# Melting of Paraffin Waves Embedded in a Porous Matrix Made by Additive Manufacturing
## 1 Introduction  
Additive manufacturing (AM) is a technology that permits the construction of a tridimensional object starting from CAD models. The actual state of the art of this technology has widened the choice of materials to be printed, opening new frontiers for heat transfer devices: not only plastic materials can now be printed, but also metallic materials, with consequent higher thermal conductivities. Many different fields of thermal sciences can have benefits for single- or two-phase applications. [PERSON] et al. [1] studied a way to enhance the heat transfer during liquid water cooling of a horizontal heated plate by means of static mixers made by additive manufacturing. Two different types of mixing devices were considered: twisted tape structures and chevron-shaped offset wings. Both the structures improved the heat transfer coefficient by two times compared to the plain channel for Reynolds numbers below 1000. [PERSON] et al. [2] investigated the feasibility of additive manufacturing to fabricate mini channel heat exchangers with longitudinal vortex generators. At a Reynolds number of 1380, the convective heat transfer coefficient can be up to three times that of the smooth channel, using distilled water as working fluid. [PERSON] et al. [3] experimentally tested tube bundles with conventional circular plain fins as well as new circular integrated pin fins and serrated integrated pin fins made by additive manufacturing during air forced convection with Reynolds number from 1600 to 6600. Fins with the new design demonstrated an improved Nusselt number compared to the conventional fins. Furthermore, the friction factor for the serrated integrated pin fins was the lowest for all the tested Reynolds numbers, and the circular integrated pin fins provided a lower friction factor compared to the conventional circular pin fins for Reynolds number lower than 4000. Another research field where additive manufacturing can exploit its benefits is related to two-phase heat transfer systems, which involve a phase transition of a material from solid to liquid or vice versa. These materials, named PCMs (phase change materials), are usually characterized by a low thermal conductivity, which may degrade the heat transfer characteristics. Thus, PCMs are often coupled with a solid insert to enhance the thermal conductivity of the block. Different ways to enhance the thermal conductivity of PCMs have been proposed in the literature. [PERSON] et al. [4] conducted an experimental study using n-eicosane as PCM embedded in a heat sink with and without internal fins for cooling portable hand-held electronic devices. Experimental results indicated that internal fins aided in lowering the temperature of the electronic device, and a higher number of fins revealed a lower temperature. A not appreciable effect of the orientation of the heat sink was observed. [PERSON] and [PERSON] [5] carried out an experimental study to characterize the thermal performance of pin-finned heat sinks implementing paraffin wax and n-eicosane as PCMs. A configuration with only PCM was tested, as well as configurations with 33, 72, and 120 pins. The best thermal performance was given by the heat sink with 72 pins. Another way to enhance the thermal conductivity of PCM is by using metal foams, which are cellular materials with randomly oriented fibers that form cells. Depending on the porosity, these cells can be open for high values of porosities. The parent material is usually characterized by a high thermal conductivity, thus increasing the overall conductivity. Among the first works related to PCM melting inside metal foams, [PERSON] et al. [6] experimentally tested aluminum foams having 5, 10, 20, and 40 PPI (pores per inch), with porosities ranging between 0.886 and 0.966, coupled with paraffin waves having melting temperatures from 25.4 to 28.9 \({}^{\circ}\)C. The heater temperature was lower in the case with foam having lower porosities, whereas the effect of the number of pores was minor.