Assessment of thermal-hydraulic performance of circular pipes equipped with Triply Periodic Minimal Surfaces

One of the greatest challenges that the scientific community is addressing in this century is to provide clean and abundant energy to satisfy the increasing worldwide demand. Many solutions are already existing or currently under development and some of them require to deal with very high temperatures and high heat loads. As an example, heat fluxes are expected to reach values up to 20 M W/m2 on some components of nuclear fusion plants. Thus, it is necessary to design very efficient heat sinks in order to respect the thermo-mechanical limitations of the structural materials. Triply Periodic Minimal Surfaces (TPMS) are three-dimensional surfaces, that can be mathematically described by sinusoidal and cosinusoidal functions. From these surfaces it is possible to generate periodic lattices that, among other applications, can be used to equip channels for heat dissipation. In recent decays, TPMS have become more and more interesting due to the advent of Additive Manufacturing that allows the production of such complex objects. They may therefore effectively substitute other porous media configurations, introducing regularity and reducing the pressure drop in the flow due to their smooth curvature. The present work aims at the study and performance assessment of four TPMS-based structures, namely Diamond, Gyroid, Lidinoid and SplitP. The first two TPMS have already been object of numerous studies, mainly in the field of innovative heat exchangers, that in- dicate better performance with respect to traditional solutions. On the contrary, there is no available literature yet regarding thermal-hydraulic applications of the others. The four structures under investigation in the present work have been inserted in a circular channel 10 cm long. Their behaviours have been analysed through numerical simulations on the Computational Fluid Dynamics software STAR-CCM+, performed considering Copper for the walls and internal solid structure and water as coolant. The dependence of hydraulic and thermal quantities on the unit cell dimension has been assessed through a parametric study for each lattice. Comparing the different TPMS, it has been found that Gyroid and Diamond produce lower pressure drops in the fluid flow, at the cost of poor thermal performances, while Lidinoid and SplitP behave exactly the opposite way. From these considerations, the possibility to combine two TPMS types has been investigated, splitting the cylindrical channel domain in two regions. Diamond or Gyroid have been inserted in the core region and the outer layer has been filled with Lidinoid or SplitP with a smaller unit cell dimension, in order to enhance heat exchange next to the wall. Four combinations have been designed and the simulations results confirmed the expectations, suggesting that an optimization process on the thickness of the outer layer could lead to interesting outcomes. For all the analysed configurations, the turbulence enhancement factor has been computed to estimate the performance improvement with respect to the smooth tube. An experimental campaign, currently ongoing at the Texas A&M University, will help in confirming and qualifying the simulated results on the different TPMS configurations.