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Superficial Velocity (superficial + velocity)

Distribution by Scientific Domains
Chemistry94%

Kinds of Superficial Velocity

  • gas superficial velocity
    		•
  • liquid superficial velocity
    		•

    Selected Abstracts

    Particle scale study of heat transfer in packed and bubbling fluidized beds

    AICHE JOURNAL, Issue 4 2009
    Z. Y. Zhou
    Abstract The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle-fluid flow, is extended to study particle-particle and particle-fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid-particle convection, particle-particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

    Experimental investigation of the hydrodynamics in a liquid,solid riser

    AICHE JOURNAL, Issue 3 2005
    Shantanu Roy
    Abstract Liquid,solid fluid dynamics has been investigated in a 6-in. (0.15 m) "cold-flow" circulating fluidized bed riser using non-invasive flow monitoring methods. Gamma-ray computed tomography (CT) was used to measure the time-averaged cross-sectional solids volume fraction distributions at several elevations. The time-averaged mean and "fluctuating" solids velocity fields were quantified using the computer-automated radioactive particle tracking (CARPT) technique. The experimental equipment, protocol of implementation, and data analysis have been discussed briefly, with particular emphasis on the specific features in the use of these techniques for studying high-density turbulent flows as in a liquid,solid riser. The experimental study examines nine operating conditions, that is, three liquid superficial velocities and three solids flow rates. The solids holdup profile is found to be relatively uniform across the cross section of the riser, with marginal segregation near the walls. The time-averaged solids velocity profiles are found to have a negative component at the walls, indicating significant solids backmixing. Detailed characterization of the solids velocity fields in terms of RMS velocities, kinetic energies, Hurst exponents, residence time distributions, trajectory length distributions, dispersion coefficients, and so forth are presented. Comparative and symbiotic analyses of the results were used to develop a coherent picture of the solids flow field. In addition, the work also serves to demonstrate the power and versatility of these flow-imaging techniques in studying highly turbulent and opaque multiphase systems. © 2005 American Institute of Chemical Engineers AIChE J, 51: 802,835, 2005 [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]

    Mechanism of Local Fluidization in Converging Packed Beds

    THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2003
    Baoquan Zhang
    Abstract The velocity profile on the bed surface of two-dimensional linear-converging beds with 15° and 30° wall angles was measured at different superficial velocities using hot-wire anemometry. The results of the velocity measurements indicate that local fluidization in the corners is caused by the geometry-induced maldistribution of fluid flow, and it occurs when the velocity in the corners exceeds the minimum fluidization velocity of particles. The results of pressure measurements within the bed show the same trend as the velocity profile, providing a qualitative verification of the velocity profile measurement. It is shown that the variation of measured pressure drop over the bed with velocity does not agree with the Ergun equation at high superficial velocities due to the severe maldistribution of fluid flow. Le profil de vitesse à la surface du lit de lits linéaires-convergents bidimensionnels avec des angles de paroi de 15 et 30∞ a été mesuré à différentes vitesses superficielles par anémométrie à fil chaud. Les résultats des mesures de vitesse indiquent que la fluidisation locale dans les coins provient de la mauvaise distribution causée par la géométrie de l'écoulement du fluide, et qu'elle survient lorsque la vitesse dans les coins excède la vitesse de fluidisation minimale des particules. Les résultats des mesures de pression dans le lit montrent la même tendance que le profil de vitesse, permettant ainsi une vérification qualitative de la mesure des profils de vitesse. On montre que la variation de la perte de charge mesurée dans le lit en fonction de la vitesse ne concorde pas avec l'équation d'Ergun à des vitesses superficielles élevées en raison de la très mauvaise distribution de l'écoulement du fluide. [source]

    Holdup and Pressure Drop in Vertical and Near-Vertical Three-Phase Up-Flow: A Collection of Flow Regimes

    ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2002
    P.L. Spedding
    Three-phase oil, water and air data are reported for vertical and near vertical +86 5° upflow in a 0 026 m i d pipe In general, the liquid holdup for near vertical flow was greater than for the corresponding vertical upflow, the exception being at low liquid and superficial velocities under 0 6 m/s and high superficial gas velocities over 20 m/s Here the liquid holdup varied being sometimes below and other times above the corresponding vertical value These variations of liquid holdup were shown to depend on the fine structure of the flow patterns present The total pressure drop and its component parts showed significantly different patterns of behaviour depending on whether the superficial gas velocity was above or below the rise velocity of a Taylor bubble The total pressure drop generally was greater for near vertical flow compared to the vertical upflow case but reflected changes in the fine structure of the flow patterns A comprehensive collection of flow regimes is included in this paper [source]

    Pressure Drop in Liquid-liquid Two Phase Horizontal Flow: Experiment and Prediction

    CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 9 2005
    D. P. Chakrabarti
    Abstract The present study is aimed at an investigation of the pressure drop characteristics during the simultaneous flow of a kerosene-water mixture through a horizontal pipe of 0.025,m diameter. Measurements of pressure gradient were made for different combinations of phase superficial velocities ranging from 0.03,2,m/s such that the regimes encountered were smooth stratified, wavy stratified, three layer flow, plug flow and oil dispersed in water, and water flow patterns. A model was developed, which considered the energy minimization and pressure equalization of both phases. [source]

    A study on un-fully developed slug flow in a vertical tube

    HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 4 2005
    Zhijia Yu
    Abstract Gas-liquid co-current vertical slug flow was studied in a vertical Plexiglas tube. Taylor bubbles and liquid slug lengths and their rising velocities were measured by means of a pair of conductivity probes under un-fully developed flow conditions. The influences of the superficial velocity of gas and liquid on slug flow parameters were examined. Using statistical analysis on the length of Taylor bubbles, the probability distribution of the length of the Taylor bubbles was obtained, which obeyed a normal distribution under a significance level of , = 0.05. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 235,242, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20063 [source]

    Viscous co-current downward Taylor flow in a square mini-channel

    AICHE JOURNAL, Issue 7 2010
    Özge Keskin
    Abstract This article presents a computational study of the co-current downward Taylor flow of gas bubbles in a viscous liquid within a square channel of 1 mm hydraulic diameter. The three-dimensional numerical simulations are performed with an in-house computer code, which is based on the volume-of-fluid method with interface reconstruction. The computed (always axi-symmetric) bubble shapes are validated by experimental flow visualizations for varying capillary number. The evaluation of the numerical results for a series of simulations reveals the dependence of the bubble diameter and the interfacial area per unit volume on the capillary number. Correlations between bubble velocity and total superficial velocity are also provided. The present results are useful to estimate the values of the bubble diameter, the liquid film thickness and the interfacial area per unit volume from given values of the gas and liquid superficial velocities. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source]

    Intensification of liquid,liquid two-phase mass transfer by gas agitation in a microchannel

    AICHE JOURNAL, Issue 8 2009
    Yuanhai Su
    Abstract In this experiment, the inert gas is used to agitate two immiscible fluids in microchannels. The mass transfer performances with or without gas agitation are investigated. 30% TBP (in kerosene)-acetic acid-water is chosen as testing system, and nitrogen as agitating gas. The superficial velocities of the immiscible liquid,liquid two phases and gas phase are varied in the range from 0.02 to 1.2 m/s, and 0 to 3.0 m/s, respectively. In microchannels, with enough gas agitating intensity, high dispersion between two immiscible liquid phases can be obtained. The overall volumetric mean mass transfer coefficients are two-folds higher than those without gas agitating, which are in the range of 3.8,30.6 s,1. Some parameters which impact on the mass transfer process, such as the mixture superficial velocity of the immiscible liquid,liquid two phases, the gas superficial velocity, the microchannel structure, the gas inlet locations and the sampling time are experimentally investigated. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

    Particle scale study of heat transfer in packed and bubbling fluidized beds

    AICHE JOURNAL, Issue 4 2009
    Z. Y. Zhou
    Abstract The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle-fluid flow, is extended to study particle-particle and particle-fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid-particle convection, particle-particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

    Transient fluidization and segregation of binary mixtures of particles

    AICHE JOURNAL, Issue 11 2000
    A. Marzocchella
    Fluidization of binary mixtures of particles belonging to group B of the Geldart classification of powders was studied. Beds tested were prepared by mixing in different proportions particles with almost equal density (,2,500 kg/m3) and dissimilar size (125 ,m silica sand and 500 ,m glass beads). Experiments were carried out using a segmented fluidization column equipped with multiple pressure transducers. Experimental procedures included continuous monitoring of pressure drop at different locations along the bed during quasi-steady or stepwise changes of gas superficial velocity, and characterization of particle-size distributions in each segment of the fluidization column after fluidization of the bed for given times. Three ranges of gas superficial velocity were recognized for each solids mixture. At low velocity the bed behaves as a fixed bed. At high velocity, it is fully and steadily fluidized. In an intermediate velocity range, transient fluidization takes place: an initially uniform fluidized bed eventually undergoes segregation, giving rise to a defluidized bottom layer rich in the coarser solids and to a "supernatant" fluidized layer where finer particles prevail. The thresholds between these velocity ranges are rather sharp and were characterized as functions of initial bed composition. Rates at which the defluidized solids layer builds up from initially uniform beds, and the ultimate compositions of the defluidized bottom and fluidized top layers are characterized for beds with different compositions at variable gas superficial velocity. [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]

    The Effect of Particle Size Distribution on Pressure Drop through Packed Beds of Cooked Wood Chips

    THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2005
    Quak Foo Lee
    Abstract The pressure drop of process liquors through columns of wood chips determines the operability, efficiency and control of both batch and continuous pulp digesters and the quality of the pulp produced from them. Pressure drop was measured through columns of industrial white spruce chips (produced with a chipping head-rig) as a function of the chip size distribution and the extent of delignification. Flow resistance depended on the porosity of the chip bed which was affected by the kappa number of the chips (which affected their flexibility) and chip size distribution, the compaction forces applied to the column, and the liquid superficial velocity. The chip beds were compressible and inelastic. Previous work from the literature using the Ergun equation to characterize pressure losses through chip beds is examined and compared with results of this work. La perte de charge des liqueurs de procédés dans des colonnes de copeaux de bois détermine l'opérabilité, l'efficacité et le contrôle des digesteurs de pâtes discontinus et continus, ainsi que la qualité de la pâte produite. La perte de charge a été mesurée dans des colonnes de copeaux d'épinette (produits avec une machine de fabrication de copeaux) en fonction de la distribution de taille des copeaux et du degré de délignification. La résistance à l'écoulement dépend de la porosité du lit de copeaux qui est influencée par le nombre de kappa des copeaux (qui influe sur leur flexibilité) et la distribution de taille des copeaux, les forces de compaction appliquées à la colonne ainsi que la vitesse de liquide superficielle. Les lits de copeaux sont compressibles et inélastiques. Un travail antérieur de la littérature scientifique faisant appel à l'équation d'Ergun pour caractériser les pertes de charge dans les lits de copeaux est examiné et comparé aux résultats du présent travail. [source]

    A Method for the Detection of Defluidized Zones in Slurry Bubble Columns

    THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003
    Ahad Emami
    Abstract The formation of defluidized zones was studied in a laboratory slurry bubble column equipped with heat transfer probes. The probes were small thermistors 2.4 mm in diameter. Dionized water and air were used as a liquid and gas phase, respectively. Solids were fine ceramic particles with mean size of 19.2 ,m and density of 2244 kg/m3. The effects of solids holdup (up to 30 wt% on gas free basis), gas superficial velocity (0.01-0.09 m/s), sparger height (0.01-0.09 m) on defluidized zones formation was studied. Cycle analysis of the local heat transfer fluctuations reliably detected the local formation of defluidized zones for each condition. La formation de zones défluidisées a été étudiée dans une colonne à bulles avec bouillie, équipée de sondes de transfert de chaleur. Les sondes étaient des petits thermistors de 2.4 mm de diamètre. De l'eau déionisée et de l'air constituaient les phases liquide et gazeuse. Les solides étaient des fines particules en céramique avec une taille moyenne de 19.2 µm et une densité de 2244 kg/m3. Les effets de la rétention solide (jusqu'à 30 % en masses sur base sans gaz), de la vitesse superficielle du gaz (0.01-0.09 m/s) et de la hauteur du distributeur de gaz ont été étudiés. L'analyse de cycle du transfert de local de chaleur a détecté de façon fiable la formation locale de zones défluidisées pour chaque condition. [source]

    Model-Aided Design of a Three-Phase Gas-Lift Reactor for Oxidation Accompanied by Catalyst Reversible Deactivation

    CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 12 2009
    Z. Gogová
    Abstract A mathematical model of a three-phase gas-lift reactor (GLR) is developed to aid the design of a target reactor for simultaneous substrate catalytic oxidation in riser and a deactivated reactivation catalyst in the downcomer section of the multifunctional reactor. In the GLR model, the hydrodynamics of a real GLR and the kinetics of glucose oxidation by air over a palladium catalyst are incorporated. The GLR model searches for the optimal geometry of the target reactor. With regard to the GLR optimal geometry, the reactor productivity is maximal for given input operational conditions. An algorithm of the GLR model is presented together with simulation results of the target GLR and with insight into the parametric sensitivity of the model. Effects of the reaction components concentrations and the gas-phase superficial velocity on the location of the target reactor optimal geometry and on the reactor productivity are discussed. [source]

    Numerical Study on Bubble Formation of a Gas-Liquid Flow in a T-Junction Microchannel

    CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 12 2009
    L. Dai
    Abstract Bubble emergence in a gas-liquid flow in a T-junction microchannel of 100,,m diameter, operated under a squeezing regime, was simulated with the computational fluid dynamics method. In general, bubble formation in channels includes three stages: expansion, collapse and pinching off. After analyzing and comparing quantitatively the three forces of pressure, surface tension and shear stress acting on the gas thread in the whole process, their effects in the different stages were identified. The collapse stage was the most important for bubble formation and was investigated in detail. It was found that the collapse process was mostly influenced by the liquid superficial velocity, and the rate and time of collapse can be correlated with empirical equations including the liquid superficial velocity, the capillary number and the Reynolds number. [source]

    A Study on Modern High Effective Random Packings for Ethanol-Water Rectification

    CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 7 2008
    S. Darakchiev
    Abstract Raschig Super-Ring is a modern and high-efficient packing used for intensification of absorption and distillation processes. The aim of this work is to characterize the efficiency of this packing applied to rectification of an important industrial system, ethanol-water, and to compare its efficiency to that of some random packings of the third generation as well as to the structured packing, HOLPACK, which is used in the ethanol production industry. The experiments were carried out in a column installation, 0.213,m in diameter with a packing height of 2.8,m. The column is heated by a number of electrical heaters (total power 45,kW), which can be switched gradually. Operation at total and partial reflux is possible. Eight types of random packings were studied: five types of Raschig Super-Ring, four metallic (with characteristic dimensions 0.5, 0.6, 0.7, and 1") and one of plastic material 0.6"; two types of packing IMTP and one plastic Ralu Flow. Some experiments were conducted at total reflux operation at vapor velocity, 0.253,0.936,m/s, and liquid superficial velocity, 4.44,·,10,4,1.63,·,10,3,m3/(m2s). Experiments at partial reflux were carried out at constant liquid superficial velocity and changeable vapor velocity as well as at constant vapor velocity and changeable liquid velocity. The results are presented as height of transfer unit, HTU, and height equivalent to a theoretical plate, HETP, as a function of the velocity of phases. [source]