水平管外蒸汽在不凝气体逸流中的冷凝和扩散流动

The presence of non-condensable gases is one of the most important reasons for the decrease of vapor condensation heat transfer efficiency. At present, a measure frequently adopted is to remove the non-condensable gases from the vapor in the condensation process. Under the effect of the slip velocity of the non-condensable gases drainage which is perpendicular to the surface of liquid film, the non-condensable gases weaken the vapor diffusion flow, reducing the condensation efficiency. By the means of experimental observations and numerical simulations for condensation over the horizontal tubes, the present project attempts to research the law of both vapor condensation and diffusion flow within the non-condensable gases drainage，understand the nonlinear evolution that the non-condensable gas drainage induces the Kelvin-Helmholtz instability, determine the correlation between the flow pattern transition point of the condensation heat transfer and the velocity of the non-condensable gases drainage, so as to achieve the condensation heat transfer enhancement by means of the non-condensable gases drainage disturbance. By setting up the micropore at the retention area of non-condensable gases, the venturi effect is formed to achieve the higher amount of the non-condensable gases drainage. In order to explore in what way the slip direction effects the amount of the non-condensable gases drainage, the molecular dynamics simulation is employed to analyze the process of vapor condensation within the non-condensable gases drainage. In addition, Two-phase boundary layer method is used to numerically analyze the synergistic function of the non-condensable gases tangential escaping out of the liquid film surface and the vapor diffusion flow on promoting vapor condensation so as to reveal the external conditions needed for the non-condensable gases drainage to promote vapor condensation. The obtained theoretical results can be verified by the experimental results from the experimental installation equipped with the direction-adjustable non-condensable gases drainage for vapor condensation over the horizontal tubes.

不凝气体的存在是蒸汽冷凝效率下降的重要原因之一，目前常采用冷凝中放空措施去除蒸汽中的不凝气体，由于存在垂直液膜表面的不凝气体逸流流速，不凝气体会减弱蒸汽扩散流动，降低冷凝效率。本项目拟采用实验观测和数值模拟的方法研究水平管外蒸汽在不凝气体逸流中的冷凝和扩散流动，理解不凝气体逸流诱导Kelvin-Helmholtz不稳定性的非线性演化，确定冷凝换热流态转变点和不凝气体逸流流速之间的关联性，实现不凝气体逸流扰动强化冷凝换热的目的。本项目拟采用在不凝气体滞留区开设微孔形成文丘里效应来获得较高不凝气体逸流量；采用分子动力学模拟含不凝气体逸流的蒸汽冷凝，探索逸流方向影响不凝气体逸流量的内因；采用两相边界层法数值分析不凝气体切向逸出液膜表面和蒸汽扩散流动的协同作用对蒸汽冷凝的促进机制，揭示不凝气体逸流促进蒸汽冷凝扩散流动的外部条件，所获理论结果采用不凝气体逸流方向可调的水平管外冷凝换热实验进行验证。

存在不凝气体是膜状冷凝衰减的重要原因之一，现有放空冷凝器中不凝气体的方法存在着减弱蒸汽流动并增加蒸汽流失的缺点。因此，不凝气体逸流和积聚对冷凝相变作用机理的研究尚属于薄弱领域。本项目基于实验研究、两相边界层法和场协同理论，围绕水平管外蒸汽在不凝气体逸流中的冷凝、不凝气体逸流对蒸汽冷凝相变三场协同的作用和不凝气体逸流改善冷凝的应用展开研究，结果发现，在圆管周角θ=135º~225º处，界面不凝气体浓度为0.75~0.87，冷凝换热系数不足300W.m2.℃，速度场和温度场间协同角为73º~89º，气液界面剪切力较低，气膜层较厚，气膜层热阻占总热阻之比几乎为1，是不凝气体的最佳逸流方向。同时，基于文丘里管内流体流动的实验和数值分析，给出不凝气体引流口的最佳几何和边界条件。由此得出结论，不凝气体逸流通过降低冷凝空间压力、减少不凝气体在液膜表面的吸附量和扩大湍流膜状冷凝区域来强化冷凝换热。在不抽真空重力热管冷凝段下方增设气库使不凝气体逸入其中可有效改善蒸汽冷凝，在1.8g/s蒸汽流量、40%的空气平均浓度和0.36Mpa的工作压力下，蒸汽凝结长度仅增加了24%，所获结果为采用不凝气体逸流增强冷凝提供了科学依据。此外，对不凝气体诱导界面KH波的演变和分子动力学对蒸汽在不凝气体中冷凝的模拟做了初步探索。

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