Experimental study on heat transfer characteristics of closed thermosyphon at different volumes and inclination angles for variable vacuum pressures

In this study, the thermal performance of vacuumed copper pipe has been experimentally investigated by using different volume values of 5 ml, 10 ml, 15 ml, and 20 ml with inclination angles of 45°, 60°, and 90° for variable vacuum pressures. For this purpose, the inner, outer diameter, and length of the vacuumed pipes filled with the distilled water were 26 mm, 28 mm, and 1500 mm, respectively. This copper pipe was vacuumed until the absolute pressure of 6.32 kPa and immersed in a heat source at a constant temperature of 82°C at the beginning of the experiments. The surface temperatures of the vacuumed pipes were measured at twenty different points by using K-type thermocouples in two different data loggers. All calculations were performed as variable saturation temperatures at variable vacuum pressures with changing the time. According to the results, the experimental boiling heat transfer coefficients showed a good agreement with the correlations in the literature. Also, the best thermal resistance was obtained in the vacuumed copper pipe with a water volume of 10 ml at the inclination angle of 90° until the end of the experiment. © 2021 The Authors.