Flow and Thermal Transport Through Unit Cell Topologies of Cubic and Octahedron Families

Abstract

Advancements in Additive Manufacturing (AM) routes have opened several opportunities for manufacturing complex porous structures which could not have been manufactured by the conventional foaming process that essentially results in unit cells represented as Tetrakaidecahedron (TKD) shape. The regular open-cell lattices belonging to the cubic and octahedron family of structures provide wide spectrum of porosity, pore-density, permeability and surface area-to-volume ratios. These complex cell topologies can potentially be manufactured additively, making the realization of above-mentioned properties possible. For efficient heat exchanger designs, the reticulated structures made from these unit cells are expected to have higher permeability, porosity, effective thermal conductivity and interstitial heat transfer coefficient. A detailed understanding of local flow and heat transport characteristics is necessary to design unit cells with such properties. To this end, four different metal foam unit cell topologies have been investigated in the present study, viz. Tetrakaidecahedron (TKD), Cube, FD-Cube and Octet. The Cube and FD-Cube unit cell topologies investigated in the present study belong to the cubic family, where the fibers are present only along the edges and face of the unit cell volume. The TKD and Octet configurations belong to the octahedron class of lattices, where complex connectivity exists within the unit cell volume. To allow for direct comparison between these three unit cells, circular fibers are chosen, which is different from the general TKD cells (tricuspid cross-sections). The porosity was fixed 0.986 which dictated the fiber dimensions of respective unit cells, with Octet fibers being thinnest. Direct simulations have been carried out with periodic boundary conditions on unit cell faces which were parallel to the flow. Such configurations when reticulated in three dimensions are representative of porous structures which are single-cell thick in the streamwise direction. These foam shapes have been proven to be very efficient as they do not allow the thermal boundary layer development. Flow and heat transfer results obtained for above boundary conditions have also been compared with the periodic boundary condition on all the faces.The direct simulation results include pressure drop per unit length and volumetric Nusselt number (corresponding to constant heat flux and constant wall temperature boundary conditions on fibers). Flow stagnation on fibers was the main contributor for both, pressure drop increase, and interstitial heat transfer enhancement. TKD unit cell had higher flow losses and thermal performance than FD-Cube. Octet (octahedron family) configuration had the highest flow losses as well as Nusselt number compared to all other unit cell topologies, because of the large surface area offered for flow stagnation.