Showing papers on "Volumetric flow rate published in 2006"

Liquid Water Removal from a Polymer Electrolyte Fuel Cell

Abstract: Liquid water transport and removal from the gas diffusion layer GDL and gas channel of a polymer electrolyte fuel cell PEFC are studied experimentally and theoretically. In situ observations of the liquid water distribution on the GDL surface and inside the gas channel were made in an operating transparent PEFC. Liquid droplet formation and emergence from the GDL surface are characterized and two modes of liquid water removal from the GDL surface identified: one through droplet detachment by the shear force of the core gas flow followed by a mist flow in the gas channel, and the other by capillary wicking onto the more hydrophilic channel walls followed by the annular film flow and/or liquid slug flow in the channel. In the former regime, typical of high gas flow rates, the droplet detachment diameter is correlated well with the mean gas velocity in the channel. In the latter regime characteristic of low gas flow rates, liquid spreading over hydrophilic channel surfaces and drainage via corner flow were observed and analyzed. A theory is developed to determine what operating parameters and channel surface contact angles lead to sufficient liquid drainage from the fuel cell via corner flow. Under these conditions, the fuel cell could operate stably under a low flow rate or stoichiometry with only a minimum pressure drop required to drive the oxidizer flow. However, when the corner flow is insufficient to remove liquid water from the gas channel, it was observed that the annular film flow occurs, often followed by film instability and channel clogging. Channel clogging shuts down an entire channel and hence reduces the cell’s active area and overall performance.

Flow-induced deformation of shallow microfluidic channels

Abstract: We study the elastic deformation of poly(dimethylsiloxane) (PDMS) microchannels under imposed flow rates and the effect of this deformation on the laminar flow profile and pressure distribution within the channels. Deformation is demonstrated to be an important consideration in low aspect ratio (height to width) channels and the effect becomes increasingly pronounced for very shallow channels. Bulging channels are imaged under varying flow conditions by confocal microscopy. The deformation is related to the pressure and is thus non-uniform throughout the channel, with tapering occuring along the stream-wise axis. The measured pressure drop is monitored as a function of the imposed flow rate. For a given pressure drop, the corresponding flow rate in a deforming channel is found to be several times higher than expected in a non-deforming channel. The experimental results are supported by scaling analysis and computational fluid dynamics simulations coupled to materials deformation models.

Preparation of highly monodisperse droplet in a T-junction microfluidic device

Abstract: In our previous work, a new flow route was developed—a so-called perpendicular shear force–induced droplet formation—in a self-designed simple T-junction microchannel device. In this work, the crossflowing rupture technique was used to prepare monodisperse droplets in a similar device and successfully prepared monodisperse droplets ranging from 50 to 500 μm with polydispersity index (σ) values of <2%. Two kinds of flow patterns of plug flow and drop flow in the T-junction microchannels could be formed. By changes in the surfactant concentration, the interfacial tension and the wetting ability varied, and the disordered or ordered two-phase flow patterns could be controlled. Evolutions of the contact angle of the oil in contact with the wall surface were explained by the adsorption of surfactant molecules to the solid–liquid interface. The increase of continuous phase flow rate and viscosity resulted in the decrease of the droplet size, and the droplet size was correlated with capillary number Ca. By comparing the variation range of drop size using the two methods, it was found that the method of perpendicular flow-induced droplet formation can control the drop size over a much wider range. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Shear force induced monodisperse droplet formation in a microfluidic device by controlling wetting properties

Abstract: Perpendicular flow is used to induce oil droplet breakup by using a capillary as water phase flow channel. It is a new route to produce monodisperse emulsions. The wetting properties of the fluids on the walls are exceedingly important parameters. Depending on the oil and water flow rates, different spatial distributions of the two phases as laminar, plugs, cobbles and drops, are obtained. The effects of two-phase flow rates on plugs and drop size are studied, and the different droplet formation mechanisms of plug flow and drop flow are discussed. Two quantitative equations utilized to predict the droplet size are developed.

Method for forming silicon carbide film containing oxygen

Abstract: A method for forming a silicon carbide film containing Si, C, O, H, and optionally N on a substrate placed in a reaction space, includes the steps of: introducing into the reaction space a precursor containing Si, C, O, and H and having at least one Si—O bond in its molecule; introducing into the reaction space an inert gas; applying RF power in the reaction space, wherein a ratio of a flow rate (sccm) of the inert gas to the RF power (W/cm2) is controlled at 30-850; and thereby depositing on the substrate a silicon carbide film containing Si, C, O, H, and optionally N.

Simulation of Fluid Flow Through a Granular Bed

Abstract: Numerical study of flow through random packing of non-overlapping spheres in a cylindrical geometry is investigated. Dimensionless pressure drop has been studied for a fluid through the porous media at moderate Reynolds numbers (based on pore permeability and interstitial fluid velocity), and numerical solution of Navier-Stokes equations in three dimensional porous packed bed illustrated in excellent agreement with those reported by Macdonald [1979] in the range of Reynolds number studied. The results compare to the previous work (Soleymani et al., 2002) show more accurate conclusion because the problem of channeling in a duct geometry. By injection of solute into the system, the dispersivity over a wide range of flow rate has been investigated. It is shown that the lateral fluid dispersion coefficients can be calculated by comparing the concentration profiles of solute obtained by numerical simulations and those derived analytically by solving the macroscopic dispersion equation for the present geometry.Copyright © 2006 by ASME

Direct Liquid Jet-Impingment Cooling With Micron-Sized Nozzle Array and Distributed Return Architecture

Abstract: We demonstrate submerged single-phase direct liquid-jet-impingement cold plates that use arrays of jets with diameters in the range of 31 to 126 mum and cell pitches from 100 to 500 mum for high power-density microprocessor cooling applications. Using parallel inlet and outlet manifolds, a distributed return concept for easy scaling to 40,000 cells on an area of 4 cm was implemented. Pressure drops < 0.1 bar at 2.5 1/min flow rate have been reached with a hierarchical tree-like double-branching manifold. Experiments were carried out with water jets having Reynolds numbers smaller than 900 at nozzle to heater gaps ranging between 3 to 300 mum. We identified four flow regimes, namely, pinch-off, transition, impingement, and separation, with different influences on heat-removal and pressure-drop characteristics. Parametric analysis resulted in an optimal heat-removal rate of 420 W/cm2 using water as a coolant. For a near optimal design with a gap to inlet diameter ratio of 1.2, we measured a heat-transfer coefficient of 8.7 W/cm2 K and a junction to inlet fluid unit thermal resistance of 0.17 Kcm2 /W (720 mum chip), which is equivalent to a 370 W/cm2 cooling performance at a junction to inlet fluid temperature rise of 63 degC, a pressure drop of 0.35 bar, and a flow rate of 2.5 1/min

Effects of surface roughness and interface wettability on nanoscale flow in a nanochannel

Abstract: Non-equilibrium molecular dynamics simulations have been carried out to investigate the effect of surface roughness and interface wettability on the nanorheology and slip boundary condition of simple fluids in a nanochannel of several atomic diameters width. The solid surfaces decorated with periodic nanostrips are considered as the rough surface in this study. The simulation results showed that the interface wettability and the surface roughness are important in determining the nanorheology of the nanochannel and fluid slip at solid–fluid interface. It is observed that the presence of surface roughness always suppresses the fluid slip for hydrophilic and hydrophobic surface nanochannels. For fluids over smooth and hydrophobic surfaces, the snapshots of fluid molecules show that an air gap or nanobubble exists at the fluid–solid interface, resulting in the apparent slip velocity. For a given surface with fixed interface wettability, the fluid velocities increase by increasing the driving force, while the driving force has no significant influence on the density structure of fluid molecules. The fluid slip and the flow rate are measured for hydrophilic and hydrophobic nanochannels. The flow rates in rough surface nanochannels are smaller than those of smooth surface walls due to the increase of drag resistance at the solid–fluid interface. The dependence between fluid slip and flow rate showed that the slip length increases approximately linearly with the flow rate for both the hydrophobic and hydrophilic surface nanochannels.

Influence of bubble size on micro-bubble drag reduction

Abstract: Micro-bubble drag reduction experiments were conducted in a turbulent water channel flow. Compressed nitrogen was used to force flow through a slot injector located in the plate beneath the boundary layer of the tunnel test section. Gas and bubbly mixtures were injected into a turbulent boundary layer (TBL), and the resulting friction drag was measured downstream of the injector. Injection into tap water, a surfactant solution (Triton X-100, 20 ppm), and a salt-water solution (35 ppt) yielded bubbles of average diameter 476, 322 and 254 μm, respectively. In addition, lipid stabilized gas bubbles (44 μm) were injected into the boundary layer. Thus, bubbles with d+ values of 200 to 18 were injected. The results indicate that the measured drag reduction by micro-bubbles in a TBL is related strongly to the injected gas volumetric flow rate and the static pressure in the boundary layer, but is essentially independent of the size of the micro-bubbles over the size range tested.

Microchannel flow control through a combined electromagnetohydrodynamic transport

Abstract: A mathematical model is developed to study the combined influences of electromagnetohydrodynamic forces in controlling the fluid flow through parallel plate rectangular microchannels. The electric double layer (EDL) effects are modelled by employing the classical Poisson–Boltzmann equation. The governing fluid flow equations are subsequently solved, in an effort to obtain closed form expressions depicting the variations in the overall flow rate as a function of various influencing system parameters. It is revealed that, with the aid of a relatively low-magnitude magnetic field, a substantial augmentation in the volumetric flow rates can be achieved. However, with magnetic fields of higher strengths, strongly opposing volumetric forces might offset any further possibilities of flow rate augmentation. Certain critical non-dimensional parameters are also identified, which can play significant roles in the overall flow augmentation mechanism.

Supercritical CO2 extraction of nutmeg oil: Experiments and modeling

Abstract: Nutmeg oil was extracted from nutmeg seed at pressures of 15–20 MPa and temperatures of 313–323 K with supercritical CO2. The effects of separation parameters such as temperature, pressure, CO2 flow rate and particle size on the extraction rate of nutmeg oil were observed. Broken and intact cells (BIC) model combined with discontinuous phase equilibrium between fluid phase and solid phase, and shrinking core model were selected to describe the extraction process. For BIC model, the initial fraction solute in broken cell to total solute in the ground particle f, dimensionless transition concentration Xc and partition coefficient K were used as fitting parameters. For shrinking core model, two effective diffusivities De were used as fitting parameters. The best fitting of De1 was from 4.33 × 10−9 to 7.69 × 10−8 m2/s and De2 was from 1.90 × 10−9 to 3.20 × 10−8 m2/s. From comparison of experimental data and models calculation, the shrinking core model could describe the experimental data well for all extraction conditions, while the BIC model could only describe the data at lower extraction yields well.

Electrospray Deposition, Model, and Experiment: Toward General Control of Film Morphology

Abstract: Poly(vinylidene fluoride) film formation with electrospray deposition has been studied with support of a droplet evaporation model. The input parameters of the model consist basically of the solvent, the solute concentration, the flow rate, and the solution conductivity. The model provides the droplet size, the solute concentration, the droplet velocity, and the shear stress of the droplet at impact as a function of the distance between the nozzle and the substrate. With some additional experimental information such as the size change of the film with spray distance and the viscosity of the solution, the growth rate of the film and the shear rate of the droplet at impact can be determined. Growth rate is shown to define distinct regimes of film formation. In those regimes, only a single factor or a limited number of factors controls the film morphology. The most important factors include the shear rate and the surface energy. It is found that at a specific range of growth rates only the shear rate determi...

Direct gas injection into saturated glass beads: Transition from incoherent to coherent gas flow pattern

Abstract: [1] The transition from incoherent to coherent buoyancy-driven gas flow is investigated in two-dimensional tanks filled with glass beads using a high-resolution optical-gravimetrical setup. Both a grain-size (dk)- and flow rate (Q)-dependent transition are observed in the gas flow pattern. Standard quasistatic criteria do not explain the experimental results, since they do not take into account the competition between stabilizing friction forces and destabilizing capillary and gravitational forces. Conceptualizing the steady state tortuous gas flow as core-annulus flow and applying Hagen-Poiseuille flow for a straight capillary, we propose a flow rate and grain-size-dependent stability criterion that accounts for the experimental results and is used to classify the experiments in a dk-Q diagram.

Atomic Oxygen Production and Exploration of Reaction Mechanisms in a He‐O2 Atmospheric Pressure Glow Discharge Torch

Abstract: This article reports on the performance of a miniature APGD-t conceived for the production of reactive species participating in bio-applications. Two operating parameters were varied: the plasma-forming gas flow rate (0.5-1.5 slm He) and the flow rate of O 2 (0-50 sccm), which was injected downstream from the plasma-forming zone, and which was used as a source of reactive species. The production of reactive species (O) was optimized, and the air entrainment in the plasma jet was minimized for a He gas flow rate of 1-1.5 slm and an O 2 /He volumetric ratio of 0.3 vol.-%. A survey of the possible reaction pathways in the afterglow plasma jet suggests that the reactive species present in the plasma afterglow, and possibly reaching a remote substrate, are metastable He(2 3 S), ground state O, OH, O 2 (a 1 Δg), O 2 (b 1 Σg + ), N, N 2 , N + 2 and O 3 .

A method for dynamic system characterization using hydraulic series resistance

Abstract: The pressure required to drive flow through a microfluidic device is an important characteristic of that device. We present a method to measure the flow rate through microfluidic components and systems, including micropumps and microvalves. The measurement platform is composed of two pressure sensors and a glass tube, which provides series resistance. The principle of the measurement is the fluid dynamical equivalent of Ohm's law, which defines the relationship between current, resistance, and voltage that are analogues to flow rate, hydraulic resistance, and pressure drop, respectively. Once the series resistance is known, it is possible to compute the flow rate through a device based on pressure alone. In addition, the dynamic system characteristics of the device-resistance and capacitance-can be computed. The benefits of this method are its simple configuration, capability of measuring flow rate accurately from the more easily measured pressure, and the ability to predict the dynamic response of microfluidic devices.