There are limitations on the Fourier–Biot equation for heat diffusion It deals with in compressible mediums, accounts for only the heat conduction without internal heat convection, and shows no expression for the internal heat generation per unit length q0 i. The convective
term can be added to the left hand side of the above equation, but it is only the convective heat contribution.We are here interested in the internal convective heat generation
depending on temperature difference as well as the external conductive and convective heat transfer.In this paper, we propose a systematically unified theory for the heat transfer mechanisms of internal convection and internal equilibrium beyond overcoming intrinsic limits on the heat diffusion equation. In the heat transfer theory, the heat fluxes are described
as functions of temperature, time, and displacement simultaneously under a postulate that temperature, time, and displacement are independent and orthogonal variables in extended
phase spaces. This is the first rigorous microscopic kinetic theory for internal equilibrium and internal convective heat transfer.The theory unifies conduction and internal convection heat
transfer, it leads to a non-equilibrium theory beyond quasi-equilibrium theories for the isothermal and isentropic processes, it works on non-equilibrium and equilibrium dynamics, and it integrates conduction and convection mechanisms. The heat transfer theory can be applied to the internal convective thermal processes at a solid–liquid boundary as an illustration