Evaluating the scale effects of metal nanowire coatings on the thermal performance of miniature loop heat pipe

In this work, the effect of metallic nanowire coating on the evaporator surface of miniature loop heat pipe is investigated. Copper Nanowires (CuNW) of three different diameters (similar to 35, similar to 70 and similar to 130 nm) and constant height of similar to 25 mu m were fabricated using electrodeposition technique on the evaporator surface. A dedicated holder was designed to fabricate nanowire coating on the interior surface of the evaporator section. These CuNW coated surfaces were characterised for their morphology, dimensions and wettability. Cavity density and cavity size of the nanowire coated surfaces were analysed using image processing technique and both were found to increase with increase in the diameter of nanowires. The micron scale cavities generated plays a vital role in generating the effective nucleation sites as well as improving the capillarity in the evaporator surface. The evaporator surface coated with CuNW was found to improve the heat transfer characteristics of miniature loop heat pipe (MLHP). As compared to the evaporator surface without any CuNW coating, an increase of nearly 2.7 times in the heat transfer coefficient values and a decrease of approximately one-third in the thermal resistance values were observed in nanowire coated surfaces. This was believed due to an increase in the area of thin-film evaporation. Enhanced characteristics of CuNW coated surfaces like improved bubble nucleation, capillarity and reduced thermal resistance made it possible to transfer a maximum heat load of 580 W with the surface temperature value of 83 degrees C. This paved a plausible way to employ these kinds of nanostructured surfaces in miniature loop heat pipes to cool high heat dissipating modem electronic equipment.