During heat pipe operation, heat supplied to the evaporator vaporizes the working fluid, which travels down to the condenser. The vapor condenses in the condenser, and the liquid is drawn back to the evaporator by the wick, using capillary forces, similar to the way that water is sucked into a sponge. For more details on how a wick operates, see How a Heat Pipe Wick Operates.
The capillary forces are dependent on the pore size of the wick, and the radii of curvature of the interface, see ΔPC, Capillary Force. The smaller the pore size, the
tighter the radii of curvature, and hence the higher the capillary pressure that can be achieved. However, the capillary flow at a given pressure drop is highest when the pores are large, so there’s a tradeoff in wick design. See Wick Properties for a definition of the different wick properties, and how they are measured.
There are three basic types of heat pipe wicks:
- Required for complicated heat pipes
- Inexpensive in comparison to the other types of heat pipe wicks
- Low-pressure drop
- Moderate capillary action
- High performance
- Moderate pressure drop
- Highest capillary action
- Suitable for higher acceleration applications
- High power in low acceleration environments (microgravity such as low earth orbit)
- Lowest pressure drop
- Lowest capillary action
Small pores provide good lift height, but generally have lower maximum power, since the liquid flow is restricted by the difficulty of getting through the small pores. This is balanced by their greater ability to operate against gravity (evaporator elevated above condenser). Larger pores usually permit large liquid flow with high maximum power, but poor lift height. Grooved wicks, with the largest pores, only operate with a slight elevation against gravity. Grooved Spacecraft Constant Conductance Heat Pipes (CCHPs) are typically tested 0.1 inch (2.54 mm) against gravity (to avoid puddle flow).
Arterial and Annular Wicks are primarily used in alkali metal heat pipes. With other working fluids, the risk of depriming the wick is greater.
Inverted Wicks: In most heat pipes, the majority of the wick is on the inner diameter of the envelope, as shown in the pictures on this webpage. In an inverted wick, the vapor space is located adjacent to the heat pipe envelope. These types of wicks are used in capillary pumped loops and loop heat pipes.