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Droplet entrainment within the evaporator to the suitable volume-filling ratio of a vertical two-phase closed thermosyphon
International Journal of Thermal Sciences ( IF 4.5 ) Pub Date : 2021-01-01 , DOI: 10.1016/j.ijthermalsci.2020.106576
T. Zhang , Z.W. Yan , L.Y. Wang , W.J. Zheng , Y.H. Su , R.C. Zheng

Abstract The present study on vertical two-phase closed thermosyphon (TPCT) is aimed to determine the suitable volume-filling ratio, as a function of geometries, heat flux and vapor temperature, in order to avoid the potential local dryout in the evaporator section and hence to maximize the performance. In the study, the droplet entrainment, which is caused by the internal counter flow during the nucleate boiling within the falling liquid film in the evaporator section, is introduced to improve the existing TPCT model, so a comprehensive model with considering all three heat transfer regimes in the evaporator is further established. The suggested lower and upper limits of volume-filling ratio (CFR and EFR) are then determined by utilizing the criterias for local dryout, flooding limit and boiling limit. Furthermore, the effects of geometries, heat flux and vapor temperature on the range of volume-filling ratio are analyzed in details, a simplified correlation of CFR is then proposed based on the numerical modeling results. Particularly, the predictions of the distribution of falling film thickness and onset of flooding are validated with the published experimental data and other numerical simulation results. It is also found that the droplet entrainment significantly influences the thickness and distribution of the falling film. Increasing vapor temperature and inner diameter and decreasing the evaporator length significantly enlarge the volume-filling ratio range (between CFR and EFR), operation envelope (corresponding to boiling limit and flooding limit) of TPCT. Increasing the condenser length increases the volume-filling ratio range and operation envelope to a small extent. The influence mechanisms of the optimum filling ratio by the heat flux and geometries are complicated; however, a correlation of CFR can be obtained in a good agreement with the numerical modeling results within ± 30% deviation for the whole limited scope.

中文翻译:

蒸发器内的液滴夹带与垂直两相封闭热虹吸管的合适体积填充比

摘要 目前对立式两相闭式热虹吸管 (TPCT) 的研究旨在确定合适的体积填充比,作为几何形状、热通量和蒸汽温度的函数,以避免蒸发器部分潜在的局部干燥和从而最大限度地提高性能。在研究中,在蒸发器部分的下降液膜内,在核沸腾期间由内部逆流引起的液滴夹带被引入以改进现有的 TPCT 模型,因此一个综合模型考虑了所有三种传热方式在蒸发器中进一步建立。然后通过利用局部干燥、溢流极限和沸腾极限的标准来确定建议的体积填充比(CFR 和 EFR)的下限和上限。此外,几何形状的影响,详细分析了容积-填充比范围内的热通量和蒸汽温度,然后基于数值模拟结果提出了CFR的简化相关性。特别是,降膜厚度分布和洪水开始的预测得到了已发表的实验数据和其他数值模拟结果的验证。还发现液滴夹带显着影响降膜的厚度和分布。提高蒸汽温度和内径,减小蒸发器长度,TPCT 的容积填充比范围(CFR 和 EFR 之间)、操作包络线(对应沸腾极限和溢流极限)显着扩大。增加冷凝器长度会在较小程度上增加容积填充比范围和操作范围。热通量和几何形状对最佳填充率的影响机制复杂;然而,在整个有限范围内,可以在 ± 30% 偏差内获得与数值建模结果非常一致的 CFR 相关性。
更新日期:2021-01-01
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