A numerical investigation into a long tube falling film evaporator in a zero liquid discharge (ZLD) process

Abstract

• The evaporation of a two-phase turbulent falling film was investigated. • The oscillating structure of the falling film flow was studied. • Two new Local and Oscillating Reynolds Numbers were introduced. • The oscillations frequency remain almost constant in the fully developed region. • Care should be taken to select Re Γ while designing falling film vertical tubes. Brine concentrators of the falling film type are essential components of zero liquid discharge (ZLD) processes. In the present work, the evaporation of a two-phase turbulent falling film in a long vertical tube has been investigated. Volume-Of-Fluid (VOF) method along with the PLIC interface reconstruction technique is utilized to simulate the two-phase flow and free surface geometry capturing. The k-ω SST turbulence model and the Lee-Tanasawa condensation-evaporation model have been used to model turbulence and mass transfer, respectively. Furthermore, the oscillating structure of the falling film flow is studied, and two new local and oscillating Reynolds numbers are introduced. Results showed that the falling film inside the tube has an oscillating nature and, after passing through an entrance and a developing region, the oscillations frequency remain almost constant. Additionally, the oscillating Reynolds number based on the local wavelength establishes the best correlation with the falling film evaporation rate.