Experimental Study On The Na-K Alloy Medium Temperature Heat Pipe

At present, considerable research and utilizations of heat pipes are concentratedon their high-efficient heat transfer performances, but little on their excellentisothermal characteristics.In the thermometry field, the calibration and verification apparatus over thetemperature range from200℃to500℃include fixed-point apparatus, oil baths, saltbaths and dry block furnaces. Due to their inconvenience, poor thermal characteristics,and pollutions to the environment, it is necessary to develop the high-precisioncomparison calibration apparatus to extend the capabilities of comparison calibrationand overcome the above-mentioned disadvantages. In view of these reasons, theNational Institute Metrology of China has designed and manufactured heat pipesusing high-purity sodium and potassium alloy as the working fluid, and carried out theresearch on them.This paper describes detailedly the sodium potassium alloy heat pipe’s designprinciples and the manufacturing processes. It includes the selection of working fluidand container materials, heat pipe processing, cleaning, vacuum degassing, and fillingprocess. Meanwhile, the heat pipe calibration furnace including design andconstruction was described. The temperature stability and uniformity of the heat pipefurnace was investigated. Additionally, further measures to improve the thermalcharacteristics were proposed.In order to develop the sodium potassium alloy heat pipe, the visual investigationon the glass-water heat pipe with a similar structure was conducted. Research showsthat over the temperature range from70℃to100℃, the temperature stability anduniformity of the water heat pipe were2.8mK and2.2mK, respectively. However, inthis range of temperature, the temperature stability and uniformity of the thermostaticbath were8.1mK and4.2mK, respectively. Therefore, the heat pipe technology cansignificantly improve the technical parameters of the thermostatic bath. In addition,the effects of the different install methods and submerged depths on the thermalcharacteristics were investigated. Research results show that when the glass-waterheat pipe was installed with brackets and wholly immersed in the liquid, whichreduced the damage to the flow field of liquid and the effect of the temperaturefluctuations of environment, it can improve the stability and uniformity of thetemperature of thermometer wells of the glass-water heat pipeBased on the research on the water heat pipe, corresponding heat pipe furnacewas designed. Also, the thermal characteristics of Na-K alloy heat pipe wereconducted. The experimental results of the sodium potassium alloy heat pipe show that the temperature stability within2hours over the range from200℃to500℃isless than5mK and the vertical temperature uniformity within12cm from the bottomof the thermometer well less than1℃. Further optimization on improvement on thetemperature uniformity will be performed..In view of the above-mentioned research, some conclusions can be drawn. First,the heat pipe technology can improve the thermal characteristics of the furnaces.Second, it is very necessary to optimize the temperature distribution of the furnace,reducing the flow of working fluid. Third, this study plays an important role inapplication of the Na-K alloy heat pipes in the thermometry field.