Nanofluids as Working Media for Loop Heat Pipes

Nanofluids have been recently investigated as new working media for two-phase thermal control systems. However, conflicting reports have emerged, in which contradictory effects of the nanoparticles on the working performance of heat pipes have been described. Some studies have shown that gold or silver nanoparticles significantly improve heat transfer performances of heat pipes. Other studies have provided evidence that metal nanoparticles have no particular effect. This study is aimed at determining whether nanofluids are good candidates as heat carriers in a Loop Heat Pipe (LHP) system. Here, a nanofluid consisting of well-characterized citrate-stabilized gold nanoparticles in water is examined. The metallic nanoparticles are functionalized with citrate ligands in order to be soluble and stable in water at room temperature. An LHP hardware set-up was developed for this investigation. There are transparent elements in the LHP compensation chamber, which enable the visualizing of the fluid flow and phase behaviors. The experimental tests with the gold nanofluid have shown the effects of nanoparticles on the LHP operation. It was found that the nanofluid was not stable at high temperatures in the metallic casing, and after 48-hours tests, there was no physical evidence of the presence of gold nanoparticles remaining in the LHP working fluid. The LHP increased performance may be due to the positive influence of the nanofluid & small amount of sodium citrate present in solution (i.e. the nanoparticle stabilizing agent) on the wettability of the wick.