Unlike solid metal, heat pipe thermal conductivity is variable and comprised of three parts: conduction into and out of the device as well as vapor transport along the length of the heat pipe. Because vapor heat transport is so efficient, heat pipe delta-T remains relatively constant regardless of length, resulting in a higher effective thermal conductivity.
Figure 1 illustrates the effect of length on heat pipe thermal conductivity where each pipe carries 25W of power. Here we see how effective thermal conductivity increases with effective length – the distance between the midpoints of the condenser and evaporator.
These results are calculated using the formula below.
Keff = Q Leff /(A ΔT)
where:
Keff = Effective thermal conductivity [W/m.K]
Q = Power transported [W]
Leff = Effective length = (Levaporator + Lcondenser)/2 + Ladiabatic [m]
A = Cross-sectional area [m2] of the heat pipe
ΔT = Temperature difference between evaporator and condenser sections [°C]
When using a CFD thermal package, model 2-phase devices as a solid object and change the material thermal conductivity to match that of the calculated value of heat pipes or vapor chambers. Those figures can be found on Celsia’s website – Heat Pipe Calculator and Heat Sink Calculator. Alternatively, start with an effective thermal conductivity of 6,000 W/(mK) and adjust until the delta-T of the two-phase device is between 4-5 degrees Celsius.
