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Void Fraction (void + fraction)

Distribution by Scientific Domains

Chemistry	68%

Selected Abstracts

A new heat balance for flow boiling

AICHE JOURNAL, Issue 8 2007
Francisco J. Collado
Abstract Recently, one of the authors suggested calculating void fraction, an essential element in thermal-hydraulics, working with the "thermodynamic" quality instead of the usual "flow" quality. However, the standard heat balance is currently stated as a function of the "flow" quality. Therefore, we should search a new energy balance between the mixture enthalpy, based on "thermodynamic" quality, and the absorbed heat. This work presents the results of such analysis based on accurate measurements of the axial profile of the cross-sectional average void fraction in the region of boiling with subcooling for water at medium and high pressures taken by Moscow Power Institute (MPI) and Argonne National Laboratory (ANL). As main results, we find that, under uniform heat flux, the mixture enthalpy suffers an abrupt reduction of its slope upon passing saturation point, and a new slip ratio could balance heat with such mixture enthalpy. © 2007 American Institute of Chemical Engineers AIChE J, 2007 [source]

Mechanics of column beds: I. Acquisition of the relevant parameters

AICHE JOURNAL, Issue 3 2003
Bee Gaik Yew
The efficiency of chromatographic columns is adversely affected by large-scale radial variations of the packing density or void ratio of the material used to prepare the bed. This heterogeneity is due to wall friction effects that take place during the preparation of the column and to seepage effects operating during the packing process and the subsequent operation of the column. The dependence of the bed's void fraction on the stress applied during its consolidation was determined, as well as its permeability at various stages of the consolidation process and the coefficient of friction between typical packing materials and the stainless steel wall of chromatographic columns. These results are required to develop and use numerical models of the volumetric response to axial compression of the bed and models of the coupled mechanical-seepage rheology of particulate materials. [source]

Mechanics of column beds: II.

AICHE JOURNAL, Issue 3 2003
Modeling of coupled stress-strain-flow behavior
The rheological behavior of a bed of packing material during its consolidation was investigated using an elastic-plastic model, the Frictional Material Model. This model takes into account the behavior of the packing material under compression stress, its consolidation, its internal friction, its friction against the column wall, and the dependence of the bed's permeability on the local void fraction. The complexity of the problem arises from the nonlinear behavior of the relationships between the critical parameters controlling the column bed behavior and the external stresses applied to this bed. Solutions of the model were calculated for combinations of axial compression and seepage stresses, the latter corresponding to the flow of the mobile-phase stream under typical conditions used in HPLC. The results demonstrate the importance of the internal angle of friction of the packing material used on the degree of radial and axial heterogeneity of the beds of chromatographic columns packed with this material. [source]

Use of Nitrogen as a Blowing Agent for the Production of Fine-Celled High-Density Polyethylene Foams,

MACROMOLECULAR MATERIALS & ENGINEERING, Issue 10 2006
John W. S. Lee
Abstract Summary: While many experiments have been performed to examine the effects of administering CO2 as a blowing agent in the foaming process, very few studies have investigated the use of N2 for this purpose. In this study, foaming experiments were conducted in extrusion using HDPE as a polymeric material and N2 as a blowing agent. Talc was used as a nucleating agent, and three different pressure-drop rates were applied to study the effects of pressure-drop rates on HDPE foams. The experimental results revealed that the void fraction of high-density foams blown with N2 was not affected by the die temperature, contrasting the situation in low-density foams. Surprisingly, it was the cell density which determined the void fraction of high-density foams. It was also found that the use of talc significantly increased the cell density and the void fraction of the foams and minimized the role played by the pressure-drop rate in cell nucleation. Effect of N2 content on the cell density of HDPE foams. [source]

Effect of ion energy on structural and electrical properties of intrinsic microcrystalline silicon layer deposited in a matrix distributed electron cyclotron resonance plasma reactor

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2010
Sanjay K. Ram
Abstract Microcrystalline silicon films were deposited in a matrix distributed electron cyclotron resonance (MDECR) plasma enhanced chemical vapor deposition (PECVD) system using pure silane, under varying substrate bias conditions. Microstructural characterization of the films shows a lower void fraction and a preponderance of nanograins in films deposited at negative bias, while in positive bias a thin incubation layer is seen with a higher void fraction. Plasma emission studies reveal higher electron temperature and more atomic H at positive bias, which lead to early onset of crystallization. The microstructural properties of the films are correlated with the dark and phototransport properties. Our study demonstrates the importance of substrate bias in controlling the ion energy and properties of films deposited in the MDECR reactor. [source]

Effect of Voids inside AP Particles on Burning Rate of AP/HTPB Composite Propellant

PROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 4 2008
Makoto KohgaArticle first published online: 21 JUL 200
Abstract Bubble contamination in an ammonium perchlorate (AP)-based composite propellant has a positive effect on the burning rate. However, the quantitative effect of the bubble contamination on the burning rate has never been revealed. In order to clarify the relationship between the increase in the burning rate and the void fraction of the propellant, propellants were prepared with fine porous AP particles (PoAP) or fine hollow AP particles (HoAPs), and their burning rate characteristics were investigated. The voids inside AP particles have the effect of increasing the burning rate. The increase in the burning rate is enhanced linearly as the void fraction increases. The effect of the void fraction on the burning rate for a propellant containing PoAP is not identical with that for a propellant containing HoAP. It was found that the effect of the void fraction on the burning rate could be estimated by the void fraction when the bubble contamination is uniform in size and shape. [source]

Destabilisation of homogeneous bubbly flow in an annular gap bubble column

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010
Fahd M. Al-Oufi
Abstract Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in an open tube and a annular gap bubble column, when operated at the same gas superficial velocity, using a porous sparger. Measurements were carried out in a vertical 0.102,m internal diameter column, with a range of concentric inner tubes to form an annular gap, giving diameter ratios from 0.25 to 0.69; gas superficial velocities in the range 0.014,0.200,m/s were investigated. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (1) the presence of the inner tube causes large bubbles to form near the sparger, which destabilise the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles, and (2) the shape of the void fraction profiles changes with gap geometry and this affects the distribution parameter in the drift-flux model. Both of these effects serve to reduce the mean gas void fraction in an annular gap bubble column compared to an open tube at the same gas superficial velocity. Des résultats expérimentaux sont présentés pour montrer qu'il existe de très grandes différences dans les fractions de vide gazeux moyennes mesurées dans un tube ouvert et une colonne à bulles à espace annulaire, lorsqu'ils sont utilisés à la même vitesse superficielle de gaz, au moyen d'un aérateur poreux. On a effectué des mesures dans une colonne verticale avec un diamètre interne de 0.102,m, avec une portée de tubes internes concentriques pour former un espace annulaire, procurant des rapports de diamètre de 0.25 à 0.69; des vitesses superficielles de gaz de 0.014 à 0.200,m/s ont été étudiées. La fraction de vide gazeux moyenne diminue avec le rapport croissant du diamètre interne à externe de la colonne à espace annulaire et la transition à la circulation hétérogène se produit à des vitesses superficielles de gaz et fractions de vide plus basses. Deux raisons sont proposées et validés par les vérifications expérimentales: (1) la présence du tube interne provoque la formation de grandes bulles près de l'aérateur, ce qui déstabilize l'écoulement à bulles homogène et réduit la fraction de vide moyenne; cet état a été confirmé en injectant délibérément de grandes bulles dans une dispersion homogène de plus petites bulles et, (2) la forme des profils de fraction de vide change avec la géométrie de l'espace qui les sépare, ce qui a des conséquences sur le paramètre de distribution du modèle à flux de dérive. Ces deux effets servent à réduire la fraction de vide gazeux moyenne dans une colonne à bulles à espace annulaire, en comparaison avec un tube ouvert à la même vitesse superficielle de gaz. [source]

Flow of Newtonian and power law liquids in tube bundles

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009
U. K. Singh
Abstract In this work, the annular (tangential) flow of Newtonian and non-Newtonian fluids in tube bundles has been studied experimentally. Extensive pressure drop data has been obtained embracing wide ranges of the Reynolds number (13,6600) and for two test modules of different geometrical arrangements, but of similar overall void fraction. Preliminary experiments suggest that the pressure drop is mainly determined by the overall void fraction of the bundle and is relatively insensitive to the detailed geometrical configuration of the bundle. A simple predictive correlation has been developed which reconciles the present results for Newtonian and power law fluids with acceptable levels of reliability. Dans le cadre de ces travaux, on a examiné de façon expérimentale l'écoulement annulaire (tangentiel) des fluides newtoniens et non newtoniens dans des faisceaux tubulaires. On a obtenu de nombreuses données de chute de pression englobant de vastes plages du nombre de Reynolds (13-6600), et pour deux modules d'essai de différentes dispositions géométriques, mais à taux de vide global similaire. Les expériences préliminaires suggèrent que la chute de pression est déterminée principalement par le taux de vide global du faisceau et est relativement insensible à la configuration géométrique détaillée du faisceau. La création d'une simple corrélation prédictive a permis de rapprocher les présents résultats pour les fluides newtoniens et des fluides en loi de puissance avec des niveaux acceptables de fiabilité. [source]

A Simple Experimental Technique to Measure Tortuosity in Packed Beds

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2006
J. M. P. Q. Delgado
Abstract In this article, a simple and inexpensive experimental technique easy to build in laboratory, for the measurement of tortuosity of a packed bed of inert particles, is described. Experimental values of the tortuosity were obtained with four different packed beds of sand. The experimental results obtained are in good agreement with the theoretical values of tortuosity in a porous media; and the data reported showed the tortuosity to increase with decrease in the void fraction of the packed bed as expected. On décrit dans cet article une technique expérimentale simple et peu coûteuse facile à concevoir en laboratoire, pour la mesure de la tortuosité d'un lit garni de particules inertes. Des valeurs expérimentales de tortuosité ont été obtenues avec quatre lits différents garnis de sable. Les résultats expérimentaux obtenus montrent un bon accord avec les valeurs théoriques de la tortuosité dans un milieu poreux; et les données signalées montrent que la tortuosité augmente avec la diminution de la fraction de vide du lit garni tel qu'attendu. [source]

Effect of packed structure on flow behaviour in a trickle bed biofilter

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2001
Meisen Li
Abstract The effects of operation conditions on the flow behaviour in gas,liquid countercurrent trickle bed biofilter were experimentally examined. In order to prevent gas channelling in the biofilter, packings with a relatively large void fraction, which have a role to maintain a high void fraction in the bed, were added. The gas and liquid velocities of the packed structure and the packings were changed, and the residence time distributions (RTDs) of the gas and liquid were measured. It was found that the addition of void supporters was very effective in the suppression of gas channelling. On a examiné de manière expérimental les effets des conditions opératoires sur les comportements d'écoulement dans un biofiltre à lit ruisselant à contre-courant gaz-liquide. Afin d'emp,cher le renardage dans le biofiltre, des garnissages ayant une fraction de vide relativement grande, qui joue un r,le pour maintenir un haut degré de vide, ont été ajoutés. Les vitesses de gaz et de liquide, la structure garnie et les garnissages ont été modifiés, et les distributions de temps de séjour (RTD) du gaz et du liquide ont été mesurées. On a trouvé que l'ajout de supports de vide était très efficace pour la suppression du renardage. [source]

Protein loading, elution, and resolution behavior in a novel device that integrates ultrafiltration and chromatographic separation

BIOTECHNOLOGY & BIOENGINEERING, Issue 2 2003
Xiao-Ping Dai
Abstract Hollow fiber membranes and chromatographic resin beads are commonly employed in a variety of bioseparation processes. A new class of integrated separation devices is being studied in which the shell side of a hollow fiber device is filled with adsorbents/chromatographic resin beads. Such devices and the corresponding separation methods integrate feed broth clarification by the microfiltration/ultrafiltration membrane with bioproduct purification by the shell-side resin beads either as an adsorbent or as beads in elution chromatography. A mathematical model has been developed for the prediction of the chromatographic behavior of such an integrated device. Simulations have been done to study the effects of axial dispersion, feed flow rate, water permeation rate, fiber packing density, and void fraction. Numerical solutions were obtained by solving the governing equations. This model can reasonably describe the concentration profiles as well as the breakthrough and elution behaviors in the integrated device. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 83: 125,139, 2003. [source]

Effects of Adsorbent Characteristics on Adiabatic Vacuum Swing Adsorption Processes for Solvent Vapor Recovery

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 11 2006
S. A. Al-Muhtaseb
Abstract The effects of the adsorbent characteristics on the performance parameters and periodic state behavior of the vacuum swing adsorption (VSA) solvent vapor recovery (SVR) processes are examined and optimized. The adsorbent characteristics studied were the adsorbent particle's porosity, density, radius and heat capacity, the packed bed void fraction, the isosteric heat of adsorption, the monolayer saturation limit of the solvent molecules on the adsorbent, the adsorbent's affinity to adsorb the solvent molecules and the mass transfer coefficient for the adsorption of the solvent molecules. It was found that the best VSA-SVR process performances can be obtained using adsorbents characterized by the minimum possible packed bed void fraction and particle porosity, with the maximum possible adsorbent heat capacity and density, adsorption monolayer saturation capacity and mass transfer coefficient, and at intermediate adsorption affinity and isosteric heat of adsorption of the solvent molecules. [source]

Destabilisation of homogeneous bubbly flow in an annular gap bubble column

A theoretical investigation of enhancement of mass transfer from a packed bed using acoustic oscillations

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2000
Raman I. Sujith
Abstract This paper describes a theoretical investigation of the possibility of improving the performance of packed bed dryers using acoustic oscillations. It was motivated by the increasing interest in the use of acoustic oscillations to improve the performance of energy-intensive, industrial processes. Sound propagation in a packed bed was modelled as propagation through a porous medium. The effect of acoustic oscillations on the drying rate was then investigated by numerically integrating the heat and mass transfer equations. The model used quasi-steady correlations for heat and mass transfer. The results show that threshold values for void fractions and sound pressure levels exist, above which significant increase in drying rate can be obtained. The increase in mass transfer decreases with effective particle diameter. On décrit dans cet article une étude théorique sur la possibilité d'arnéliorer la performance de séchoirs à lits garnis par des oscillations acoustiques. Cette étude est motivée par l'intér,t grandissant pour les oscillations acoustiques en vue d'améliorer la performance de procédés industriels énergivores. La propagation du son dans un lit garni a été modélisée comme une propagation dans un milieu poreux. Puis on a étudié l'effet des oscillations acoustiques sur la vitesse de séchage en intégrant numériquement les équations de transfert de chaleur et de matière. Le modéle emploie des corrélations quasi stationnaires pour le transfert de chaleur et de matière. Les réultats montrent que des seuils existent pour les fractions de vide et les niveaux de pression sonore, au-dessus desquels une augmentation significative de la vitesse de séchage peut ,tre obtenue. L'augmentation du transfert de matière diminue avec le diamètre effectif des particules. [source]

Modeling O2 transport within engineered hepatic devices

BIOTECHNOLOGY & BIOENGINEERING, Issue 1 2003
Randall E. McClelland
Abstract Predicting and improving oxygen transport within bioartificial liver (BAL) devices continues to be an important engineering challenge since oxygen is one of the critical nutrients necessary for maintaining hepatocyte viability and function. Such a computational model would not only help predict outcomes but it would also allow system modifications to be analyzed prior to developing experimental protocols. This would help to facilitate future design improvements while reducing both experimental time and capital resource costs, and is the focus of the current study. Specifically, a computational model of O2 transport through collagen and microporous collagen ECMs is analyzed for hollow fiber (HF), flat plate (FP), and spheroid BAL designs. By modifying the O2 boundary conditions, hepatocyte O2 consumption levels, O2 permeability of the ECM, and ECM void fractions, O2 transport predictions are determined for each system as a function of time and distance. Accuracy of the predictive model is confirmed by comparing computational vs. experimental results for the HF BAL system. The model's results indicate that O2 transport within all three BAL designs can be improved significantly by incorporating the enhancement technique. This technique modifies a diffusion-dominant gel ECM into a porous matrix with diffusive and convective flows that mutually transport O2 through the ECMs. Although tortuous pathways increase the porous ECM's overall effective length of O2 travel, the decreased transport resistances of these pathways allow O2 to permeate more effectively into the ECMs. Furthermore, because the HF design employs convective flow on both its inner and outer ECM surfaces, greater control of O2 transport through its ECM is predicted, as compared with the single O2 source inputs of the flat plate and spheroid systems. The importance of this control is evaluated by showing how modifying the O2 concentration and/or transfer coefficients of the convective flows can affect O2 transport. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 82: 12,27, 2003. [source]