Larger Heat Pipes = Lower Thermal Conductivity?

Engineers often question why larger-diameter heat pipes have lower effective thermal conductivities than their smaller-diameter counterparts. The answer illustrates a common confusion when evaluating heat pipes using this metric vs the temperature rise (Delta-T).

Below is the equation for effective thermal conductivity (keff). Where: Q = Power (watts), L = effective length of heat pipe, A = cross-sectional area (meter sq.), Delta-T = temperature rise of the heat pipe.

Increasing the diameter of a heat pipe (all else held constant), or switching to the most likely vapor chamber, increases its cross-sectional area, decreasing Delta-T. However, this decrease is more than offset by the increase to A, cross-sectional area. ?The larger (A * Delta-T) denominator causes a lower effective thermal conductivity (W/mk).

The following table is generated using the following shared inputs for each diameter:

Q = Fixed at 22 watts as this is the Qmax of the 4mm heat pipe.

• L-actual = 200mm (0.2m)
• L-effective = 150mm (0.15m)
• Evaporator Length = 20mm
• Condenser Length = 75mm

Effective thermal conductivity is mainly used for CFD modeling while the delta-T is best used to choose the correct heat pipe diameter given the heat pipe meets Qmax and thermal budget requirements and fits in the allotted space.