New insights toward the influence of agglomeration on dust dispersion and related effects on the ignitability and explosivity of sub-micronic powders
Résumé
Ignition and explosion characteristics of combustible dusts are mainly under the dependence of two kind of parameters: the physico-chemical properties of the powder and the parameters affecting the process. Within these latest parameters, turbulence has a great impact on both dispersion level of the dust and flame propagation. In addition, turbulence end to affect agglomeration level of the dust, which have both an impact on the dispersion level and on the ignition and explosion characteristics which are measured in standard apparatus such as the Modified Hartmann tube (MikeIII) for ignition sensitivity or 20 L sphere for explosion severity. Until recently, based on previous studies carried out in the 70's, it was thought that flammability risks were decreasing as the agglomerates sizes were lowered. If this observation seemed available for microsized powders, it was however shown by Trunov, in the case of aluminum nanoparticles that agglomeration could make ignition easier than for deagglomerated nanoparticles. This effect seems to be mainly due to local self-heating of the agglomerates. Agglomeration plays then a major role on the reliability of safety characteristics measurements and its impacts on ignition and explosion severity needs therefore to be thoroughly understood. In that context, we carried out a CFD study in order to better assess phenomena pertaining to the agglomeration and fragmentation of particles dispersed by a standard apparatus in a Mike II apparatus. This preliminary analysis was focused on the characterization of biphasic turbulent flows within a modified Hartmann tube (Mike III) in order to study the interdependence between agglomerate and flow conditions. Due to the apparent worsening effects of nanoagglomerates, this study focused mainly on sub-micronic particles in order to catch relevant phenomena from an industrial safety perspective. In the near future, the relevance of the dispersion system of the 20 L explosion sphere will be assessed as well following a similar methodology in order to analyze the level of confidence of explosion characteristics measurements of agglomerated nanopowders.