Treating Insulated Boundary Nodes as Interior Nodes:
The Mirror Image Concept
One way of obtaining the finite difference formulation of a node on an insulated
boundary is to treat insulation as “zero” heat flux and to write an energy
balance, as done in Eq. 5–23. Another and more practical way is to treat the
node on an insulated boundary as an interior node. Conceptually this is done by replacing the insulation on the boundary by a mirror and considering the
reflection of the medium as its extension (Fig. 5–17). This way the node next
to the boundary node appears on both sides of the boundary node because of
symmetry, converting it into an interior node. Then using the general formula
(Eq. 5–18) for an interior node, which involves the sum of the temperatures of
the adjoining nodes minus twice the node temperature, the finite difference
formulation of a node m 0 on an insulated boundary of a plane wall can be
expressed as