CFD Analysis of Falling Film Wettability in MED Desalination plants
Authors: Tahir Furqan , Mabrouk Abdelnasser , Koc Muammer 
Published: June 2019

In Gulf Cooperation Council (GCC) countries, 56% of the market is captured by thermal based desalination (both MED and MSF), while in Qatar 91% is based on thermal desalination. Among the thermal desalination technologies, Multi Effect Desalination (MED) operates at lower specific power consumption and is considered more energy-efficient than the Multistage Flash (MSF) distillation. The lower energy consumption of MED plants is due to high overall heat transfer coefficient and low pumping power. The most common problem in MED evaporators is fouling of tubes, which reduces heat transfer and performance. Horizontal falling film evaporators are the most common evaporator type used in MED; in which liquid film is maintained outside the tube and part of it evaporates producing vapors. Improperly maintained liquid film leads to fouling i.e. formation of salts on the surface of tubes, which affects heat transfer properties of the surface. So in order to minimize fouling, tubes should be completely wet; especially column based on second row in triangular pitch configuration and bottom row tubes, which receive lesser mass flux. Wettability study of falling film evaporator in 3-D domain is carried out using commercial CFD tool i.e. Ansys fluent v18.0. A column of three tubes having OD 19.05 mm, vertical pitch of 24.7 mm and 20 mm section is considered in this study. Brine inlet has an opening of 1 mm ×  1 mm and mass flux (Wetting rate Γ) is varied from 0.03 kg/m-s to 0.2 kg/m-s. This mass flux variation include all flow regimes namely droplet, column and sheet modes. For multiphase flow, volume of fluid (VOF) model is selected, as this model is used where two phases are separated in the flow regime. Wall adhesion model is incorporated for film and tube contact. Conservation of mass, momentum and energy equations were solved in a transient manner via SIMPLE algorithm for pressure-velocity coupling. All equations were discretized in 2nd order and residual for convergence criteria was set to 10-6 for better accuracy. The time step was taken 5 × 10-6 s, to ensure that the global courant number is below 0.25 and iterations for each time step were 40. Initially mesh dependency check was carried out and then the model was validated against available data in the literature. CFD results show that at lower mass flux i.e. 0.03 kg/m-s, tubes are not completely wet and there are dry patches, which are more susceptible to fouling. As the mass flux is increased flow regime changes from droplet to column and then to sheet mode. Increasing mass flux diminishes dry patches problem but also at higher mass flux, film is detached from tube surface causing lower portion of tube to remain dry. There is no fixed critical value for flow regime transformation as it depends on many factors like tube Outer Diameter (O.D.), tube pitch, nozzle type, nozzle diameter and solution properties etc. Moreover, the effect of surface roughness was also taken into consideration and it was concluded that more roughness results in better film stability and enhanced wettability.