Bulk Fluid Velocity Research Articles

Induction electrohydrodynamics micropump for high heat flux cooling

Induction electrohydrodynamics (EHD) has been investigated as a possible means of pumping liquids through microchannel heat sinks for cooling microprocessors. A pump utilizing induction EHD has been microfabricated and tested. The experimental results matched the predictions from correlations to within 30%. Based on this, a micropump has been designed which is miniaturizable to a level where it can be integrated into the microchannels. The micropump utilizes a vibrating diaphragm along with induction EHD for pumping. The vibrating diaphragm does not cause any net flow by itself but causes high local bulk fluid velocities which lead to an increase in the power drawn from the electrodes and an increase in efficiency of EHD, both of which lead to a higher flow rate. The performance of the pump is predicted using an experimentally validated numerical model. The numerical model solves the three-dimensional transient fluid flow and charge transport problem due to simultaneous actuation of EHD and the vibrating diaphragm. Numerical results for micropumps integrated into trapezoidal microchannels are presented. The results indicate that the proposed micropump design has significant potential for microelectronics cooling applications: It is easy and inexpensive to fabricate, needs no added space, and can achieve the high flow rates needed.

Electroosmotic flow velocity measurements in a square microchannel

Experiments were performed using a microparticle image velocimetry (MPIV) for 2D velocity distributions of electroosmotically driven flows in a 40-mm-long microchannel with a square cross section of 200×200 μm. Electroosmotic flow (EOF) bulk fluid velocity measurements were made in a range of streamwise electric field strengths from 5 to 25 kV/m. A series of seed particle calibration tests can be made in a 200×120×24,000-μm untreated polydimethyl siloxane (PDMS channel incorporating MPIV to determine the electrophoretic mobilities in aqueous buffer solutions of 1× TAE, 1× TBE, 10 mM NaCl, and 10 mM borate. A linear/nonlinear (due to Joule heating) flow rate increase with applied field was obtained and compared with those of previous studies. A parametric study, with extensive measurements, was performed with different electric field strength and buffer solution concentration under a constant zeta potential at wall for each buffer. The characteristics of EOF in square microchannels were thus investigated. Finally, a composite correlation of the relevant parameters was developed in the form of u* = 5.46;varsigma {kern 1pt} ^{{*0.739}} DN^{{ - 2.132}} ME^{{*0.9996}}_{x} within ±1% accuracy for 99% of the experimental data.

Simulations of magnetorheological suspensions in Poiseuille flow

Particle-level simulations are conducted to study magnetorheological fluids in plane Poiseuille flow. The importance of the boundary conditions for the particles at the channel walls is examined by considering two extreme cases: no friction and infinite coefficient of friction. The inclusion of friction produces Bingham fluid behavior, as commonly observed experimentally for MR suspensions. Lamellar structures, similar to those reported for electrorheological fluids in shear flow, are observed in the post-yield region for both particle boundary conditions. The formation of these lamellae is accompanied by an increase in the bulk fluid velocity. The slip boundary condition produces higher fluid velocities and thicker lamellar structures.

Boundary-layer exchange by bubble: A novel method for generating transient nanofluidic layers

Unstirred layers (i.e., Nernst boundary layers) occur on every dynamic solid-liquid interface, constituting a diffusion barrier, since the velocity of a moving liquid approaches zero at the surface (no slip). If a macromolecule-surface reaction rate is higher than the diffusion rate, the Nernst layer is solute depleted and the reaction rate becomes mass-transport limited. The thickness of a Nernst boundary layer (δN) generally lies between 5 and 50μm. In an evanescent wave rheometer, measuring fibrinogen adsorption to fused silica, we made the fundamental observation that an air bubble preceding the sample through the flow cell abolishes the mass-transport limitation of the Nernst diffusion layer. Instead exponential kinetics are found. Experimental and simulation studies strongly indicate that these results are due to the elimination of the Nernst diffusion layer and its replacement by a dynamic nanofluidic layer (δν) maximally 200–300nm thick. It is suggested that the air bubble leads to a transient boundary-layer separation into a novel nanoboundary layer on the surface and the bulk fluid velocity profile separated by a vortex sheet with an estimated lifetime of 30–60s. A bubble-induced boundary-layer exchange from the Nernst to the nanoboundary layer and back is obtained, giving sufficient time for the measurement of unbiased exponential surface kinetics. Noteworthy is that the nanolayer can exist at all and displays properties such as (i) a long persistence and resistance to dissipation by the bulk liquid (boundary-layer-exchange-hysteresis) and (ii) a lack of solute depletion in spite of boundary-layer separation. The boundary-layer-exchange by bubble (BLEB) method therefore appears ideal for enhancing the rates of all types of diffusion-limited macromolecular reactions on surfaces with contact angles between 0° and 90° and only appears limited by slippage due to nanobubbles or an air gap beneath the nanofluidic layer on very hydrophobic surfaces. The possibility of producing nanoboundary layers without any nanostructuring or nanomachining should also be useful for fundamental physical studies in nanofluidics.

Influence of Bulk Fluid Velocity on the Efficiency of Electrohydrodynamic Pumping

Abstract The efficiency of conversion of electrical power into fluidic power in an electrohydrodynamic (EHD) pump depends on the bulk fluid velocity. An analytical formulation is developed for calculation of the efficiency of an EHD pump, with and without the presence of a superimposed flow due to an externally imposed pressure gradient. This formulation is implemented into a numerical model, which is used to investigate the effect of bulk fluid velocity on the efficiency of the EHD action. In particular, the net flow due to the combined action of EHD and a positive or negative external pressure gradient is computed. Both ion-drag pumps and induction EHD pumps are considered. Pumps based on the ion-drag principle that are studied include a one-dimensional pump, a two-dimensional pump driven by a stationary potential gradient, and another driven by a traveling potential wave. Two-dimensional repulsion-type and attraction-type induction pumping caused by a gradual variation in the electrical conductivity of the fluid is also investigated. The efficiency of EHD pumps exhibited a strong dependence on bulk fluid velocity: for the two-dimensional steady ion-drag pump, for example, the efficiency increased from less than 2% to 22% under the influence of an external pressure gradient. The corresponding increase in efficiency for a two-dimensional repulsion-type EHD pump was from 0.26% to 24.5%.

Adsorption Dynamics of Toluene in a Packed Bed Charged with the Polymeric Adsorbent

Experimental and theoretical studies were made on the adsorption of toluene by polymeric adsorbent in an isothermal condition of 298K. A mathematical model was developed to simulate the column dynamics of a pure adsorption system. To represent the mass-transfer rate inside the adsorbent particle, the linear driving-force approximation model was applied. By optimizing the breakthrough curve data with a dynamic model, an empirical correlation was proposed to represent the overall mass-transfer rate for the toluene-polymeric adsorbent system. The result with this correlation, which is a function of the partial pressures for adsorbate and interstitial bulk fluid velocities, showed that the prediction agreed well with the experimental data of breakthrough curves.

Influence of Surface Tension-Driven Convection on Cyclic Voltammograms of Langmuir Films of Redox-Active Amphiphiles

Simulations were conducted to elucidate the effects of Marangoni convection on the cyclic voltammograms of Langmuir films of redox-active amphiphiles. Oxidation and reduction of the amphiphiles occur at a line electrode that contacts the Langmuir film, and oxidation is assumed to introduce an electrostatic contribution to the surface equation of state of the film. A boundary element method is used to compute the bulk fluid velocities that are induced by the corresponding surface tension gradients and a finite element method used to compute the surface transport of the surfactants. When compared with the case of purely diffusional transport processes, the simulation results reveal that when Marangoni convection is included, the cyclic voltammograms become asymmetric. The anodic peaks tend to be larger and sharper than the cathodic and the ratio of cathodic peak to anodic peak is about 0.808. The results of the simulation are compared to past reports of cyclic voltammograms of Langmuir monolayers of a ferrocenyl amphiphile, and good agreement is found.

Comment [on “The Parker Challenge”

Fundamental to considering the relative merits of a (B, V) versus an (E, j) description of a plasma, one must first define the bulk fluid velocity and characterize the dissipation mechanism responsible for fluid cohesion. While the equations of magnetohydrodynamics (MHD) owe a heritage to studies of Coulomb‐collisionally dominated plasma, many geospace regions of interest are, in fact, Coulomb‐collisionless. Yet, the fluid equations of collisional MHD give a remarkably good and predictive description of this plasma. Still, collisionless plasma have qualities not captured by the collisional MHD paradigm, and their recognition requires an interpretation of the bulk fluid velocity that distinguishes the frozen‐in frame and the flux‐defined fluid velocity of conventional MHD.

Particle tracking techniques for electrokinetic microchannel flows.

We have applied particle tracking techniques to obtain spatially resolved velocity measurements in electrokinetic flow devices. Both micrometer-resolution particle image velocimetry (micro-PMV) and particle tracking velocimetry (PTV) techniques have been used to quantify and study flow phenomena in electrokinetic systems applicable to microfluidic bioanalytical devices. To make the flow measurements quantitative, we performed a series of seed particle calibration experiments. First, we measure the electroosmotic wall mobility of a borosilicate rectangular capillary (40 by 400 microm) using current monitoring. In addition to this wall mobility characterization, we apply PTV to determine the electrophoretic mobilities of more than 1,000 fluorescent microsphere particles in aqueous buffer solutions. Particles from this calibrated particle/ buffer mixture are then introduced into two electrokinetic flow systems for particle tracking flow experiments. In these experiments, we use micro-PIV, together with an electric field prediction, to obtain electroosmotic flow bulk fluid velocity measurements. The first example flow system is a microchannel intersection where we demonstrate a detailed documentation of the similitude between the electrical fields and the velocity fields in an electrokinetic system with uniform zeta potential, zeta. In the second system, we apply micro-PIV to a microchannel system with nonuniform zeta. The latter experiment provides a simultaneous measurement of two distinct wall mobilities within the microchannel.

Predicting thermal breakthrough in heterogeneous media from tracer tests

Analysis of the conservation equations demonstrates that the ratio of fluid-to-temperature velocities is a constant, even in heterogeneous media. Because tracers move at the bulk fluid velocity, tracer test analysis can be used to predict temperature declines at production wells that result from injection. Tracer histories are transformed into predicted temperature histories through a simple variable transformation. Examples demonstrate that the method works well for single-phase flow in heterogeneous (but porous) media. In the presence of strong permeability correlation (layering), the method's accuracy degrades somewhat; the onset of cooling, however, is still predicted reasonably accurately.

A theoretical study on the effect of surface roughness on mass transport and transformation in biofilms

This modeling study evaluates the influence of biofilm geometrical characteristics on substrate mass transfer and conversion rates. A spatially two-dimensional model was used to compute laminar fluid flow, substrate mass transport, and conversion in irregularly shaped biofilms. The flow velocity above the biofilm surface was varied over 3 orders of magnitude. Numerical results show that increased biofilm roughness does not necessarily lead to an enhancement of either conversion rates or external mass transfer. The average mass transfer coefficient and Sherwood numbers were found to decrease almost linearly with biofilm area enlargement in the flow regime tested. The influence of flow, biofilm geometry and biofilm activity on external mass transfer could be quantified by Sh-Re correlations. The effect of biofilm surface roughness was incorporated in this correlation via area enlargement. Conversion rates could be best correlated to biofilm compactness. The more compact the biofilm, the higher the global conversion rate of substrate. Although an increase of bulk fluid velocity showed a large effect on mass transfer coefficients, the global substrate conversion rate per carrier area was less affected. If only diffusion occurs in pores and channels, then rough biofilms behave as if they were compact but having less biomass activity. In spite of the fact that the real biofilm area is increased due to roughness, the effective mass transfer area is actually decreased because only biofilm peaks receive substrate. This can be explained by the fact that in the absence of normal convection in the biofilm valleys, the substrate gradients are still largely perpendicular to the carrier. Even in the cases where convective transport dominates the external mass transfer process, roughness could lead to decreased conversion rates. The results of this study clearly indicate that only evaluation of overall conversion rates or mass fluxes can describe the correct biofilm conversion, whereas interpretation of local concentration or flow measurements as such might easily lead to erroneous conclusions.

Mass transfer coefficients within anaerobic biofilms: effects of external liquid velocity

This work concerns mass transport in anaerobic biofilms, formed under upflow liquid velocities similar to the prevailing conditions in anaerobic reactors used for wastewater treatment. During biofilm formation under liquid velocities of 1.5 and 13.2 m/h, internal mass transfer coefficients were routinely measured. Mass transfer coefficients attained pseudo steady-state values between 2–4×10 −3 m/h, no dependence being observed between bulk flow and internal mass transport rates. However, a transient variation in the liquid velocity from 1.5 up to 13.2 m/h, imposed after the biofilm had reached the steady-state, increased the internal mass transport by 20% on average. This result suggests that periodic changes in the bulk fluid velocity can be used as a tool to increase the transport of soluble substrates inside already formed biofilms, although the effect seems to be limited.

An experimental investigation of capsules of anomalous shape conveyed by liquid in a pipe

This paper deals with the effect of bulk fluid velocity, diameter and length ratios, apparent density and volumetric concentration on the capsule/liquid velocity ratio, pressure gradient of the capsule flow in the pipe and on the position of capsules. The position and shape deformation of semi-rigid capsules contributed to the explanation of the effect and distribution of the transversal hydrodynamic forces, acting on a capsule. Measurements were carried out in a horizontal straight plexiglass pipe of inner diameter 0.05 m and water was used as a carrier liquid. It was found that the semi-rigid capsule trains reached lower velocities than the rigid ones for the regime given by Reynolds number ( Re) lower than 5×10 4, whereas the opposite holds for the region where the Re exceeds 10 5. The semi-rigid capsule trains have about 20% lower pressure gradient than trains consisted of identical solid cylindrical capsules for the regime given by Re lower than 5×10 4, for the higher flow velocity this effect gradually disappears.

Beam misalignments and fluid velocities in laser-induced thermal acoustics

Beam misalignments and bulk fluid velocities can influence the time history and intensity of laser-induced thermal acoustics (LITA) signals. A closed-form analytic expression for LITA signals incorporating these effects is derived, allowing the magnitude of beam misalignment and velocity to be inferred from the signal shape. It is demonstrated how instantaneous, nonintrusive, and remote measurement of sound speed and velocity (Mach number) can be inferred simultaneously from homodyne-detected LITA signals. The effects of different forms of beam misalignment are explored experimentally and compared with theory, with good agreement, allowing the amount of misalignment to be measured from the LITA signal. This capability could be used to correct experimental misalignments and account for the effects of misalignment in other LITA measurements. It is shown that small beam misalignments have no influence on the accuracy or repeatability of sound speed measurements with LITA.

A MATHEMATICAL MODEL OF SUPERCRITICAL METHYLAMINE (SCM) DELIGNIFICATION OF WOOD

An unsteady-state mathematical model describing supercritical fluid (SCF) delignification of wood in a packed bed was developed. Equations describing intraparticle supercritical methyl-amine reaction with lignin, and subsequent transport of the methylamine-lignin complex into the bulk phase were derived and solved using orthogonal collocation. Model results for the case of supercritical methylamine are presented and compared with experimental results. Simulation results correctly predicted the experimental results. Sensitivity of methylamine extraction of lignin was evaluated by varying bulk fluid velocity, mass transfer coefficient, and particle diameter. Delignification was a strong function of particle diameter and a relatively weaker function of fluid velocity and mass transfer coefficient. The Thiele modulus and Biot number showed that delignification was reaction rate limited for wood particle diameter values ≤ 0.1 cm, but delignification was limited by intra-particle mass transfer for wood particle diameter values ≥ 1.0 cm.

Equilibria and dynamics for mixed vapors of BTX in an activated carbon bed

AbstractExperimental and theoretical studies were made on the adsorption of benzene, toluene, p‐xylene, and their binary and ternary mixtures by activated carbon in an isothermal condition of 303 K. Experimental isotherms for pure components were measured by two kinds of experimental techniques: the static volumetric method and breakthrough data analysis. Breakthrough data analysis shows that both the extended Langmuir equation and the ideal adsorbed solution theory could explain mixture isotherm data with good accuracy. In addition, a mathematical model was developed to simulate the column dynamics of pure and mixed‐vapor adsorption systems. To represent the mass‐transfer rate inside the adsorbent particle, the linear driving‐force approximation model was applied. By optimizing the pure‐ and the binary‐component breakthrough curve data with a dynamic model, an empirical correlation was proposed to represent the overall mass‐transfer rates for the BTX vapor‐activated carbon system. Results with this correlation, which is a function of the partial pressures for each adsorbate and interstitial bulk fluid velocities, showed that the prediction agrees well with the experimental data of ternary breakthrough curves.

Influence of Fluid Velocity and Cell Concentration on the Transport of Motile and Nonmotile Bacteria in Porous Media

The effect of fluid velocity on the transport of motile and nonmotile bacteria was studied in saturated soil columns using radiolabeled cells. According to colloid filtration theory, decreasing the bulk fluid velocity in a porous medium increases the number of collisions of passive colloids with particles and, therefore, should result in increased colloid retention in porous media. However, for motile cells, there was a variation in cell retention significantly different from that predicted by filtration theory at low fluid velocities, leading to the conclusion that filtration theory is not applicable for this motile bacterial strain at low fluid velocities. As the pore velocity was decreased from 120 to 0.56 m/day, the fractional retention of motile cells (Pseudomonas florescens P17) decreased by 65%, and the collision efficiency (α) defined as the ratio of particles that attach to soil grains to particles that collide with the soil (calculated using a filtration equation) decreased from 0.37 (120 m/day) to 0.003 (0.56 m/day). For passive colloids, the fractional retention (if α is a constant equal to 0.01) would increase by more than 800% over this same velocity range. To support our conclusion that cell motility was the factor producing this change from filtration theory, we rendered P17 cells nonmotile and tested this strain and a second nonmotile strain [Burkholderia (Pseudomonas) cepacia G4] under the same conditions. Collision efficiencies for both nonmotile suspensions were constant. For nonmotile P17, α was equal to 0.018 ± 0.003 (0.56−590 m/day). Over a wide velocity range for nonmotile G4, α was equal to 0.22 ± 0.067 (11−560 m/day). Swimming cells were presumably able to avoid sticking to soil grains at low fluid velocities, but at high fluid velocities, cell motility did not reduce attachment. Two additional factors known to affect cell transport (solution ionic strength and cell concentration) were also examined with these two strains in porous media. Decreasing the ionic strength from 4.14 to 0.0011 mM (at a constant pH) decreased cell retention for motile P17 by 39 ± 12%, but this is less of a reduction than is typically observed for nonmotile strains. Increasing the cell concentrations of motile P17 increased the overall retention of cells, suggesting that previously deposited cells provided a more favorable surface for adhesion than the native soil (ripening). In contrast, increasing the cell concentrations of G4 resulted in lower retention, suggesting that deposited cells provided a less favorable collector surface (blocking). These results need to be further investigated with other motile and nonmotile species. However, our results do suggest that wider dispersal of cells during bioaugmentation than previously thought possible may be achieved by using a combination of motile cells, low pumping velocities, and low ionic strength solutions. Optimal cell concentrations to use for in situ bioaugmentation of contaminated soil will depend on the adhesion of the bacterial strains for soil grains and with each other, but in general blocking-type cells are capable of greater dispersal at higher concentration than ripening-type cells.

Nonclassical Theory of Turbulent Collisionless Plasmas

A transport theory is developed for the proton component of collisionless plasmas; nonclassical is taken to mean that Maxwellian distributions and Coulomb collisions are nonexistent or cannot be assumed outright. Novel features include a self-consistent description of the coupling between nonthermal distributions and the bulk fluid velocity. This theory combines results from several well-established research disciplines in the field of space plasma theory: turbulence, particle acceleration, and fluid dynamics. Key to the present treatment is basing the plasma description on the ensemble average of the proton distribution function, and complementing it with the bulk fluid velocity, for a complete description of the proton component of turbulent collisionless plasma. The information contained in the distribution and fluid velocity encompasses that in a conventional MHD description of velocity, density, and pressure. This approach also provides transport coefficients, viscosity and conductivity, and these can be used to fill out a conventional MHD description. Motivation for this work is to supply a description of collisionless plasmas appropriate for investigating nonthermal effects in the creation of stellar coronae and acceleration of stellar winds. The basic assumption of this approach is that magnetic fluctuations in the turbulent collisionless plasma will tend to isotropize the ensemble average of the proton distribution. This average may be identified observationally as the same time average used to construct the average fluid velocity and magnetic field of standard solar wind turbulence theory. Expansion about isotropy replaces the expansion about a Maxwellian in classical plasma models, allowing closure of the moment equations. Because isotropization need not proceed in the usual flux-defined fluid frame, one finds contributions to the fluid pressure from a fluid-frame mass flux driven by gradients and fluid acceleration. Such terms must, in general, accompany any fluid description of the solar wind or other collisionless plasma in which dissipation of fluctuations affects the dynamics of the average fluid variables. The equation describing the proton distribution reduces to the cosmic-ray transport equation at high particle speeds and describes heating of the distribution due to shears, compressions, and accelerations at low proton speeds. This description thus unifies our understanding of dissipation in collisionless plasmas over the range of particle speeds.

Fast flow phenomena in a toroidal plasma

The bulk fluid velocity is measured spectroscopically with 10 μs time resolution in the Madison Symmetric Torus (MST) reversed-field pinch (RFP) [Fusion Technol. 19, 131 (1991)], a diagnostic capability used to study the fast flow dynamics associated with locked modes and the RFP dynamo. The phase velocity of the tearing modes and the fluid velocity accelerate between sawtooth events, reaching a maximum speed of about 20 km/s in a few ms. Both slow down at the sawtooth crash in ≊100 μs. This deceleration time scale is as calculated for the tearing modes from the action of electromagnetic torque on the magnetic islands, but is much faster than expected from the viscous torque on the bulk fluid. In the RFP, correlated fluctuations in the tearing modes and fluid velocity probably also generate current via the ‘‘RFP dynamo,’’ 〈ũ×B̃〉, where u is the bulk fluid velocity. Initial data indicate a possible increase in 〈ũφB̃r〉 during sawtooth events, coincident with toroidal flux generation.

Influence of aggregate size on solute transport as measured using computed tomography

Application of computed tomography (CT) using selected tracers for in-situ monitoring of solute transport in soil cores is useful for identifying the influence of soil structure on solute movement. The objective of this study was to apply X-ray CT techniques for measurement of solute breakthrough curves for selected soil aggregates and to determine the two-dimensional velocity distributions within soil cores. Soil material from the A horizon of a Mexico silt loam (Udollic Ochraqualf) was brought to the laboratory, passed through selected sieves, and packed in soil cores (76 mm × 76 mm). Solute breakthrough experiments were conducted using a 1 % KI solution with the cores held in an apparatus used to control the bulk fluid velocity. Shapes of average breakthrough curves measured using CT generally compared well with curves measured from effluent samples. Aggregate sizes of 0.50–1.00 and 1.00–2.00 mm gave dispersion coefficients of 0.37 and 1.13 mm 2/s, respectively, using a pore water velocity of 0.11 mm/s. Aggregate sizes of 0.25–0.50 and 1.00–2.00 mm gave dispersion coefficients of 0.042 and 0.123 mm 2/s, respectively, using a pore water velocity of 0.05 mm/s. Techniques were developed for calculating the pore water velocity distribution as a function of location within the soil core. Velocities can be calculated at approximately 17,000 locations within a 76-mm diameter soil core, thus achieving a level of detail not previously obtained.