Effect Of Fluid Shear Research Articles

Simulation Study on the Dynamic Behaviors of Water-in-Oil Emulsified Droplets on Coalescing Fibers.

Although increasing superwetting membranes have been developed for separating oil-water emulsions based on the "size-sieving" mechanism, their pores are easily blocked and fouled by the intercepted emulsified droplets, which would result in a severe membrane fouling issue and a sharp decline in flux. Instead of droplet interception, a fiber-based coalescer separates oil/water emulsions by inducing the emulsified droplets to coalesce and transform into layered oil/water mixtures, exhibiting an ability to work continuously for a long time with high throughput, which makes it a promising technology for emulsion treatment. However, the underlying mechanism of the separation process is not well understood, which makes it difficult to further improve the separation performance. Hence, in this work, the dynamic behaviors of water-in-oil emulsified droplets on the surface of the coalescing fiber were numerically investigated based on the phase-field model. The attachment, transport, and detachment behaviors of droplets on fibers were directly observed, and the effects of fiber wettability, orientation, arrangement, and fluid speed were studied in detail. First, it was observed that the droplets will move downstream along the fiber surface under the effect of fluid shear, and the large droplets tend to coalesce with their downstream small droplets on the same fiber surface because they move faster compared to the small droplets. Second, it was found that the emulsified droplet will spontaneously transport to the intersection of two angled fibers under the drive of asymmetric Laplace pressure, which demonstrated that the emulsified droplets tend to gather at the intersection of fibers when permeating through a coalescing medium. Third, it was found that the detachment behaviors of droplets from the fiber surface are strongly affected by their size, fiber wettability, and fluid velocity. In addition, the results of our simulation show that the backside of two closely attached fibers can further inhibit the detachment of droplets. We truly believe that our research results are of significance to optimize the parameters of a fiber-based coalescer for separating oil-water emulsions and to develop novel oil/water separators.

The effect of fluid shear on oscillating bubbly flows

The impact of fluid shear on the bubble distribution in channel flows with periodically oscillating pressure gradient is examined by direct numerical simulations. Equal-sized and nearly circular bubbles are placed randomly in the channel at the initial time. In the absence of shear, the bubbles form columns spanning the width of the channel, but a strong enough shear breaks up the columns, leading to a more random bubble distribution. The effect of the nondimensional shear rate on the flow can be divided into low shear rate, moderate shear rate, and high shear rate regimes. The flow dynamics is also influenced by the Reynolds and the Euler number, and when these numbers decrease, the low shear rate regime, with stable tilted columns of bubbles, becomes smaller. Comparison of results for two- and three-dimensional flows shows that the dynamics observed in two-dimensional flows is also found in three dimensions.

The Effect of Shear Rate on Aggregation and Fragmentation of Asphaltene Aggregates

This paper investigates the shear rate influence on asphaltene aggregation in heptane-toluene mixture through experiments and theoretical modeling at constant temperature and pressure. The experiments were performed within a Couette apparatus at different shear rates (129.14 – 1142.39 s−1). The size distributions of asphaltene aggregates in time were measured. In all tests, the average diameter of asphaltene aggregates initially increases up to a maximum and then decreases to a steady-state value. The existence of steady state size is believed to occur when the rates of aggregation and fragmentation become equal. A fractal population balance model was used and the aggregation kernel was assumed to be a linear combination of orthokinetic and perikinetic kernels. The Particle Swarm Optimization (PSO) algorithm was employed for estimating the model parameters including orthokinetic and perikinetic collision efficiencies, and , respectively, fractal dimension, , and breakage coefficient, . The results showed that the fractal dimension, the collision efficiency of orthokinetic mechanism and breakup rate coefficient increase with increasing of shear rate. However, the value of perikinetic collision efficiency decreases with the increase in shear rate. Totally, the effect of fluid shear on fragmentation was found to be higher than that on aggregation of asphaltene particles.GRAPHICAL ABSTRACT

Flow accelerated degradation of organic clear coat: The effect of fluid shear

Flow accelerated organic coating degradation has been received attention, but the degradation mechanism is still not completely understood. In this study, we focus on investigating the influence of fluid shear on the degradation of organic coatings. A commercially available epoxy based clear coating was exposed to the laminar flow, and a 3.5 wt% NaCl solution was employed as the working fluid with a variety of flow rates. The change in the composition of the working fluid during the coating immersion was characterized by Fourier Transform Infrared Spectrometer (FTIR) and conductivity measurement. The effect of fluid shear is also demonstrated by the change in coating thickness. The barrier properties of organic coatings were monitored inline by Electrochemical Impedance Spectroscopy (EIS) measurement. The time evolution of coating resistance, capacitance, and permittivity obtained from the equivalent circuit model was studied to demonstrate the internal composition change of the coatings due to the flowing fluid. In this study, the experiments and analysis confirmed that flowing fluid promotes the release of coating materials from the coating into the surrounding working fluid while it accelerates the transport of water and ions into the coating.

Helical swimming can provide robust upwards transport for gravitactic single-cell algae; a mechanistic model

In still fluid, many phytoplankton swim in helical paths with an average upwards motion. A new mechanistic model for gravitactic algae subject to an intrinsic torque is developed here, based on Heterosigma akashiwo, which results in upwards helical trajectories in still fluid. The resultant upwards swimming speed is calculated as a function of the gravitactic and intrinsic torques. Helical swimmers have a reduced upwards speed in still fluid compared to cells which swim straight upwards. However a novel result is obtained when the effect of fluid shear is considered. For intermediate values of shear and intrinsic torque, a new stable equilibrium solution for swimming direction is obtained for helical swimmers. This results in positive upwards transport in vertical shear flow, in contrast to the stable equilibrium solution for straight swimmers which results in downwards transport in vertical shear flow. Furthermore, for strong intrinsic torque, when there is no longer a stable orientation equilibrium, we show that the average downwards transport of helical swimmers in vertical shear flow is greatly suppressed compared to straight swimmers. We hypothesise that helical swimming provides robustness for upwards transport in the presence of fluid shearing motions.

Effect of fluid shear and particles collision on the oxidation of ferrous iron by Acidithiobacillus ferrooxidans

In this paper, the experiments were conducted in a concentric cylinder shear device. The effect on oxidation activity of ferrous iron by Acidithiobacillus ferrooxidans was tested under different shear conditions achieved through regulating the rotating speed of the inner cylinder. The results suggested that the fluid shear rate had little adverse effect on the oxidation activity of At. ferrooxidans. In this study, silicon dioxide powder was added to the medium to investigate the effect of particles collision on the oxidation activity of bacterial cells, and it was found that the oxidation rate of ferrous iron by At. ferrooxidans was greatly decreased with the increase of silicon dioxide concentration.

MEMBRANELESS DIALYSIS: PERFORMANCE OF SECONDARY SEPARATORS

Membraneless separation of blood components does not discriminate solutes or allow adequate volume transport without a secondary separator (SS). SS’s control primary membraneless separators (PS’s) by recycling molecules to be preserved. SS’s also control PS performance by removing water, thereby raising the osmotic pressure of the recycle stream high enough to transport water in the PS in the absence of transmembrane pressure. SS’s may work by reaction, absorption, or selective membrane transport, and must be small and able to sustain performance for many hours. The performances of small dialysis devices as SS’s with respect to ultrafiltration (UF) and solute transport over in vitro measurement times up to 48 hours, using bovine plasma are discussed. UF rates corresponding to removal of 10 kg/wk water and dialysis rates corresponding to 200 g/wk urea were examined in small hollow-fiber modules with 500 cm2 area. Modest increases in SS transmembrane pressure and elevations of sheath fluid blood urea nitrogen (BUN) were seen when fixed loads of urea were imposed over long times. Changes in performance may be implemented by elevated shear rates (made possible when low cell counts are achieved in the fluid sent to the SS) and by changes in module area. Reported test data are also of interest because very little information is currently available in any environment to estimate the long-term, uninterrupted performance of dialysis fibers, and the effect of fluid shear on performance decrement.

The effect of fluid shear and co-adsorbed proteins on the stability of immobilized fibrinogen and subsequent platelet interactions

The conformation adopted by the plasma protein fibrinogen upon its adsorption onto synthetic surfaces has been implicated to play an important role in determining the blood compatibility of biomaterials. It has recently been shown that adsorbed fibrinogen undergoes biologically significant conformational changes with increasing residence time on the surface of selected biomaterials. The purpose of this study was to examine the effects of co-adsorbed proteins and shear forces on such timedependent functional changes in fibrinogen adsorbed onto polyethylene (PE), polytetrafluoroethylene (PTFE), and silicone rubber (SR). Fibrinogen was adsorbed onto these materials for 1 min and then allowed to 'reside' on these surfaces for up to 2 h prior to assessing its biological activity. Changes in fibrinogen reactivity were determined by measuring the adhesion of 51Cr-labeled platelets and the ability of blood plasma to displace previously adsorbed fibrinogen. The magnitude of platelet adhesion to substrates adsorbed with pure fibrinogen increased in the presence of shear, compared with static conditions; at the lowest shear rate of 200 s-1, samples exhibited a 20-fold increase in adhered platelet levels. In contrast, at a higher shear rate of 1000 s-1, the three polymers supported minimal levels of platelet attachment. Surfaces pre-adsorbed with 10% plasma did not promote a significant increase in the number of adherent platelets with increasing shear when compared with the pure fibrinogencoated substrates. The presence of shear also significantly altered the materials' ability to retain fibrinogen. Under static conditions, the amount of fibrinogen retained following incubation in blood plasma increased on all materials with increasing fibrinogen residence time. However, the materials varied distinctly in their ability to retain adsorbed fibrinogen with increasing fibrinogen residence time, shear rate, and nature of the co-adsorbed proteins. Thus, the results from this study indicate that fluid shear, residence time of the adsorbed protein, nature of the co-adsorbed proteins, and surface chemistry of the material all play important roles in influencing platelet–surface interactions and that they act in a complex manner to influence the biocompatibility of a material.

Effect of fluid shear and irradiance on population growth and cellular toxin content of the dinoflagellate Alexandrium fundyense

The potential for in situ turbulence to inhibit dinoflagellate population growth has been demonstrated by experimentally exposing dinoflagellate cultures to quantified shear flow. However, despite interest in understanding environmental factors that affect the growth of toxic dinoflagellates, little is known of the effect of shear on the growth of toxin‐producing dinoflagellate species. Cultures of the dinoflagellate, Alexandrium fundyense, a producer of toxins responsible for paralytic shellfish poisoning, were exposed to quantified laminar shear generated in Couette flow for 1–24 h d−1 over 6–8 d. Shear stress in all experiments was 0.003 N m−2, similar to levels expected in near‐surface waters on a windy day. Net population growth decreased with shear exposures ≫1 h d−1 and became negative with exposures ≫12 h d−1. Cellular toxin content at the end of each experiment was measured by a receptor‐binding assay that used [3H]saxitoxin. Toxin cell−1 of cultures sheared for ≫1 h d−1 increased up to three times that of control cultures. Cellular toxin content increased significantly as growth rate of sheared cultures decreased. However, varying culture growth rate using irradiance had no significant effect on toxin cell−1. Because shear stress levels used in this study were plausible for near‐surface turbulent flows, oceanic turbulence may inhibit population growth and increase cellular toxin content of A. fundyense. However, in natural populations it would be difficult to distinguish the effect of turbulence on toxin content from other influences on toxin variability, particularly if volume‐or mass‐specific, rather than cell‐specific, measures of toxin are used.

Lipid transport aspects of atherogenesis.

In this review we shall examine the current understanding of events that lead to the incipient formation of the early foam cell lesion in atherogenesis and its localization. Particular emphasis will be placed on the intimal transport mechanisms that lead to the growth of extracellular lipid liposomes in the intima, since there is now substantial evidence that this growth is the triggering event in the complex sequence of processes that leads to the recruitment of blood borne monocytes into the subendothelial intima and their subsequent conversion to macrophages. The role of the endothelium, intimal proteoglycans and internal elastic lamina (IEL) in modulating the transport of low density lipoproteins (LDL) in the subendothelial space will be analyzed and a new hypothesis for the co-localization of liposome formation, cellular level endothelial leakage and monocyte entry described. The possible modifications of LDL in the liposomes that facilitate the conversion of monocytes into foam cells is summarized. We also discuss the fluid dynamic aspects of intimal transport and the relationship of fluid shear stress to the localization of cellular level endothelial leakage of LDL. The effect of fluid shear on other endothelial cell functions has been recently reviewed in [1].