| As the international community's requirements for environmental protection become more stringent,the pressure of refrigerant replacement is increasing.This is not only a challenge for refrigeration industry,but also a national action,even a global environmental protection action.Therefore,the refrigeration industry is eager to seek new developments and new ways to achieve green and low carbon targets.Magnetic refrigeration?MR?is universally acknowledged as an alternative solution for vapor-compression refrigeration to eliminate the usage of hydrofluorocarbons?HFCs?.At present,there are still some problems in the research and application of MR at room temperature,such as low operating frequency,large heat transfer loss,low heat regeneration efficiency,complex system,small temperature span and cooling capacity.As a result,it is of great significance for the development and application of MR at room temperature to study and explore on the novel mechanism of heat regeneration cycle.Firstly,based on the theoretical cycle of MR,the principle and performance of each cycle type in practical application is analyzed,the advantages and disadvantages of the existing MR cycle is summarized,and the main factors affecting the performance of MR are investigated.Then a composite MR system with multiple cycle modes is designed based on the active magnetic regenerator?AMR?.The multiple cycle modes include AMR series cycle,AMR parallel cycle and AMR cascade cycle.By constructing the corresponding experimental setup served as the same system benchmark,a set of tests is conducted to investigate the operation characteristics and cooling performance of the three different cycle modes under various conditions.The experimental results reveal that the specific cooling power?SCP?decreases with the increase of system temperature span for all the three cycle modes.The variation trends are generally in linear negative correlation.Compared to the widely used AMR parallel cycle in the MR prototypes,the AMR cascade cycle increases the temperature span by 57%,which proves that the AMR cascading cycle is also a promising system design for MR at room temperature.Secondly,concerning the inherent problems of MR system based on conventional AMR cycle,a novel micro-unit regeneration?MUR?cycle for MR is proposed.The conventional AMR cycle uses heat transfer fluid for heat regeneration,leading to frequency limitation of reciprocating flow,large temperature difference of heat regeneration and large loss of fluid-to-solid heat transfer.In order to regenerate heat at the correct time and in the reasonable space and get rid of the inertia thinking of using heat transfer fluid for heat regeneration,the regeneration method of direct solid-to-solid heat transfer and the system design of“exchange space for time”are adopted and the MUR cycle is then proposed.The cycle characteristics and heat transfer characteristics of the MUR cycle are investigated by means of magnetic-thermodynamic cycle analysis and numerical simulation.The effects of geometric parameters and operating conditions on the MUR cooling performance are also comprehensively studied.The results show that the MUR cycle has high efficiency in heat regeneration and has advantage in establishing large cycle temperature span.The theoretical maximum cycle temperature span can be up to 50.9 K,which provides good cooling performance and high energy efficiency.Thirdly,considering that the heat transfer rate is an important factor affecting the cooling performance of the MUR cycle,the heat transfer and heat regeneration processes in MUR model are studied and optimized from the aspects of heat transfer structure and heat transfer mode.Through heat transfer optimization theory analysis,optimization method investigation,experiment validation and numerical simulation,the effects of different heat transfer structures,heat transfer modes,heat transfer characteristics and their interrelationships on the cooling performance of the MUR cycle are studied and compared.By using copper blocks for heat regeneration,the maximum SCP of MUR cycle obtained by the topology optimization structure increases by an average 391%compared to the structure without heat transfer optimization;By using Peltier elements for rapid heat regeneration in the MUR cycle,the maximum SCP can be up to 160.9 W kg-1,and the increase rate of cooling performance ranges from 82%to 149%compared to the heat regeneration dependent on just heat conduction.Finally,based on the above aspects of theoretical and experimental results,the overall system scheme and key component schemes of the novel fully solid state MR are designed from the perspective of practical application and engineering requirements.By means of permanent magnet system simulation and three-dimensional dynamic grid simulation,the parameters of magnetic field and the optimal configuration of the fully solid state MR are investigated.The feasibilities of heat transfer structure optimization and several potential system optimization schemes are demonstrated.The system simulation results considering various practical factors show that the magnetic field strength in the magnet gap is evenly distributed,and the average magnetic field strength is 0.65 T,which satisfies the operation requirement of the MUR cycle.In addition,the system temperature spans can reach 14.1 K and 19.0 K respectively under two different rotation modes of intermittent rotation and continuous rotation,which is also in line with the practical cooling requirements.In this thesis,the mechanism of MUR cycle is studied.The theoretical model of MUR cycle for MR and the methods of heat transfer enhancement for solid state MR are creatively proposed.The explorations in the theoretical model of the MUR cycle,the design and optimization of the MUR cycle based MR system,the heat transfer enhancement of the micro-unit regenerator and the application potential improvement of fully solid state MR have been made and are of theoretical significance and practical application value,providing new research methods to promote the progress of MR technology. |
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