Structure of packed beds probed by Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) volume-visualisation in combination with image analysis techniques are used to characterise the structure within the inter-particle space of unconsolidated and consolidated packed beds of ballotini for column-to-particle diameter ratios of 9, 14 and 19. The beds are characterised using two approaches. First, radial distributions of the voidage are calculated. The reduced radial distribution function of the void space in a plane perpendicular to the axis of the bed is also used to investigate correlated structures within the void space. For all column-to-particle diameters, the correlated structure extends up to 6.5 diameters into the packing material and is seen to increase upon consolidation. Second, the inter-particle space is segmented into individual pores, defined as a portion of the void space bounded by a solid surface and planes erected where the hydraulic radius of the void space exhibits local minima. Statistical distributions of the characteristics of these pores, such as radius, surface area, volume and coordination to other pores, are obtained. Upon consolidation, there is an increase in the number of small pores and decrease in the relative size of constrictions within the pore space. This significant change in the detailed pore structure of the bed will act along with the decrease in overall porosity also observed upon consolidation to create a structure, which will influence the characteristic hydrodynamics associated with the bed.

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