Summary

In recent years, magnetic refrigeration has been considered one of the most promising alternative technologies for refrigeration at around room temperature. In many research groups around the world, the research on this technology is conducted in two fronts: the development of magnetocaloric materials (refrigerants) and of prototypes. The Brayton cycle is one of the most largely employed thermodynamic cycles in magnetic refrigeration for applications at around room temperature. In this cycle, the most important component is the active magnetic regenerator (AMR), since it enables the attainment of temperature spans between the hot and cold reservoirs larger than those achieved purely by magnetizing and demagnetizing the magnetocaloric material. Thus, the chief objective of this dissertation is the development and subsequent experimental analysis of a reciprocating AMR with parallel plates made of gadolinium (Gd). The study was carried out in two parts. In the first, the magnetocaloric effect (MCE) of Gd was determined experimentally as a function of temperature, for a magnetic field of 1.65 T, by means of a direct measurement of the adiabatic temperature change. This preliminary phase of the work was fundamental for a better understanding of the influence of phenomena such as magnetization and demagnetization on the MCE. In the second part, a thermodynamic analysis of the reciprocating AMR was carried out using water as the working fluid. The dependent variables of this analysis are the cooling capacity and the temperature span. They are both evaluated in terms of the mass low rate, m_ , the utilization factor, and the temperature of the hot reservoir, TCF , which is kept equal to the ambient temperature in all tests. These independent variables were varied so as to and the best operating point for the AMR. As a result, a maximum temperature span of 4.45 K was measured for  0.38 and T =CF = 296.15 K. The maximum cooling capacity was 3.9 W, for m_ = 18 kg/h ..1 and TCF = 296.15 K. The results obtained in both parts are in line with trends observed in the literature. However, the results achieved in the second part are considered to be modest, possibly due to thermodynamic losses taking place in the cycle. In this sense, the thermodynamic losses have been mapped and evaluated qualitative in order to assist future designs of AMR prototypes. Among the most signicant losses are those associated with the demagnetization factor, non-uniformities in the magnetic held, heat transfer to the environment and heat generation by means of the Joule eschect due to eddy currents. Together, the losses act toward decreasing both the MCE and the temperature procle in the regenerator, resulting in a smaller temperature span and refrigeration capacity.

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