modeling a heat pipe for a natural convection system?

[Anonymous]

I have a system I'm trying to model which uses a heat pipe to get energy from a hot zone to a radiator.

My system produces approx 18W of waste heat when running full power which only occurs periodicly, 45s on/180+ seconds off.

Usually I model heat pipes by balancing the thermal conductivity so that the ends of the heat pipe are only a few degress delta as my heat pipe is usually oversised for my application. In this case my heat pipe is undersized in certain orientations. Heat pipes always have high thermal conductivities, hwoever, their power capacity diminishes based on several factors including the orientation of the evaporator section compared to the condensor. In my case my heat pipe has to be derated to approx 4-5W of max power throughput.

I need some decent transient results so I don't think putting a 4W constant thermal sink where my heat pipe interface would work well.

Is there any way to model a material which has high conductivity but limits overall power transfer?

For those who aren;t familiar with heat pipes here's an overview

http://www.thermacore.com/thermal-basics/heat-pipe-technology.aspx

Thanks!