Potential Energy (entransy) In Thermal Science And Its Application

Thermal science, as a branch of physics, shares little common features with other branches and even the two core heat-related subjects, heat transfer and thermodynamics, are not closely related on the one hand. There are more phenomenological laws and empirical relations in heat transfer compared with other developed branches of physics such as mechanics and electrics on the other hand. These facts urge us to rethink the nature of heat and its related properties. In this thesis, the concept of potential energy has been introduced to the thermal science based on review and analogous analysis of the fundamental concepts among mechanics, electrics and thermal science. Based on their common features on the irreversibility of transfer process, optimization methods for thermal processes are completed and developed so as to improve the energy utilization efficiency.By analyzing the common features of potential energy in mechanics and electrics, it can be found that potential energy is the product of coupled extensive quantity and intensive quantity (potential). Considering the expression of entransy in heat transfer is the product of extensive quantity of internal energy and intensive quantity of temperature, it can be coined as transfer potential energy. The concept of potential energy can also be extended to heat to work conversion processes, which represents the ability of heat to work conversion. The dissipation rate of transfer potential energy and conversion potential energy is the measure of irreversibility of heat transfer and heat to work conversion processes respectively.The concept of transfer potential energy (entransy) and its dissipation has been extended to the radiative heat transfer optimization. For problems with complex boundary conditions, the principle of minimum weighted average thermal resistance has been presented to optimize heat transfer processes. Similarly, based on the definition of conversion thermal resistance, the principle of minimum conversion thermal resistance has also been developed to optimize thermodynamic systems, which can be stated as: the thermodynamic process has the maximum power output under a certain constraint when the conversion thermal resistance is minimized. The concept of thermal resistance and its minimum principle can strengthen the relations between the subject of heat transfer and thermodynamics.The improvements of Clausius entropy and entropy balance equation have been proposed. Starting from the first law of thermodynamics, entropy can be defined by state functions directly. Compared with the original definition in terms of process quantity, the improved one makes it unnecessary to invent reversible processes for entropy change calculation of a system in irreversible processes. Moreover, the distinction of reservoir entropy flux from system entropy flux provides a convenient way to separate and calculate internal and external entropy generations of a system.For a closed system reversibly brought into equilibrium with the environment the process can be divided into two sequent sub-processes: the heat interaction and the work interaction of the system and the environment. In the heat interaction, the system develops work indirectly while in the work interaction, the system develops work directly. The macroscopic physical meaning of entropy is a measure of unavailability in the heat interaction only between a system and the environment. This kind of explanation is more definite than that in the present literature where the work interaction is not excluded. Entropy is the product of extensive quantity of heat and intensive quantity of the reciprocal of temperature, and the latter is the potential of unavailability. Thus, the essence of entropy is the unavailable potential energy during the heat interaction between the system and the environment.The expression of entropy has been deduced with the assumption of constant heat capacity that entropy is proportional to the logarithm of temperature. This result indicates the relationship between the macroscopic and microscopic definition of entropy.