Investigação experimental do comportamento térmico de termossifões aplicados a coletores solares utilizando nanofluido de óxido de grafeno

Abstract

Thermosyphons are passive gravity-assisted heat transfer devices utilizing the latent vaporization heat of their working fluid operating in a two-phase cycle. These devices can transfer large amounts of heat with small temperature gradients. Among other applications, thermosyphons are commonly used in high pressure vacuum solar collectors, with the aim of improving the thermal efficiency of these heat exchangers. Thermosyphons are composed of three parts, the evaporator, which has the function of absorbing heat and transferring it to the working fluid so that it evaporates and, by pressure difference, it ascends to the evaporator; the condenser, whose function is to exchange heat with the cooling fluid so that the working fluid can condense and return to the evaporator; and finally, the adiabatic section. The objective of this Work of Conclusion Course was to experimentally analyze the influence of the application of graphene oxide nanofluid, in the volumetric concentrations of 5 and 10%, on the thermal performance, under different values of working slope and filling ratio, aiming at the establishment of optimal conditions. For this purpose, thermosyphons with an evaporator length of 1600mm, 40mm adiabatic section and 35mm condenser were built from a 1/4" seamless outer diameter copper tube. First, the behavior of the working fluids was evaluated (distilled water, graphene oxide nanofluid with 5 and 10% in volumetric concentrations), with different volumetric flow rates of the cooling fluid in the thermosyphon condenser (1.00, 0.75, and 0.50L/min), under different heat loads (35, 55, and 75W), with RP 50% and inclination of 25°, due to the latitude of the city of Ponta Grossa/PR, which is 25° 5' 40'. It was concluded that for the different thermal loads the conditions of 0.75 L/min and the 5% graphene oxide nanofluid presented the best conditions. For working slope (17, 25, and 35°) analysis, it is concluded that 35° would have the lowest thermal resistance for the conditions already established. In the third stage, the filling ratio was varied (30, 40, and 50%), where 40% FR had the lowest thermal resistance. Given the defined parameters, an experimental comparative study of the thermal behavior between 5% graphene oxide nanofluid and distilled water with such parameters was performed and it was concluded that for small heat loads the nanofluid has considerably lower thermal resistance, but for loads higher thermal values are very close.