Channel Geometry

Knowing how wall shear stress develops at the membrane surface is extremely useful when trying to reduce concentration polarization and fouling. Newly developed as well as manufactured ceramic membranes exhibit various channel geometries (cylindrical, square, triangular, etc). Mass transport characteristics depend on the geometry that conditions hydrodynamic conditions. The goal of this work is to study the influence of the channel geometry on the wall shear stress for various operating parameters (tangential velocity, transmembrane pressure…). Numerical simulations are performed for various inlet velocities for different channel geometries. The wall shear stress along the channel perimeter as a function of the shape and the cross section of the channel are studied. The influence of the geometry on the membrane performances is also studied. The simulated shear stress is employed to correlate experimental results. The results of this comparison show that mass transfer resistance depends on the wall shear stress alone, regardless of the flow rate, the shape or section of the channels.

This paper describes the design and fabrication of a polyjet-based three-dimensional (3D)-printed fluidic device where poly(dimethylsiloxane) (PDMS) or polystyrene (PS) were used to coat the sides of a fluidic channel within the device to promote adhesion of an immobilized cell layer. The device was designed using computer-aided design software and converted into an .STL file prior to printing. The rigid, transparent material used in the printing process provides an optically transparent path to visualize endothelial cell adherence and supports integration of removable electrodes for electrical cell lysis in a specified portion of the channel (1 mm width × 0.8 mm height × 2 mm length). Through manipulation of channel geometry, a low-voltage power source (500 V max) was used to selectively lyse adhered endothelial cells in a tapered region of the channel. Cell viability was maintained on the device over a 5 day period (98% viable), though cell coverage decreased after day 4 with static media delivery. Optimal lysis potentials were obtained for the two fabricated device geometries, and selective cell clearance was achieved with cell lysis efficiencies of 94 and 96%. The bottleneck of unknown surface properties from proprietary resin use in fabricating 3D-printed materials is overcome through techniques to incorporate PDMS and PS.

This volume of the annual hydrologic data report of Kentucky is one of a series of annual reports that document hydrologic data gathered from the U.S. Geological Survey's surface-and groundwater data-collection networks in each State, Puerto Rico, and the Trust Territories. These records of streamflow, groundwater levels, and water quality provide the hydrologic information needed by State, local, and Federal agencies and the private sector for developing and managing our Nation's land and water resources.

One of the major problems that will affect the performance of a membrane separation process is the occurrence of concentration polarisation phenomenon and should be minimised whenever possible. It was found in previous literatures that the spacers in a spiral-wound membrane (SWM) module can help to reduce this phenomenon but at the expense of higher energy consumption. In this work, the concentration polarisation behaviour in a spacer-filled membrane channel were studied using computational fluid dynamics (CFD) approach. A total of twelve spacer shapes were studied and it shows that spacer shapes can affect the concentration polarisation phenomenon in a membrane channel. The results show that modification of square spacer shapes can provide a good balance between concentration polarisation control and power consumption.

The impact of construction of dams and reservoirs on alluvial rivers extends both upstream and downstream of the dam. Downstream of dams, both the water and sediment supplies can be altered leading to adjustments in the river channel geometry and ensuing changes in riparian and aquatic habitats. The wealth of pre and post-regulation data on the Middle Rio Grande, New Mexico, provides an excellent case study of river regulation, channel adjustments, and restoration efforts. Cochiti Dam was constructed on the main stem of the Rio Grande in 1973 for flood control and sediment retention. Prior to dam construction, the Rio Grande was a wide, sandy braided river. Following dam construction, the channel bed degraded and coarsened to gravel size, and the planform shifted to a more meandering pattern. Ecological implications of the geomorphic changes include detachment of the river from the floodplain, reduced recruitment of riparian cottonwoods, encroachment of non-native saltcedar and Russian olive into the floodplain and degraded aquatic habitat for the Rio Grande silvery minnow. Recent restoration strategies include removal of non-native riparian vegetation, mechanical lowering of floodplain areas, and channel widening.

Knowing how wall shear stress develops at the membrane surface is extremely useful when trying to reduce concentration polarization and fouling. Newly developed as well as manufactured ceramic membranes exhibit various channel geometries (cylindrical, square, triangular, etc). Mass transport characteristics depend on the geometry that conditions hydrodynamic conditions. The goal of this work is to study the influence of the channel geometry on the wall shear stress for various operating parameters (tangential velocity, transmembrane pressure…). Numerical simulations are performed for various inlet velocities for different channel geometries. The wall shear stress along the channel perimeter as a function of the shape and the cross section of the channel are studied. The influence of the geometry on the membrane performances is also studied. The simulated shear stress is employed to correlate experimental results. The results of this comparison show that mass transfer resistance depends on the wall shear stress alone, regardless of the flow rate, the shape or section of the channels.

This paper presents a finite element, three dimensional numerical model of flow in porous ceramic
ultrafiltration membrane system together with experimental validation. The modelling is based on the computational fluid
dynamics (CFD) technique. Major difficulty that arises during CFD modelling is appropriate and precise description of the
porous media in terms of Navier-Stokes equation. Based on pressure-flow experimental measurements we present the
approach for calculating the essential components of porous media flow resistance which are necessary for proper
description of membrane process. The detailed description of a membrane was applied which accounts for support and
membrane layer respectively. Moreover own procedure applied is presented, that is written using C language, for
calculation of flow parameters. The model presented is in very good accordance with experimental results.

The increasing adoption of ultra-low pressure (ULP) membrane systems for drinking water treatment in small rural communities is currently hindered by a limited number of studies on module design. Detailed knowledge on both intrinsic membrane transport properties and fluid hydrodynamics within the module is essential in understanding ULP performance prediction, mass transfer analysis for scaling-up between lab-scale and industrial scale research. In comparison to hollow fiber membranes, flat sheet membranes present certain advantages such as simple manufacture, sheet replacement for cleaning, moderate packing density and low to moderate energy usage. In the present case study, a numerical model using computational fluid dynamics (CFD) of a novel custom flat sheet membrane module has been designed in 3D to predict fluid flow conditions. The permeate flux through the membrane decreased with an increase in spacer curviness from 2.81 L/m2h for no (0%) curviness to 2.73 L/m2h for full (10...

The increasing adoption of ultra-low pressure (ULP) membrane systems for drinking water treatment in small rural communities is currently hindered by a limited number of studies on module design. Detailed knowledge on both intrinsic membrane transport properties and fluid hydrodynamics within the module is essential in understanding ULP performance prediction, mass transfer analysis for scaling-up between lab-scale and industrial scale research. In comparison to hollow fiber membranes, flat sheet membranes present certain advantages such as simple manufacture, sheet replacement for cleaning, moderate packing density and low to moderate energy usage. In the present case study, a numerical model using computational fluid dynamics (CFD) of a novel custom flat sheet membrane module has been designed in 3D to predict fluid flow conditions. The permeate flux through the membrane decreased with an increase in spacer curviness from 2.81 L/m2h for no (0%) curviness to 2.73 L/m2h for full (10...

Despite intensive research, fouling remains a severe problem in membrane filtration. It is often controlled by applying turbulent flow which requires a higher energy consumption. So-called turbulence promoters or static mixers can be inserted into the flow channel of tubular membranes. They deflect the fluid, induce vortices, enhance particle back-transport and increase the shear rate at the membrane surface, thus mitigating fouling. However, little is known how

The red microalga Porphyridium cruentum is exploited industrially for its exopolysaccharides (EPS) and pigments production. EPS produced by P. cruentum are partially released and dissolved into the surrounding environment, they can be recovered from the culture medium after removing the cells. This paper presents a parametric study of the ultrafiltration of EPS solutions on organic membrane. The EPS solutions were produced in conditions representative of an industrial production. They were filtered at lab-scale on a flat, PES 50 kDa MWCO membrane in a complete recirculation mode of permeate and retentate. Permeate flux-transmembrane pressure (TMP) curves were established up to the limiting flux for the filtration of solutions with various values of concentration in EPS (0.10-1.06 kg GlcEq m), fluid tangential velocity (0.3-1.2 m s) and temperature (20°C and 40°C). The reversible and irreversible parts of fouling were evaluated for each experiment and the critical flux was determined...

Application of ultrafiltration membranes for removal of humic acids is investigated below. Membrane filtration processes were compared using two different setups: circular flow and stirred dead end flow. The transmembrane pressure, temperature, feed concentration, pH, ionic strength and shear stresses applied on the membrane surfaces were kept constant whilst the permeate flux and solute rejection were measured during the experiments with both setups. It was shown that the rejection (both the observed and the true rejection) in the case of circular flow was higher than in the case of dead end flow. The mass transfer coefficients were determined for both setups. In the case of stirred dead end, it ranged in from (2.14-4.72) x 10-6 m/s; however, for circular cross flow system, the mass transfer coefficients were found in the range (2.24-3.22) x 10-5 m/s. Comparison of the mass transfer coefficients obtained for both systems showed that it was significantly higher for circular flow systems as compared with stirred dead end system at similar operating conditions. Energy consumed per volume of purified water by circular flow system (0.345 kW) was found to be much lower when by stirred dead end system (0.955 kW). This proved that the performance of circular flow system was more efficient in terms of rejection, mass transfer coefficient and energy consumption.

The time-varying flow field in spacer-filled channels of spiral-wound membrane (SWM) modules is mainly due to the development of fouling layers on the membranes that modify the channel geometry. The present study is part of an approach to tackling this extremely difficult dynamic problem at a small spatial scale, by uncoupling the fluid dynamics and mass transfer from the fouling-layer growth process. Therefore, fluid dynamics and mass transfer are studied for a spacer-filled channel whose geometry is altered by a uniform deposit thickness h. For this purpose, 3D direct numerical simulations are performed employing the "unit cell" approach with periodic boundary conditions. Specific thickness values are considered in the range 2.5-10% of the spacer-filament diameter D as well as other conditions of practical significance. The qualitative characteristics of the altered flow field are found to be very similar to those of the reference geometry with no gap reduction. For a given flow rate, the pressure drop, time-average wall-shear stresses and mass-transfer coefficients significantly increase with increasing thickness h due to reduced channel-gap, as expected. Correlations are obtained, applicable at the "unit cell" scale, of the friction factor f and Sherwood number Sh, which exhibit similar functional dependence of f and Sh on the Reynolds and Schmidt numbers as in the reference no-fouling case. In these correlations the effect of channel-gap reduction is incorporated, permitting predictions in the studied range of fouling-layer thickness (h/D) = 0-0.10. The usefulness of the new results and correlations is discussed in the context of ongoing research toward improved modeling and dynamic simulation of SWM-module operation.

The role of dilute suspensions in fouling a ultrafilter tubular membrane module is studied in detail for a wide range of wall permeation flux conditions. The inlet flow profiles are assumed to be either uniform (plug flow) or parabolic (fully developed) shape. It is assumed that the particles are neutrally buoyant and the concentrations are so low that it does not influence the fluid flow. Furthermore, the particle-particle interactions and the forces of interaction between the particle and the membrane wall are assumed to be unimportant. The governing equations of motion for the fluid are solved by a finite difference scheme. To compute the particulate fouling, the equations of motion for the particles are solved by the fourth-order Runge-KuttaGill method. Results are presented for both hydrodynamics and membrane fouling by dilute suspensions for conditions such as the effect of assumed inlet velocity profiles, and a wide range of wall permeation flux conditions. 0376-7388/88/$03.50 0 1988 Elsevier Science Publishers B.V.

The role of dilute suspensions in fouling a ultrafilter tubular membrane module is studied in detail for a wide range of wall permeation flux conditions. The inlet flow profiles are assumed to be either uniform (plug flow) or parabolic (fully developed) shape. It is assumed that the particles are neutrally buoyant and the concentrations are so low that it does not influence the fluid flow. Furthermore, the particle-particle interactions and the forces of interaction between the particle and the membrane wall are assumed to be unimportant. The governing equations of motion for the fluid are solved by a finite difference scheme. To compute the particulate fouling, the equations of motion for the particles are solved by the fourth-order Runge-KuttaGill method. Results are presented for both hydrodynamics and membrane fouling by dilute suspensions for conditions such as the effect of assumed inlet velocity profiles, and a wide range of wall permeation flux conditions. 0376-7388/88/$03.50 0 1988 Elsevier Science Publishers B.V.

The effect of Dean vortices on the filtration flux and efficiency of crossflow microfiltration processes in sinusoidal curved and helically coiled tubular membranes was investigated by several research groups. Experimental results are presented showing an enhanced filtrate flux and efficiency of the filtration process. CFD simulations were performed, revealing two important effects of secondary flow: an increased wall shear stress causing a reduced particle deposition and an enhanced mass transfer between the boundary layer and the bulk phase.

The aim of this work is to characterize more accurately, through experimental or theoretical approaches, the hydrodynamics of three particular systems usually used for flux enhancement: shear-enhanced filtration with a vibrating membrane module, gas/liquid two-phase flows and Dean vortices. The correlation between local hydrodynamics and permeate flux obtained during crossflow filtration of a yeast suspension under different operating conditions confirmed the importance of the wall shear stress in these three systems.

Computational¯uid dynamics is used to investigate designs for rotating membrane disk ®lters. Simulations have been run for the case of water permeating through a membrane disk rotating in a pressurized housing. The water was assumed to be Newtonian, incompressible, non-fouling and isothermal. A ±4 model was used to describe turbulent¯ow in the vessel surrounding the rotating disk. Similar to a nonporous disk, the rotation of the membrane disk induces a recirculating¯ow pattern of the¯uid within the vessel. However, the centrifugal force acting on the permeate may locally increase the permeate side pressure above the feed side pressure resulting in a negative local transmembrane pressure. Hence, a portion of the membrane is subject to a reversed¯ow of permeate which reduces effectiveness of membrane area and may damage the membrane. This`back pressure' phenomenon can be avoided by a careful choice of the operating conditions and design parameters. The propensity for`back pressure' is higher when the membrane is more permeable but can be reduced by increasing the feed¯ow rate or decreasing the disk diameter (i.e. the membrane area). #

The use of turbulence promoter can effectively enhance the permeate flux in crossflow filtration of particulate suspensions. Various Researchers have investigated the influence of static turbulence promoters on permeate flux improvement of membrane based filtration processes. The flow field generated by these promoters induces hydraulic turbulence and increases the wall shear stress in the membrane, which leads to enhanced scouring of the membrane surface and therefore to the permeate flux enhancement. Turbulence Promoters such as paddle mixers, static mixers, kenic (series of helical elements) mixers, rods with rings, glass beads, moving balls, detached strips and mesh like spacers can be placed near the membrane surface to promote turbulence. The main reason for prevention to a wide range of commercial use is the increase of pressure drop by using a static turbulence promoter can cause significant variation of trans-membrane pressure along the membrane. This paper reviews the use of turbulence promoters in membrane filtration, apparatuses used in turbulence promoter aided membrane filtration.

Development of membranes technologies led industrials and researchers to focus on the way to optimize filtration processes in terms of quality and costs of production. In this way, this paper presents a study about determination of low fouling operating conditions in dead-end ultrafiltration of natural organic matter which leads to the definition of sustainable conditions for filtration by limiting formation of irreversible layer. The reversibility of fouling is studied in respect to a break in the driving force and the effect of the duration of the break is underlined by comparing reversibility after an instantaneous rinsing and a 24 hours soaking. The existence of a critical filtered volume (below which the mass accumulation is reversible and above which a significant irreversible fouling exists) is experimentally demonstrated. Results show that it is possible to avoid the formation of irreversible fouling layer when operating parameters (the pair filtered volume and applied pressure) are suitably chosen.

The transient membrane fouling during the concentration by cross-flow ultrafiltration of soy protein extracts subjected to electroacidification is examined by combining experimentation with computational fluid dynamics (CFD) modeling. Transient reversible (water removal) and irreversible (chemical removal) membrane fouling resistances, permeate flux, protein concentration and protein concentration dependent viscosity were obtained experimentally. A detailed fouling resistance model to describe the reversible and the irreversible fouling resistances was developed in terms of the microscopic local transient and spatial pressure difference, permeate velocity, protein concentration and initial fouling resistance conditions. This fouling resistance model is used in a boundary condition for the permeate velocity when solving the momentum and protein concentration continuity equations with CFD. The model estimates agree with experimentally measured permeate flux, protein concentration and transient irreversible and reversible fouling resistances. In particular, the model estimated accurately the transient reversible and irreversible fouling resistances, a limitation of most previously published models. The model shows considerable axial variation of the reversible fouling resistance and the protein concentration at the membrane surface which supports the inadequacy of the film theory and the assumptions for constant properties. In contrast, the irreversible fouling resistance remains relatively constant with axial position suggesting protein adsorption.

Membrane processes have been intensely developing during the last decades, and mainly in dairy industry. Considering the feed effluent complexity, concentration polarization phenomenon and fouling are accentuated limitations for the development of membrane dairy filtration processes. Knowledge of the wall shear stress developed at the membrane surface is fundamental to reduce those phenomena. In this work, the variation of the wall shear stress on cylindrical, square, triangular and hybrid channels by numerical simulation for various operating parameters was studied. Predictions were established for different commercial ceramic membranes and predict the geometry that tends to better mass transport efficiency by enhancing hydrodynamics conditions. Numerical simulations are performed over a typical range of Reynolds numbers inside different channel geometries under laminar and turbulent conditions. Consequently, this paper intended to enhance the performances of these processes by maximizing the average wall shear stress on the membrane surface by numerical simulation. A comparison with experimental results was realized and a good agreement was obtained. Given those conclusions, a new membrane according to the whole CFD results consistent with experimental results was designed.

Coiling membranes help to create in a flow Dean vortices which enhance flux permeation and bring improvement to membrane filtration processes. The Dean number enables prediction of the appearance of those vortices. As it increases, a secondary flow appears with one pair of vortices. However, this flow profile needs a certain length to be fully developed. For membrane processes, it is useful to know this developing length so one can calculate the exact membrane area under such Dean shear stress. Developing angles or lengths were already simulated for a parabolic and a uniform inlet flow for a torus. In this study, we determined numerically the developing length for different kinds of tori and for woven or helical tubes. The simulations featured parameters of wide ranges of validity: internal diameter (0.25-1 mm), coil diameter (4-32 mm), Reynolds number (0-400) and pitch (2-32 mm) for the helical shape.

Keywords: Dean flows; developing length; torus; helical tube; instability

Knowing how wall shear stress develops at the membrane surface is extremely useful when trying to reduce concentration polarization and fouling. Newly developed as well as manufactured ceramic membranes exhibit various channel geometries (cylindrical, square, triangular, etc). Mass transport characteristics depend on the geometry that conditions hydrodynamic conditions. The goal of this work is to study the influence of the channel geometry on the wall shear stress for various operating parameters (tangential velocity, transmembrane pressure...). Numerical simulations are performed for various inlet velocities for different channel geometries. The wall shear stress along the channel perimeter as a function of the shape and the cross section of the channel are studied. The influence of the geometry on the membrane performances is also studied. The simulated shear stress is employed to correlate experimental results. The results of this comparison show that mass transfer resistance depends on the wall shear stress alone, regardless of the flow rate, the shape or section of the channels.

Keywords: computational fluid dynamics; wall shear stress; channel geometry; ceramic membrane; fouling