Heat pipe is a device for transferring heat with high heat conduction ratio. It transfers heat energy by vaporization and condensation of a fluid with little temperature reduction. A heat pipe has three section in general including evaporator, adabatic and condenser sections. When heat reaches evaporator, the fluid evaporates and it forms a different pressure in the pipe. The different fluid pressure causes the vapor moves through the pipe and reach condenser. In condenser section, the vapor condenses and its latent heat releases and then the fluid returns from inside wick by capillary pressure to evaporator section. This study has been conducted to show the effect of using three water based nanofluids (water+ Al2O3), (carbon nanotube+water) and (water+diamond) on the heat operation of heat pipe. We also applied a new correlation for viscosity of nanofluids developed by bakhshan et al..Finally the graphs of velocity, temperature, pressure and heat resistance of heat pipe were obtained with using the nanofluids and carbon nanotube and has been compared with conventional operation of heat pipe which uses from pure water.
A mathematical model of a cylindrical heat pipe using TiO2, CuO, and Al2O3 nanofluids was solved analytically by shafahi et al., . The thermal performance of cylindrical heat pipes utilizing the above nanoﬂuids as the working ﬂuid has been investigated. They obtained the velocity, pressure, temperature and maximum heat transfer limit for different nano- particle concentration levels and sizes. Their results show that the thermal performance of a heat pipe is improved and temperature gradient along the heat pipe and thermal resistance across the heat pipe are reduced when nanoﬂuids are utilized as the working ﬂuid.
Kyu hyung Do et al.  investigated a mathematical model was developed for quantitatively evaluating the thermal performance of a water-based Al2O3 nanoﬂuid heat pipe with a rectangular grooved wick. Their results show that, thin porous coating layer formed by nanoparticles suspended in nanoﬂuids is a key effect of the heat transfer enhancement for the heat pipe using nanoﬂuids. Also, the effects of the volume fraction and the size of nanoparticles on the thermal resistance of nanoﬂuid heat pipes were studied. Furthermore the experimental results showed that the thermal resistance of the nanoﬂuid heat pipe tended to decrease with increasing the nanoparticle size.
Zhenhua et al.  carried out an experimental study on the application of nanofluids in an inclined miniature grooved heat pipe using water-based CuO nanofluid as the working fluid. The working ﬂuid consisted of CuO nanoparticles with diameter of 50 nm and pure water. The experimental results showed that with increasing of the inclination angle, the heat transfer coefficient of the evaporator and the condenser section will increase. The evaporation heat...