Mechanism Analysis And Experimental Research Of Micro-unit Regeneration Magnetic Refrigeration Cycle At Room Temperature

To cope with the climate crisis of global warming,exploring green and environmental friendly refrigeration technologies has become an urgent issue for the refrigeration industry today.New refrigeration technologies based on the Caloric effect use solid materials with special thermal effect as refrigerant,which has the characteristics of zero Global Warming Potential(GWP)and quiet operation.Among them,room temperature magnetic refrigeration(MR)technology is considered as an important research direction and technical scheme to alleviate the pressure of climate change.At present,most of the MR prototypes are based on the traditional active regenerative cycle.However,active regenerative cycle generally suffers from low operating frequencies,high regeneration losses,complex flow path systems and small output cooling capacity and temperature span values.Therefore,it is of great significance to construct a fully solid-state MR cycle model from the angle of irreversible losses minimization.Through in-depth mechanism analysis and experimental investigation of cycle's internal regenerative characteristics and refrigeration performance,the application of room temperature MR technology can be further promoted.In this dissertation,the thermodynamic analysis of room temperature MR cycle has been carried out.The entropy generation theory has been introduced to evaluate the thermodynamic performance of different MR cycles from the perspective of energy"quality".Based on the second law of thermodynamics,the expressions of heat transfer and flow resistance entropy generation rates in the traditional active magnetic regeneration(AMR)cycle and the novel micro-unit regeneration(MUR)cycle have been obtained respectively.The process with the greatest irreversible losses in the cycle has been found and the effective ways to improve the cycle's thermodynamic performance have been identified.Under the condition of the same temperature span,the irreversible losses of two different cycles have been compared.The results show that the total entropy production rate of the MUR cycle is smaller than that of the AMR cycle,with a total entropy production rate reduction of approximately 52%.Based on the entropy generation minimization analysis,the MUR cycle is more efficient in energy utilization.The analysis of the mechanism of irreversible losses in the MUR cycle shows that an efficient enhanced heat transfer structure inside the micro-unit lattice can lead to higher energy efficiency.Therefore,this dissertation adopts the insertion of high thermal conductivity materials to enhance the heat transfer performance inside the MCM lattices.The enhanced heat transfer mechanism has been analysed and the paired lattice's heat transfer characteristic has been verified by experiments.By means of numerical simulation of the 3D system model,the heat transfer and cooling characteristics have been compared and analysed with/without optimisation of the internal structure,and the cycle's performance parameters under different operating parameters have been further compared.The simulation results show that the maximum no-load temperature span of the parallel-plate MUR system is 155%higher than that of the gadolinium-only MUR system.An efficient enhanced heat transfer structure between paired MCM lattices is another key point to further reduce irreversible losses in solid-state MUR magnetic refrigeration cycle.In this dissertation,the thermoelectric modules(TEM)have been used as the enhanced heat transfer medium between the high and low temperature MCM lattices.A three-dimensional solid-state magnetic refrigerator based on MUR cycle and TEM elements has been modelled in detail.In this model,the simplification of the TEMs is based on the theoretical analysis,and the rationality of the simplified model has been verified from the performance and heat transfer perspectives.The system cooling performance and the heat transfer mechanism have been compared with/without the optimisation of heat transfer structure between paired MCM lattices.Meanwhile,the influence of different internal heat transfer structures and voltage intensity on the system performance have been further analysed.The simulation results can provide a reference for the prototype's experimental research.The temperature span obtained by Gd-only MUR system with TEM elements is increased by approximately 210%compared to that without TEM elements.After further optimisation of the internal heat transfer structure,a maximum system temperature span of 21.5 K can be obtained for the parallel-plate MUR system with TEM elements.Based on the above theoretical analysis and simulation studies,and combined with consideration of practical factors,the design scheme and assembly rules of the solid-state MUR room-temperature magnetic refrigeration prototype have been determined.The experimental investigation can be divided into the following three steps:first,a proof-of-principle test platform has been built to experimentally verify the feasibility of the MUR cycle.The cold and hot water heat exchanger have been placed at the cold and hot ends of the system,respectively.The cooling/heating capacity in the heat exchanger has been conducted to MCMs which rotating to the cold/hot end.The simulation of magnetization at the hot end and demagnetization at the cold end can be realized by this means.Then,the experimental study on the MUR magnetic refrigeration system working in the intermittent rotating mode has been carried out.The relationship between the system temperature span and different structural and operating parameters has been obtained.The structural parameters include the lattice number of MCM,the heat transfer structure inside the MCM lattice,and the heat transfer structure between paired MCM lattices.And the operating parameters include regenerative time and input voltage intensity.In addition,the heat transfer characteristics within and between the MCM lattices have been analysed in detail.Finally,based on the experimental test results,the practical constraints of the solid-state MUR room-temperature magnetic refrigeration prototype have been explored.Several follow-up solutions and directions for system performance improvement have been proposed.The proof-of-principle experimental results show that the maximum temperature difference between the cold and hot ends is 126%higher than the temperature drop of the MCM which absorbing cooling capacity at the cold end.The experimental test results of the solid-state MUR room temperature magnetic refrigeration prototype show that the optimization of transfer structure within and between the MCM lattice can be regarded as two effective ways to improving the system's cooling performance.The temperature span obtained by the parallel-plate(graphene)MUR system has been increased by about 36%compared to the Gd-only MUR system.Meanwhile,the temperature span of the parallel-plate(graphene)MUR system with TEM elements has been increased by about 100%compared to that without TEM elements.Under the condition of input voltage of 1 V and regenerative time t_lattice(28)25s,the temperature span of the prototype system is 3.2 K,which is 7 times larger than the demagnetization temperature change((35)T_MCM(28)0.4K)of the MCM in the prototype.This dissertation is based on the MUR room-temperature magnetic refrigeration cycle.The research focuses on the theoretical analysis of irreversible losses,the enhanced heat transfer methods within the cycle,the design and construction of the solid-state MUR magnetic refrigeration system,the proof-of-principle of MUR cycle and the performance test of the fully solid-state magnetic refrigeration prototype.The above research work has theoretical significance and application value,providing technical support to promote the development of solid-state room temperature magnetic refrigeration technology.