Presentation
Heat pipes are high efficiency heat transfer devices. They consist basically of a metal tube closed in both extremities, with a porous structure covering its internal wall. The porous structure can be a metallic fiber, a syntherized porous piece, or small grooves on the internal walls of the tube. The tube is evacuated and filled with a certain amount of working fluid, so the liquid volume is smaller than the total internal volume of the tube. The liquid is absorbed by the porous structure. When heat is applied in one of the extremities, called evaporator, the liquid evaporates. This vapor flows on to the other extremity, called condenser, which is externally cooled. The vapor then condenses on the tube internal walls, and the liquid is absorbed by the porous structure. The liquid then flows through capillarity back to the evaporator extremity, thus completing a cycle.
The thermal resistance of a heat pipe is quite low when compared with other heat transfer devices because the internal heat transfer phenomena occur through phase changes, which are not associated to temperature drops. In other words, the temperature variation between the extremities in a heat pipe is close to none. If a solid metal bar was to be used for the same end, the thermal resistance would be up to 1000 times higher than that of the heat pipe, because conduction would be the only heat transfer phenomena in place, which is always associated to temperature gradients. The illustration below shows a thermosyphon heat pipe.
The main difference between a heat pipe and a two-phase thermosyphon is that the thermosyphon does not have a porous structure to pull back the condensed liquid to the evaporator. It uses the gravitational force instead. For that reason, thermosyphons cannot operate in the horizontal orientation, against gravitaty, or in zero-gravity evironments (i.e. spacecrafts). In those cases, conventional heat pipes must be used. The advantage of thermosyphons is their simplicity, because no porous structure is used. Thermosyphons have been used increasingly in industry applications, including in Brazil.
