Assessment of the thermal performance of a thermosyphon heat pipe using zirconia-acetone nanofluids

In the present work, an experimental investigation was conducted to quantify the heat transfer coefficient, thermal resistance and the thermal performance of a thermosyphon heat pipe charged with zirconia-acetone nanofluid. The thermosyphon was designed to operate at high heat fluxes such as 200 kW/m2 (∼200 sun) similar or larger than the evacuated solar tube heat pipes. The zirconia-acetone nanofluid was prepared and stabilized by adding a surfactant, homogenizing with sonication and setting the pH of the nanofluid with a buffer solution. The thermosyphon was fabricated with an oxygen-free copper with the internal and outer diameters of 10.7 mm and 12 mm, respectively and the axial length of 280 mm. The thermal performance of the heat pipe was assessed for various heat fluxes (1 kW/m2-200 kW/m2), filling ratio (20%–75%), tilt angle (5°–70°) and the mass fraction of the nanofluid (0.025%–0.1%). Results showed that the presence of the nanofluid decreases the total thermal resistance of the heat pipe reaching the minimum value at the largest heat flux applied to the evaporator. Also, the heat transfer coefficient of the evaporator was increased. Likewise, the zirconia-acetone nanofluid enhanced the boiling heat transfer mechanism and the geyser boiling, which resulted in the enhancement in the thermal performance of the heat pipe due to the increase in the heat transfer coefficient of the heat pipe by 36.3% at wt. % = 0.1 in comparison with the pure acetone. The optimum tilt angle and filling ratio values were 65° and 60%, respectively, in which the highest heat transfer coefficient was achieved. A correlation was also developed using a dimensionless analysis to predict the Kutateladze number as an index for the thermal performance of the heat pipe.