With the increasing use of Computational Fluid Dynamics to investigate multiphase flow scenarios, modelling surface tension effects has been a topic of active research. A well known associated problem is the generation of spurious velocities (or currents), arising due to inaccuracies in calculations of the surface tension force. These spurious currents cause nonphysical flows which can adversely affect the predictive capability of these simulations. In this paper, we implement the Continuum Surface Force (CSF), Smoothed CSF and Sharp Surface Force (SSF) models in OpenFOAM. The models were validated for various multiphase flow scenarios for Capillary numbers of 10 − 3 –10. All the surface tension models provide reasonable agreement with benchmarking data for rising bubble simulations. Both CSF and SSF models successfully predicted the capillary rise between two parallel plates, but Smoothed CSF could not provide reliable results. The evolution of spurious current were studied for millimetre-sized stationary bubbles. The results shows that SSF and CSF models generate the least and most spurious currents, respectively. We also show that maximum time step, mesh resolution and the under-relaxation factor used in the simulations affect the magnitude of spurious currents.
The resulting electrical potential of a reverse electrodialysis is reduced both due to ohmic and non-ohmic resistances. The nonohmic resistance is mainly controlled by concentration polarization which is a considerable challenge in a membrane based processes and is a result of accumulation or depletion of specific ions adjacent to the ionic exchange membranes compared to the bulk solution. This phenomenon effectively reduces the driving force across the membrane, hence affects the performance of the process. The present work aims to present a numerical model based on coupled Navier-Stokes and Nernst-Planck equations to predict flow and pressure drop as well as concentration and electrical potential for optimizing the performance of the system, using OpenFOAM. The model is demonstrated in a flat and spacerfilled channel for different Reynolds number. The results reveal that reducing the Reynolds number and introducing flow promoters such as cylindrical corrugations in a dilute solution channel reduces the resistivity of a RED unit cell, hence increasing the produced electrical potential. However, introducing cylindrical corrugations in a concentrated solution channel has an adverse effect on the resistivity, leading to an unfavorable resistivity increment.
A Helmholtz free energy description of the four nucleation mechanisms used to explain the bubble nucleation in electrochemical systems is presented. The mechanisms are compared based on the nucleation energy barrier and critical nuclei radius. The theoretical analysis sheds light on the effect of parameters like contact angle on the electrode surface and pre-existing gas bubbles on nucleation energy barrier. A free energy based description of surface tension (planar interface) is also obtained from the thermodynamic framework.
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