For the 2015 Formula SAE race, Swansea University Race Engineering worked closely with additive manufacturing expert Renishaw to redesign the intercooler of its race car.
The aim was to reduce the component’s weight and size, in order to increase the speed of the car. To achieve this, the team used a form of additive manufacturing, metal powder bed fusion.
This technique uses a powerful high precision laser to fuse fine metallic powders in order to form highly complex functional components. The design of the parts is optimized using 3D CAD and the components themselves can be built from a wide range of metal powders melted in a tightly controlled inert atmosphere, in layers with a thickness ranging from 20?m to 100?m.
The location of the intercooler is essential because the part requires flowing ambient air to cool the boost air into the engine. In a Formula Student race car, the intercooler is situated behind the driver, above the engine an unsuitable position in terms of heat transfer.
The new high-performance heat exchanger needed to have excellent heat transfer and low pressure drop through the system. The wider range of geometries made available by additive manufacturing meant the team could experiment with more innovative designs that enabled better airflow. Although additive manufacturing has previously been used in Formula 1, the current project is unique due to the large scale and cross flow nature of a car intercooler.
Overcoming design challenges
The team’s objectives were to ensure a high density of boost air for the engine, while still maintaining a good mass flow rate of air. The intercooler also needed to be as light as possible.
“Design had a massive impact on the end product, even more so than the material, which is something we didn’t originally expect,” explained Dr Nicholas Lavery, director of the Materials Advanced Characterisation Centre (MACH1) at Swansea University.
“Although our students had to go back to the drawing board many times, they were very pleased with the end product. It’s also interesting when results are unexpected; it either means you’ve muddled up your calculations or you’re on to something good. Luckily, in this case, it was the latter.”
To identify the best alternative, students tested four intercooler core designs. The different iterations reflected the design principles and capabilities of different manufacturing methods. The first core design, used as a benchmark, was a conventionally manufactured aluminium alloy, AlSi10Mg core with a simple vertical design on the ambient side and a sine wave shape on the boost side.
The second core was designed from 316L stainless steel using metal powder bed fusion. The design was identical to the first one, but was used to test how the added surface roughness available with additive manufacturing impacted heat transfer and pressure drop.
The real benefit of additive manufacturing came when the students started experimenting with the design freedom of the technology. The third intercooler core had a lattice of three dimensional star shapes to form the mesh a design only made possible by additive manufacturing machines. The mesh was generated using additive manufacturing software and although it took several iterations, the team knew that they were on to a winner.
The fourth and final core used the same three dimensional star lattice structure used in the mesh core, but also made the strut cross-section finer, which meant a higher surface area and density.
Although superior to casting, the surface finish was not as smooth as a manufactured surface. The tests found that the higher surface roughness of additively manufactured intercoolers outperformed conventional counterparts.
This counter intuitive result could be due to better air mixing, altered flow conditions or a combination of both.
Mastering material and software challenges
After the students had identified the most efficient design, they examined potential materials and originally tried titanium Ti6Al4V, a lighter alternative to stainless steel, but with lower thermal conductivity.
Renishaw then offered to manufacture the part using aluminum alloy AISi10Mg, a material considered ideal due to its high thermal conductivity and light weight. Renishaw used the students’ designs to manufacture the intercooler.
Handling the designs was one of the most complex aspects of the project, mostly due to their large size. However, it was the CAD functions that enabled the most efficient positioning of the lattices to allow maximum heat efficiency, thus increasing the capabilities of the intercooler.