The Research On Exergy Transfer Of Forced Convective Heat Transfer

Convective heat transfer is widely used in many heat exchange equipments. The convective heat transfer process usually contains momentum transfer and heat transfer, the common essence of which is energy transfer and conversion. In fact, energy transfer and conversion are inevitably accompanied with exergy transfer and conversion. 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. Furthermore, it is pointed out that no matter condition the entropy generation or exergy destruction is greater than zero in convective heat transfer from the viewpoint of thermodynamics, but from the viewpoint of exergy transfer (profit), the exergy variation derived from convective heat transfer for working fluids may be less than zero in a certain condition, which has no significance in engineering. Thus, theoretically and practically, it is of importance to study the rules of exergy transfer and conversion and find the change range in the process parameters and configuration, where the exergy variation derived from convective heat transfer for working fluids is greater than zero. Up to date, research on exergy transfer has been mostly limited to conduction heat transfer, such as the exergy transferring process for steady-state and unsteady-state conduction. But for the research on exergy transfer of forced convective heat transfer through a duct, it has been done by establishing a differential equation of exergy transfer, which is very complex and is not convenience to apply in engineering. 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 forced convective heat transfer through a duct with constant wall temperature/ constant heat flux for thermally and hydrodynamic fully developed laminar and turbulent flow from the viewpoint of exergy transfer, which is different from that in preceding literatures. By introducing convective exergy transfer coefficient, exergy flux and defining the local and mean exergy-transfer Nusselt number, their new generalized expressions are derived and the equation of convective exergy transfer rate have been provided. By reference to a smooth duct, the effects of Reynolds number, Prandtl number, non-dimensional wall temperature, non-dimensional heat flux and different cross-sectional position in the duct on exergy transfer process are analyzed.