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Entropy generation is recognized as a common measurement of the irreversibility in diverse processes, and entropy generation minimization has thus been used as the criterion for optimizing various heat transfer cases. To examine the validity of such entropy-based irreversibility measurement and its use as the optimization criterion in heat transfer, both the conserved and non-conservative quantities during a heat transfer process are analysed. A couple of irreversibility measurements, including the newly defined concept entransy, in heat transfer process are discussed according to different objectives. It is demonstrated that although thermal energy is conserved, the accompanied system entransy and entropy in heat transfer process are non-conserved quantities. When the objective of a heat transfer is for heating or cooling, the irreversibility should be measured by the entransy dissipation, whereas for heat-work conversion, the irreversibility should be described by the entropy generation. Next, in Fourier's Law derivation using the principle of minimum entropy production, the thermal conductivity turns out to be inversely proportional to the square of temperature. Whereas, by using the minimum entransy dissipation principle, Fourier's Law with a constant thermal conductivity as expected is derived, suggesting that the entransy dissipation is a preferable irreversibility measurement for heat transfer.

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The correlation model of gas–liquid interfacial tension (surface tension) of nonaqueous mixtures containing ionic liquids (ILs) is still in its infancy. The purpose of this work is to develop a modified Hildebrand–Scott equation based on local composition concept and the group surface area parameters of UNIFAC model as well as to evaluate the capability of this model. The surface tension of 33 IL–cosolvent binary systems were correlated by the modified Hildebrand–Scott equation with two energy parameters and an overall average relative deviation of only 0.92%. The ILs in the study included imidazolium-based, pyridinium-based, and isoquinolinium-based ones, and the cosolvents included methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, acetonitrile, and tetrahydrofuran. Using the energy parameters obtained from the surface tension of a given IL–cosolvent binary system at 298.15 K, the surface tension of the same system at other different temperatures was predicted by the modified Hildebrand–Scott equation, with an overall average relative deviation of 1.06%.

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Based on the mass-energy relation of Einstein's relativity theory, the mass of solid lattices is divided into two parts: the rest mass of solid atoms and the equivalent mass of thermal vibration energy of lattices. The latter is exactly the equivalent mass of the phonon gas in a solid. The vibration energy of the solid lattices includes the thermal energies consisting of the rest mass of the solid lattices and the equivalent mass of the phonon gas. The state equations for the lattice rest mass and the phonon gas are deduced based on the state equation of solids. The heat conduction is just the motion of the phonon gas in a solid. The conservation equations for the phonon gas motion are established. It is found that the conservation equation of phonon gas momentum degenerates to the Fourier's conduction law when the inertial force of the phonon gas can be ignored. The physical nature of the Fourier's law is the balance between the driving force and the resistance for the motion of the phonon gas. Under ultrahigh heat flux conditions where the inertial force is too high to be ignored, the Fourier's law is no longer valid even under the steady condition.