-2 COMPF2 OZone SFIRE-4 CCFM CFAST OZone FDS SMARTFIRE SOFIE PD7974-1 PD7974-1 Simple equations for hand calculations Spreadsheet Simple equations Computer models

Figure 1 Options for fire modelling in compartments

Generally, the factors influencing the severity of a compartment fire can be summarised as follows:

• Fire load type, density and distribution
• Combustion behaviour of fire load
• Compartment size and geometry
• Ventilation conditions of compartment
• Thermal properties of compartment boundary

The occurrence of flashover in a compartment fire imposes a transition to the fire development. Therefore, many fire models are classified under pre- or post-flashover, except for the computational fluid dynamic (CFD) models, which cover both phases.

There are a number of options available to calculate the fire severity as follows:

The level of complexity increases from simple fire models to field models as shown in Figure 1. Basically, the first four fire models can be considered as simple models, whereas the zone and CFD models are advanced models. The input parameters for each of these models are quite different with the advanced models requiring very detailed input data and simple models requiring little input.

In the simple fire models, the gas temperature of a compartment is taken as uniform and represented by a temperature-time relationship. The smoke movement and fire spread cannot be considered. They are more suitable for modelling post-flashover fires.

The advanced fire models are normally theoretical computer models that simulate the heat and mass transfer process associated with a compartment fire. They can predict compartment gas temperatures in much more detail. The smoke movement and fire spread may be taken into account. As reflected in their names, a zone model may present the gas temperature into single or different zones, whereas a CFD model provides a space/field dependent gas temperatures distribution.

Each of the fire models will be discussed in the section.

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