No heat can cross a symmetry line, and thus symmetry lines can be treated
as insulated surfaces and thus “mirrors” in the finite difference formulation.
Then the nodes in the middle of the symmetry lines can be treated as interior
nodes by using mirror images. Six of the nodes are boundary nodes, so we will
have to write energy balances to obtain their finite difference formulations. First
we partition the region among the nodes equitably by drawing dashed lines between
the nodes through the middle. Then the region around a node surrounded
by the boundary or the dashed lines represents the volume element of the node.
Considering a unit depth and using the energy balance approach for the boundary
nodes (again assuming all heat transfer into the volume element for convenience)
and the formula for the interior nodes, the finite difference equations
for the nine nodes are determined as follows: