【作者】 董彬；

【导师】 赵军； 王世学；

【作者基本信息】 天津大学 ， 热能工程， 2012， 博士

【摘要】 蒸气的凝结现象在自然界和人类生活中普遍存在，蒸汽凝结传热也是核动力、蒸汽动力、石油化工和制冷空调等领域中所广泛遇到的相变传热过程。Marangoni凝结起因于混合蒸气凝结时液相的局部浓度差，这种凝结不同于膜状和珠状凝结，并在特定的工况条件下能获得数倍于纯水蒸气凝结传热系数的强化换热效果。本文以混合蒸气的Marangoni凝结传热过程作为研究对象，通过实验研究和理论分析，对Marangoni凝结传热特性曲线、凝结液膜形态特征及其不稳定性、液相气相热阻的变化特征以及不凝性气体对传热的影响规律等进行了研究。本文搭建了具有高气密性的凝结传热实验系统，可以实现在较大过冷度范围内凝结传热特性的测量和凝结过程的高速动态可视化观测与记录。对氨-水混合蒸气的凝结传热特性曲线的测定和分析表明：①凝结传热特性随表面过冷度显示出有峰值的非线性变化规律；②存在一个最优氨蒸气浓度（0.38%左右）使传热系数和热流密度达到峰值。这在以往的文献中未见报道；③在特定的过冷度范围内，非常微量的氨的添加可使水蒸气的凝结传热大大强化；④传热系数随蒸气流速的增大而增大，但其峰值点所对应的过冷度与蒸气流速无关。Marangoni凝结发生时，传热系数和凝结形态与混合蒸气浓度及凝结表面过冷度有紧密联系。对凝结形态图片的测量和统计结果显示液珠尺寸是一个不连续的变量，液珠的尺度分布与Rosim-Rammler分布函数符合较好。本文通过引入液膜覆盖率的概念，解释了Marangoni凝结传热特性随过冷度的非线性变化规律。实验表明，酒精-水二元体系的凝结传热系数要远远大于氨-水二元体系。本文通过对凝结液膜不稳定性及液相、气相热阻变化规律的研究发现，凝结液膜的不稳定性和气相扩散热阻共同决定了Marangoni凝结传热特性。通过定量计算，解释了不同二元体系的差异对浓度差Marangoni凝结传热效果的影响程度，结合可视化的实验结果，对浓度差Marangoni凝结传热的机理进行了分析。此外，还进行了不凝性气体对Marangoni凝结传热的影响规律和作用机制的实验研究和分析。结果表明，不凝气含量较低时，很少不凝性气体的增加就可造成凝结传热系数的大幅下降。在过冷度范围不同的区域，不凝性气体对凝结传热的影响规律不同，在传热系数的峰值点处，不凝性气体含量的增加，引起传热系数的下降最为明显。更多还原

【Abstract】 Steam condensationexists generally in nature and human’s life, and steamcondensation heat transfer is the common phase change heat transfer process in thefields of nuclear power, steam power, petrochemical, refrigeration and airconditioning. The so-called Marangoni condensation is caused by the localconcentration gradient. Marangoni condensation is different from the filmcondensation and dropwise condensation and the heat transfer may be greatlyenhanced in certain working conditions. In this paper, the characteristic curves ofMarangoni condensation heat transfer, the shape and unstable feature of condensatefilm, thethermal resistance of condensate film and vapor film and the effect of thenon-condensable gas on heat transfer were investigated.A condensation heat transfer experimental system with high air tightness wasestablished to obtain the Marangoni condensation heat transfer characteristics and thecondensate images. The characteristic curves of ammonia-water Marangonicondensation heat transfer showed a nonlinear variation along with the surfacesubcooling and had peak values. There was an optimal ammonia concentration rangenear0.38%that heat transfer coefficient (HTC) and heat flux reached maximu. Incertain surface subcooling range, the addition of a vary little amount of ammonia tothe steam could greatly enhance the condensation heat transfer. The HTC increasedalong with the vapor velocity and the surface subcooling corresponding to the peakvalues of HTC had nothing to do with the vapor velocity.The Marangoni condensation HTC and condensate mode were influenced by theconcentration and surface subcooling. The measurement and calculation on the dropsize of the ethanol-water Marangoni condensation revealed that the drop size was adiscontinuous variable and the drop size distribution showed a good agreement withthe Rosin-Rammler model. The nonlinear feature of the Marangoni condensation heattransfer characteristic curves was explained from the point of view of the liquid filmcoverage.The experimental results proofed that he HTC of ethanol-water binary systemwere higher than that of the ammonia-water binary system. It could be attributed tothe difference of condensate instability and the diffusion thermal resistance between the two binary systems. Through the quantitative calculation of condensate instabilityand diffusion thermal resistance of the two binary systems, the mechanism of theMarangoni condensation caused by the concentration gradient was analysised.The effect of non-condensable gas on the characteristic of Marangonicondensation heat transfer was studied. The results showed that the HTC couldgreatly decreased by a little bit increase of the non-condensable gas in the range of thelow concentration of non-condensable gas. The influence of the non-condensable gason the HTC was very obvious in the peak value range of HTC.更多还原