Thermal analysis of thermosyphon (HVAC)) systems using different refrigerants

Abstract

Most of the comparison studies that carried out on thermosyphon are focused on different parameters and factors that affect the thermosyphon performance thus to identify the most efficient system. Therefore, less attention was given towards comparison of the different structures of thermosyphon. Hence, this research sought to compare and identify the best performing thermosyphon structure among Straight shape, C-shape and Close looped system in terms of thermal performance when subjected to different factors namely fill ratio and inclination angle by using R410A as working fluid. Another primary focus of this research was to address the ability of the best performing thermosyphon structure in real life situation and its ability to reheat the air and prevent excessive dehumidification by using different refrigerants namely R410A, R32 and R134a. The thermal performances of R410A filled thermosyphon subjected to low heat flux from 1200W/m2 to 5000W/m2 and evaporator wall temperatures between 20 °C and 50 °C with fill ratios 1.00 and 0.75 and at different inclinations of 45°, 68° and 90° were investigated by using Straight shape, C-shape and Close looped thermosyphon. The performance of the thermosyphon which is determined from its heat transfer capability was found to be dependent on inclination angle and fill ratio. Experimental result shows that heat transfer coefficient increases as the heat input increases, while thermal resistance decreases exponentially with increasing input power for all three structures. Change in fill ratio and inclination angle at various heat input contributed to a better thermosyphon performance. From the investigated experimental set up, heat transfer was highest for fill ratio 1.00 and inclination angle of 68° compared to inclination angles of 45° and 90° for all three investigated structures. Straight shape thermosyphon observed to have better overall heat transfer and smaller thermal resistance compare to C-shape and Close loop thermosyphon due to its geometry which has less resistance for refrigerant circulation in its shape. As straight shape being the best performing structure its ability to reheat the air and prevent excessive dehumidification was investigated by using different refrigerants and fill ratios. From the test, it was observed that fill ratio 1.00 exhibits the largest temperature difference between entering and leaving air compared to other fill ratios of 0.75, 0.50 and 0 for all the refrigerants at same input power. In overall, it was found that R32 refrigerant filled thermosyphon had best performance compared to R134a and R410A refrigerants as it has the smallest difference in temperature between evaporator and condenser section.