真空条件下水平单管降膜蒸发液膜厚度及流场模拟研究
发布时间：2019-05-09 00:52
【摘要】：水平管降膜蒸发器和传统的换热器相比,具有传热温差小、传热系数高、可低温传热等优点,广泛应用在海水淡化、食品加工、制冷工程、石油冶炼和化学工程等众多领域。本文研究的是真空状态下水平单管降膜蒸发传热传质过程,该过程中管内是蒸汽冷凝过程,管外是降膜蒸发过程。管内管外都属于气液两相流范畴,且都存在相变潜热交换,情况较为复杂。建立水平管降膜蒸发冷凝管内外换热模型,对管内蒸汽冷凝换热热阻、管壁导热热阻、污垢热阻和管外降膜蒸发换热热阻进行了分析计算,分析计算出在四个热阻中管外降膜蒸发换热热阻占总热阻比重最大,且是管内的两倍左右,因此研究管外降膜蒸发换热机理对于水平管降膜蒸发器的换热性能的提高具有重要意义。建立了水平管降膜蒸发器管外物理模型和单元仿真模型,加载了单元仿真模型的传热边界条件,利用VOF模型以及ANSYS FLUENT自带蒸发冷凝模型对不同喷淋密度和传热温差下的管外降膜蒸发流动传热传质情况进行了数值模拟,得到不同喷淋密度下的管外降膜蒸发传热系数,与相同条件,同种工况下的实验数据进行对比,发现实验结果与数值模拟结果相差在15%之内,验证了所建立的数学模型以及VOF模型加蒸发冷凝模型模拟管外降膜蒸发传热传质的准确性。模拟结果显示随着喷淋密度增大,传热温差增大,喷淋密度由0.013 s)kg/(m?增大到0.058 s)kg/(m?,管外降膜蒸发换热系数由716 2k)W/(m?增大到2698k)W/(m2?,增大了4倍左右。对真空条件下水平管外降膜蒸发过程中液膜厚度以及流场流动换热特性进行了数值模拟及分析,具体包括:液膜内液体x方向速度分布规律、液膜内液体y方向速度分布规律以及水平管圆周不同位置液膜厚度分布规律,还研究了不同喷淋量下的液膜厚度分布。研究发现,在最开始液体与管壁发生撞击,液膜发生堆积所以液膜厚度较厚,然后在θ取5o~30o范围内时,液膜在管壁表面铺展开来,液膜厚度逐渐变薄,且降低幅度较大;然后在θ取30o~90o范围内时由于重力以及管壁表面剪切力作用,液膜厚度逐渐缓慢减小,且液膜厚度达到最小值;θ为90o~155o时液膜厚度有较小幅度的增大;θ为155o~180o时由于管壁两侧液膜合流相互撞击以及液膜脱离管壁,液膜在此范围内出现堆积,液膜厚度急速增大。因此,喷淋密度为0.4 kg/(m·s)、蒸发温度为80℃、管壁温度为85℃时水平单管降膜蒸发液膜最薄位置出现在90°,液膜最薄为0.001m,液膜最厚的位置出现在θ为180°的位置为0.006m,是最薄液膜厚度的6倍。随着喷淋量增加,液膜厚度增厚发生在沿管壁圆周方向,但是不同区域厚度增加幅度会有不同,0o~90o区间厚度增加比90o~180o更为明显,产生这种现象可能是由于喷淋量增加,增强了换热,强化了传质过程的发生。
[Abstract]:Compared with the traditional heat exchanger, the horizontal tube falling film evaporator has the advantages of small heat transfer temperature difference, high heat transfer coefficient and low temperature heat transfer. It is widely used in many fields such as seawater desalination, food processing, refrigeration engineering, petroleum smelting and chemical engineering. In this paper, the heat and mass transfer process of falling film evaporation of horizontal single tube in vacuum state is studied, in which steam condensation process is in the tube and falling film evaporation is in the outer part of the tube. Both inside and outside the tube belong to the category of gas-liquid two-phase flow, and there is phase transition latent heat exchange, which is more complex. The internal and external heat transfer model of falling film evaporation condensing tube in horizontal tube is established. The heat transfer resistance of steam condensation, heat conduction of pipe wall, fouling heat resistance and outside falling film evaporation heat transfer resistance of tube are analyzed and calculated. It is analyzed and calculated that the heat transfer resistance of falling film evaporation outside the tube accounts for the largest proportion of the total thermal resistance in the four thermal resistances, and is about twice as much as that in the tube. Therefore, it is of great significance to study the heat transfer mechanism of falling film evaporation outside the tube for improving the heat transfer performance of horizontal tube falling film evaporator. The physical model and unit simulation model of horizontal tube falling film evaporator are established, and the heat transfer boundary conditions of the unit simulation model are loaded. The heat and mass transfer of falling film evaporation flow under different spray density and heat transfer temperature difference was numerically simulated by using VOF model and ANSYS FLUENT evaporation condensation model, and the heat transfer coefficient of falling film evaporation outside tube under different spray density was obtained. Compared with the experimental data under the same conditions and the same working conditions, it is found that the difference between the experimental results and the numerical simulation results is less than 15%. The accuracy of the established mathematical model and VOF model plus evaporative condensation model to simulate the heat and mass transfer of falling film evaporation outside the tube is verified. The simulation results show that the heat transfer temperature difference increases with the increase of spray density, and the spray density is from 0.013 s) kg/ (m? When it increases to 0.058 s) kg/ (m 鈮，
本文编号：2472354
[Abstract]:Compared with the traditional heat exchanger, the horizontal tube falling film evaporator has the advantages of small heat transfer temperature difference, high heat transfer coefficient and low temperature heat transfer. It is widely used in many fields such as seawater desalination, food processing, refrigeration engineering, petroleum smelting and chemical engineering. In this paper, the heat and mass transfer process of falling film evaporation of horizontal single tube in vacuum state is studied, in which steam condensation process is in the tube and falling film evaporation is in the outer part of the tube. Both inside and outside the tube belong to the category of gas-liquid two-phase flow, and there is phase transition latent heat exchange, which is more complex. The internal and external heat transfer model of falling film evaporation condensing tube in horizontal tube is established. The heat transfer resistance of steam condensation, heat conduction of pipe wall, fouling heat resistance and outside falling film evaporation heat transfer resistance of tube are analyzed and calculated. It is analyzed and calculated that the heat transfer resistance of falling film evaporation outside the tube accounts for the largest proportion of the total thermal resistance in the four thermal resistances, and is about twice as much as that in the tube. Therefore, it is of great significance to study the heat transfer mechanism of falling film evaporation outside the tube for improving the heat transfer performance of horizontal tube falling film evaporator. The physical model and unit simulation model of horizontal tube falling film evaporator are established, and the heat transfer boundary conditions of the unit simulation model are loaded. The heat and mass transfer of falling film evaporation flow under different spray density and heat transfer temperature difference was numerically simulated by using VOF model and ANSYS FLUENT evaporation condensation model, and the heat transfer coefficient of falling film evaporation outside tube under different spray density was obtained. Compared with the experimental data under the same conditions and the same working conditions, it is found that the difference between the experimental results and the numerical simulation results is less than 15%. The accuracy of the established mathematical model and VOF model plus evaporative condensation model to simulate the heat and mass transfer of falling film evaporation outside the tube is verified. The simulation results show that the heat transfer temperature difference increases with the increase of spray density, and the spray density is from 0.013 s) kg/ (m? When it increases to 0.058 s) kg/ (m 鈮，
本文编号：2472354
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