Numerical Study On Heat Transfer Characteristics Of Nanofluidic Gravity Heat Pipes

As a kind of high efficient heat transfer component, gravity heat pipe whose internal complex heat and mass transfer is the focus of the research about the heat transfer mechanism of heat pipe, has already been widely used in field of heat exchange. The evaporation and condensation process of nanofluids that act as a new variety of heat transfer medium in the heat pipe is needed to acquire, which has a great important significance for understanding the impact on the heat pipe. Based on the overview of construction, working principle of the heat pipe and the research status of nanofluids, heat pipe filled with nanofluids and the numerical research about heat pipe, a numerical model for the heat and mass transfer process inside heat pipe is built to analyze and study the startup of heat pipes filled with pure water and nanofluids respectively.A gravity heat pipe, which has the characteristics of 9.52 mm outer diameter, 0.6mm wall thickness, a 100mm-long evaporator, a 50mm-long adiabatic and a 100mm-long condenser, was selected as the main research object in the paper. The transient numerical model was developed based on the VOF and UDF technique. Under the condition of the40 W, 60 w, 80 W heating power, the process of evaporation and condensation for heat pipes filled with pure water and nanofluids are simulated respectively. Meanwhile, the wall temperatures of evaporator, adiabatic and condenser, thermal resistance and effective thermal conductivity of heat pipes filled with pure water and nanofluids are calculated and analyzed respectively.The results of research shows that the phase transition processes in the evaporator and condenser of heat pipe filled with pure water are nucleate boiling and filmwise condensation respectively. Meanwhile, the quantities, size, shape and position of vapor bubble produced by boiling in the evaporator change along with the variation of the heating power at the same time. With the increase of heating power, the time which the formation of continuous liquid film needs becomes short and the thickness of liquid film also increases. The variation tendency of the evaporator wall temperature indicates that the time, which start-up of heat pipes filled with pure water needs, is with value of 19 s, and the wall temperature increases with the heating power increasing. The variation tendency of the condenser wall temperature illustrates that the heat pipes filled with pure water start to work in advance after the heating power increases to a certain extent, and the fastest time is around 3s.Moreover, the thermal resistance of heat pipes filled with pure water, which is with the minimum value of 0.552K/W, decreases with the heating power increasing, and effective thermal conductivity of heat pipes filled with pure water, which with the maximum value of2.07×106W/m?K-1, rises with the heating power increasing.It is found that the shape and position of vapor bubble produced by boiling in the evaporator of heat pipes filled with Al2O3 nanofluids change compared with heat pipes filled with pure water at the same time. The thickness of liquid film is obviously increases under the same heating power except for 40 W heating power. The wall temperatures of evaporator,adiabatic and condenser of heat pipes filled with Al2O3 nanofluids and pure water have the same variation tendency, which indicates that starting method of heat pipe has not been changed by Al2O3 nanofluids. However, the time that the start-up of heat pipes filled with Al2O3 nanofluids needs, is 3s faster than the time that heat pipes filled with pure water needs,and the heat pipes also start to work in advance. Meanwhile, compared to the heat pipes filled with pure water, the evaporator wall temperature of heat pipes filled with Al2O3 nanofluids is with highest drop of 0.61%, and the condenser wall temperature is with highest raise of 0.36%, which proves that heat pipes filled with Al2O3 nanofluids has the better isothermal characteristics than he heat pipes filled with pure water. After the Al2O3 nanofluids is used as the working fluids, thermal resistance is with maximum decline ratio of 9.34% and effective thermal conductivity is with maximum rise ratio of 3.47%, which both occurs under the condition of 60 W heating power. Moreover, the stability of heat pipe improves with the concentration increasing, and it can be found that the best heat transfer performance was appeared when the concentration of nanofluids was 6%.