In this paper, the heat transfer performance of gravity heat pipes with anodic aluminum oxide (AAO) wall surface is studied. The main purpose is to study the effects of the length and diameter of AAO nanotubes on the temperature distribution, overall thermal resistance, and dry-out occurrence of gravity heat pipes charged with acetone under different input heat powers. AAO nanotubes were first grown by anodizing the inner wall surface of the evaporator section of aluminum alloy gravity heat pipes. The influences of AAO nanotube length and diameter on the temperature distribution, overall thermal resistance, and dry-out occurrence were then investigated by a thermal performance test system. Experimental results show that increasing the AAO nanotube length could result in reduced temperature variation between the evaporator section and the condenser section, leading to reduced thermal resistance, and delayed dry-out occurrence at higher heat inputs. In addition, increasing the AAO nanotube diameter could also cause decreases in temperature variation and overall thermal resistance, but it could not have a significant effect on the occurrence of dry-out phenomenon. Based on these results, it can be concluded that, if the anodic oxidation treatment is applied to the inner wall surface of the evaporator section of a gravity heat pipe, its heat transfer performance could be significantly improved. The maximum temperature difference and overall thermal resistance of the processed heat pipe are 46.12% and 58.68% lower than those of the unprocessed heat pipe, respectively; moreover, compared to the unprocessed heat pipe, the maximum applicable input heat power to avoid dry-out occurrence can be increased up to about 40%. Such a study could be used for cooling purposes in a wide range of applications such as passive cooling of electronic devices, highly efficient heat recovery, and cleanroom air conditioning.
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