TY - JOUR
T1 - Impact of surface tension and viscosity on falling film thickness in multi-effect desalination (MED) horizontal tube evaporator
AU - Tahir, Furqan
AU - Mabrouk, Abdelnasser
AU - Koç, Muammer
N1 - Publisher Copyright:
© 2019 Elsevier Masson SAS
PY - 2020/4
Y1 - 2020/4
N2 - 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.
AB - 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.
KW - CFD
KW - Falling film thickness
KW - MED evaporator
KW - Surface tension
KW - Viscosity
UR - http://www.scopus.com/inward/record.url?scp=85076673123&partnerID=8YFLogxK
U2 - 10.1016/j.ijthermalsci.2019.106235
DO - 10.1016/j.ijthermalsci.2019.106235
M3 - Article
AN - SCOPUS:85076673123
SN - 1290-0729
VL - 150
JO - International Journal of Thermal Sciences
JF - International Journal of Thermal Sciences
M1 - 106235
ER -