Optimal Design And Experimental Investigation Of Active Magnetic Regenerator For Room Temperature Applications

Room-temperature magnetic refrigeration(MR)is universally acknowledged as an alternative solution to the elimination of HFCs.To obtain higher cooling capacity of the MR system under large temperature span is of great significance for the study and application of MR system.Within limited magnetic field intensity,heat regeneration is adopted by MR cycle due to the small adiabatic temperature change of Magnetocaloric material(MCM),so as to accumulate the limited adiabatic temperature variation through magnetizing and demagnetizing process,and thus to obtain a reasonable output of cooling capacity under large temperature span.In this project,we have researched the restrictive factors of magnetic regenerator under large temperature span for MR system,determined the irreversible loss of heat regeneration process caused by irreversible factors,and also studied on reasonable methods to accumulate limited temperature difference of MR regenerative cycle.By developing the optimized micro magnetic reverse cycle,establishing the energy efficiency evaluation index of MR system,and analyzing the parameter matching and irreversible factors of the active magnetic regenerator(AMR),it is indicated that the MR regenerative cycle has great potential in refrigeration.However,with the available HTFs and optimizing methods of the regenerating process,it is still difficult to reduce the inherent irreversible exergy loss of the AMR.Therefore,the heat regenerating technology in AMR needs to be improved,where dividing the regenerating process of magnetic reverse regenerative cycle into multistage micro regenerating cycle is effective for reducing the irreversible exergy loss during heat regenerating process.Based on above research,a new micro regenerating cycle is built in this paper,while the magnetic regenerative cycle is re-illustrated from the aspects of efficient regeneration mechanism,regenerator design and regenerator optimization.The main purposes of this paper are as follows.Firstly,the theoretical cycle of MR is established and analyzed to accurately evaluate the potential of MR regenerative cycle and build efficient MR regenerative cycle.Secondly,by analyzing the energy of MR system and employing exergy performance and exergy efficiency in MR system evaluation,objective and standard value are provided for magnetic regenerator optimization.Thirdly,the magnetic-thermal coupling numerical model of two-dimensional fluid reciprocating AMR is established while regenerator is optimized via micro cycle theory,so as to explore the potential of performance enhancement of AMR.Fourthly,the magnetic-thermal coupling numerical model of two-dimensional fluid unidirectional AMR is established to study the regeneration mechanism of AMR cycle,and analyze the irreversible factors causing cooling capacity reduction under large temperature span.Then,on account of the hardness in completely removing the inherent irreversible factors of AMR,micro regenerative cycle is adopted while a new regenerator executing this cycle is designed,and thus provide a feasible solution for obtaining higher cooling capacity under large temperature span by numerically optimizing the structure of the new regenerator.Moreover,whether the thermal properties and fluid-solid coupling heat transfer properties of the nonpolar molecules will change under magnetic field energy are experimentally tested,so as to develop theoretical basis for refining and optimizing the regenerator.It is indicated in the research of the establishment and optimization of the magnetic Brayton/Ericsson cycle that,under specific cooling and heating source temperature,by controlling the temperature before magnetizing and demagnetizing(T1/T3)of the MCM,the heating capacity can be used to recover the imbalance regenerative heat of the material during the MR cycle,and reduce the exergy loss of cooling capacity caused by the imbalance regenerative heat,and consequently the optimized magnetic reverse refrigeration cycle can be obtained.When analyzing the performance of the optimized theoretical cycle of magnetic reverse refrigeration,a cooling exergy of 30W,approximate a cooling capacity of 450W,is achieved by a magnetic refrigetation system with 1.3L first-order MCM,namely Gd,and a magnetic field of 1.5T under a temperature span of 20K.Faced with the bottleneck of improving the performance of available MCM and the magnetic field intensity of permanent magnet,the potential of the magnetic regenerator should be noticed,while the optimization of MR regenerating cycle is effective for improving the regenerating efficiency.Analyzing the energy composition of the MR system according to the second law of thermodynamics to establish suitable evaluating indicators MR,which is the precondition for optimizing the MR regenerating cycle,is another objective achieved in this paper.So far,the heat and cold source temperature of tested MR in different studies are un fixed.Thus,when taking account of both the quantity and quality of the energy input and output of the MR system,the exergy indicator is adopted for the system(regenerator)in this paper,so as to evaluate the performance of the system(regenerator)in a more accurate way.By experimentally testing the rotary MR system,the exergy performance/exergy efficiency is proved to be quantifiable and comparable,which also provides a optimizing index for the study on regenerator optimization.Furthermore,the two-dimensional magnetic-thermal coupling numerical model is established for the AMR with flat plate or porous media.After experimentally testing the validity of the model,the interior regenerating mechanism of the aAMR is clarified:solve the time inconsistency(reciprocating flow)or space inconsistency(unidirectional flow)during the regenerating process of the magnetic reverse cycle by pumping the HTF used as media.According to the regenerating mechanism,by analyzing the irreversible factors of theAMR,it is determined that the heat transfer temperature difference between HTF and MCM,the exergy loss caused by residual HTF in the AMR during the switch of hot/cold blow and by the thermal short-circuiting between heat/cold source of the plate,and the imbalance regenerative heat are the factors that restricts the output of cooling exergy of the AMR under large temperature span.The irreversible loss can be partially but not entirely reduced by increasing the thermal conductivity of the HTF,decreasing the specific heat capacity of the HTF,and decreasing the porosity of the plate.However,since the imbalance regenerative heat is inherent to the AMR,to recover the imbalance regenerative heat will cost a partial sacrifice in cooling capacity,which results in an irreconcilable contradiction between large temperature span and higher output of cooling exergy.Thus,breakthrough in regenerating design is required to obtain higher output of cooling exergy with MR system under large temperature span.As for new regenerator design,based on the optimized magnetic reverse cycle theory,a micro regenerating cycle is put forward in this paper and a regenerator executing this cycle is designed,as to achieve efficient regenerating motivated by small temperature span.During the regenerating process,the MCM is rotated to remove the space inconsistency.It is proved by numerical simulation that high output of cooling capacity under small temperature span can be obtained by employing regenerating system with single Curie temperature MCM,while the imbalance regenerative heat can be effectively reduced by controlling the mass ratio of the two regenerators in the series system consists of multiple/single Curie temperature MCM,and thus increase the output of cooling capacity.The largest cooling exergy of a series micro regenerator system under a temperature span of 20K is 21.42W,while the cooling capacity it provides to a cold source of 280K at an environmental temperature of 300K can exceed 280W.Meanwhile,the largest cooling exergy under a temperature span of 40K is 4.81W,while the cooling capacity it provides at an environmental temperature of 300K can exceed 30W,also the EER and R*can be improved to 7.0 and 0.8,respectively,which provides a practical technology for realizing higher cooling capacity with MR system under large temperature difference.Finally,to study the heat transfer process of fluid under the interior magnetic field of the AMR,experiments on the characteristics of heat transfer/flow resistance of the HTF under magnetic field are carried out,and lays the foundation for accurate design of regenerator.By conducting characteristic experiments on flow boiling heat transfer,flow resistance and visualization experiment of pool boiling bubble dynamics of non-polar molecule R141b and R134a in the magnetic field,and test of the IR and the proton spin-lattice relaxation time of R141b after magnetic treatment,the experiment results indicate that the magnetic field energy with an intensity of 1.5T cannot change the molecular structure of the non-polar molecules,while its influence on the flow resistance and boiling heat transfer intensity of the fluid is negligible.