We perform a numerical study of the mass transfer in Kelvin cell structures. We reach qualitative agreement with experimental works on foams. New correlations are presented. The Lévêque analogy is verified.

Polyhedral open cell lattice catalyst substrates are proposed based on results of numerical simulations and recent advances in Additive Manufacturing (AM) techniques. Detailed simulations have compared different polyhedral structures in terms of mass transfer (aiming at optimal reactivity in the mass transfer limited domain) and flow through resistance. The simulations have taken into account dimensional limits given by the possibilities of AM techniques. Comparisons with state of art honeycombs have been also used in order to identify the optimal shape. Substrates with these optimal characteristics have been manufactured out of Al2O3 with Stereolithography. Subsequently, these substrates have been coated and used for measurements of C3H6 oxidation in a model gas reactor. Measurements have focused in determining oxidation efficiency at different gas hourly space velocities as well as light-off behaviour. Simulation results show that the optimal open cell structures are comprised by a cubic elementary cell rotated by 45° so that one spatial diagonal of the cube is aligned to the main gas flow. Higher porosities and smaller strut diameters improve the reactivity to pressure drop trade off. However, given the current manufacturing limitations, it is not possible to produce structures with strut diameters lower than 0.5 mm. This results in high porosity but low specific surface area (i.e ε=0.95 and Sv=400m 2 /m 3 ). Thus, reaching a target conversion requires higher overall catalyst volume. The simulations show that for a series of geometrical parameters the open cell structures can reach identical conversion in respect to the honeycombs with only a fraction of the overall surface area and thus a fraction of the noble metals, while the overall dimensions are in the same order of magnitude and the pressure drop can reach lower levels. Measurements in the model gas reactor confirm the mass transfer advantages of the polyhedral structures as predicted by the simulations. Measurements also show that the polyhedral lattices have very similar light-off behaviour in spite the four times lower surface area. NomenclatureA: Cross section of catalyst AM:Additive Manufacturing CPSI:Cells Per Square Inch, commercial characterization of honeycomb catalyst substrates Cubic:Additive Manufactured (AM) catalyst substrate consisting of cubes as elementary cells aligned with the main flow Cubic45:AM catalyst substrate consisting of cubes as elementary cells rotated by 45° so that one spatial diagonal of the cube is aligned to the main gas flow Dij:Diffusivity of specie i in a gas j dc:Wetted width of a (square) honeycomb channel Dc:Inner width of a (square) honeycomb channel, difference to dc is the coating thickness ds:Strut diameter ghsv:Gas hourly space velocity through the catalyst, it corresponds to the ratio between the gas volume flow rate and the catalyst volume HC:Honeycomb catalyst substrate (conventional) K:Mass transfer coefficient Kelvin:AM catalyst substrate consisting of Kelvin cells (tetrakaidekahedral) as elementary cells

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Open cell foams are considered promising catalytic substrates providing high surface area and a tortuous structure resulting in enhanced mass transfer characteristics. CFD analysis, recently, has focused in pointing structures with favourable reactivity-flow resistance characteristics. In order to reduce the geometrical complexity and computational efforts, foams have been modelled as regular (polyhedric) open cell structures. In this study a comprehensive comparison of real foams with equivalent Kelvin cell lattices is performed in CFD. Therefore 4 typical foams (two ceramic and two metallic) have been chosen. Geometric properties have been accessed with Micro-Tomography scans. Randomised Kelvin cell lattices have been generated matching porosity and specific surface area of the scanned real foams. Geometric, momentum and mass transfer characteristics of real foams and Kelvin cell lattices are analysed with CFD. Kelvin cell lattices showed similar behaviour in respect to their real foam equivalents, had though clearly better reactivity-pressure drop trade-offs. Based on the results presented best performances as a catalyst can be expected by 3D printed, additive manufactured, high porosity polyhedric structures