Jiangfeng Guo

Multi-Objective Optimization of Heat Exchanger Design by Entropy Generation Minimization

In the present work, a multi-objective optimization of heat exchanger thermal design in the framework of the entropy generation minimization is presented. The objectives are to minimize the dimensionless entropy generation rates related to the heat conduction under finite temperature difference and fluid friction under finite pressure drop. Constraints are specified by the admissible pressure drop and design standards. The genetic algorithm is employed to search the Pareto optimal set of the multi-objective optimization problem. It is found that the solutions in the Pareto optimal set are trade-off between the pumping power and heat exchanger effectiveness. In some sense, the optimal solution in the Pareto optimal set achieves the largest exchanger effectiveness by consuming the least pumping power under the design requirements and standards. In comparison with the single-objective optimization design, the multi-objective optimization design leads to the significant decrease in the pumping power for achieving the same heat exchanger effectiveness and presents more flexibility in the design process.

The Entropy Generation Minimisation based on the Revised Entropy Generation Number

In the present work, an improved Entropy Generation Minimisation (EGM) approach aiming at minimising the revised entropy generation number which is the non-dimensionalised entropy by the ratio of heat flow to the input temperature of the cold fluid is developed for a plate-fin heat exchanger design with multiple design variables with the help of genetic algorithm. It is found that the approach can decrease the total fan power dramatically and improve the exchanger effectiveness simultaneously. Finally, this approach is applied to a heat recovery system where the heat exchanger works as a component of the system.

Thermodynamic Analysis and Optimization Design of Heat Exchanger

In order to address the contradiction between the limited fossil fuel reserves and sharp increase of huge energy demand from the world economy and people’s daily lives, there is an urgent need to develop energy saving measures. Heat exchanger as a device for heat transfer from one medium to another is widely applied in power engineering, petroleum refineries, chemical industries, food industries, and so on. Therefore it is of great value to improve the heat exchanger performance and save energy in heat exchange processes. Recently with the aim of reducing the unnecessary heat dissipation in heat exchange processes, we have studied thermodynamic analysis and optimization design of heat exchangers. Firstly based on the genetic algorithm and the improved entropy generation number which avoids the ‘entropy generation paradoxes’ induced by the original entropy generation number, we proposed an improved entropy generation minimization approach for heat exchanger optimization design. Secondly, we found that the entransy is a state variable and the second law of thermodynamics can be described by the entransy and entransy dissipation, this work places the entransy dissipation theory on a solid thermodynamic basis. Thirdly, based on the entransy dissipation theory we derived the expression of the local entransy dissipation rate for heat convection, developed variational principles for heat transfer and showed that this principle is compatible with the Navier–Stokes–Fourier equations. Fourthly, based on the entransy dissipation theory, we proposed a heat exchanger performance evaluation criterion called the entransy dissipation number and established a principle of entransy dissipation equipartition for heat exchanger optimization designs. Finally, we developed an entransy dissipation minimization approach for heat exchanger optimization design and applied it to the tube-and-shell heat exchanger optimization design.

Performance analysis of an irreversible regenerative intercooled Brayton cycle

The influences of the number of heat transfer units distribution, cycle pressure ratio, specific heat ratio, etc. on the performance of an irreversible regenerative closed intercooled Brayton cycle are investigated in terms of thermal efficiency, thermodynamic efficiency, dimensionless power, entropy generation number and Ecological Coefficient Of Performance (ECOP). Different performance criteria correspond to the different optimal number of heat transfer units distribution schemes, generally, the NtuR fraction is the largest, followed by NtuH fraction and NtuL fraction, the NtuI is the smallest in the optimal distribution schemes. There exist the optimal specific heat ratios, the optimal cycle pressure ratios and the optimal intercooling pressure ratios for the maximum thermal efficiency, the maximum thermodynamic efficiency and the maximum ecological coefficient of performance in some situations. In general, the optimal specific heat ratio relatively increases as the cycle pressure ratio decreases and slightly increases as the intercooling pressure ratio increases. The optimal cycle pressure ratio increases and the optimal intercooling pressure ratio slightly decreases as the specific heat ratio decreases.

A New Criterion for Assessing Heat Exchanger Performance

The principle of minimum entropy production has played an important role in the development of non-equilibrium thermodynamics. Inspired by this principle, Bejan derived the expression of the local entropy production rate for heat convection and established the entropy production minimization approach for the heat exchanger optimization design. Although one can obtain the entropy production distribution in the heat exchanger numerically, it can not directly been employed to examine the heat exchanger performance. Tondeur and Kvaalen found that the entropy production uniformity is closely related to the heat exchanger performance. In the present work, based on Tondear and Kvaalen’s work, an entropy production uniformity factor is defined, which quantifies the uniformity of the local entropy generation distribution in heat exchanger. Numerical results of the heat transfer in a rectangular channel show that the larger entropy production uniformity factor implies less irreversible loses. Therefore, this factor can serve as a thermodynamic figure of merit for assessing the heat exchanger performance.Copyright