大长径比垂直换热管外瞬态池沸腾的研究

通过实验研究了在瞬态条件下垂直换热管外池沸腾的流动和传热特性，获得了两相流动的可视化结果，展现了池沸腾沿垂直管流型的演变过程。分析了换热管于大尺度受限空间和开放空间的池沸腾换热特性的差异，受限空间下整体传热性能优于开放空间，而受限空间下上部与底部位置的传热系数的比值小于开放空间；随着热负荷的增加，开放空间与受限空间的差异化减小。此外，针对饱和池沸腾进行了与实验模型相对应的数值模拟，开放空间的池沸腾流场呈现出分层、渐进和传导性的对流特性；而受限空间下，受限边界的内外水域间形成了上下循环的对流机制。

Study of transient pool boiling on vertical tube with large length-diameter ratio

Through experiments, the flow and heat transfer characteristics of pool boiling outside the vertical heat exchange tube under transient conditions were studied, and the visualization results of two-phase flow were obtained, showing the evolution process of pool boiling along the vertical tube flow pattern. The differences of pool boiling are analyzed between the large scale restricted space and the open space. Analysis of overall heat transfer in restricted space is superior to that in open space. However, the heat transfer coefficient ratio between the upper location and bottom position is less than that of the open space. The more heat load increases, the less time difference between open space and restricted space is. Meanwhile, numerical simulations corresponding to the experimental model are carried out to investigate the saturated pool boiling. The flow fields of pool boiling in the open space presents a hierarchical, progressive, and conductive circulation. The thermal stratification is formed in open space. While the setting of restricted space forms the circulation convection mechanism between the large pool and the restricted space.