In my problem, I have defined 2 domains; solid domain (spheres) and fluid domain (the space between them).
In sub domain in solid domain I have defined heat source. Indeed each sphere is a fuel element. Now I have a problem with boundary condition in solid domain (fuel domain). As you can see in the following picture there are several spheres (fuel elements) and in boundary conditions in Heat Transfer section, I don't know which option is the best to my problem. I selected "heat transfer coefficient " but there is a problem to define "coefficient" and outside temperature. I have the value of power density so I can calculate the energy for each sphere and also for all spheres. My problem is in order to calculate "heat transfer coefficient" should I consider all spheres? or I should consider each sphere and calculate its thermal resistance?
Many thanks in advance.