Experimental and analytical study of active magnetic regenerators

From an energy-saving and environmentally friendly perspective, magnetic refrigeration at room temperature is an emerging alternative to conventional vapor compression cooling. The recent progress toward commercialization of magnetic refrigeration is hampered by its relatively low temperature span and cooling power. The active magnetic regenerator (AMR) is an essential component to implement the thermal energy transfer and scale up the temperature span for a magnetocaloric system. Under a magnetic refrigeration cycle, the AMRs involve the coupling of heat and mass transfer, magnetocaloric effect and parasitic losses, which complicate the AMR design. In this dissertation, test apparatus, numerical models and analytic models were developed to visualize the intrinsic thermodynamic process in an AMR. These analysis tools facilitate material characterization, thermal-hydraulic evaluation, cooling performance assessment, and stability analysis. The relations of heat transfer effectiveness, cooling capacity, and transient temperature profile were analytically quantified, as well as numerically and experimentally validated. The thesis has found that the solid heat storage term of the effectiveness has a clearer impact on both the conjugate heat transfer and cooling capacity of AMRs than the entire effectiveness does. For a harmonic cycle, the amplitude and phasing of local temperature are affected by utilization, relative fluid displacement, and number of transfer unit. The slope of average temperature profile is weakly dependent of uneven distribution of heat capacity of magnetocaloric material (MCM) in a fluid oscillating process. However, the magnetocaloric effect would significantly change the slope of average temperature profile in an AMR cycle. The results of experimental, numerical and analytical work obtained in this thesis will improve the efficiency of future AMR designs. The challenges for AMRs are also related to shaping porous geometries from often-brittle MCMs. In this dissertation, freeze-cast lamellar channel AMRs, 3D printed double corrugated AMRs, tape cast triangular microchannel AMRs, and packed particle bed AMRs of different types are characterized experimentally and numerically. The channel surface quality was found to have great impact on heat transfer and cooling performance, as well as the stability. The triangular microchannel AMR obtained a good balance between cooling performance and flow resistance. The non-bonded AMRs with stabilized first order MCMs are promising due to relatively high cooling performance compared to epoxy-bonded AMRs. After investigating the potential of these preliminary AMRs, further suggestions for the next generation AMRs are provided.