Nathalie Bardin-Monnier

Combining CFD Simulations and Piv Measurements to Optimize the Conditions for Dust Explosion Tests

Two studies were developed in parallel on the dust dispersion in a tube during a flammability test. On the one hand, an experimental set-up composed of a Particle Image Velocimetry (PIV) system, a high speed video camera and a laser diffraction sensor was used to characterize the dust cloud, notably the mean velocity of the particles, the root-mean-square velocity and the turbulence intensity. On the other hand, a Computational Fluid Dynamics (CFD) simulation was developed by using an Euler-Lagrange approach. Good agreements were obtained between particle velocities and turbulence levels measured by Particle Image Velocimetry and those determined by simulations. The relation between the initial turbulence and the homogeneity of the dust dispersion has also been discussed. Three stages have been identified during the dust dispersion: a first phase of turbulence intensity increase due to the presence of powerful air jets, a second phase of the turbulence decrease during which the velocity vectors are less oriented and the dust cloud tends to be more uniform and a third represented by the particles settling. The relevance of the electrode positioning as well as the choice of the ignition delay tv in order to perform reproducible flammability tests have also been discussed. In the short term, these results will improve our predictive models on dusts explosions. In the medium term, this study will advocate modifications of the existing procedures/standards in order to define, ab initio, the suspension characteristics which will better correspond to actual industrial conditions or, which will lead to the worst case scenario.

Experimental Study and Modelling of the Pyrolysis of Organic Dusts: Application to Dust Explosions

Pyrolysis gases have been obtained by heating wheat starch thanks to a modified Godbert-Greenwald oven under argon atmosphere. The gases have been analyzed by micro-gas chromatography. The explosivity of both starch and pyrolysis gases have been compared. Totally different explosive behaviours have been observed for the gases and for pure dusts, which highlights the influence of the pyrolysis step on the explosivity of organic particles with diameters greater than 30 mu m. However, tests carried out with pyrolysis gases and glass beads mixtures also demonstrate that the impacts of heat transfer and turbulence/combustion interactions should not be neglected. In addition, a one dimensional finite-volume scheme has been developed to represent the pyrolysis of a single starch particle. The model has been validated for particles exposed rapidly at high temperatures ( 1,000 K). The characteristic times of the pyrolysis have then been determined, which confirms the importance of the primary gasification of the particle in the rate-limiting step of the dust explosion. These results also provide a perspective for understanding the specific behaviour of gas/dust hybrid mixtures towards explosion.

Initial Pressure Drop for Fibrous Media

Pressure drop is one of the most important elements in the choice of filters. The goal of any new design for a filter media is, in general, to minimize this parameter and, consequently, minimize the expenditure of energy for a given efficiency. A filter’s pressure drop is partly related to the internal structure of the fibrous media (thickness, fiber size distribution, packing density, binders, etc.) and also to its external structure, i.e. the shaping of the media (pleating). This chapter, dedicated to the initial pressure drop across fibrous media (i.e. in the absence of filtration and, thus, of any clogging), presents different approaches to evaluating pressure drop as well as studying the influence of the non-homogeneity of media and the impact of pleating on this parameter.