Performance Optimization For Heat Engines With Nonlinear Heat Transfer Working Fluids

Finite-time thermodynamics as an extension and generalization of classic thermodynamics has become a new field of modern thermodynamic theory.Finite–time thermodynamics mainly focuses on the performance optimization of heat engines,which has the great potential in using energy sources effectively and developing advanced technologies.The trade-off criterion method based on the analysis of the objective function constructed not only can give the working regime where the compromise between the maximum efficiency and the minimum lost energy can be fulfilled,but can provide an unified theoretical framework for optimizing the performance of heat engines.This method has been widely used in the performance optimization of macro-heat engines and micro-heat devices.In the first chapter,we firstly introduce the fundamental results of the classic thermodynamics including the basic concepts of thermodynamics,the four laws of thermodynamics,and the relation between the second law of thermodynamics and the Carnot engine.Then,the contents and the development of Finite-time thermodynamics are introduced.Within the framework of Finite-time thermodynamics,we carry out the following works on the performance optimization of heat engines in this thesis:1.The heat transfer process inevitably occurs in the operation of real heat engines.In this thesis,a low-dissipation heat engine with generic heat transfer process is proposed based on the low-dissipation Carnot model.The formulas for the power and the efficiency of heat engine with generic heat transfer law are derived,and the low-dissipation heat engine performance is also optimized by the trade-off optimization method,which offers an unified scheme to understand the behaviors of heat engines with generic heat transfer processes.Furthermore,the characteristics of the power as well as the efficiencies for thermal engines with the different heat transfer processes are discussed in detail,and it is found that the power and the efficiency without heat transfer process are independent of heat leak,but are related to contact time,heat dissipation and Carnot efficiency.The power output of heat engine monotonically increases as Carnot efficiency increases,but the large contact time ratio and the large dissipation ratio make it difficult to provide the big power output.2.We thoroughly investigate the low-dissipation heat engines and generalize the dissipation term to the quadratic order case,thus a nonlinear dissipation heat devicesis proposed.The dimensionless formulas of the output(input)power and the efficiency(coefficient of performance)for the nonlinear dissipation heat engines(refrigerators)are derived in terms of characteristic parameters for heat devices and the dimensional analysis.Based on the trade-off criterion,the optimal performance of the nonlinear dissipation heat devices is discussed in depth,and some system-specific properties for the nonlinear dissipation heat devices under the trade-off optimization are also elucidated.Finally,both conclusions and prospects are presented in Chapter 4 of this thesis.Our results may provide practical insights for designing actual heat engines and refrigerators.