Performance evaluation of an additively manufactured freeform wick for heat pipe applications

Evaporation rates of working fluid from porous structures are of great importance to heat pipe applications. This study presents the evaluation of the evaporation/boiling process of water from a stainless steel 316L wick structure fabricated through 3D-printing technology. The fabrication process that is employed to provide the test sample, yields relatively high repeatability within the wick structure. An experimental facility was developed to observe the characteristic processes of flat heat pipe operating conditions. The internal volume of the test cell is 40 × 20 × 6 mm3. A uniform heat flux was applied to the base plate near one end, while the other end was cooled by means of a cooling water jacket. The thermal performance of the 3D-printed wick, filled with water as working fluid, is tested for different heat inputs 0.75–82.5 kW/m2. Evaporation thermal resistances were determined for different heat loads and filing ratios. Additionally, the wick surface was visualized during evaporation and boiling, which allows for correlating the thermal performance with the observed regime. It is found that nucleate boiling from the wick substrate leads to a substantially increased thermal performance compared to evaporation from the liquid-free surface at the top of the wick. The 3D-printed wick features a minimum evaporator resistance of 0.086 K/W. Experimental results are combined into an evaporation/boiling curve specifically for 3D-printed wicks. The transition between the evaporation and boiling regimes is found to be dependent on the heat flux and filling ratio, and is reasonably well predicted by the bubble-nucleation criterion available in the literature. Altogether, the experimental results verify that 3D printing is a promising technology to fabricate freeform porous structures for heat pipe applications. Compared to flat heat pipes with a screen mesh, grooved wick or composite wick, a 3D-printed wick yields a higher thermal performance.