Optimal Analysis On The Performance Of Two Kinds Of Irreversible Refrigeration Cycles

The refrigeration technology has been widely applied to many fields, and theoretical investigation of refrigeration cycle is one of important subjects for the study of refrigeration technology. Some scholars have explored the optimal theory of refrigerator cycles by using optimal control theory. For most refrigeration applications, single-stage refrigeration systems are adequate. However, when the span of temperature between the cooled space and the heat sink is considerable and the heat leak is inevitable, combined refrigeration systems have to be used. There have been a lot of studies on the combined refrigeration systems in series. The combined refrigeration systems in parallel connection are rarely investigated. For the latter, it is necessary and important to explore the effect of heat leak on the cycle performance. On the other hand, magnetic refrigeration is a promising and forthcoming refrigeration technology because of its characteristic in high efficiency and no pollution. The theoretical investigation of magnetic refrigeration cycles has received an increasing attention. For the existing study of magnetic refrigeration cycles, the temperatures of heat reservoirs have been thought as constant. In fact, for a refrigerator, the heat capacities of heat reservoirs are usually finite. Therefore, it is very significant to investigate the optimal performance characteristics of a magnetic Ericsson refrigerator cycle with finite heat capacities.In Chapter 1, the development of refrigeration technology is summarized.A new active thermal potentiostatting system is established in chapter 2, in which the influences of finite-rate heat transfer and the heat leak are taken into account. Mathematical expressions of the main performance parameters are derived. Using the method of the optimal control theory, the minimum total power input of the system with non-zero cooling rates is calculated and the temperatures of the working fluid in the isothermal processes are optimized. The optimal allocation of the heat-transfer areas is also discussed in detail.In Chapter 3, an irreversible magnetic Ericsson refrigerator cycle is put forward, in which the finite heat capacities of heat reservoirs, thermal resistances, inherent regenerative losses, additional regenerative losses and the internal irreversibility are taken into account. Based on thermodynamic equations of a paramagnetic material, the performance characteristics of the cycle are investigated. The optimal equations between the cooling load and the coefficient of performance and between the cooling load and the power input are derived. Moreover, the maximum cooling load and the other corresponding performance parameters are determined and the optimal operating region of the cycle is discussed and evaluated.The results obtained in the present thesis are closer to the performance characteristics of actual refrigerators than those in literature and are helpful to the optimal design and performance improvement of refrigerators.