Convection in a hollow cylinder between solid domains

Hey everyone,

I'm trying to simulate the heat distribution of three electrical power cables with a constant load current buried in soil with COMSOL Multiphysics 5.1.

The geometry consists of a rectangular soil and three cables in same depth with 1m outer distance between them. Each cable is laid into a protection pipe and each pipe is laid into a layer of high thermal conductivity material which is linked to the surrounding soil. It is assumed that all cylindric domains (cable, pipe, high conductive material) are arranged concentric to each other.

I'm working with the Laminar Flow Module using the Heat transfer in Solids and Fluids with stationary solver. Solid Heat transfer is already working.

My question is: How can I simulate the convection in the cylindrical protection pipe filled with air between the outer cable layer and the inner protection pipe layer ?

With Heat Flux Node it is not possible as I don't have a constant temperature on the outside (Text) thus I can only apply this Heat Flux to the limiting domains of my model (outside of the rectangular soil).

As I'm mainly interested in the thermal heating due to convection and not in the velocity or pressure distribution I hope there is another possible way to do so.

Thanks in advance

Jan