Optimum entropy generation structure and partial efficiencies of finite-time carnot heat engines under performance limit conditions

To explore the entropy production structure during an irreversible Carnot engine cycle consisting of the partial entropy generations within the hot- and cold-end fluids arisen from the external and internal irreversibilities, we establish the partial thermal efficiency and the partial entropy transfer efficiency concepts and their relationship between them by splitting the entire engine into the tandem construction connected by an intermediate reservoir at the weighted temperature. We derive the novel lower and upper bounds of the optimum internal and external entropy transfer efficiencies (entropic efficiencies) at maximum ecological function connecting with the optimum linear irreversible entropy generation structure that reaches the performance limits, which shows a better correlation with the operating data of power plants than the low-dissipation (LD) model at maximum power or ecological function. We obtain two bounds of the instantaneous power at an arbitrary thermal (entropic) efficiency for the LD engines, which are identical to the previous average power results. We also clarify four physical interpretations of the Curzon and Ahlborn's effectiveness (defined as the ratio of the actual and ideal Carnot efficiencies), and recover the Augulo-Brown's thermal efficiency at maximum ecological function and the thermal and entropy transfer efficiency limits of the LD model. These conclusions can be extended to the thermal devices including refrigerators and heat pumps.