On the basis of linear non-equilibrium thermodynamic theory, exergy transfer analyses of laminar and turbulent forced convection are conducted in terms of external flows over a flat plate. Two kinds of non-dimensional concepts involving the definition of the local and mean exergy transfer Nusselt number and non-dimensional exergy flux are incorporated into exergy transfer analysis. The new analytical expressions for the local and mean exergy transfer Nusselt number and non-dimensional exergy flux are adopted to describe the exergy transfer characteristics over a flat plate. By taking air as working fluid, the influences of flat plate geometry, Reynolds number, and other operating parameters on the exergy transfer Nusselt number and non-dimensional exergy flux are examined. It is shown how the flow geometric parameters and Reynolds number, etc., may be selected in order to maximize the exergy utilization associated with a specific external convection process. In addition, the results obtained from exergy transfer analysis are compared with those obtained from traditional heat transfer analysis.
On the basis of the first and second laws of thermodynamics, the general expression of the number of entropy generation units of three-fluid heat exchangers with three thermal communications was derived. The effect of several non-dimensional design parameters on the number of entropy generation units of three-fluid heat exchangers was thoroughly discussed. Furthermore, the detailed comparisons of results have been given for the arrangement of the parallel flow and the counter flow. It is shown that the variation tendencies of the number of entropy generation units with the ratio of the thermal resistances, ratio of the thermal capacities, and number of heat transfer units for the parallel-flow arrangement are different from those of the counter-flow arrangement. There exists an extremum of the number of entropy generation units for the counter-flow arrangement. In addition, the entropy generation for the counter flow is mostly smaller than that of the parallel flow under the same conditions.