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Six industry leaders (nTopology, Ansys, EOS, Stress Engineering Services, North Star Imaging, and Synopsys) took on a challenge to develop a clean slate rapid design of an advanced heat exchanger, leveraging each of the advanced capabilities that their companies offer.
The aim of the project was to improve the energy efficiency and system performance of the heat exchanger while using less material in less space, as well as combine design, simulation, CT inspection, and testing to help with quality control of this new, high-cost product. The final model displayed significant reductions in parts, pressure drop, and increased heat transfer.
Most new heat exchangers applications are space-constrained, with increasing thermal requirements driving a need for smaller, more efficient designs. However, there has been relatively little innovation in common heat exchanger designs, which typically use inefficient tube and shell geometries. By using an implicit geometry, a heat exchanger design using complex surfaces provides significant gains in terms of performance.
The project saw nTopology and Ansys collaborate on generating and simulating a new heat exchanger design. nTopology use implicit geometry technology to create triply periodic minimal surfaces (TPMS) that are beneficial for heat exchangers due to their smooth topology for flow dynamics. In addition, TPMS designs inherently isolate two regions (such as hot and cold), and have a high strength to weight ratio, making them ideal for light-weighting.
The redesigned heat exchanger with the TPMS geometry is much smaller than legacy designs, achieving a reduction in parts from 40 to just one, with an 81% decrease in total mass.
For the re-design, nTopology defined a heat exchanger infill with the TPMS geometry, achieving a reduction in parts from 40 to just one, with an 81% decrease in total mass. Different designs were created, and tested using Ansys software, first with Ansys Discovery Live for real-time simulations of fluid flow in different design iterations. Ansys Fluent CFD simulations were then employed to compare the performance of a legacy heat exchanger design and the TPMS heat exchanger. The study showed impressive results, including an 85% reduction in volume, an 11.7x increase in heat transfer per unit volume, and a 9.4x increase in heat transfer per unit mass.
Design validation results of the compact heat exchanger design.
Ansys optimized the build process from the design using simulation tools to evaluate factors such as the best orientation, custom support structure design, and other inputs to help reduce the risk of printing effects, and to minimize the build time. Process simulation in Ansys software also helped with this process in terms of predicting factors such as distortion, porosity, and microstructure, as well as the specific process parameters of the chosen EOS M290 printer.
Build process analysis (left), and the final part (right).
For the build, EOS carried out some further preprocessing, before setting up the print, including remote monitoring to ensure that no problems were taking place. A suitable print was achieved on the first attempt, taking around 62 hours for two parts, followed by post-processing steps including heat treatment to create a high-quality, 3D-printed component.
The part was CT-scanned by North Star Imaging to create an image dataset for import to Simpleware software to compare it’s as-designed and as-built performance. Simpleware software was used to segment the CT data to identify solid and fluid domains, and to carry out defect detection through visual inspection of pores, holes, and cracks. Surface models (STL files) of the solid and fluid regions were then created and overlaid with the as-designed STL for surface deviation analysis. With this approach, it is straightforward to identify differences between the designs as the result of manufacturing.
Visualization of the CT image data.
Surface deviation analysis of the as-designed versus as-built part in Simpleware software.
In the case of the heat exchanger, there was very little deviation between the two models, represented by some small areas at the base of the heat exchanger where printing had caused some rounding and excess powder. Overall, though, the printed part was very true to the nTopology design. This image-based workflow can also be opened up to automation using Machine Learning to rapidly import, segment, and obtain critical measurements for the user, reducing the risk of inter-operator differences.
The next step in the workflow involved seeing how the as-built part performed compared to the as-designed version. Simpleware software was used to generate a multi-domain Finite Element (FE) mesh with conforming interfaces ready for import to Ansys software.
Automatically generated multi-domain FE mesh in Simpleware software.
The model in Ansys was then subjected to large deformation plasticity analysis to validate burst pressure. Material properties were assigned based on the Ansys Additive Materials library, and comparisons made between the scanned nTopology mesh and the original CAD mesh. Submodeling of the mesh was also carried out to further analyze the performance of the heat exchanger without the need for significant computational resources.
According to the simulations, as-built and as-designed performance is very similar, with marginal improvements in deformation for the former version possibly being the result of local thickening. To increase confidence in the heat exchanger performance, Stress Engineering Services, Inc. performed a range of full-scale physical tests, including of environmental exposure and shock and vibration, as well as heat transfer and pressure drop. When compared to another heat exchanger, the TPMS design significantly outperformed it, notably in terms of flow rate. Moreover, the TPMS model exceeded the burst pressure requirement of 350 psi by >14x, demonstrating that it is suitable for real-world applications.
Burst pressure test simulation.
The part was then installed into Stress Engineering’s full scale heat transfer flow testing setup to measure the pressure drop and Overall Heat Transfer Coefficient. Hot and cold test fluid were sent through the heat exchanger to measure the heat exchanger’s performance. Thermal Imaging was used to visualize the heat distribution in the heat exchanger during testing. The test results of the decrease in the hot fluid temperature as a function of the hot or cold fluid flow rate show, that the reduction in the hot fluid temperatures decreases as the flow rate increases with the highest measured change in hot fluid temperature of around 50°F at roughly 1 GPM cold fluid flow rate. The performance of the 3D printed heat exchanger is compared to a traditional commercially available brass shell and tube heat exchanger.
Heat transfer flow testing with thermal image data.
After heat transfer flow testing, the heat exchanger was then subjected to pressure testing. The pressure was gradually increased in stages all the way up to failure (loss of pressure). A sophisticated Digital Image Correlation (DIC) system was used to measure the deformation in the heat exchanger during the pressure testing. The DIC system uses optical imaging to measure surface displacements and calculate strain. Similar to that of a strain gage however it is the entire visible surface of the test article that can be measured. These strains can then be compared to the Finite Element Analysis for refinement of the analytical model.
Proof pressure test and pressure to failure (burst) test.
To establish a workflow towards validation, the heat exchanger prototype that underwent burst testing was re-imaged to visualize the regions in which the failure occurred. 4 regions in which cracks had formed near the base of the heat exchanger were located that corresponded with the locations where failure was detected in the burst test. While the heat exchanger prototype performance far exceeded expectations in performing the purpose of this post-test imaging and visualization were to identify regions where the design could be revisited and strengthened if higher levels of performance would be required.
Visualization of the post burst test heat exchanger. Red areas highlight regions of failure where cracks were found.
The redesign of the heat exchanger was highly successful at using TPMS to improve performance whilst reducing size to account for space limitations. Compared to legacy designs, the TPMS part achieved significant improvements, reducing volume by 85%, total mass by 81%, and producing an 11.7x increase in heat transfer per unit volume, a 9.4x increase in heat transfer per unit mass, and a 7.9x increase in surface area per unit volume. In addition, the TPMS design saw a 9.1x reduction in pressure drop on its hot side, and a 1.16x increase in pressure drop on the cold side. Reducing a heat exchanger with 40 or more parts to just one part represented a similar success for the project.
With this workflow, each partner demonstrated how combining the latest technologies for design, inspection, additive manufacturing, and simulation, it is possible to re-design and validate the performance of crucial components. This approach has great potential to reduce material costs, speed up design-to-part processes, and increase efficiency in multiple areas.
Watch the entire 6-part webinar series "A Journey Through Advanced Manufacturing" on-demand, presenting the full design, simulation, fabrication, testing, and quality assurance workflow.
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