Estudo do coeficiente de transferência de calor por convecção para escoamento em golfadas no evaporador de um termossifão de vidro

Abstract

The thermossyphon is a device with large potential in sustainable applications, either by its capacity to be used as a solar thermal collector for residential water heating, or by its applicability as a waste heat recovery device for industrial utilization. In order to design a thermossyphon, the convective heat transfer coefficient shall be known. This parameter depends on how the work fluid flows inside the evaporator. In this work, the objective is to study the convective coefficient for flows with phase change in slug flow form. In order to achieve it bench tests have been conducted using a glass thermossyphon, using distilled water as work fluid and an electrical resistor as heat supply. Beyond the temperature and pressure data measured by internal and external sensors, the flow pattern inside the device was filmed with a high speed camera. The images were then processed in order to obtain flow parameters such as the mixture quality and the bubble velocity. Until the authors’ knowledge, this is a pioneer approach when it comes to thermossyphons. The data gathered by the sensors and image processing were used in correlations found in the literature, with which was possible to determine the convective coefficient in the evaporator, ranging from 3.742 to 10.936 W/m2-K. It is recommended the development of specific models for the slug flow case in thermossyphons, given its complexity and the fact there are no specific models for this phenomenon. The experimental dada obtained in this work, as well as the new proposed analysis, can feed posterior studies for the obtainment of new correlations to estimate the convective coefficient in thermossyphon evaporator section.