The heat pipe is a passive cooling device that transfers heat from a hot source to a heat sink using fluids as a working medium. Working medium evaporation and condensation are key factors for designing an efficient heat pipe. Many researchers highlight nanofluids, a mixture of base fluid and nanoparticles, as a new working medium for more efficient heat pipes. The present research aimed to investigate heat pipe long-term performance using water-based nanofluids as working medium. Nanofluids with 1 and 3?vol% Al_(2)O_(3) of 20–70?nm particle diameter in water were prepared and characterized. It has been seen in our previous study that the heat pipe performance is enhanced by an average of 26%; however, this enhancement was not sustained over long use and raised a concern about the long-life homogeneity of the nanofluid due to the liquid evaporation. Therefore, we investigated used nanofluid characteristics to determine whether it stays suspended in the base fluid as dispersed particles, or it agglomerates, then aggregates in bigger sizes and then precipitates. The dried heat pipe’s porous medium is cut-out after several uses and is scanned by electron microscope (SEM) at different operation heat loads. Some aggregated nanoparticles have been seen on the wick surface, which caused a capillary and thermal resistance. Also, a sample of the used nanofluid is dried and scanned by SEM, and it shows similar particles aggregation to those observed on the surface of the porous medium. This study showed heat pipe performance improvement because of the heat transfer enhanced features of nanofluids technology, and it justifies the performance decay after long-term use.
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