Thin Channel Flow Research Articles

Rheophysical Characteristics of Hydrocarbon Flow in Microcracks

The rheophysical aspects of the non-Newtonian behavior of oil-water emulsion during flow in thin channels are considered experimentally. Using the microchannel model it is established that the nonlinear rheological effect in the flow of oil-water emulsion in micro-slits is mainly caused by the value of the electrokinetic potential of the system, by reducing of which it is possible to significantly weaken the non-Newtonian nature of the fluid. To regulate the electrokinetic potential, antistatic additives were used, the optimal concentration of which was established experimentally. Based on the Bingham model, rheological parameters of oil-water flow were estimated at different micro-slit clearances, in the absence and presence of an antistatic additive. It is established that a reduction in the electrical potential leads to a significant decrease in the yield shear stress during the fluid flow in the microchannel.

A rheophysical study of the non-newtonian behavior of water flow in thin channels

The development of low-permeable hydrocarbon reservoirs is becoming an increasingly urgent task, and therefore, the study of the laws of fluid movement in subcapillary pores and microcracks is a crucial scientific and technical problem. The previous experimental studies revealed that a viscous liquid during flow in low-permeable reservoirs exhibits an anomalous non-Newtonian character, accompanied by a violation of the linearity of the filtration process, and, consequently, Darcy's law. It was also established that starting from a certain critical size of the opening of the crack, the flow of a Newtonian fluid (water, viscous oil) becomes non-Newtonian, with the manifestation of an initial pressure gradient and flow locking. In this research work, rheophysical aspects of the non-Newtonian behavior of water during flow in thin rectangular channels are considered experimentally. Using the microchannel model, it is established that the nonlinear rheological effect in the flow of water in micro-slits is mainly caused by the value of the electrokinetic potential of the system, by reducing of which it is possible to significantly weaken the non-Newtonian nature of the fluid. To regulate the electrokinetic potential of the fluid system, an antistatic additive was used, the optimal concentration of which was established experimentally. The optimal concentration is defined to be 0.006 %. Based on the Bingham model, the rheological parameters of water flow were estimated at different micro-slit clearances changed in the range of 10÷25 micrometers, in the absence and presence of an antistatic additive. It is also established that a reduction in the electrical potential of the fluid flow leads to a significant decrease in the yield shear stress during the flow of water in the microchannel

Effects of nonlocal hydromechanics in a flow in thin channels

An approach to viscous friction is described as nonlocal momentum exchange between different layers of a fluid. The Navier−Stokes equations are replaced by pseudo-differential equations hyperbolic in time. In this case, instead of zero velocity on the boundary, a nonlocal nonlinear boundary condition is set in the form of the velocity dependence of the coefficient before the intensity of the momentum exchange with the boundary. The non-newtonian character of the viscosity of water is shown in experiments with thin insulin needles and explained by the nonlinear character of the momentum exchange of water with the boundary. The calculations agree very well both with our experiments and with the experiments of other authors. Calculations show that the flow decreases more than one-and-a-half times in comparison with the Poiseuille flow for channels with a diameter of 360−390 μm, which is confirmed in experiments.

THE FORMATION OF PROTECTIVE IRON CARBONATE FILMS AND THE CONTROL OF CO2CORROSION IN TURBULENT FLOW

The formation of a protective layer of iron carbonate (FeCO3) can reduce the rates of corrosion and prolong the useful life of carbon steel. However, turbulent flow conditions in this layer can easily be damaged and thus compromise the protection of the steel. In this work, will be presented a methodology based on the chemical aspects of the mechanism of formation of iron carbonate layer in a Thin Channel Flow Cell (TCFC). Electrochemical techniques were used to measure the corrosion rate and corrosion potential on the surface of steel API X65 exposed to aqueous solution of 1 wt.% NaCl purged with CO2 at 2 atm, pH 6.6 and 80°C in turbulent flow conditions. The surfaces and cross sections of the samples were characterized by means of Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDS) analysis. The results confirm the nucleation and growth of iron carbonate layer: the extension of supersaturation of the solution and the corrosion rate have decreased, and the corrosion potential has increased. The surface analysis showed dense and uniform FeCO3 layer with c.a. 20 µm thick after 120 hours in turbulent flow conditions.

Surface Platinum Electrooxidation in the Presence of Oxygen

Understanding the influences of Pt surface oxides is crucial for elucidating the oxygen reduction reaction (ORR) in fuel cells, particularly with an eye toward the design of next-generation Pt-based electrocatalysts with improved activity. Although gaseous O2 is always present during the ORR, the majority of previous Pt surface oxide studies were conducted in the absence of O2 due to experimental limitations. In this study, multiple in situ techniques were applied to study the ORR on platinum in the presence of O2. The thin channel flow Pt electrode and the electrochemical quartz microbalance (EQCM) techniques on Pt polycrystalline electrodes suggest that the influence of O2 in the electrolyte on mass change and charge transfer is negligible. The oxide formation followed a logarithm-growth behavior initiating as early as 0.2 s after potential steps. This suggests that no slow conversion of oxide species (e.g., between OHads and Oads) takes place. Despite the negligible effect of O2 on the measured oxide c...

Methods of measuring rheological properties of interfacial layers (Experimental methods of 2D rheology)

Current methods of studying the rheological properties of interfacial layers at the interfaces of fluids are reviewed. This area of research includes two-dimensional 2D rheology. Regardless of the similarities between the parameters of rheological properties of two-dimensional and bulk (three-dimensional) systems, when measuring surface properties, it is necessary to reformulate the main experimental methods to allow for the different dimensions of surface and bulk characteristics of material. Parameters of shear and dilational (measured upon expansion-compression) properties of interfacial layers are distinguished, and the latter are considered to be independent parameters of a system. The most attention was given to the rotational methods of measuring shear viscosity and the components of the complex 2D elastic modulus, as well as to measuring surface tension upon harmonic changes of the bubble (droplet) surface area, which allows characteristics of the dilational behavior of thin liquid films to be determined. Both groups of methods are widely used in laboratory practice and realized in the form of a number of original and commercial instruments. Dilational measurements of interfacial layers can also be performed with oscillations of a movable barrier on a Langmuir trough. In addition, methods based on the propagation of capillary waves across the surface of a liquid, as well as rarer methods of capillary flow in thin channels forced by either a surface tension gradient or the motion of the interface, are considered.

Characterisation of a simple electrochemical detector for high-performance liquid chromatography and flow-injection analysis based on carbon microcylinder electrodes

A simple thin channel flow cell, modified to incorporate carbon microcylinder indicator electrodes, was tested as an amperometric detector in high-performance liquid chromatography (HPLC) and flow-injection analysis (FIA). The hydrodynamics of the cell were studied by means of steady state electrochemical experiments at varied flow rates. The signal dependence on flow rate has proven that for 30 μm diameter microcylinder electrodes, convective diffusion prevails and no radial diffusion effects are observed. Nevertheless, mass transfer rates are enhanced for microcylinders placed perpendicular to the analyte flow. In HPLC and FIA experiments with typical electroactive analytes, the microcylinder-based detector gave good quality responses and, when operated at potentials near the mass transfer control region, exhibited an improved sensitivity and signal-to-noise (S/N) ratio, resulting in lower detection limits (less than 10 ppb) than a macroelectrode indicator. This is in complete agreement with simple considerations for the analyte signal and background current at thin electrodes with their smaller dimension parallel to the flow velocity in a channel cell.

A general approach to the derivation of peak area flow dependence in FIA and HPLC amperometric detection

The dependence of the peak area, Q, on the analyte volumetric flow rate, U, in flow injection analysis (FIA) and high performance liquid chromatography (HPLC) with amperometric detection, was studied for typical electroactive species and in a wide flow rate range. Based on the hydrodynamics of thin channel flow cells (as established by steady state experiments) and simple dispersion theory considerations, a linear relationship between log Q and log U with a −2/3 slope has been derived in a general manner (irrespective to the type of dispersion) for amperometric detectors operated in the limiting current potential region. In the case of mixed mass transfer and kinetic control the variation of Q with U is more complicated but the peak area is still smoothly decreasing with the flow rate. These predictions were found to be in reasonable agreement with experiment for a few indicative systems both in FIA and HPLC experiments. On the contrary, the non-steady state current corresponding to the peak maximum of FIA and HPLC exhibited local maxima and the former could not be described by any of the equations proposed for the dispersion in FIA experiments. The practical implications of the form of integrated signal, Q, dependence on flow rate for FIA and HPLC amperometric detection are also discussed.

Designing Blood Oxygenators

Extracorporeal blood oxygenators are used to provide cardiopulmonary support during open heart surgery. In the study reported here, mass transfer correlations were determined for commercially available blood oxygenators. Two configurations used commercially, flow outside and across bundles of hollow fibers and flow in thin channels between parallel flat sheet membranes, were investigated. Water and glycerol/water mixtures were used as a substitute for blood. Diffusion of oxygen into and out of these solutions was studied. For flow across bundles of hollow fibers, the mass transfer correlations derived here are in agreement with analogous correlations for crossflow heat exchangers. However, for flow in thin channels, the rate of mass transfer is often less than predicted from theory. This compromised mass transfer can be explained by considering slight variations in the thickness of the blood flow channels. The mass transfer correlations developed here could be used to design better blood oxygenators.

Mass and momentum transfer in commercial blood oxygenators

In modern blood oxygenators, microporous membranes (flat sheet or hollow fibre) are used to separate the blood and gas phases. Blood flows on one side of the membrane while gas (usually oxygen or air) flows on the other side. Since the membranes used are hydrophobic (e.g., polypropylene, Teflon), the pores are gas filled resulting in a negligible membrane mass transfer resistance. The major resistance to gas transfer is due to the blood side mass transfer boundary layer. In this study mass transfer and friction factor correlations for commercially available hollow fibre and flat sheet blood oxygenators have been determined experimentally. Water was used as a substitute for blood. The diffusion of oxygen into and out of water has been studied. Two different flow configurations used commercially have been investigated: flow outside and across woven hollow fibre bundles and flow in thin channels. The results for flow across bundles of woven hollow fibres may be compared to analogous results for flow in cross flow heat exchangers. The results obtained here can be used to further optimize the design of blood oxygenators.

Mass and momentum transfer in blood oxygenators

Mass transfer and friction factor correlations for commercially available hollow fibre and flat sheet blood oxygenators have been determined experimentally. Water was used as a substitute for blood. The diffusion of oxygen into and out of water has been studied. Three different flow configurations have been investigated: flow inside hollow fibres, flow outside and across woven hollow fibre bundles and flow in thin channels. For flow inside the fibres, the results obtained are in close agreement with analogous theoretical correlations. The results for flow across bundles of woven hollow fibres may be compared to analogous results for flow in cross flow heat exchangers. For flow in thin channels, the results obtained here are in agreement with the analogous theoretical heat transfer correlation developed by Lévêque. However, at Graetz numbers less than 10, the results deviate from the predictions of Lévêque. This deviation cannot be predicted by extensions to the Lévêque solution or from the more rigorous analysis by Graetz. However it is possible to predict the observed deviation by considering slight polydispersity in the thickness of the thin liquid flow channels.

Studies on protein transmission in thin channel flow module: the role of dean vortices for improving mass transfer

This study is mainly comprised of evaluating mass transfer coefficient from the volumetric flux and protein transmission data in a thin channel flow module. Dilute solutions of lysozyme were ultrafiltered through hydrophilic membrane (cellulose acetate type) with molecular weight cut off (MWCO) of 30,000. Experiments were performed in thin channel flow (TCF10) configuration and data for transmission and volumetric flux obtained as a function of pressure and cross flow velocity. The results were analysed keeping into account the formation of Dean vortices and it was seen that the presence of Dean vortices gave better performance in terms of mass transfer. The mass transfer coefficient obtained with Dean vortices was comparable with the other commercial cross flow system involving secondary flow like Taylor vortices. A new mass transfer correlation has also been presented and compared with the reported correlations in literature.

Diffusion anormale pour le gaz de Knudsen

We show how a rough boundary described by a diffusive boundary condition may generate a diffusion process in a fluid. At variance with previous results obtained by probabilistic argument ( see [2]), our proof relies entirely on functional analysis as in the paper by Golse [3]. We prove the evolution of the density of a free molecular flow in thin channels bounded by parallel plane surfaces is described by a diffusion equation, on a time scale of ρ(ε) ~ ε 2¦ log ε 2¦ in the limit as the distance between the plates tends to 0.

Postseismic deformation and stress diffusion due to viscoelasticity and comments on the Modified Elsasser Model

Finite element techniques have been used to investigate the far‐field deformation and stress changes due to asthenospheric viscoelastic relaxation following a dip‐slip earthquake. The diffusion of extensional stress toward the subduction zone following a thrust earthquake on land is qualitatively consistent with the modified Elsasser model as proposed by Rydelek and Sacks (1988, 1990) to explain the coupled occurrence of land and sea earthquakes near Japan. However, the magnitude of the diffusing tensional signal is significantly smaller. The nominal model consists of a partially faulted elastic lithosphere overlying a viscoelastic substrate. Other models consider thin channel flow, rupturing of the entire elastic lithosphere, and changes in the depth of faulting. While some of these changes have significant impact on the magnitude and spatial features of the stress and deformation field, the far‐field stress remains small. Numerical experiments demonstrate that the assumptions of the modified Elsasser model accentuate uniaxial deformation. When these assumptions are replicated in the finite element calculations, reasonable agreement between the models is achieved.

A visible wavelength solid-state LDA and application to thin channel flow

A visible wavelength solid-state LDA is described. The instrument has been designed for measurements in narrow gaps. Poiseuille and Couette flow measurements are demonstrated.

The Diffusion Limit of Free Molecular Flow in Thin Plane Channels

The subject of this paper is free molecular flow in thin channels bounded by parallel plane surfaces on which Maxwell’s boundary condition applies. With tools from probability theory, it is proved that in the limit as the domain width h tends to zero, the evolution of the density is described by a diffusion equation, on a timescale of $1/(h\log h^{-1} )$, and with a diffusion coefficient of $(2 - \alpha )\sqrt T /(2\alpha \sqrt \pi )$ ($\alpha $ is the accommodation coefficient and T is the surface temperature). The logarithmic factor in the timescale is geometry dependent; in thin cylinders of diameter h, the timescale is $1/h$, as Babovsky has proved in [Journal of Statistical Physics, 44 (1986), pp. 865–878]. Numerical calculations indicate that the diffusion limit is closely approximated even at fairly large values of h.

Mechanical effects on rates of hemolysis.

Empirical studies were conducted on how rates of hemolysis were influenced by mechanical factors, such as shear, turbulence, agitation, pressure fluctuation, and bubbling, in five types of experimental apparatus: rotating cylinder, rotating disks, agitated vessel, pressure pulsator, and bubble columns. Our data indicate, at least qualitatively, that an important mechanical factor controlling rates of hemolysis in practical devices, in which blood does not flow in thin channels or capillaries, is the turbulent stress in the bulk blood, rather than the shear at the blood-solid interface.

Rat peritoneal macrophage adhesion to hydroxyethyl methacrylate-ethyl methacrylate copolymers and hydroxystyrene-styrene copolymers.

Macrophage adhesion to a wide variety of substrates has been measured, but no systematic study of the influence of specific substrate chemical properties on adhesion is available. These studies were conducted using two series of materials, copolymers of hydroxyethyl methacrylate (HEMA) and ethyl methacrylate (EMA) and copolymers of hydroxystyrene and styrene, to determine the effect of a single chemical property, polar character, on adhesion. Rat peritoneal macrophages were allowed to contact polymer substrates for periods ranging from 1 to 240 min before being subjected to a shear stress of 60-120 dynes/cm2 in a thin-channel flow cell. Percentage adhesion was calculated from the number of cells that remained adherent to the substrate after 30 s of applied shear stress. Macrophages remained adherent to 100% EMA and all hydroxystyrene-styrene copolymer surfaces after only 1 min of contact. In copolymers of the HEMA-EMA series, the time required to attain peak adhesion levels increased with increasing substrate hydrophilicity (increasing HEMA content). Cells did not attach to the 20% EMA/80% HEMA copolymer and the 100% HEMA polymer. The results demonstrate that there is a time delay between contact and adhesion of the cells to surfaces of increasing hydrophilicity within the HEMA-EMA series and no time delay with the hydroxystyrene-styrene series. The time delay is thought to be a function of the excluded volume provided by polymers that are able to undergo significant chain rotation and or swelling in the solvent, water. Small excluded volumes present in copolymers of high EMA content and all hydroxystyrene-styrene copolymers offer little or no resistance to formation of adhesive bonds by macrophages, whereas copolymers with large excluded volumes (high HEMA content) prevent contact and/or adhesion. A mechanism based on the net excluded volumes of both the cell and substrate surface macromolecule is proposed to explain this phenomenon.

Blood ultrafiltration: A design analysis

Ultrafiltration of blood finds application in the conventional dialysis and adsorber-ultrafilter artificial-kidney systems. The ultrafilters used for these processes are either flat-membrane thin-channel flow systems or bundles of capillary-like hollow fibres. The design analysis of hollow-fibre and thin-channel ultrafilters is carried out by assuming 2-dimensional and axisymmetric flow of a Newtonian fluid accompanied by convective and diffusive mass transfer across the semipermeable membrane wall. The wall permeation velocity is assumed to be small and (a) decaying exponentially along the flow direction and (b) proportional to the transmembrane pressure difference. The ultrafilter is simulated for the operation of (a) a conventional dialyser and (b) and ultrafilter as used in the adsorber-ultrafilter artificial kidney. The parametric analysis of the ultrafilter is studied.