Standard Thermosyphons are very effective in transferring heat when the evaporator is located below the condenser. Most thermosyphons are straight tubes, with counter-current flow: vapor flowing up, and liquid draining down in the walls of the tube. This simple design allows thermosyphons to be fabricated relatively inexpensively.
The flooding limit is one of the limits to the maximum power that a thermosyphon can carry, especially at lower temperatures, when the vapor velocity is high. In this limit, the shear of the vapor traveling to the condenser prevents the liquid film on the wall from traveling back to the evaporator. This limit can be bypassed with a Loop Thermosyphon.
Figure 1. Loop Thermosyphon Operation
In a Loop Thermosyphon, the vapor and liquid travel in the same direction around the loop, eliminating the flooding limit, and allowing much higher powers; see Figure 1. Heat is supplied to a wicked evaporator, located at or near the bottom of the loop. The heat vaporizes the working fluid, which exits from the top of the evaporator and travels through the vapor line to the condenser. The vapor condenses, and liquid drains from the bottom of the condenser, which is located above the liquid line. The amount of working fluid in the loop is chosen so that liquid builds up in the liquid line below the condenser, generating a hydrostatic head that prevents vapor from exiting from the bottom of the evaporator.
Loop thermosyphons are often used in high-power and high-heat-flux applications. One loop thermosyphon, shown in Figure 2, removed 4,390W over a 10 cm2 area, corresponding to a heat flux of 465Wcm2. Details of the wick are shown in Figure 3 and Figure 4. A large block of copper was used around the evaporator wick in order to fit all of the cartridge heaters required.
The most common application for loop thermosyphons is cooling high power electronics systems. The thermal resistance of loop thermosyphon cold plates can be less than 0.005 °C/W. A second benefit is that the condenser can be located remotely from the cold plate, and attached using quick disconnects.
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