| The warming climate and construction of infrastructure in cold regions have disrupted the thermal equilibrium of permafrost zone,leading to its degradation.A gravity heat pipe,also known as a two-phase closed thermosyphon(TPCT),has advantages such as simple structure and significant heat transfer capabilities in single directions.TPCTs have been shown to efficiently transfer heat from permafrost to the atmosphere,playing an important role in protecting permafrost and stabilizing cold region engineering.However,most research on TPCTs in permafrost zone has focused on their engineering applications,with insufficient attention given to their own enhanced heat transfer capabilities.Given the current situation of increasing climate warming and permafrost engineering,it is of great significance to find the best design and construction parameters for TPCTs suitable for protection of permafrost engineering.The study improved the heat transfer performance testing and monitoring system for large-scale TPCTs that can simulate negative temperature conditions in permafrost,and selected three types of TPCTs with different filling rates(20%,30%,40%)to conduct experiments under different temperature differences(2 °C,4 °C,10 °C,and 13 °C),inclination angles(0～90°),and length ratios(2,3,4)conditions.The study comprehensively analyzed the influence of these factors on TPCTs heat transfer performance and evaluated their heat transfer performance under multiple operating conditions.The main conclusions of this paper are as follows:(1)Based on the monitoring of the temperature inside and outside the TPCT,it was found that the temperature distribution of the TPCT changed similarly at different filling rates,but the magnitude of temperature changes differed.A significant temperature drops regions appeared along axial direction inside TPCT,which were mainly concentrated near the gas-liquid interface of the working medium.A clear pattern of axial temperature distribution inside TPCT with increasing inclination angle was also observed.When the inclination angle was 0°,the temperature distribution inside the TPCT was stable.When the inclination angle was between 10～50°,temperature fluctuations occurred in the evaporator section,while the temperature in the condenser section smoothly decreased.In the range of 60～90°,the TPCT exhibited a cooling trend along the axial direction from bottom to top,but the magnitude in the condenser section was greater.When the inclination angle of the TPCT is between10～50°,there would be two temperature drops.The first temperature drop was related to the filling rate,while the position of the second temperature drop was related to the length ratio.This position gradually shifts towards the condenser section with increasing inclination angle.(2)Based on calculations of axial temperature differences under different experimental conditions,it was found that the axial temperature differences of the TPCT under different filling rates and length ratios show a consistent trend as the inclination angle increases.The minimum temperature difference was reached at10°～20°,indicating that the isothermal performance of the TPCT was optimal within the range.Under the current experiment conditions,the TPCT with a filling rate of 30%and a length ratio of 3 has the best isothermal characteristics when the inclination angle is 10°.(3)Based on monitoring of the heat flux density,it was found that the heat flux density of the TPCT increased with the increasing temperature difference.TPCTs with filling rates of 30% and 40% had relatively high heat flux density.The distribution of heat flux density of TPCTs with different length ratios is similar.The heat flux density of the evaporator and the condenser sections both showed a trend of increasing first and then decreasing with the increasing inclination angle,and increasing the length of the evaporator section weakened its heat flux density while increasing the length of the condenser section strengthened its heat flux density.Comparing different inclination angles,the maximum heat flux density of the evaporator section appeared between10°～20°,while the maximum heat flux density of the condenser section occurred at an inclination angle of 20°.(4)The total thermal resistance of TPCTs under different operating conditions was calculated based on temperature and heat flux density monitoring results,and the heat transfer efficiency was analyzed.The results showed that the TPCT with a filling rate of 30% had the smallest thermal resistance and the highest heat transfer efficiency compared to the other two operating conditions.Under the optimal filling rate condition,TPCTs with different length ratios had different inclination angles at which the smallest total thermal resistance occurred.The optimal inclination angles for length ratios of 2,3,and 4 are 40°,10°,and 50°,respectively.Moreover,the TPCT with a length ratio of 4 and an inclination angle of 50° had the smallest thermal resistance and the highest heat transfer efficiency.Overall,the study provides new data support for optimizing the heat transfer performance of TPCTs in different operating conditions in permafrost regions,and can also provide reference for the structural parameter design and layout of TPCTs in cold region engineering. |
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