CFD as a tool for modelling membrane systems

Computational fluid dynamics (CFD) is a computer-based numerical method used to analyse systems that involve fluid flow and/or heat and mass transfer (Versteeg & Malalasekera, 2007). CFD bridges the two different approaches for solving engineering problems before the computer era, theoretical and experimental; it relies on mathematical models while being easy to adapt to almost any realistic condition (Anderson & Wendt, 1995). Another feature of CFD is its versatility, as it allows the analysis of systems for a variety of applications such as chemical reactions (Salehi et al., 2016), aerodynamics (Snel, 2003), dispersion of pollutants (Chu et al., 2005), blood flows (Byun & Rhee, 2004), among many others

Concentration polarisation minimisation in membrane channels through electro-osmotic mixing

Reverse osmosis (RO) promises to play an increasingly crucial role in water supply, especially via desalination. One of the major problems faced by RO technology is the decline in membrane performance due to concentration polarisation (CP) and fouling. CP increases the osmotic pressure gradient across the membrane, hence reducing the net driving pressure gradient. Moreover, CP increases the probability of fouling. An electro-osmosis technique is proposed in this thesis which has the potential to reduce CP because it induces the movement of fluid in the vicinity of membrane, thus improving mixing within the boundary layer and enhancing mass transfer. Computational Fluid Dynamics (CFD) is used to simulate steady and unsteady electroosmotic flow (EOF) in 2D unobstructed and obstructed channels. First, a mathematical simplification of EOF is developed that reduces the required computational load while retaining the model’s accuracy and physical meaning. It is shown that EOF can be mimicked using a slip velocity. The results from CFD are found to be in good agreement both with published data and with more rigorous simulation approaches. For steady EOF in unobstructed channels, the spatial variation in slip velocity is found to be the driver for mass transfer enhancement. For uniform-unsteady EOF in unobstructed channels, a sinusoidal time-varying electro-osmotic slip velocity has negligible effect on the time-averaged hydrodynamics and mass transfer, because the effect is nullified within the time oscillation period. Nevertheless, there are still benefits for using unsteady EOF for fouling reduction/prevention, as increases in slip velocity ii frequency and amplitude increase the maximum wall stress which is a proxy for fouling reduction. For unsteady EOF in spacer-filled channels, the simulation results show that an oscillating slip velocity has the potential to induce vortex shedding. This occurs when a resonant slip velocity frequency is used for Reynolds numbers near the transition from steady to unsteady flow

Simulation of reverse osmosis process: Novel approaches and development trends

Reverse osmosis is an essential technological separation process that has a large number of practical applications. The mathematical simulation is significant for designing and determining the most effective modes of membrane equipment operation and for a deep understanding of the processes in membrane units. This paper is an attempt at systematization and generalizing the results of the investigations dedicated to reverse osmosis simulation, which was published from 2011 to 2020. The main approaches to simulation were analyzed, and the scope of use of each of them was delineated. It was defined that computational fluid dynamics was the most used technique for reverse osmosis simulation; the intensive increase in using of molecular dynamics methods was pointed out. Since these two approaches provide the deepest insight into processes, it is likely that they will further be widely used for reverse osmosis simulations. At the same time, for the simulation of the membrane plant, it is reasonable to use the models that required the simplest solutions methods. The solution-diffusion model appears to be the most effective and flexible for these purposes. Therefore, this model was widely used in considering the period. The practical problems solved using each of the considered approaches were reviewed. Moreover, the software used for the solution of the mathematical models was regarded

Transport processes and instabilities induced by electric fields acting on fluidic interfaces

Electrohydrodynamics (EHD) describes the area of research, which studies the interactions of fluid motion and electric fields. In liquids with non-negligible conductivity, charged regions are confined to thin layers closest to boundaries, where EHD effects are most pronounced. In the present work, different phenomena that involve the actuation of fluidic interfaces by electric fields are studied. Electro-osmosis describes the fluid flow due to electric fields acting on charged regions close to the interface of a fluidic domain. When a liquid is deposited above a microstructured superhydrophobic surface, additional charges can be brought to the enclosed gas-liquid interface by placing a gate electrode below the surface. In this work, the production of a superhydrophobic surface with both micro- and nano-scales is described. In addition to inducing charges, a gate electrode exerts a force on the gas-liquid interface, pulling it in between the structures. Experimentally, the wetting state stability is characterized using reflection microscopy, revealing a continuous range of wetting states at dual-scale surfaces. By using non-constant electro-osmotic flow, complex height-averaged flow fields can be induced in a Hele-Shaw cell, which is characterized by a small distance between the parallel bounding walls compared to a characteristic lateral length scale. The governing equations for of the flow field are derived, accounting both for stationary and oscillatory electric fields. The electro-osmotic flow field is characterized above a single disc-shaped gate electrode in a microfluidic channel, using particle tracking velocimetry. In addition, using proof-of-principle experiments, the ability to create complex flow patterns is demonstrated. In order to use flow shaping in biochemical applications, a height-averaged transport model for a passive species is derived using a perturbation method, accounting for advection, diffusion and sample dispersion. The effects of sample dispersion are represented by a non-isotropic dispersion tensor. The reduced-order model shows good agreement to three-dimensional simulations, and potential applications are discussed. Electric fields lead to forces on fluidic interfaces, and in this work, two different EHD instabilities at an interface between a dielectric and a conducting liquid are investigated. Upon application of a spatially homogeneous, harmonically oscillating electric field, a resonant response of the interface can be observed above a critical amplitude. An experimental setup with a circular domain is used to observe the spatial structure of the instability, which is extracted from light-refraction at the liquid-liquid interface. The resulting dominant wavelengths and instability modes show good agreement to an analytical model. Furthermore, the role of the domain boundary is investigated. Upon applying a spatially inhomogeneous, but time-constant electric field, the interface exhibits EHD tip streaming above a critical voltage, emitting droplets into the dielectric phase. The presence of conducting droplets alters the spatial structure from a Taylor cone located centric below the pin electrode to a surface depression, where the interface moves away from the electrode and cones emerge from the rim. By experimentally characterizing a submerged electrospray and using additional numerical modeling, it is shown that the droplets induce a flow in the dielectric liquid, which is responsible for the change of the spatial structure of the instability

Flow-3D CFD model of bifurcated open channel flow: setup and validation

Bifurcation is a morphological feature present in most of fluvial systems; where a river splits into two channels, each bearing a portion of the flow and sediments. Extensive theoretical studies of river bifurcations were performed to understand the nature of flow patterns at such diversions. Nevertheless, the complexity of the flow structure in the bifurcated channel has resulted in various constraints on physical experimentation, so computational modelling is required to investigate the phenomenon. The advantages of computational modelling compared with experimental research (e.g. simple variable control, reduced cost, optimize design condition etc.) are widely known. The great advancement of computer technologies and the exponential increase in power, memory storage and affordability of high-speed machines in the early 20th century led to evolution and wide application of numerical fluid flow simulations, generally referred to as Computational Fluid Dynamics {CFD). In this study, the open-channel flume with a lateral channel established by Momplot et al (2017) is modelled in Flow-3D. The original investigation on divided flow of equal widths as simulated in ANSYS Fluent and validated with velocity measurements

Advanced Topics in Mass Transfer

This book introduces a number of selected advanced topics in mass transfer phenomenon and covers its theoretical, numerical, modeling and experimental aspects. The 26 chapters of this book are divided into five parts. The first is devoted to the study of some problems of mass transfer in microchannels, turbulence, waves and plasma, while chapters regarding mass transfer with hydro-, magnetohydro- and electro- dynamics are collected in the second part. The third part deals with mass transfer in food, such as rice, cheese, fruits and vegetables, and the fourth focuses on mass transfer in some large-scale applications such as geomorphologic studies. The last part introduces several issues of combined heat and mass transfer phenomena. The book can be considered as a rich reference for researchers and engineers working in the field of mass transfer and its related topics