How do I define a symmetric boundary condition for a thermal solution?
In thermal problems, planes of symmetry behave adiabatically. That is, no
heat transfer occurs across them. In order to model this behavior, section
your model along the plane of symmetry. After meshing the model, do not apply
any boundary conditions to the nodes in the plane of symmetry. For thermal
analysis, nodes on external surfaces behave adiabatically when boundary
conditions are not prescribed.