The Research On Factors Influencing Heat Transfer Limit Of Gravity Heat Pipe

As a high-efficiency heat-exchange unit, the gravity heat pipe has advantages of simple structure, low cost and high reliability etc., and has widely been used in the respect of the civilian energy conservation. So, the research method combining simulation and experiment was employed to study the factors influencing heat transfer limit of the gravity heat pipe. In the theoretical research, the numerical simulation of gas-liquid flow in the gravity pipe was accomplished by Fluent, and five factors influencing the heat limit of the gravity pipe were studied, such as the waveforms of the liquid membrane, heat pipe operating temperature, the adiabatic section’s height, the adiabatic section’s diameter, as well as the working fluid types. In the experimental research, the gravity heat pipe and the test platform for its heat transfer limit were personally designed by our group. The main material of this gravity heat pipe is copper, and its height is 2.3 m, and the diameters of the adiabatic section are 6.35 mm and 9.52 mm. A visual liquid lens was mounted on the adiabatic section in order to observe the gas-liquid flow. In the experiment, heating power could be regulated by the controlled silicon and the experimental data was collected by the computer. It were studied that the effects of the different heat pipe operating temperature, working fluid types and diameter of the adiabatic section etc. on the heat limit of gravity pipe. The research results of this thesis together with practical conclusion based on above investigating process are listed as follows,(1) With the help of simulation software, the analysis result of heat transfer limit could be achieved,(1) the analysis in the effects of the waveform of the liquid membrane on the heat transfer limit showed that this waveform was formed by amplitude and wave length and the limit of the heat pipe had a positive correlation with the amplitude or the wave length, which meant that the limit would increase along with the increase of amplitude or wave length.(2) The analysis in the effects of the operating temperature on the heat transfer limit showed that the limit of three types of heat pipes also had a positive correlation with the operating temperature, and the ratio, which was obtained by data fitting, between the heat limit of heat pipe and latent heat of vaporization was proportional to the product of gas-liquid density.(3) The analysis in the effects of the different adiabatic section’s diameter on the heat transfer limit showed that although the maximum heat transfer of the larger diameter heat pipe was superior to the smaller one, the heat flux of the smaller one was greater than the larger one, and the ratio between the heat flux and latent heat of vaporization was almost proportional to the product of the gas-liquid density.(4) The analysis in the effects of the height of the heat pipe insulation on the heat transfer limit showed that the extreme heat-transfer value of heat pipes with three different working fluids performed a similar variation when the height changed, which illustrated that the extreme value would not change too much along with the increase of the height.(2) With the help of experimental platform, the heat transfer limit of heat pipe has been studied. The comparison between the experimental data and the simulative data found that there was always a certain difference between them under different operating conditions, and the difference was within the range of 7% to 18%. But this difference was acceptable in the allowable error range, which meant that all data of the heat transfer limit achieved by simulation was under reasonable range and verified the reasonability of simulation on this thesis.