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Impact of surface tension and viscosity on falling film thickness in multi-effect desalination (MED) horizontal tube evaporator
International Journal of Thermal Sciences ( IF 4.5 ) Pub Date : 2020-04-01 , DOI: 10.1016/j.ijthermalsci.2019.106235
Furqan Tahir , Abdelnasser Mabrouk , Muammer Koç

Abstract Falling film evaporators are extensively used in many applications because of their higher heat and mass transfer coefficient over low temperature difference. Falling film thickness, which depends on liquid spray density and thermophysical properties, is an important parameter in determining heat and mass transfer performance as it represents thermal resistance. In multi-effect desalination (MED) evaporators, the thermophysical properties such as surface tension and viscosity change due to operating temperature differences, i.e. higher in the first evaporator/effect and lower in the last evaporator/effect. The surface tension variation with temperature is often neglected in most of the film thickness characterization and modeling studies. Therefore, it is needed to analyze surface tension effects distinctly on film thickness. In this study, a 2D CFD model was developed by implementing volume of fluid (VOF) multiphase model to distinguish liquid and gas phases. The effects of viscosity and surface tension were analyzed separately, and it was found that with constant surface tension, viscosity effects account for 66.1% variation in film thickness for operating temperature range of 85 °C–5 °C. However accounting both viscosity and surface tension dependence, the results showed 72% increment in the film thickness. In addition, CFD results exhibited lower conduction thermal resistance of 0.2 m2 K/W at 85 °C against 0.4 m2 K/W at 5 °C, which reflects better evaporator performance at higher temperature. Hence, more focus and detailed analysis are recommended given in increasing the first effect temperature from 65 °C to 85 °C as it would improve thermal performance due to lower thermal resistance rather than decreasing last effect temperature from 40 °C to 5 °C as the thermal resistance would increase.

中文翻译:

多效淡化 (MED) 水平管蒸发器中表面张力和粘度对降膜厚度的影响

摘要 降膜蒸发器由于其在低温差下具有较高的传热和传质系数而被广泛应用于许多应用领域。降膜厚度取决于液体喷雾密度和热物理特性,是决定传热和传质性能的重要参数,因为它代表热阻。在多效脱盐 (MED) 蒸发器中,表面张力和粘度等热物理特性会因操作温度差异而发生变化,即在第一个蒸发器/效中较高,而在最后一个蒸发器/效中较低。在大多数薄膜厚度表征和建模研究中,表面张力随温度的变化通常被忽略。因此,需要明确分析表面张力对膜厚的影响。在本研究中,通过实施流体体积 (VOF) 多相模型来开发二维 CFD 模型以区分液相和气相。分别分析了粘度和表面张力的影响,发现在表面张力恒定的情况下,在 85°C-5°C 的工作温度范围内,粘度影响占薄膜厚度变化的 66.1%。然而,考虑到粘度和表面张力的相关性,结果显示薄膜厚度增加了 72%。此外,CFD 结果显示出较低的传导热阻,在 85 °C 时为 0.2 m2 K/W,而在 5 °C 时为 0.4 m2 K/W,这反映了更高温度下更好的蒸发器性能。因此,
更新日期:2020-04-01
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