In the case of shell conduction that is applied on a boundary wall, the original wall is treated as a coupled wall, where the surface that is adjacent to the fluid cells is referred to as the “outer” surface,and the temperature of the surface that is away from the adjacent fluid cells is referred to as the “external temperature (shell)”. The surface that is in the middle of the cells grown for the wall is referred to as the “inner” surface. See Figure 13.4: A Case for Shell Conduction (p. 753). Note that the definition for the inner surface of a shell conduction model differs from the definition used for thin walls without
shell conduction