Understand flames to avoid industrial accidents

In this image, we see the simulation of a flame during an explosion, gray, closed by a series of obstacles, purple. This type of simulation provides a better understanding of how flames propagate around industrial buildings and limit the damage from accidental explosions.

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

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

These explosions occur after a loss of containment of a flammable fluid, which spreads in a large volume over the industrial area and mixes with the oxygen in the surrounding air. In contact with an energy source in the area where the cloud has spread, for example an electric arc, a combustion reaction can be initiated and extended until all the fuel has been consumed.

After the mixture is ignited, a flame (also called a “reaction zone”) spreads inside the air / fuel mixture. Depending on the circumstances, this flame can have two types of propagation regimes:

  • deflagration, obtained when the source of ignition is of low energy: the flame propagates at a speed inferior to the one of the sound.

  • detonation, which requires significant initial energy: then the flame spreads at supersonic speed and causes much greater damage.

It is possible that a flame initially in the explosion mode makes the transition to a detonation regime after its acceleration. The acceleration of the flame can be caused by many factors, including the level of turbulence and the presence of obstacles in the propagation zone.

In the latter case, two effects can be observed. First, the deformation of the flow around the obstacles causes 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 wrinkles further increase the total area of ​​the flame. 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 explosion to detonation, which is much more violent.

Understand the spread of flames in industrial facilities

To limit these devastating effects, it is important to understand the mechanism of acceleration of flames that spread within the obstacles of an industrial site. This will provide for the construction of sites whose layout limits the spread of flames and the harmful effects of the explosion.

Before modeling a complete factory, simplified configurations such as the one shown in the figure allow accurate statistical analysis of the impact of the shape, layout, and degree of obstruction of obstacles. They also allow you to check the quality of the numerical models used by comparing them with experiments that are done on smaller scales than those of a factory.

The image shows a 5 meter diameter premixed, gray methane / air flame spreading through a series of violet obstacles. In the image, the structure of the flame is represented in the tetrahedral mesh that is used to divide space into small volumes in which the equations of evolution of mass, velocity and temperature are solved using the OpenFoam mechanical software. digital fluids. This configuration has a spherical symmetry that allows accurate statistics of events along the trajectory of the flame when it encounters obstacles.

Once the different models have been validated, calculations can be implemented on realistic industrial configurations that will increase the safety of the sites in question and the new sites to be built.

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