Summary

This work investigated the performance of a two-phase loop thermosyphon (TPLT), using carbon dioxide as refrigerant fluid and operating under typical conditions of a Free-Piston Sitrling Cooler (FPSC). The basic characteristic of the problem inhabits in the transport of a heat transfer rate of 600W from a refrigerator cabinet to a cylindrical surface that corresponds to the cold head of a FPSC. The evaporator of the TPLT cools the air inside the cabinet, whereas the condenser transfers the heat absorbeb on the evaporator to the cold head of the FPSC. This work is divided into two parts, an experimental approach and a numerical one. In the experimental approach a test bench was constructed in such a way that it can control and measure the operating conditions of the condenser and the evaporator. To simulate the cabinet air, a closed circuit of wind tunnel type was constructed, with controlled temperature and airflow rate. To simulate the operating conditions of the cold cold head of a FPSC a heat exchanger was coupled to the internal surface of the condenser. In this heat exchanger, the evaporation of a fluid occurs leading to an uniform temperature condition in the inner surface of the condenser. During the experiments there had been varied the refrigerant charge, the temperature difference between the hot and cold heat sources, the height difference between the condenser and the evaporator, the airflow rate and the tubing diameter. In the numerical part, a model was developed applying the principles of mass, momentum and energy conservation for each component. The two-phase regions were considered as an homogeneous flow. The single-phase and two-phase heat transfer coefficients were determined through available experimental correlations in literature. The experimental results had shown that there is a refrigerant charge where the TPLT operates with a maximum refrigeration capacity. It can be noticed that the increase of the temperature difference slightly increases the refrigeraton capacity, however the overall thermal conductance drops. Moreover it was verified that the airflow strongly affects the performance of the system. The results generated from the numerical simulations, when compared with the experimental results, fitted 85% of the simulated points inside an error band of ± 20%, which was considered acceptable given the simplicity of the model and the complexity of the problem studied.

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