Study On Exergy Transfer Characteristic Of Forced Convective Heat Transfer For External Flow

As we know, energy has two properties of quantity and quality, and exergy is the measurement of energy quality. With energy sources being hard up and people's consciousness of economizing energy sources increasing, exergy analytical method has greatly developed in supervising all kinds of energy using process, which can make energy quantity and quality to be used rationally. Because exergy has characteristic of energy and it can be transformed and transferred, so study on exergy transfer is rising recent years. Energy is conservative in its transfer and conversion process, while exergy is 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. Compared with exergy analytical method, exergy transfer has dynamic characteristic in its transfer process and can open out the essential of energy depreciation even more. So study on exergy transfer can provide new theoretical basis for economizing energy sources. Presently, research on exergy transfer has been mostly limited to conductive heat transfer, such as the exergy transferring process for steady-state and unsteady-state conduction. And it is relatively scarce for the research on exergy transfer of forced convective heat transfer. But convective heat transfer is widly used in many heat exchange equipments. Up to date, the research of forced convective heat transfer for external flow, such as the flow on a flat plate or through a duct vertically, is merely based on heat transfer characteristic, but the research of exergy transfer characteristic has not yet been accomplished. So the study on the exergy transfer characteristic of forced convective heat transfer for external flow is very necessary.This paper has mainly studied the exergy transfer characteristics of forced convective heat transfer on a flat plate and through a duct vertically with constant wall temperature . At the same time, the flow on a flat plate is differentiated by laminar flow and turbulent flow. By introducing convective exergy transfer coefficient and defining the local and mean exergy-transfer Nusselt number, the equation of convective exergy transfer are derived. Through the numerical calculation of the exergy transfer characteristics of forced convective heat transfer for external flow, it is discussed that the dependence relations of the convective exergy transfer performance on Reynolds number, wall temperature and different wall position, etc. Besides, the exergy transfer characteristic has been compared with that of the heat transfer. On the other hand, this paper has renewedly defined the fin transfer efficiency from the view of exergy transfer. The conventional definition of fin efficiency is the ratio of the actual heat transfer quantity to the ideal heat transfer quantity. And the ideal heat transfer quantity is defined as the heat transfer quantity when the temperature of the whole fin is equal to that of the fin base, which has maximal temperature difference, namely, maximal heat transfer quantity under this condition. However, it should be pointed out that the temperature difference irreversibility of heat transfer is also maximal here, namely, it is not the thermodynamic ideal state from the thermodynamic viewpoint. Therefore, the conventional definition of fin efficiency is unreasonable. At the same time, the influence of flow resistance is not also considered, so the conventional definition of fin efficiency should be realized and defined again. This paper has newly defined the fin transfer efficiency from the viewpoint of exergy transfer, namely, it's the ratio of the actual exergy transfer quantity to the ideal exergy transfer quantity. Furthermore, the new expression of the fin exergy transfer efficiency is derived and the effect of fin size, Reynolds number and dimensionless number on the exergy transfer efficiency of pin fin and straight fin are discussed and the relevant numerical calculation results are given. Besides, the results of fin exergy transfer efficiency are compared with that of fin heat transfer efficiency. The research reported in this paper provides the new theoretical basis to perfect the research on exergy transfer of forced convective heat transfer for external flow, and has important academic guiding significance and great engineering practical values.