Heat pipes are high-efficient heat transfer devices and have been widely applied in various thermal systems. Since heat pipes utilize the phase change of the working fluid to transport the heat, the selection of working fluid is of essential importance to promote the thermal performance of heat pipes. Owing to the heat transfer enhancement effect of nanofluid in the single phase and phase change heat transfer, nanofluid in heat pipes have been used as the working fluid to enhance their heat

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... sfer performance. Under appropriate operating conditions, nanofluid will exhibit high thermal conductivity and stability and are increasingly being used in many heat transfer applications in industrial fields. In recent years, the studies on nanofluid mainly focused on its thermal conductivity, and on forced convection and boiling heat transfer mechanisms. In this work various experiments are carried out to study the behavior of nanofluid to improve the performance of a circular heat pipe. The circular copper tube is used as heat pipe, with pure water and hybrid nanofluid (Al2O3 + CuO) with water base as working fluids. An experimental setup is designed and constructed to study the effects of the charged volume ratio of the working fluid (Water, 1%). The effect of volume fraction of Nanoparticles in the base fluid on the thermal resistance is investigated. INTRODUCTION: A heat pipe is an excellent heat conductor; one end of a heat pipe is the evaporationsection, and the other end is the condensation section. When the evaporation section is heated, the liquid in the heat pipe evaporates rapidly. This vapor releases its heat at the condensation section, which has a small vapor pressure difference, and condensesback into liquid. The condensed liquid in the condensation section then flows back to the evaporation section along the inner wall of the heat pipe and undergoes endothermic evaporation in the evaporation section. The heat transfer of a heat pipe uses a workingfluid that changes phases in a continuous endothermic and exothermic cycle, giving the heat pipe excellent heat transfer performance.Replacing the traditional working fluid with a working fluid with a high heat transfer performance is worth considering.The common types of heat pipes primarily include two-phase closed thermosyphons, pulsating heat pipes, and oscillating heat pipes.Capillary-driven two-phase systems offer significant advantages over traditional single-phase systems.