Comprehensive Study of 3D Liquid Flow Fields in Additive Manufactured Structures for SMART Reactors Using Large-Scale Vertical Magnetic Resonance Imaging and Computational Fluid Dynamics

Triply Periodic Minimal Surface (TPMS) structures have emerged as a new class of porous materials with variable geometries and favourable transport properties, making them promising for reactor internals in chemical engineering. However, experimental data on internal TPMS flow behaviour are still limited. To address this gap, the flow behaviour in additively manufactured TPMS structures is analysed using three-dimensional Magnetic Resonance Imaging (MRI) velocimetry in a large-bore vertical 3 T MRI system, in cylindrical columns of 38 mm diameter and Reynolds numbers between 50 and 300. Three different TPMS geometries are investigated, and consistency between Computational Fluid Dynamics (CFD) simulations and experimentally measured MRI velocity fields is established through cross-validation. The MRI system provides fully three-dimensional velocity fields with a divergence deviation below 6 %. MRI revealed distinct flow features: the Gyroid TPnS exhibited pronounced channelling, while the Schwarz-Diamond TPSf showed merge-split behaviour, achieving a 46 % increase in lateral mixing compared to the Gyroid TPnS structures. Numerical simulations reproduce the flow features and show agreement with the MRI data. The combined methodology demonstrates the suitability of MRI velocimetry for the experimental validation of CFD simulations and establishes a robust foundation for future studies of heat and mass transfer, as well as reactive flow, in structured reactor systems.

💡 Research Summary

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This paper presents a comprehensive investigation of liquid flow within additively manufactured Triply Periodic Minimal Surface (TPMS) structures using a combination of three‑dimensional magnetic resonance imaging (MRI) and computational fluid dynamics (CFD). The authors focus on three TPMS geometries—standard Gyroid, Gyroid rotated by 45°, and Schwarz‑Diamond—each fabricated with 70 % porosity, a unit‑cell size of 10 mm, and integrated into a cylindrical column of 38 mm diameter and 100 mm length. The structures were produced in a single step by laser‑based powder bed fusion, ensuring seamless walls and realistic reactor conditions.

Experiments were conducted in a large‑bore vertical 3 T MRI scanner. Phase‑contrast velocity encoding captured full three‑dimensional velocity fields over a 20 mm field of view, using 1 mm thick slices (20 slices total). Reynolds numbers ranging from 50 to 300 were examined, covering laminar to transitional flow regimes. The MRI system was validated through mass‑flow rate comparison, pressure‑drop measurements, and a divergence analysis that confirmed the velocity field remained within 6 % of a divergence‑free condition, demonstrating reliable mass conservation.

CFD simulations were performed with ANSYS Fluent. High‑resolution unstructured meshes (average cell size ≈ 0.3 mm, y⁺ < 1) were generated directly from the STL files of the printed parts. Boundary conditions matched the experimental setup: inlet average velocity corresponding to the target Reynolds numbers, zero‑gauge pressure at the outlet, and no‑slip walls. Laminar models were used for Re ≤ 200, while a k‑ε turbulence model handled higher Reynolds numbers. The CFD results were cross‑validated against MRI data, showing agreement within 5 % for pressure drop, average axial velocity, and shear‑strain based mixing metrics.

Key flow phenomena emerged from the combined analysis. The standard Gyroid exhibited pronounced “channeling”: a dominant axial pathway with high central velocities and limited lateral mixing. Rotating the Gyroid unit cell by 45° disrupted this alignment, reducing channeling and enhancing cross‑stream exchange without a substantial increase in pressure loss. The Schwarz‑Diamond geometry displayed a characteristic “merge‑split” pattern, where fluid streams repeatedly bifurcate and recombine, leading to a 46 % higher lateral mixing index compared with the unrotated Gyroid. These observations were corroborated by CFD, which reproduced the same vortex structures and mixing behavior.

The study highlights the importance of geometric design parameters—surface‑area‑to‑volume ratio, porosity, and unit‑cell orientation—in governing internal flow and mixing in TPMS‑based reactor internals. The MRI‑CFD workflow provides a robust validation framework: MRI offers non‑invasive, high‑resolution, three‑dimensional velocity data, while CFD supplies detailed insight into pressure fields and local shear that are difficult to measure experimentally. Their agreement confirms that CFD can be trusted for predictive design of TPMS reactors, and MRI can serve as an experimental benchmark for future studies involving heat transfer, multiphase flow, or catalytic reactions.

In conclusion, the authors demonstrate that large‑scale vertical MRI is capable of delivering accurate, divergence‑controlled velocity fields within complex porous structures, and that CFD models can faithfully replicate these fields. The comparative analysis of Gyroid, rotated Gyroid, and Schwarz‑Diamond geometries establishes clear links between structural topology and mixing performance, providing actionable guidance for the design of next‑generation “SMART” reactors that require precise control of flow, heat, and mass transport. This integrated methodology paves the way for systematic optimization of TPMS‑based reactor components across a broad range of chemical engineering applications.