Energy transfer from one body to another is central to life in the
universe. Heat transfer is a discipline of thermal science and engineering
that concerns the exchange of thermal energy [1–11]. It is
one of the most fundamental subjects dealing with thermodynamic
energy transfer processes of heat, work, and chemical energy.
Boiling, condensation, melting, and solidification are exact
examples of heat transfer during phase transitions.
Even though heat transfer as one of three energy transfer processes
has been comprehensively studied since its inception, its
entire understanding is challenging due to the very complex characteristics
[12–23]. Heat transfer is classified into the three modes
of heat conduction, convection, and radiation in addition to equilibrium.
The heat diffusion theory for conductive heat transfer is
well established, but a rigorous theory for thermal internal convective
heat transfer does not exist. As a consequence, reliable theoretical
methods are still lacking, and the extrapolation beyond
experiments plays the major role at the present state of the art.
However, design, operation, and optimization of involved devices
require accurate predictions of heat transfer between a solid surface
and a fluid.
There are limitations on the Fourier–Biot equation for heat diffusion
[9]
r ðjrTÞ qcp@T=@t þ q0
i ¼ 0: ð1Þ
It deals with incompressible 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 (qcpva rT) 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