Influence of turbulence on flame propagation during dust explosion

Dust explosions represent a major risk in all industries dealing with combustible dust. When these powders have sufficiently fine dimensions, they can easily be suspended in the air; in the presence of an ignition source, an explosion may occur. These accidents cost a lot in terms of human lives and financial losses. The measures necessary to avoid it must be implemented.

In order to safeguard the industry from this risk, this phenomenon has to be understood. Safety parameters are important as they allow understanding the level of risk present in an industrial plant and allow designing safety systems that limit the consequences in case of explosion. Models describing flame propagation in the case of gas explosions are now available. With such models, consequences of explosions can be predicted in real accident scenarios. These models can be adapted for the explosions of organic dusts, as these two phenomena exhibit similar combustion mechanisms. Unfortunately the adaptation of the gas flame propagation models is not possible for metal dust; indeed the combustion reaction in this case occurs at the surface of the particles (heterogeneous reaction). Unfortunately, metal dusts give rise to very strong explosion phenomena and are classified as particularly dangerous substances. To construct models suitable for metal dusts experimental data are needed, resulting from studies which aim to a greater understanding of the phenomenon.

This work consists of an experimental study on the influence of dispersion-inducedturbulence on aluminum dust flame propagation. To have an explosion of dust, at the moment of ignition, a solid combustible-comburent suspension has to be present. The solid combustible in our case is aluminum powder. The comburent is instead the oxygen of the air.

Dust suspension is an experimental challenge: in order to use the experimental data for 9the construction of a numerical model, it is important first to quantify the degree of homogeneity of this suspension; obtaining a homogeneous cloud is preferable. Furthermore, the dust dispersion generates a certain degree of turbulence within the suspension. This turbulence level influences the propagation of the flame, and it is, therefore, necessary to be able to quantify it. The characteristics of the suspension evolve due to the sedimentation of dust, so the phenomenon is non-stationary, and its characterization is complicated.

The experimental study was carried out in the laboratories of the Institute of Risk Sciences of IMT Mines Alès, where a prototype was built specifically to investigate this phenomenon. Within this prototype, a suspension of dust is generated using a special dispersion system.

In the first phase of the work the suspension was characterized: the level of homogeneity in terms of concentration and the level of turbulence were studied. Once the characteristics of the suspension were known, deflagration tests were conducted in the same prototype. The main parameters relating to the explosion were defined and the link between the initial turbulence of the suspension and the characteristics of the explosion was investigated.