# Design and Additive Manufacturing of TPMS Heat Exchangers
## 1 Introduction  
Heat exchangers (HXs) are present in all major industrial fields, being an essential component of most engineering systems. The design of an HX is a balance between maximizing the surface area in order to transfer heat and minimizing the pressure drop of the HX. In the aviation and automotive industries, compact HXs were widely used thanks to their relatively small volumes and consequently low weights and high thermal efficiency [1]. It is very important to reduce the weight of HXs, by acting on the size/weight, while performance in terms of thermal efficiency must reach high levels [2]. In aeronautical and aerospace applications, the most commonly used HXs are the finned plate type due to their compactness, high efficiency, and ease of system integration. Most of the finned plates in compact HXs are parallel to each other. It is difficult to fabricate other geometries of the fins due to conventional manufacturing capabilities [3]. Modern additive manufacturing (AM) techniques provide new ways to manufacture HXs with complex geometries. Laser Powder Bed Fusion (L-PBF), previously called Selective Laser Melting (SLM), is a typical AM process that uses a laser beam as a heat source to fuse metal powders. The starting point of the process is the creation of a 3D CAD model, which is then sliced numerically into several finished layers by the software. For each sliced layer, a hatch pattern is defined and computed as a laser path. The path data is exported to a laser scanner. The laser beam is manipulated by the laser scanner to the layer to fuse the CAD model onto the layer of powder. The process is repeated layer by layer, and the melted powder particles solidify to form a component [4; 5]. L-PBF is the most widely used metal AM technology in the world and is a promising technology for the printing of HXs with complex geometries. The L-PBF process also has challenges. It is difficult to create a high-quality, defect-free thin wall or thin features in general with a thickness of less than 200-300 \(\upmu\)m. In addition, conventional manufacturing methods such as stamping have the capability to fabricate plate fins with typical thicknesses ranging from 46 to 200 \(\upmu\)m [6; 7]. The use of triply periodic minimal surfaces (TPMSs) has been proven to be a possible method to enhance the heat transfer and, consequently, the efficiency of a TPMS HX [8]. Figure 1 shows some examples of triply periodic minimal surfaces (TPMSs), such as gyroid, diamond, Schwarz, SplitP, etc. Due to their unique capability in providing high surface area-to-volume ratios and operating at high turbulent modes and high structure stiffnesses, these structures have been explored recently in relation to heat exchangers (HXs) and have achieved unprecedented performances, by taking advantages of additive manufacturing (AM) technology [9; 10]. The TPMS-walled HXs, manufactured via AM, have great potential for industrial development. The relationships between heat transfer, pressure drop, and type of structure still need to be properly evaluated [11; 12]. These TPMS-based core structures are not easy to design using existing CAD systems, especially in the case of HXs with complex 3D geometries [13]. In this paper, the commercial software nTop 4.23.2 (www.ntop.com) was used to model the TPMS HXs. Recently, studies about TPMS HX have used polymer composites that have some limitations: (a) low thermal conductivity, (b) failure at high operating pressure, or (c) high porosity in 3D printing. [PERSON] reported a microarchitected gyroid lattice liquid-liquid compact heat exchanger realized via stereolithography additive manufacturing as a single ready-to-use unit. The experimental results showed a 55% increase in exchanger effectiveness for the additively manufactured gyroid lattice heat exchanger in comparison to a common heat exchanger [14]. The Reynolds number in the 3D printed compact heat exchanger was relatively small. The turbulence in the compact HX should have been much higher.