Building fire physics and smoke control

A fire in a building can cause considerable destruction and damage. Yet fire dynamics are not always well understood.

A real-life anecdote will suffice to demonstrate the importance of this type of analysis: when certain buildings with large volumes equipped with natural smoke extraction are commissioned, a smoke test is generally carried out for educational purposes. To avoid damaging the just-completed volume, very low thermal outputs are used.

A few years ago, for an emblematic building, and while the test was going perfectly well, the client's representative was upset by the fact that the layer of smoke took up a third of the free height, violently questioning the team who had designed the system: if for a few kilowatts the layer of smoke took up a third of the height, for triple the power, and therefore still well below what a fire would generate, the volume would be completely smoked out!

This is not the case, of course, and we shall see that in such a case, the maximum possible fire will lead to a layer of smoke less than half the height of the ceiling. Nevertheless, it's hard to explain the physics of smoke layers calmly in the heat of a real test, which takes place at night, with many people who all have an opinion!

The aim of this article is precisely to set out the problem, and then to use thermodynamic and physical methods to characterize the major quantities that influence fire dynamics, as this understanding is an essential prerequisite for the reasoned and sensible use of numerical fire simulation.

The plan is as follows: firstly, safety issues are presented. Next, the basic thermodynamics are reviewed and applied to the case of fire in buildings, providing an initial understanding of the phenomenon. Two canonical configurations, one with mechanized extraction and the other with natural smoke extraction, are then presented in detail, before introducing simulation software and the smoke extraction engineering approach. Finally, the consequences of changes in the building stock on fire dynamics are outlined.