Development of Closed Two-Phase Thermosyphon for Decay Heat Removal System

为了获得比传统单管热虹吸管(简称STT)更好的换热性能，开发了一种特殊类型的热虹吸管，称为环状并联热虹吸管。LPT由两个STT组成，顶部和底部由两个U型管连接。这项研究的目的是获得关于LPT的传热特性的基础知识，以便将其应用于核反应堆的被动热衰变去除系统。重点考察了充注比和系统压力对冷凝器换热的影响，以及冷凝器内流动脉动对凝结换热的影响。实验采用水和乙醇混合工质，乙醇的系统压力为23~144kPa，水的系统压力为12~57kPa，充注比为0.3~0.9，冷却液温度为25~50℃。实验结果表明：蒸发器部分的壁面温度分布比STT更为均匀，换热系数与STT相近。除不凝性气体外，LPT获得的最大热流密度是STT的三到四倍。但包括气体在内，它会下降到比STT更低的值。下降的原因尚不清楚，仍有待调查。底部U形管内流体脉动对冷凝换热的影响较小，实验前预期的流体流量脉动不能减小不凝性气体对冷凝换热的影响。

A special type thermosyphon, a looped parallel thermosyphon referred to LPT, has been developed for obtaining better heat transfer performance than a conventional single tube thermosyphon referred to STT. The LPT consists of two STTs joined each other at top and bottom by two U-tubes. The purpose of this study is to obtain fundamental knowledge regarding to heat transfer characteristics of the LPT in order to apply it to a passive heat decay removal system of a nuclear reactor. Effects of fill charge ratio and system pressure on the heat transfer and effect of flow fluctuation in the condenser part on condensation heat transfer in the presence of non-condensable gas are mainly examined. Experiments are conducted with water and ethanol working fluids in wide ranges of system pressures of 23 to 144 kPa for ethanol and 12 to 57 kPa for water, charge ratio of 0.3 to 0.9 and coolant temperatures of 25 to 50 ℃. Experiments using a STT are also conducted to compare with the heat transfer characteristics of the LPT on the same conditions.Major findings of the experiments are as follows : In the evaporator part, the wall temperature profile is more uniform than that in the STT and the heat transfer coefficient is similar between LPT and STT. The maximum heat flux obtained in the LPT excluding a noncondensable gas is three to four times larger than that in STT for both working fluids. But including the gas it falls down to a lower value than that in STT. The reason of the decrease is not clear and has been remained to be investigated. The effect of fluid fluctuation in the bottom U tube on the condensation heat transfer is minor and the effect of the noncondensable gas on the condensation heat transfer can not be minimized by the fluid flow fluctuation which was expected before the experiments.