Fire modelling – what is its purpose?

Numerical Calculation (fire modelling) has become increasingly popular since the beginning of the 1990s. These numerical tools are particularly useful for the following cases:

  • Simulate the effects of a fire (and its behaviour) while designing a building;
  • Reproduce a fire as part of an investigation after a loss to validate, for example, the scene of a fire or the damages; and
  • Reproduce the results of a fire or fire tests when you know the parameters (materials, extraction system, fire area, etc.).

Let’s concentrate on fire modelling during the building design. We now know that this kind of modelling is extremely useful. Modern-day computers are powerful and can carry out a large amount of iterative computing in a reasonable (calculation time) period. This is useful when working with architectural designs and building mechanical systems that sometimes do not align with regulatory frameworks (i.e. NBC). In fact, the acceptable solutions that comply with NBC do not require fire modelling. This practice is appropriately part of the process of presenting alternatives to the Authorities having Jurisdiction, namely RBQ.

Fire modelling by numerical calculations is a tool that is complex and is used to predict the physical and chemical interactions of a fire. The model represents a compartment and specific predetermined scenarios.

This type of process assists designers in decision-making while developing an architectural design and/or the building mechanical system (ventilation and exhaust). Naturally, the goal of the design decision process involves comparing the proposed performance with this alternative solution with the minimum performance level required by the NBC. This use of fire modelling is ‟qualitative” since we are qualitatively comparing several different scenarios using the same analysis tool.

There are three broad categories of tools used while carrying out fire modelling in compartments:

  • Analytical models;
  • Modelling of different zones; and
  • Computational Fluid Dynamics (CFD).

In general, CFD fire modelling is more comprehensive and better represents what we can expect to see in a fire in a compartment. This style of fire modelling allows you to estimate a wide range of information that is useful for designers, including:

  • The interface height between fresh air and smoke;
  • The displacement and speed of smoke or gas;
  • The time it takes to set off the sprinkler system;
  • The detection time of fire or smoke;
  • The temperatures of materials and of structural elements;
  • The changes to the location and power of the fire area;
  • The concentration of combustion products;
  • Etc.

Several kinds of software have been designed for CFD fire modelling. For example, the NIST (National Institute of Standard and Technologies, the equivalent of the NRCC (National Research Council Canada) in the US developed Fire Dynamics Simulator (or FDS) software. This software dating from the late 1990s, was designed specifically to simulate the dynamics of fluids caused by gravity flow in fire conditions. This software has been validated and verified by independent bodies that include testing organizations, university laboratories, insurance companies, fire services, etc. Fire modelling using software like FDS must be carried out while respecting the limits of the software. For example, FDS software is less accurate for simulating the phenomenon of re-radiation of hot smoke close to the floor of the compartment, but it effectively simulates the room temperature.

Specialized professionals in fire safety can determine the best fire modelling software to use in a particular situation. In collaboration with other professionals involved in the project, they can determine the relevant inputs.

Both users and designers must bear in mind that fire modelling is a ‟digital” representation of a fire in a compartment and not an absolute reality. It is impossible to predict the behaviour of a fire with certainty. The data entered into fire modelling software can influence the final result. Therefore, we suggest using fire modelling software as a comparison tool.

When fire modelling is used for comparative analysis purposes, we then compare the acceptable situation (minimum NBC requirements) and the proposed solution in order to reach a conclusion concerning the comparative performance level. The proposed solution shall at least be as effective or improve the level of fire safety. Technorm’s team of professionals can help you evaluate the performance of alternative solutions and its professionals are able to execute this type of analysis with sophisticated equipment.

By Patrick LeBlanc, P.Eng., Fire Safety group

By |2019-06-14T16:45:14+00:00March 13th, 2019|News|0 Comments