Unlike solid metals, heat pipe thermal conductivity is variable, consisting of three key components: conduction into the heat pipe, conduction out of the heat pipe, and vapor transport along its length. Vapor transport is exceptionally efficient, allowing heat pipes to maintain a relatively consistent delta-T. This results in a much higher heat pipe effective thermal conductivity compared to solid materials like copper or aluminum.
How Length Impacts Heat Pipe Thermal Conductivity
Figure 1 demonstrates the effect of length on heat pipe thermal conductivity when transporting 25W of power. As the effective length increases—the distance between the midpoints of the condenser and evaporator—the effective thermal conductivity also rises significantly.
This relationship is calculated using the following formula:
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 of the heat pipe [m²]
- ΔT = Temperature difference between evaporator and condenser sections [°C]
Modeling Heat Pipe Thermal Conductivity in CFD
When simulating heat pipes in computational fluid dynamics (CFD) software, model the two-phase device as a solid object. Adjust the material’s effective thermal conductivity to match the calculated value for the specific heat pipe or vapor chamber. If precise data is unavailable, a starting value of 6,000 W/(m·K) is recommended. Fine-tune this value until the delta-T of the device falls between 4–5°C for a typical 100-150W application.
For precise effective thermal conductivity and design tools, visit Celsia’s website to access Heat Pipe Calculator (shows data for heat pipes) and Heat Sink Calculator (shows data for a vapor chamber).
