Essence
Ansys Employee

I have a couple of questions. Please help me understand your query better.

You mentioned SiO2 and copper. Hence, I assume SiO2 at the core and copper around it (as shown in the figure). Is that right? If it is, then I do not see any fluid domain around cylinder for natural convection to occur.

If my above assumptions are right, then it would be better to use Ansys Fluent instead of Ansys Mechanical. You need to create a fluid domain enclosing  the cylinder.