Thermal management of complex systems is one of the most common engineering challenges. From small electronic devices to large industrial installations, transferring, storing and dissipating excessive heat are common problems that can find widely diverse solutions.
While bigger industrial heat exchangers still resort to conventional manufacturing techniques, Metal Additive Manufacturing is finding more and more applications in smaller, more compact designs such as electronic devices, renewable energy applications, refrigeration cycles, fuel cells, ICE vehicles and ultimately electric cars. The main benefit offered by the technology is the increased design freedom, that should allow for more efficient heat transfer, whilst reducing the number of required parts. This results in the use of less space and material, allowing for lighter devices that can manage higher thermal loads and power densities.
There are several ways to obtain a more efficient heat exchanger leveraging Additive Manufacturing strengths. By far the most common is to increase the surface available for heat exchange through the use of lattice structures, pin fins and micro-channels. All these features allow for a higher heat transfer at the expense of an increase in pressure drop across the heat exchanger. Lattice structures, in particular, can result detrimental to the overall efficiency of the system if not properly designed, due to the increased resistance opposed to the fluid flow. Pin fins, on the other hand, are almost always beneficial: whilst only marginal improvements are expected for circular and elliptical fins, diamond, air-foil and rectangular fins offer the highest heat transfer rate per unit pressure drop.
Another way to improve heat exchangers efficiency is to increase the turbulence of the flow in internal channels. Generating vortices within an axial flow, it is possible to create a secondary, orthogonal velocity field that reduces the boundary layer thickness and promotes heat exchange. Such vortices can be created by a twisting tube or by swirl generators. Using Selecting Laser Melting technology (SLM/DMLS) the latter can be realised together with the main body of the heat exchanger, without the need for assembly and virtually at no extra cost, ensuring improvements to the heat transfer coefficient of up to 40% with only marginal pressure drops. Such a solution was recently used by Purdue University to design an award-winning Heat Sink.
One last heat transfer enhancement can be obtained with a rough surface approach. While an improvement of up to 70% in the heat transfer coefficient can be obtained, such an approach greatly increases the friction coefficient resulting once again in a higher pressure drop. Such an effect becomes even more relevant for micro-channels with a small hydraulic diameter.
Depending on the specific application and overall geometric layout, these enhancements can help designers achieve their target performance, taking full advantage of the design freedom offered by Metal AM. Integrating this knowledge into a basic understanding of Design for Additive Manufacturing (DfAM) enables the design of cost-effective and high-performance thermal management devices. Co-design, in particular, becomes even more important in the design of heat exchangers: according to the orientation of the part inside the build volume, internal features such as channels and cavities must be designed not to require support structures. Only by sharing ideas and constraints from the early stages of the design project, the manufacturability of the part can be ensured.
Fabio Baiocchi
fabio.baiocchi@additivalab.com
