Phase change heat transfer characteristics of an additively manufactured wick for heat pipe applications

This study presents the phase change heat transfer characteristics of an additively manufactured wick for heat pipe applications. For this study, a flat heat pipe is considered that is made of stainless steel 316L utilizing a fabrication process that yields a wick structure with multi-scale features. Using laser powder bed fusion, relatively large pore structures, similar to screen-mesh structures, are laser molten with micro-scale laser-sintered features on top of this - we refer to these as multi-scale features in this paper. To characterize the phase change processes at heat pipe operating conditions, an experimental facility was developed. For this aim, a test cell with an internal volume of 40 x 20 x 6 mm(3) was fabricated. Different filling ratios with water as working fluid and heat inputs ranging from 0.75 to 82.5 kW/m(2) were tested. Additionally, a thermal network model to predict the overall heat transfer performance of an additively manufactured flat heat pipe is presented. The model is based on the heat conduction through the wick and wall in both axial and radial directions. Validation by comparison to experimental data shows an excellent agreement for the operating temperature (adiabatic or vapour temperature). The fabricated wick structure exploits enhanced evaporation heat transfer; thin film evaporation occurs due to the multi-scale sintered powder features on top of the fabricated wick surface. Additionally, the optimal filling ratio for which the device has maximal thermal performance is determined. The evaporation superheat is determined and discussed for different filling ratios. Experimental results show the optimum thermal performance of the flat heat pipe for a filling ratio of 110%. Compared to conventional wick structures, additively manufactured wick design exhibits improved thermal performance at higher heat fluxes. The experimental results suggest that additive manufacturing is a promising technology to fabricate freeform porous structures for heat pipes in general.