Standard heat pipes can operate in any orientation, since they use a wick to return liquid from the condenser to the evaporator with capillary forces. In contrast, a Thermosyphon (alt. Thermosiphon) uses gravity to return liquid, so the evaporator must be located below the condenser. Thermosyphon operation is shown in Figure 1. Heat supplied to evaporator vaporizes the working fluid, which travels up to the condenser, vaporizes, and drains back down the sides of the envelope. The adiabatic and condenser sections are typically bare, while the evaporator may have a wick for start-up.
Figure 1. Thermosyphon Operation.
Thermosyphons also act as diode heat pipes. When heat is applied to the condenser, there is no condensate, and hence no way to form vapor and transfer heat to the evaporator.
Table 1 shows the differences between a thermosyphon and a heat pipe. The major benefits of a thermosyphon is that it can carry more power than a heat pipe with the same diameter, and the thermosyphon length is essentially unlimited. With the exceptions of grooved heat pipes for space, heat pipes can generally have an adverse elevation of less than 1 foot (25 cm). This can be seen by running ACT’s heat pipe calculator with different elevations, and noting that the heat pipe power gradually reduces to zero as the elevation increases.
In contrast, thermosyphons can operate as long as they are tilted so that gravity returns the condensate, and can be as long as desired. In some cases, thermosyphons greater than 50 m long have been fabricated for geothermal applications.
Table 1. Differences Between a Heat Pipe and a Thermosyphon.
|Liquid Returns by||Capillary Wick||Gravity|
|Orientation||Any||Evaporator Lower than Condenser|
|Length||Typically< 1 foot (25 cm)||Unlimited|
Figure 2 shows a titanium/water thermosyphon with a 2 m long condenser that was fabricated and tested at ACT. The eventual application is heat rejection from a Lunar heat pipe radiator for a NASA mission. These heat pipes have a proprietary wick in the evaporator that allows the water in the thermosyphons to be frozen and thawed multiple times without damaging the thermosyphons.
A Loop Thermosyphon can be used instead of a thermosyphon in high-power and high-heat-flux systems. In a loop thermosyphon, the vapor and liquid travel through separate lines, eliminating the shear stresses that limit thermosyphon power through the flooding limit.