Constructal optimization of a plate condenser with fixed heat transfer rate and effective volume in ocean thermal energy conversion (OTEC) system is performed based on constructal theory. Optimizations of entropy generation rate ( ${\dot{S}}_{\mathrm{g}}$ ) in heat transfer process and total pumping power ( $P_{\mathrm{sum}}$ ) due to friction loss are two conflicting objectives for a plate condenser. With the conventional optimization method, the plate condenser is designed by taking a composite function (CF) considering both ${\dot{S}}_{\mathrm{g}}$ and $P_{\mathrm{sum}}$ as optimization objectives, and employing effective length, width, and effective number of heat transfer plates as design variables. Effects of structural parameters of the plate condenser and weighting coefficient of CF on design results are investigated. With a multi-objective genetic algorithm, the plate condenser is designed by simultaneously optimizing ${\dot{S}}_{\mathrm{g}}$ and $P_{\mathrm{sum}}$ , and the Pareto optimal set is obtained. The results demonstrate that CFs after primary and twice-constructal optimizations are respectively reduced by 7.8% and 9.9% compared with the initial CF, and the effective volume of the plate condenser has a positive impact on the twice minimum CF. Furthermore, the Pareto optimal set can provide better selections for performance optimizations of plate condensers.

A marine condenser with exhausted steam as the working fluid is researched in this paper. Constructal designs of the condenser are numerically conducted based on single and multi-objective optimizations, respectively. In the single objective optimization, there is an optimal dimensionless tube diameter leading to the minimum total pumping power required by the condenser. After constructal optimization, the total pumping power is decreased by 42.3%. In addition, with the increase in mass flow rate of the steam and heat transfer area and the decrease in total heat transfer rate, the minimum total pumping power required by the condenser decreases. In the multi-objective optimization, the Pareto optimal set of the entropy generation rate and total pumping power is gained. The optimal results gained by three decision methods in the Pareto optimal set and single objective optimizations are compared by the deviation index. The optimal construct gained by the TOPSIS decision method corresponding to the smallest deviation index is recommended in the optimal design of the condenser. These research ideas can also be used to design other heat transfer devices.

Power output ( $P$ ), thermal efficiency ( $\eta$ ) and ecological function ( $E$ ) characteristics of an endoreversible Dual-Miller cycle (DMC) with finite speed of the piston and finite rate of heat transfer are investigated by applying finite time thermodynamic (FTT) theory. The parameter expressions of the non-dimensional power output ( $\overline{P}$ ), $\eta$ and non-dimensional ecological function ( $\overline{E}$ ) are derived. The relationships between $\overline{P}$ and cut-off ratio ( $\rho$ ), between $\overline{P}$ and $\eta$ , as well as between $\overline{E}$ and $\rho$ are demonstrated. The influences of $\rho$ and piston speeds in different processes on $\overline{P}$ , $\eta$ and $\overline{E}$ are investigated. The results show that $\overline{P}$ and $\overline{E}$ first increase and then start to decrease with increasing $\rho$ . The optimal cut-off ratio ${\rho }_{opt}$ will increase if piston speeds increase in heat addition processes and heat rejection processes. As piston speeds in different processes increase, the maximum values of $\overline{P}$ and $\overline{E}$ increase. The results include the performance characteristics of various simplified cycles of DMC, such as Otto cycle, Diesel cycle, Dual cycle, Otto-Atkinson cycle, Diesel-Atkinson cycle, Dual-Atkinson cycle, Otto-Miller cycle and Diesel-Miller cycle. Comparing performance characteristics of the DMC with different optimization objectives, when choosing $\overline{E}$ as optimization objective, $\eta$ improves 26.4% compared to choosing $\overline{P}$ as optimization objective, while $\overline{P}$ improves 74.3% compared to choosing $\eta$ as optimization objective. Thus, optimizing $E$ is the best compromise between optimizing $P$ and optimizing $\eta$ . The results obtained can provide theoretical guidance to design practical DMC engines.

Using finite time thermodynamic theory, an irreversible steady-flow Lenoir cycle model is established, and expressions of power output and thermal efficiency for the model are derived. Through numerical calculations, with the different fixed total heat conductances ($U_T$) of two heat exchangers, the maximum powers ($P_{\max }$), the maximum thermal efficiencies (${\eta }_{\max }$), and the corresponding optimal heat conductance distribution ratios ($u_{L_P(opt)}$) and ($u_{L_{\eta }(opt)}$) are obtained. The effects of the internal irreversibility are analyzed. The results show that, when the heat conductances of the hot- and cold-side heat exchangers are constants, the corresponding power output and thermal efficiency are constant values. When the heat source temperature ratio ($\tau$) and the effectivenesses of the heat exchangers increase, the corresponding power output and thermal efficiency increase. When the heat conductance distributions are the optimal values, the characteristic relationships of $P-u_L$ and $\eta -u_L$ are parabolic-like ones. When $U_T$ is given, with the increase in $\tau$, the $P_{\max }$, ${\eta }_{\max }$, $u_{L_P(opt)}$, and $u_{L_{\eta }(opt)}$ increase. When $\tau$ is given, with the increase in $U_T$, $P_{\max }$ and ${\eta }_{\max }$ increase, while $u_{L_P(opt)}$ and $u_{L_{\eta }(opt)}$ decrease.