【作者】 张世一；

【导师】 金滔；

【作者基本信息】 浙江大学 ， 制冷及低温工程， 2011， 硕士

【摘要】 液氮等低温液体的饱和温度远低于室温,且其汽化潜热小,由于漏热等原因易发生沸腾。与水、甲醇等常规液体沸腾实验相比,低温液体沸腾实验由于设有为减少环境漏热影响而添加的杜瓦等设备使得装置结构更为复杂,观测过程也更加困难。虽然常温液体沸腾研究成果通常可以扩展到低温液体,但其中涉及一些经验参数的适用性还有待验证。核态沸腾中的高热流密度区域,温差小并伴随大换热系数,换热效果明显优于其他区域；而且由于该区域气泡大量产生、合并,流场和温度场极为复杂,目前研究还存在许多问题。在此背景下,本文对低温液体高热流密度沸腾开展了实验观测及传热模型改进的研究。本文首先对高热流密度沸腾时厚液层的蒸发过程进行分析,研究厚液层蒸发传热模型及其相关参数,发现常规液体的经验公式或其中的某些相关参数对液氮的适用性值得进行针对性的验证。在此基础上,根据液氮及测量对象的特点,在综合分析文献中的主要实验测量手段后,决定采用可视化技术和控制热流方法对沸腾表面进行观测,设计并搭建了实验台。通过实验获得传热模型中的高热流密度沸腾区起点、合体气泡脱离频率、临界热流密度等参数,讨论了沸腾表面材料和表面直径对临界热流密度及其对应过热度的影响规律。基于厚液层传热模型,结合本文的实验结果,推算出了厚液层初始厚度和活化核心密度,进而分析了表面材料对活化核心密度的影响方式。将实验中获得的各参数与厚液层传热模型的基本方程相结合,建立起较完整的液氮高热流密度区沸腾传热模型。采用该模型对液氮中高热流密度沸腾过程进行模拟分析,计算结果与实验值基本上都能够很好地吻合。更多还原

【Abstract】 Cryogenic liquid with a saturation temperature much lower than room temperature and small latent heat of vaporization is apt to boil in case of heat leak or internal heat resource. Compared with the liquids with near ambient temperature, such as water and methanol, the experiments on the boiling of cryogenic liquids, especially the observation procedure, are more complex and difficult due to the obligatory insulation structures for reducing heat leakage from the surroundings. Even though the results from common liquid’s boiling can be extended to the cryogenic liquids, there are many empirical parameters still requiring the verification of its applicability. The high heat flux boiling in nucleate boiling can achieve better heat transfer efficiency than other regions due to its large heat transfer coefficient with a small temperature difference. The flow and temperature field is very complex due to the production and coalescence of the large numbers of bubbles. There are still many problems to resolve. Therefore, the high heat flux boiling of the cryogenic liquids was scheduled to be studied in this work.Firstly, the evaporation process of macro-layer in the high heat flux boiling was analyzed, followed by an investigation of macro-layer evaporation model and the parameters in the model. It was found that the applicability of the physical model and empirical parameters for boiling process still needs further verification for the cryogenic liquid such as nitrogen. On this basis, according to the characteristics of the liquid nitrogen and the measured objects, the visualization and heat flux control methods were carried out to monitor the boiling phenomena on the heated surface, referring to the relevant literatures. The experimental program was then developed, and the experiment apparatus was designed and built. The parameters in the macro-layer evaporation model, such as the starting point of high heat flux boiling, the detachment frequencies of coalesced bubbles and the critical heat flux, was obtained from the measuring results. The influences of the boiling surface materials and the surface diameter on the critical heat flux and the corresponding superheat were discussed. The thickness of macro-layer and the active site density were then calculated from the equations of macro-layer evaporation model with the help of the experimental results. The influence of the boiling surface materials on the active site density was also analyzed. Finally, a boiling heat transfer model of liquid nitrogen at high heat flux was established, with which the calculation results can fit well with the results from experiments.更多还原