Experimental study on heat transfer properties of gravity heat pipes in single/hybrid nanofluids and inclination angles

In order to meet the working requirements of heat pipes under complex working conditions and high heat flux density, and propose corresponding thermal management solutions. In this paper, the single and hybrid nanofluids (TiO2-H2O, Al2O3-H2O, Al2O3+TiO2-H2O) with mass fraction of 5% under the gradient of gravity heat pipe at 50°, 60°, 70°, 80°, 90° are studied. The changes in thermal resistance, thermal efficiency and equivalent heat transfer coefficient of water were compared with deionized water. The results are as follows: The overall efficiency of gravity heat pipe ranges from 70% to 85%. Compared with deionized water, the nanofluid can effectively improve the heat transfer. At different inclination angles, the hybrid nanofluid with the same mass fraction shows the reverse trend compared with the single nanofluid by decreasing the thermal resistance of the heat pipe and increasing the heat transfer efficiency. At an inclination angle of 70°, the heat transfer efficiency of the nanofluid Al2O3+TiO2-H2O is 6.8% weaker compared to TiO2-H2O and 4.5% weaker compared to Al2O3-H2O. At a tilt angle of 60°, the heat transfer capacity of Al2O3+TiO2-H2O nanofluid increases by 7.3% compared to TiO2-H2O and by 5.6% compared to Al2O3-H2O.