As shown in Fig. 2, the heat transfer theory deals with the four regions of non-equilibrium, irreversible quasi-equilibrium, reversible equilibrium and equilibrium. The theory affords governing equations for non-equilibrium, quasi-equilibrium, and equilibrium processes which include the internal convection and external conduction mechanisms. Fig. 3 demonstrates relationships among these processes. Non-equilibrium reactions include internal convection
processes, and equilibrium reactions denote internal equilibrium processes. Quasi-equilibrium reactions of external convection and diffusion lead to isothermal and isentropic processes.
In the following sections, governing equations and solutions in these four regions are discussed