Experimental study on long-distance anti-gravity loop heat pipe with submicron-scale porous structure

Although a loop heat pipe (LHP) is capable of transporting heat over a long distance under anti-gravity conditions in principle, it has not been demonstrated yet. This paper reports an experimental study of an anti-gravity type LHP that can transport heat for a few meters against gravity. For a LHP to generate a capillary force of several hundred kPa, a submicron-scale porous material made of glass was selected as the primary wick. The wick properties such as pore radius, permeability, and wettability with the working fluid were evaluated. A flat evaporator-type LHP with a few meters of vapor and liquid transport lines was designed, fabricated, and tested. Firstly, a preliminary experiment was conducted with water as a working fluid under horizontal conditions. The length of the vapor and liquid lines were both 10 m. It was found that there was a large heat leak from the evaporator to the compensation chamber. In addition, the vapor did not reach the condenser because the vapor was condensed in the vapor line. Secondly, experiments were conducted under the horizontal (0 m), 2 m, and 4 m anti-gravity conditions. In order to reduce the heat leak, a hydrophilic polytetrafluoroethylene wick was installed at the rear side of the primary wick. The vapor and liquid transport length were 6.5 m, and the ethanol was selected as a working fluid. These experimental results demonstrated that the LHP could transport heat under horizontal and 2 m anti-gravity conditions, whereas the LHP cannot transport heat under 4 m anti-gravity conditions. Finally, a LHP with a vapor and liquid transport line length of 4 m was fabricated, which was able to operate under 4 m anti-gravity conditions.