Experimental Investigation Of Heat Transfer Enhancement In An Inclined Axial Miniature Grooved Heat Pipe Using Nanofluids

An experimental investigation was carried out to study the heat transfer enhancement of an inclined axial miniature grooved heat pipe using Cu nanofluid as the working fluid. The effects of mass concentration of Cu nanoparticles, operating pressure, inclined angle of heat pipe, nanoparticle sort and cooling condition were discussed. The results showed in terms of axial temperature distribution, heat transfer coefficients of evaporator and condenser sections, critical heat flux and total heat resistance of the heat pipe. Besides, the characteristics of heat pipe inner surface after experiment was analyzed.The experimental results indicate that:(1) The temperature distribution along heat pipe appears to be more uniform owing to the reduction of temperature for using nanofluid compared with water. The coefficients of heat transfer and critical heat flux increase obviously while total heat resistant decrease remarkably when substituting Cu nanofluid for water as the working fluid of miniature heat pipe.(2) The mass concentration of nanoparticles has apparent influences on both heat transfer coefficients and critical heat flux. The mass concentration of 1.0 wt% corresponds to the optimum heat transfer.(3) The minimum operating pressure of 7.45 kPa corresponds to the maximum heat transfer enhancement and the maximum critical heat flux enhancement. At that working pressure, the heat transfer coefficients of evaporator section averagely enhance by 60%, total heat flux decrease by 25% and the critical flux can be enhance by 55% when substituting the 1.0 wt% Cu nanofluids for water. (4) The influence of gravitation is obvious in the axially direction. Therefore the effect of inclination angle on the performance of miniature grooved heat pipe is obvious. There exist a optimize angle 75°corresponds to the maximum heat transfer coefficient and critical heat flux. The heat transfer coefficients of evaporator section averagely enhance by 70%, total heat flux decrease by 33% and the critical flux nearly can be double when the heat pipe was inclined.(5) The enhancement ratio is different when using different nanoparticles. Cu nanofluid has better enhancement effect than CuO nanofluid because its thermal conductivity is much higher.(6) With certain cooling condition, heat pipe operating temperature increase with the input power. When heat flux is definite, the lower cooling water temperature is, the lower heat pipe operating temperature. The average temperature of evaporator section decrease and critical heat flux increase when using Cu nanofluid as working fluid of the heat pipe.(7) The startup time is shorter when deionized water is substituted by 1 wt% Cu nanofluid, and the critical heat flux of startup unsteadily is lower than startup steadily.(8) An extremely thin porous layer forms on the inner surface of the heat pipe after the experiment. The existence of the porous layer can reduce the solid-liquid contact angle hence increase critical heat flux.Experimental results show that nanofluid can greatly increase the heat transfer characteristics of the inclined miniature grooved heat pipe at low operating pressure. Nanofluid is a potential new kind of working fluid of heat pipe to enhance the heat transfer efficiency.