Images of science: Understanding flames to avoid industrial accidents

In this image, we see the simulation of a flame during an explosion, in grey, enclosed by a series of obstacles, in purple. This type of simulation makes it possible to better understand how flames spread around industrial constructions and to limit the damage of accidental explosions.

Industrial chemical sites have the potential to be the scene of destructive events, with catastrophic human and material tolls. Disasters like those of Beirut in 2019 in Lebanon,AZF in 2001 in France, of Port Hudson in 1970 in the United States or Buncefield and 2005 in the United Kingdom show that no one is safe from an industrial accident that leaves deep scars in society.

Very different causes can be at the origin of these industrial disasters. The Port Hudson and Buncefield disasters were linked to explosions of an uncontained gas cloud (or unconfined vapour cloud explosion in English).

Ces explosions take place following a loss of containment of a flammable fluid, which spreads in a large volume around the industrial site and mixes with the oxygen in the surrounding air. Upon contact with an energy source in the area where the cloud has spread, for example an electric arc, a combustion reaction can initiate and spread until all the fuel has been consumed.

Following ignition of the mixture, a flame (also called a “reaction zone”) propagates within the air/fuel mixture. Depending on the circumstances, this flame can have two types of propagation schemes :

• the Blastobtained when the ignition source is low energy: the flame propagates at a speed lower than the speed of sound.
• the detonationwhich requires significant initial energy: the flame then propagates at supersonic speed and causes much greater damage.

It is possible that a flame initially in the deflagration regime makes the transition to a detonation regime. following its acceleration. Flame acceleration can be caused by many factors, including the level of turbulence and the presence ofobstacles in the propagation area.

In the latter case, two effects can be observed. First of all, the deformations of the flow around the obstacles cause an “elongation” of the flame, which increases its surface on a large scale. In addition, the generation of a wake at the level of the obstacles encountered by the flame will lead to the production of turbulence, which increases the transport of mass and energy in the flow. In those regions characterized by high levels of turbulence, small-scale wrinkling further increases the total flame area. However, the speed of the flame depends directly on its surface. As a result, the presence of obstacles in the flame propagation zone can lead to its acceleration and a rapid change in the propagation regime from deflagration to detonation, which is much more violent.

–