Research On Preparation Of Room-temperature Magnetic Refrigeration Materials And Hybrid Magnetic Refrigerators

Magnetic refrigeration is expected to be the future cooling technology due to its higher energy-efficiency and lower environmental impact than conventional gas compression refrigeration.The feasibility of applying this technology highly depends on the design and synthesis of suitable magnetic cooling materials,the better material processing ways and the development of prototype machine.In this thesis,we conducted a study on the magnetic refrigeration,including a processing method of room-temperature magnetic refrigerant materials,a room-temperature hybrid refrigerator that combines the active magnetic refrigeration effect with the Stirling cycle refrigeration effect and a low-temperature hybrid refrigerator that combines the active magnetic refrigeration effect with the Gifford-McMahon cycle refrigeration effect.The main results are summarized as follows:1.Research on La?Fe,Si?₁₃/phenolic resin magnetocaloric composite materialsWe have successfully fabricated plate-shaped La?Fe,Si?₁₃/phenolic resin composite magnetic refrigerants with a thickness of 0.5 mm.The crystal structure of La?Fe,Si?₁₃remains unchanged when covering with the cured phenolic resin.The magnetocaloric effect of La?Fe,Si?₁₃ was unaffected.It is particularly worth mentioning that the La(Fe_11.6-xCo_x)Si_1.4C_0.15/phenolic resin composites have excellent thermal conductivities with values reach up to 3.1 Wm^-1K^-1.This work represents a simple and energy-efficient process for industrial manufacture.2.CFD numerical simulation for room-temperature hybrid magnetic refrigeratorAnsys Fluent software is applied in this dissertation.Based on the physical model of hybrid refrigerator and the theories of magnetocaloric effect,CFD model of hybrid magnetic refrigerator was established.This dissertation described the internal heat transfer mechanism of Stirling and magnetic refrigeration effect at active regenerator.Some parameters of the model such as working frequency were analyzed and the best phase angle was figured out in order to couple these two cooling effects positively.Simulation results show that Stirling and magnetic cooling effects can couple positively at phase angle of 60°.The results also show that increasing the system frequency can enhance the cooling performance of the system.3.Experimental study on room-temperature hybrid magnetic refrigeratorA room-temperature hybrid refrigerator that combines the active magnetic refrigeration effect with the Stirling cycle refrigeration effect is studied here.Based on Stirling cycle refrigerator,the prototype consisted with two coaxial Halbach adjustable rotating permanent magnets to provide magnetic field.Regular Gadolinium sheets were refilled to the regenerator and comparing experiments were carried out.Testing results show that Stirling and magnetic cooling effects can couple positively at phase angle of 60°.With an operating pressure of 5.5 MPa and a frequency of 2.5 Hz,a no-load temperature of273.8 K was reached in 9 minutes.For the hybrid operation,cooling powers of 56.4 W was achieved over temperature span of 12 K.The cooling power improves by 28.5%if compared with that exploiting only the Stirling cycle refrigeration effect.We also tested the performance of the regenerator packed with plate-shaped La?Fe,Si?13/phenolic resin composite magnetic refrigerants.The results show that the maximum cooling power of 41W was achieved over a temperature span of 30 K.4.Experimental study on low-temperature hybrid magnetic refrigeratorA low-temperature hybrid refrigerator that combines the active magnetic refrigeration effect with the refrigeration effect is studied here.In the apparatus,a Gifford-McMahon cycle refrigerator uses ErNi grains as the second stage regenerator and the regenerator is put in a magnetic field,which is provided by a Halbach-type rotary permanent magnet assembly.Testing results show that Gifford-McMahon and magnetic cooling effects can couple positively at phase angle of 60°.At the cryocooler reciprocating frequency of 1 Hz the minimum no-load temperature was 3.5 K for ErNi second stage regenerator.The maximum refrigeration capacity at 5.0 K was 0.87 W.