Simulation of flow and heat transfer in 3D printable triply periodic minimal surface heat exchangers (2020)
Type of ContentTheses / Dissertations
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
3D printing allows the manufacture of novel processing equipment including heat exchangers. A wide variety of different heat exchanger designs exist, and the usage of each depends on the application for which they are purposed. These designs are sensitive not only to the conditions at which they are operated, but also to shape effects caused by their internal structures. Triply periodic minimal surface (TPMS) structures were used as the internal structure of the heat exchangers studied in this work. TPMS structures tested include the Schoen gyroid, Schwarz diamond and Schwarz primitive. Shape effects cannot be easily predicted ahead of time without testing, but physical testing can be expensive when assessing the effects of many different parameters. Since 3D printing of metallic parts is a relatively difficult, developing field, numerical simulations are employed to reduce testing time and save on material costs. The lattice Boltzmann method was used as the main means of simulating the flow and heat transfer in these structures.
Designs of heat exchangers with TPMS internal structures and corresponding inlets and outlets were created and saved as stereolithography (STL) files. These files were used to store information about 3D structures and were translated into simulation domains for carrying out computational fluid dynamic (CFD) calculations. The design was not straightforward as it required a semi-automatic procedure to seal off some of the channels to ensure separation of the two heat transfer fluids. A procedure was developed to check that the design was physically feasible, that all walls were sufficiently thick, and that there was no leakage.
The lattice Boltzmann methods was set up in the Palabos environment to solve conjugate heat transfer problems in three dimensions. The problems were defined with a fluid lattice and a thermal lattice, the latter existing in the two fluid channels and the thermally conductive walls. The method was verified using previously published solutions to similar problems, in both steady and unsteady situations.
Once the method was verified, heat transfer in different representative elemental volume TPMS structures and in entire heat exchangers was simulated, while altering key parameters to determine their effects. Obtaining a stable solution within all parts of both lattices proved to be difficult. This stage aimed to increase the efficiency of the TPMS heat exchangers.
The major results of this investigation were that at the low Reynolds and Péclet numbers studied, the influence of TPMS shape effects on the heat transferred was relatively minor. The choice of geometry had a far greater influence on the pressure drop experienced by the fluid. As such, less tortuous designs were favourable when considering a heat transfer per unit pressure drop metric. The Schwarz primitive was the best performing TPMS by this metric, though it was surpassed by a flat plate structure, designed specifically to minimise the pressure drop. When pumping is no concern and consequently the pressure drop does not matter, the gyroid was found to perform slightly better in overall heat transfer ability, as measured by the Nusselt number.
Although the current work was limited to low Reynolds numbers, future testing of TPMS designs should be carried out at higher Reynolds numbers approaching the turbulent region. At higher Reynolds number it is likely for TPMS to exhibit more significant shape effects. However, it is also likely that the lattice Boltzmann method with struggle to obtain a stable solution. In this work, only a small subset of popular TPMS geometries, in various orientations were studied. It is possible a more efficient TPMS shape than the gyroid exists, that has not been tested.
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