The Research On Exergy Transfer Characteristic Of Heat Exchanger

The process of energy transfer and conversion is widely used in the field of industries. Thus, enhancing the cognition of its processes and discussing the basic rules of energy transfer processes are very important for controlling and improving its processes. Due to there are two classes attributes which contain the conservation of quantity and the differences of quality in the energy transfer processes. So, in the research of energy transfer processes, except study the transfer rules of its quantity, the transfer rules of its quality should be discussed. Analyse the energy transfer processes by combine the quantity and quality in order to accurately evaluate the value of energy. Energy is conservative in its transfer and conversion process, while exergy is known to be non-conservative due to the irreversibility of its transfer process. As a result, exergy transfer must have the rules of its own, which are different from those of energy transfer. Thus, theoretically and practically, it is of importance to study the rules of exergy transfer and conversion. Up to date, research on exergy transfer has been mostly limited to conduction heat transfer, such as the exergy transferring process for steady-state, unsteady-state conduction and exergy transfer of forced convective heat transfer through a duct. But for the research on exergy transfer of heat exchangers is less. As an extension of the previous works, the objective of this paper is based on the first, second thermodynamic laws and the non-equilibrium thermodynamics theory, to evaluate the exergy transfer characteristics of heat transfer through a heat exchanger with the condition of considering the flow friction or not for three flow patterns of counter flow, parallel flow and cross flow heat exchangers which operating above/below the surrounding temperature from the viewpoint of exergy transfer, which is different from that in preceding literatures. Exergy transfer effectiveness, local and mean exergy transfer coefficient, exergy transfer units number etc. are firstly derived and its equations have been provided. The effects of heat transfer units number, the ratio of cold to hot fluids heat capacity rate and flow patterns etc. on exergy transfer processes of heat exchangers are discussed systemically. The relationship of exergy transfer units number and exergy transfer effectiveness are analyzed. In addition, contraste the exergy transfer effectiveness and exergy transfer coefficient for a finite pressure drop and those for zero pressure drop when the objective medium are ideal gas and incompressible liquid. Finally, exergy transfer effectiveness with heat transfer effectiveness is compared.On the other hand, the fouling always consists in many thermal equipments. It is of more important to analyze quantificationally the effect of fouling on the performance of exergy transfer in heat transfer equipment. The paper analyses the effect of fouling on exergy transfer process of heat exchanger operating above/below the surrounding temperature when not considering flow friction. The effects of heat transfer units number, the ratio of cold to hot fluids heat capacity rate and flow patterns etc. on exergy transfer effectiveness and local and mean exergy transfer coefficient of heat exchangers for considering fouling or not are discussed. The research reported in this paper provides a valid way to analyze quantificationally the effect of fouling on energy transfer process performance.