| Due to its high efficient thermal conductivity,gravity heat pipe is widely used in many fields.In this paper,the conventional gravity heat pipe with a total length of 500mm,an outer diameter of 22mm and a wall thickness of 1mm and the gravity heat pipe built-in spring in evaporation section are experimentally studied,and the conventional gravity heat pipe is simulated by CFD.Study contents and conclusions are as follows:(1)The start-up characteristics and steady-state heat transfer performance of gravity heat pipe were analyzed by experiments.The influence of five different heating powers,four different filling rates and five different cooling water flows on the heat transfer performance of gravity heat pipes was discussed and analyzed.The results show that with the increase of heating power,the starting time of gravity heat pipe decreases,the starting temperature and working temperature increase,the heat transfer coefficient of evaporative section increase and the overall thermal resistance of gravity heat pipe decrease.When the heating power is constant,the heat transfer coefficient in the evaporation section increases first and then decreases with the increase of liquid filling rate.When the liquid filling rate is 0.5,the maximum value of the heat transfer coefficient in the evaporation section is 1161.8W/(m~2·°C),and the minimum value of the total thermal resistance is 0.185°C/W compared with other liquid filling rates.At this point,the heat release and efficiency indexes are better than other liquid filling rates.At the same heating power,the heat transfer coefficient of evaporation section,the temperature difference between cooling water inlet and outlet and the efficiency of heat transfer decrease with the increase of cooling water flow.The thermal resistance does not change significantly with the cooling water flow.The lowest thermal resistance occurs when the heating power is300W and 16L/h,and the value is 0.1849°C/W.(2)The conventional gravity heat pipe was simulated by CFD at 1:1 with the experimental model.VOF model is adopted to track the interface of gas-liquid two phases.By comparing the simulation results with the experimental values,the results show that the average relative deviation between the temperature of evaporation section,adiabatic section and condensing section of the gravity heat pipe and the experimental values is within 6%when the liquid filling rate is 50%.Through numerical simulation of evaporation and condensation,the simulation results are in good agreement with the visualization experiment in the bubble formation form.When the heating power is 100W and the filling rate is 50%,the wall temperature of the evaporation section will decrease with the increase of the inclination Angle.When IA is 60°,the thermal resistance is at least 0.6097K/W,and the heat transfer performance of the gravity heat pipe is the best.(3)The structural modification experiment of the gravity heat pipe was carried out by means of the built-in spring in the evaporation section.The copper spring with the same length as the evaporation section,the wire diameter of 0.8mm and the outer diameter of 14mm was added in the evaporation section to enhance the heat transfer in the evaporation section.The experimental results show that under low power,the operating temperature of the built-in spring is higher than that of the conventional heat pipe.The addition of the built-in spring in the evaporation section of the gravity heat pipe is not conducive to the start process of the gravity heat pipe,resulting in a higher operating temperature.When the heat transfer is stable,the built-in spring gravity heat pipe has more uniform wall temperature distribution and higher evaporative heat transfer coefficient than the conventional heat pipe,with a maximum increase of 59.9%and a maximum decrease of 12.2%compared with the conventional heat pipe,resulting in higher heat transfer efficiency. |
