“Traditionally, heat exchangers flow hot fluid and cold fluid through straight pipes, mainly because straight pipes are easy to manufacture,” says Xiaoping Qian, a professor of mechanical engineering at UW–Madison. “But straight pipes are not necessarily the best geometry for transferring heat between hot and cold fluids.”

Additive manufacturing enables researchers to create structures with complex geometries that can yield more efficient heat exchangers. Given this design freedom, Qian set out to discover a design for the hot and cold fluid channels inside a heat exchanger that would maximize heat transfer.

He harnessed his expertise in topology optimization, a computational design approach used to study the distribution of materials in a structure to achieve certain design goals. He also incorporated a patented technique, called projected undercut perimeter, that considers manufacturability constraints for the overall design.

With an optimized design in hand, Qian worked with colleague Dan Thoma, a professor of materials science and engineering at UW–Madison, who led the 3D printing of the heat exchanger using a metal additive manufacturing technique called laser powder bed fusion.

From the outside, the optimized heat exchanger looks identical to a traditional version with a straight channel design—but their internal core designs are strikingly different. The optimized design has intertwining hot and cold fluid channels with intricate geometries and complex surface features. These complex geometric features guide fluid flow in a twisting path that enhances the heat transfer.

Collaborator Mark Anderson, a professor of mechanical engineering at UW–Madison, conducted thermal-hydraulic tests on the optimized heat exchanger and a traditional heat exchanger to compare their performance.

The optimized design was not only more effective in transferring heat but also achieved a 27% higher power density than the traditional heat exchanger. That higher power density enables a heat exchanger to be lighter and more compact—useful attributes for aerospace and aviation applications.

While previous research has used topology optimization to study two-fluid heat exchanger designs, Qian says this work is the first to harness topology optimization and impose manufacturability constraints to ensure the design can be built and tested.

“Optimizing design on the computer is one thing, but to actually make and test it is a very different thing,” Qian says.

“It’s exciting that our optimization method worked. We were able to actually manufacture our heat exchanger design. And, through experimental testing, we demonstrated the performance enhancement of our optimized design. The excellent work performed by the students, postdoctoral researchers and scientists in the three research groups made this advance possible.”

Sicheng Sun, a recent Ph.D. graduate from Qian’s research group, is the first author on the paper. Additional co-authors include Tiago Augusto Moreira, Behzad Rankouhi, Xinyi Yu and Ian Jentz, all from UW–Madison.

The researchers patented their projected undercut perimeter technique through the Wisconsin Alumni Research Foundation.

For more information: International Journal of Heat and Mass Transfer.

Image: A rendering of a topologically optimized unit cell for a heat exchanger core. The optimized design has hot and cold fluid channels with intricate geometries and complex surface features.

Introduced in the 1990s, LPBF uses metal powder and lasers to 3D print metal parts. But porosity defects remain a challenge for fatigue-sensitive applications like aircraft wings. Some porosity is associated with deep and narrow vapor depressions which are the keyholes.

The formation and size of the keyhole is a function of laser power and scanning velocity, as well as the materials’ capacity to absorb laser energy. If the keyhole walls are stable, it enhances the surrounding material’s laser absorption and improves laser manufacturing efficiency. If, however, the walls are wobbly or collapse, the material solidifies around the keyhole, trapping the air pocket inside the newly formed layer of material. This makes the material more brittle and more likely to crack under environmental stress.

Sun and his team, including Professor Anthony Rollett from Carnegie Mellon University and Mechanical Engineering Professor Lianyi Chen from the University of Wisconsin-Madison, developed an approach to detect the exact moment when a keyhole pore forms during the printing process.

“By integrating operando synchrotron x-ray imaging, near-infrared imaging, and machine learning, our approach can capture the unique thermal signature associated with keyhole pore generation with sub-millisecond temporal resolution and 100 percent prediction rate,” Sun said.

In developing their real-time keyhole detection method, the researchers also advanced the way a state-of-the-art tool — operando synchrotron x-ray imaging — can be used. Utilizing machine learning, they additionally discovered two modes of keyhole oscillation.

“Our findings not only advance additive manufacturing research, but they can also practically serve to expand the commercial use of LPBF for metal parts manufacturing,” said Rollett.

“Porosity in metal parts remains a major hurdle for wider adoption of LPBF technique in some industries. Keyhole porosity is the most challenging defect type when it comes to real-time detection using lab-scale sensors because it occurs stochastically beneath the surface,” Sun said. “Our approach provides a viable solution for high-fidelity, high-resolution detection of keyhole pore generation that can be readily applied in many additive manufacturing scenarios.”

For more information: Science Magazine

Image: UVA materials science and engineering postdoctoral fellow Zhongshu Ren (left) and Tao Sun display the results of their research.