How a Heat Pipe Wick Operates

When a heat pipe operates, heat applied in the evaporator vaporizes the working fluid, which travels to the condenser and condenses.  The condensate is returned passively to the evaporator by capillary forces.  As discussed in the capillary limits page, for the heat pipe to function, the maximum capillary force that the wick can generate must be greater than the sum of the vapor pressure drop, the liquid pressure drop, and the gravitational head.

The pressure difference between the liquid and vapor at any location on the interface is known as the capillary pressure, ΔP_c.

ΔP_c = P_Vapor – P_Liquid

The capillary pressure depends on surface tension and two radii of curvature of the liquid/vapor interface, measured perpendicular to each other:

where:

σ                      Surface tension, N/m
r₁ and r₂           are the radii of curvature (m)

Due to these capillary forces, liquid drops and vapor bubbles both have a pressure higher than the surrounding fluid.  The smaller the drop or bubble, the smaller the radii of curvature, and the higher the pressure difference between inside and outside.  In a heat pipe, the core of the heat pipe is filled with vapor, while the liquid is retained in the wick by capillary forces; see Figure 2.

The heat pipe wick allows the capillary force to vary along the length of the heat pipe, returning liquid from the condenser to the evaporator.  For sintered and screen wicks, the two radii of curvature are identical, so the capillary pressure equation reduces to:

 

where r_c is the pore radius.

One of the radii is infinite for grooves, so the equation becomes:

 

 

The pressures and curvatures are shown schematically in Figure 3.  The vapor pressure drops from its initial value as the vapor travels towards the condenser, while the liquid pressure drops as the liquid travels counter currently back to the evaporator.  Therefore, the vapor pressure is always higher than the liquid pressure (except at the end of the condenser), so a capillary pressure exists along the entire heat pipe. To support this pressure difference, the liquid/vapor interface is curved, with the curvature increasing along the length of the heat pipe, from the condenser to the evaporator.

 

[1] These figures are taken from Dr. Jentung Ku’s excellent Heat Pipe Short Course and are used with his permission.