Showing papers on "Volumetric flow rate published in 2005"

Microfluidic flow focusing: drop size and scaling in pressure versus flow-rate-driven pumping.

Abstract: We experimentally study the production of micrometer-sized droplets using microfluidic technology and a flow-focusing geometry. Two distinct methods of flow control are compared: (i) control of the flow rates of the two phases and (ii) control of the inlet pressures of the two phases. In each type of experiment, the drop size l, velocity U and production frequency f are measured and compared as either functions of the flow-rate ratio or the inlet pressure ratio. The minimum drop size in each experiment is on the order of the flow focusing contraction width a. The variation in drop size as the flow control parameters are varied is significantly different between the flow-rate and inlet pressure controlled experiments.

Two-Phase Modeling and Flooding Prediction of Polymer Electrolyte Fuel Cells

Abstract: A newly developed theory of liquid water transport in hydrophobic gas diffusion layers is applied to simulate flooding in polymer electrolyte fuel cells ~PEFCs! and its effects on performance. The numerical model accounts for simultaneous two-phase flow and transport of species and electrochemical kinetics, utilizing the well-established multiphase mixture formulation to efficiently model the two-phase transport processes. The two-phase model is developed in a single domain, yielding a single set of governing equations valid in all components of a PEFC. The model is used to explore the two-phase flow physics in the cathode gas diffusion layer. Multidimensional simulations reveal that flooding of the porous cathode reduces the rate of oxygen transport to the cathode catalyst layer and causes a substantial increase in cathode polarization. Furthermore, the humidification level and flow rate of reactant streams are key parameters controlling PEFC performance and two-phase flow and transport characteristics. It is also found that minimization of performance limitations such as membrane dry-out and electrode flooding depends not only on material characteristics but also on the optimization of these operating parameters.

Stability of parallel flows in a microchannel after a T junction.

Abstract: In this work, the flow of immiscible fluids in microchannels is studied. Flow pattern diagrams obtained in microfluidic chips are presented. Monodisperse droplets or parallel flows are obtained depending on the flow rate values of the aqueous phase and the oil phase. Transition from droplet regime to parallel flows cannot be described in terms of capillary numbers. Using confocal microscopy and high speed imaging, it was shown that droplets are formed through a blocking-pinching mechanism ruled by flow rate conservation. Conditions for parallel flow stability are quantified.

Characteristic periodic motion of polymers in shear flow.

Abstract: The motion of both free and tethered polymer molecules as well as rigid Brownian rods in unbound shear flow is found to be characterized by a clear periodicity or tumbling frequency. Periodicity is shown using a combination of single molecule DNA experiments and computer simulations. In all cases, we develop scaling laws for this behavior and demonstrate that the frequency of characteristic periodic motion scales sublinearly with flow rate.

Membrane Cleaning in the Dairy Industry: A Review

Abstract: Membrane separation processes have become part of the set of basic unit operations for dairy process design and product development. These processes are employed in a variety of separation and concentration duties, but in all cases, the membranes must be cleaned regularly to remove both organic and inorganic material deposited on the surface from the fluid stream being processed. Cleaning is a vital step in maintaining the permeability and selectivity of the membrane and is necessary to return the plant to its original capacity, to minimize risks of bacteriological contamination, and to produce acceptable products. Caustic-, acidic-, and enzyme-based cleaners may be used for membrane cleaning and are usually formulated with additives to best match the specific cleaning duty. Cleaning generates significant volumes of wastewater and reduces membrane life and plant productivity, so each regime must be optimized with respect to concentration, order and duration of cleaning steps, temperature, pressure, and flow rate. This article reviews the key mechanisms governing cleaning performance and suggests directions by which further optimization may be achieved.

Role of hydrodynamic drag on microsphere deposition and re-entrainment in porous media under unfavorable conditions.

Abstract: Deposition and re-entrainment of 1.1 μm microspheres were examined in packed glass beads and quartz sand under both favorable and unfavorable conditions for deposition. Experiments were performed at environmentally relevant ionic strengths and flow rates in the absence of solution chemistry and flow perturbations. Numerical simulations of experimental data were performed using kinetic rate coefficients to represent deposition and re-entrainment dynamics. Deposition rate coefficients increased with increasing flow rate under favorable deposition conditions (in the absence of colloid-grain surface electrostatic repulsion), consistent with expected trends from filtration theory. In contrast, under unfavorable deposition conditions (where significant colloid-grain surface electrostatic repulsion exists), the deposition rate coefficients decreased with increasing flow rate, suggesting a mitigating effect of hydrodynamic drag on deposition. Furthermore, the re-entrainment rate was negligible under favorable con...

Flow pattern visualization in a mimic anaerobic digester using CFD.

Abstract: Three-dimensional steady-state computational fluid dynamics (CFD) simulations were performed in mimic anaerobic digesters to visualize their flow pattern and obtain hydrodynamic parameters. The mixing in the digester was provided by sparging gas at three different flow rates. The gas phase was simulated with air and the liquid phase with water. The CFD results were first evaluated using experimental data obtained by computer automated radioactive particle tracking (CARPT). The simulation results in terms of overall flow pattern, location of circulation cells and stagnant regions, trends of liquid velocity profiles, and volume of dead zones agree reasonably well with the experimental data. CFD simulations were also performed on different digester configurations. The effects of changing draft tube size, clearance, and shape of the tank bottoms were calculated to evaluate the effect of digester design on its flow pattern. Changing the draft tube clearance and height had no influence on the flow pattern or dead regions volume. However, increasing the draft tube diameter or incorporating a conical bottom design helped in reducing the volume of the dead zones as compared to a flat-bottom digester. The simulations showed that the gas flow rate sparged by a single point (0.5 cm diameter) sparger does not have an appreciable effect on the flow pattern of the digesters at the range of gas flow rates used.

Fabrication of a peristaltic PDMS micropump

Abstract: This paper presents fabrication and drive test of a peristaltic PDMS micropump actuated by the thermopneumatic force. The micropump consists of the three peristaltic-type actuator chambers with microheaters on the glass substrate and a microchannel connecting the chambers and the inlet/outlet port. The micropump is fabricated by the spin-coating process, the two-step curing process, the molding process using negative photoresist, etc. The diameter and the thickness of the actuator diaphragm are 2.5 mm and 30 μm, respectively. The meniscus motion in the capillary tube is observed with a video camera and the flow rate of the micropump is calculated through the frame analysis of the recorded video data. The maximum flow rate of the micropump is about 0.36 μL/s at 2 Hz for the zero hydraulic pressure difference, when the three-phase input voltage is 20 V.

CFD predictions for flow‐regime transitions in bubble columns

Abstract: This work evaluates the ability of multiphase computational fluid dynamics (CFD) models to predict known flow regimes in air–water bubble columns. An initial grid-resolution study shows that grid spacing of 0.25 cm or smaller must be used for adequate resolution. The ability of the two-fluid model to predict homogeneous- and transitional-flow behavior is analyzed next, and the flow predictions are found to be highly dependent on the model formulation (that is, bubble-induced turbulence, drag, virtual mass, lift, rotation, and strain). At low gas velocities, homogeneous flow is observed for only a particular set of force models. At higher gas velocities, the same set of models yields reasonable predictions of transitional flow for small columns. Bubble size and liquid coflow also affect flow structures and flow stability at high gas flow rates. Scale-up to larger column diameters is studied for both the homogeneous- and transitional-flow regimes. In the homogeneous regime, the flow behavior is found to be independent of column diameter. However, because of neglect of coalescence, transition to churn-turbulent flow is not observed at high gas velocities for large column diameters. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Effects of Si content in DLC films on their friction and wear properties

Abstract: Diamondlike carbon (DLC) has been well known as a very hard and very low-friction material The most critical issue of DLC is to improve its adhesion To improve the adhesion, some other elements, such as Si, have been included in DLC films We have studied the effects of Si content on friction and wear properties of DLC films made by our bipolar pulse PBII system The content of Si was changed by varying the flow rates of tetramethylsilane (TMS) from 00 to 35 sccm while the flow rate of toluene and the total pressure was kept constant Friction coefficients (FCs) were measured with a JIS-SUJ2 (AISI 52100) ball of 3-mm diameter The length of sliding was 2 mm, the speed was 10 mm/s, and the load was 49 N for 40-min tests Without TMS flow, the friction coefficients (FCs) were the largest, 035, and the DLC film came off easily With the increase of TMS flow up to 10 sccm, the FC decreased to 014, and there was almost no peeling off For 20 sccm, the FC was 018, and there was a lot of debris on the DLC surface Hardness gradually decreased with the increase of TMS flow up to 10 sccm, and it decreased drastically for high flow rates or high Si content Internal stress (compressive) of the films decreased with the increase of TMS flow Si content in the film corresponded almost linearly to the TMS flow rate One to two percent of Si doping is very suitable for improving the adhesion of films and reducing internal stress while maintaining the surface hardness of DLC films

Discrete Particle Simulation of Solid Flow in a Model Blast Furnace

Abstract: This paper reports a numerical study of solid flow in a model blast furnace under simplified conditions by means of discrete particle simulation (DPS). The applicability of the proposed DPS approach is validated from its good agreement with the experiment in terms of solid flow patterns. It is shown that the DPS is able to generate a stagnant zone without any need for any arbitrary treatment, and capture the main features of solid flow within the furnace at a microscopic level. The results confirm that the solid flow in a blast furnace can be divided into four different flow regions. However, the flow is strongly influenced by the front and rear walls in a 2D slot model furnace whereas the predicted stagnant zone decreases significantly with wall sliding friction. In a 3D model with periodic boundary conditions incorporated, a smaller stagnant zone is obtained. The effects of solid flow rate, particle properties such as sliding and rolling friction coefficients on the solid flow are also investigated. The results are analysed in terms of solid flow patterns, solid velocity field, porosity distribution and normal force structure. The implication to blast furnace operation is discussed.

The influence of growth conditions on the microstructure and critical currents of TFA-MOD YBa2Cu3O7 films

Abstract: The influence of three processing parameters, temperature, gas flow rate and water pressure, on the YBa2Cu3O7 film growth on LaAlO3 single-crystal substrates from trifluoroacetate precursors has been investigated and the optimal film processing conditions to achieve high critical currents have been determined. We have found that the growth conditions maximizing the critical current density are those where the nucleation of a-axis oriented grains is minimized, as determined by μ-Raman spectroscopy. Under these conditions the normal state resistivity is very near to that of single crystals because a vanishingly small film porosity is achieved. Transmission electron microscopy analysis of films quenched from the growth temperature gives some hints for understanding the mechanism linking the film porosity with the concentration of a-axis grains. A cross-linked influence of different processing parameters, such as temperature and water pressure, or water pressure and gas flow, has been demonstrated. The optimal growth temperatures are 790–830 °C, but at these growth temperatures, the critical current density is still dependent on the gas flow rate and water pressure. The optimal processing ranges are a compromise between two different competing phenomena influencing the quality of the films: inhomogeneous film formation due to HF gas stagnancy at small nominal growth rates (low gas flow rate or water pressure) and perturbed crystallinity at high gas flow rates or water pressures.

Turbulent rotating disk flow with inward throughflow

Abstract: The evolution of the entrainment coefficient K of the rotating fluid in a rotor–stator cavity with an inward through flow and pre-rotation is studied according to the flow parameters. Measurements are obtained in water for a turbulent Batchelor type of flow with two separated boundary layers on the rotating and stationary disks by means of a laser Doppler anemometer, and the results are compared to those performed using pressure transducers. We show that the entrainment coefficient K depends on a local flow rate coefficient Cqr according to a 5/7 power law whose coefficients depend on the boundary conditions. A theoretical analysis confirms this behaviour of K.

The Debye-Hückel approximation: Its use in describing electroosmotic flow in micro- and nanochannels

Abstract: In this work we consider the electroosmotic flow in a rectangular channel. We consider a mixture of water or other neutral solvent and a salt compound, such as sodium chloride, and other buffers for which the ionic species are entirely dissociated. Results are produced for the case where the channel height is much greater than the width of the electric double layer (EDL) (microchannel) and for the case where the channel height is of the order or slightly greater than the width of the EDL (nanochannel). At small cation, anion concentration differences the Debye-Huckel approximation is appropriate; at larger concentration differences, the Gouy-Chapman picture of the electric double emerges naturally. In the symmetric case, the velocity field and the potential are identical. We specifically focus in this paper on the limits of the Debye-Huckel approximation for a simplified version of a phosphate-buffered saline (PBS) mixture. The fluid is assumed to behave as a continuum and the volume flow rate is observed to vary linearly with channel height for electrically driven flow in contrast to pressure-driven flow which varies as height cubed. This means that very large pressure drops are required to drive flows in small channels. However, useful volume flow rates may be obtained at a very low driving voltage. In the course of the solution, we establish the relationship between the wall mole fractions of the electrolytes and the zeta potential. Multivalent electrolyte mixtures are also considered.

CFD Simulations of Coupled, Countercurrent Combustor/Reformer Microdevices for Hydrogen Production

Abstract: Two-dimensional computational fluid dynamics (CFD) simulations are used to study spatially segregated, multifunctional, microchemical devices for hydrogen production. In particular, coupling between homogeneous propane combustion and catalytic ammonia decomposition on a Ru catalyst is studied in a microdevice consisting of alternating combustion and decomposition channels as a function of flow rate and materials conductivity in the countercurrent flow configuration. It is found that the high temperatures generated via homogeneous combustion lead to high conversions in short contact times and thus to compact devices. Different performance measures are evaluated to assess the operability of the device. Sufficiently high ammonia flow rates serve a dual purpose by lowering device temperatures and enabling the production of larger flow rates of hydrogen. Finally, it is shown that device operation is limited only to high-conductivity materials and fast ammonia flows.

Bioprocess intensification in flow-through monolithic microbioreactors with immobilized bacteria.

Abstract: Microporous polymers (with porosity up to 90%) with a well- prescribed internal microstructure were prepared in monolithic form to construct a flow-through microbioreactor in which phenol-degrading bacteria, Pseudomonas syringae, was immobilized. Initially, bacteria was forced seeded within the pores and subsequently allowed to proliferate followed by acclimatization and phenol degradation at various initial substrate concentrations and flow rates. Two types of microporous polymer were used as the monolithic support. These polymers differ with respect to their pore and interconnect sizes, macroscopic surface area for bacterial support, and phase volume. Polymer with a nominal pore size of 100 μm with phase volume of 90% (with highly open pore structure) yielded reduced bacterial proliferation, while the polymer with nominal pore size of 25 μm with phase volume of 85% (with small interconnect size and large pore area for bacterial adhesion) yielded monolayer bacterial proliferation. Bacteria within the 25 μm polymer support remained monolayered, without any apparent production of extracellular matrix during the 30-day continuous experimental period. The microbioreactor performance was characterized in terms of volumetric utilization rate and compared with the published data, including the case where the same bacteria was immobilized on the surface of microporous polymer beads and used in a packed bed during continuous degradation of phenol. It is shown that at similar initial substrate concentration, the volumetric utilization in the microreactor is at least 20-fold more efficient than the packed bed, depending on the flow rate of the substrate solution. The concentration of the bacteria within the pores of the microreactor decreases from 2.25 cells per μm2 on the top surface to about 0.4 cells per μm2 within 3 mm reactor depth. If the bacteria-depleted part of the microreactor is disregarded, the volumetric utilization increases by a factor of 30-fold compared with the packed bed. This efficiency increase is attributed to the reduction of diffusion path for the substrate and nutrients and enhanced availability of the bacteria for bioconversion in the absence of biofilm formation as well as the presence of flow over the surface of the monolayer bacteria. © 2005 Wiley Periodicals, Inc.

Flow rate measurement device

Abstract: PROBLEM TO BE SOLVED: To provide a flow rate measurement device wherein variation of a flow velocity measurement characteristic in each product is reduced at low cost. SOLUTION: A flow passage unit 20 forms a flow passage communicating a gas inflow port 11 and a gas outflow port 12. A multilayer unit 24 is stored inside an intermediate flow passage 22 constituting the flow passage unit 20, and forms a measurement flow passage wherein flow velocity measurement by a flow velocity sensor is performed. Rubber bands 26a, 26b wound on the outer face of the multilayer unit 24 closes up a gap C between the inner face of the intermediate flow passage 22 and the outer face of the multilayer unit 24 to prevent a flow of a gas to the gas outflow port 12 by passing through the gap C. COPYRIGHT: (C)2006,JPO&NCIPI