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Large Bubbles (large + bubble)
Selected AbstractsHeat transfer for Marangoni-driven boundary layer flowHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 2 2002David M. Christopher Abstract Marangoni convection induced by variation of the surface tension with temperature along a surface influences crystal growth melts and other processes with liquid,vapor interfaces, such as boiling in both microgravity and normal gravity in some cases. This paper presents the Nusselt number for Marangoni flow over a flat surface calculated using a similarity solution for both the momentum equations and the energy equation assuming developing boundary layer flow along a surface. Solutions are presented for the surface velocity, the total flow rate, and the Nusselt number for various temperature profiles, Marangoni numbers, and Prandtl numbers. For large bubbles, the predicted boundary layer thickness would be less than the bubble diameter, so the curvature effects could be neglected and this analysis could be used as a first estimate of the effect of Marangoni flow around a vapor bubble. © 2002 Scripta Technica, Heat Trans Asian Res, 31(2): 105,116, 2002; DOI 10.1002/htj.10019 [source] Effect of rheological behavior of epoxy during precuring on foamingJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2008Osamu Takiguchi Abstract In this study, the effect of rheological behavior of epoxy during precuring on foaming was investigated. Dynamic time sweep test of epoxy/curing agent (100/1, w/w) was conducted. The viscosities as a function of time showed extremely rapid increase from the order of 102,103 to 106Pa · s at a certain time, followed by slow increase of the viscosities. Dynamic frequency sweep test of precured epoxy with curing agent was conducted at 90°C. The critical gelation time was obtained by using rheological criterion proposed by Winter and Chambon. We found that the slopes of G,(,) and G,(,) decreased with increasing precuring time. Correspondingly, tan , showed a change from negative to positive slope at a critical time. By using the results, the critical gelation time was determined as t = 895,935 s. Samples of epoxy/curing agent/blowing agent (100/1/0.5) were precured for 960,1620 s. And then precured samples were foamed at 230°C for 300 s to decompose chemical blowing agent. The formed bubble size distribution becomes sharp with increase of the precuring time. There are roughly two sizes of bubbles when precured for relatively short time (t < 1080 s) before foaming: large bubbles (>100 ,m) and small ones (,30 ,m). On the other hand, foams precured for long time (t > 1200 s) before foaming, large bubbles disappear, and the average diameter of the bubble becomes small while the porosity is low. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Simulation of a slurry-bubble column reactor for Fischer-Tropsch synthesis using single-event microkineticsAICHE JOURNAL, Issue 8 2009Gisela Lozano-Blanco Abstract A single-event microkinetic model for Fischer-Tropsch synthesis including the water-gas shift reaction has been implemented in a one-dimensional, two-bubble class, heterogeneous model with axial effective diffusion to study the performance of a commercial slurry bubble column reactor. Mass balance equations are solved for every species in the reaction network in the large bubbles, small bubbles, and slurry phase, whereas the energy balance is applied to the slurry phase. The catalyst concentration profile is described by a sedimentation-dispersion model. The combination of microkinetics that generate net production rates for the individual reaction products and hydrodynamics allows describing detailed concentration profiles along the reactor axis as a function of operating conditions and design parameters. As example, the effects of catalyst loading, syngas feed flow rate, inlet temperature, or hydrogen to carbon monoxide inlet ratio on the individual hydrocarbons are investigated. To our knowledge, no reactor model in literature is able to describe detailed compositions at the level described by the reactor model developed in this work. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Dispersed oil,water,gas flow through a horizontal pipeAICHE JOURNAL, Issue 5 2009K. Piela Abstract An experimental study of three-phase dispersed flow in a horizontal pipe has been carried out. The pressure drop over the pipe strongly increases with increasing bubble and drop volume fraction. Because of the presence of drops the transition from dispersed bubble flow to elongated bubble flow occurs at a lower gas volume fraction. The gas bubbles have no significant influence on the phase inversion process. However, phase inversion has a strong effect on the gas bubbles. Just before inversion large bubbles are present and the flow pattern is elongated bubble flow. During the inversion process the bubbles break-up quickly and as the dispersed drop volume fraction after inversion is much lower than before inversion, a dispersed bubble flow is present after inversion. (When inversion is postponed to high dispersed phase fractions, the volume fraction of the dispersed phase can be as high as 0.9 before inversion and as low as 0.1 after inversion.) © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Detecting regime transitions in slurry bubble columns using pressure time seriesAICHE JOURNAL, Issue 7 2005Keshav C. Ruthiya Abstract Changes in the coherent standard deviation and in the average frequency of measured pressure time series with gas velocity, are proposed, as unique and unambiguous criteria to mark flow regime transitions in slurry bubble columns. In a 2-dimensional (2-D) slurry bubble column, pressure time series are measured at different gas velocities simultaneously with high-speed video recording of the gas-liquid flow. The frequency of occurrence and the average diameter of the large bubbles are determined from video image analysis. The gas velocity where the first large bubbles are detected, with an average diameter of 1.5 cm, and with a frequency of occurrence of one bubble per s, is designated as the first regime transition point (transition from the homogeneous regime to the transition regime). At this point, the coherent standard deviation of the measured pressure fluctuations clearly increases from zero. The gas velocity where the average diameter and the frequency of occurrence of the large bubbles become constant, is designated as the second regime transition point (transition from the transition regime to the heterogeneous regime). From this point onward, the slope of the coherent standard deviation of the measured pressure fluctuations clearly decreases with gas velocity, while the average frequency becomes constant. These clear changes with gas velocity in the coherent standard deviation, and in the average frequency are also demonstrated in a 3-D slurry bubble column. © 2005 American Institute of Chemical Engineers AIChE J, 2005 [source] Destabilisation of homogeneous bubbly flow in an annular gap bubble columnTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010Fahd 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] 2D Slurry Bubble Column Hydrodynamic Phenomena Clarified with a 3D Gas,Liquid ModelTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003Jeroen H. J. Kluytmans Abstract The gas hold-up in a 2D bubble column is modelled using a 3D gas hold-up model. The influence of the scale of 2D bubble columns on several parameters, for instance, transition gas hold-up, transition gas velocity, and bubble rise velocities, is investigated and related to 3D bubble columns. By adapting the rise velocity of the large bubbles of an existing 3D bubble column model (Krishna et al., 2001a), the gas hold-up in both the homogeneous and the heterogeneous regime can be described satisfactorily. By adjusting the transition points only, it is also possible to describe the gas hold-up in systems containing small amounts of carbon particles and electrolyte. The smallest dimension of the 2D slurry bubble column, the column thickness, influences the location of the regime transition point. In the heterogeneous regime, however, it is only the largest column dimension, the column width, that influences the gas hold-up. These observations together enable proper 2D/3D bubble column comparison in future studies. Dans cette étude, la rétention de gaz dans une colonne à bulles en 2D est modélisée à l'aide d'un modèle de rétention de gaz en 3D. L'influence de l'échelle des colonnes à bulles 2D sur plusieurs paramètres, comme la rétention de gaz de transition, la vitesse de gaz de transition et les vitesses de montée des bulles, est étudiée et reliée aux colonnes à bulles 3D. On montre qu'en adaptant la vitesse de montée des bulles larges fournie par un modèle de colonnes à bulles 3D existant (Krishna et al., 2001a), la rétention de gaz tant en régime homogène qu'hétérogène peut être décrite de manière satisfaisante. En ajustant seulement les points de transition, il est également possible de décrire la rétention de gaz dans des systèmes contenant de petites quantités de particules de carbone et d'électrolyte. On a trouvé que la plus petite dimension de la colonne à bulles à suspensions 2D, soit l'épaisseur de la colonne, influence la position du point de transition de régime. Cependant, dans le régime hétérogène, c'est seulement la plus grande dimension de la colonne, soit la largeur de la colonne, qui influence la rétention de gaz. Toutes ces observations vont permettre des comparaisons adéquates des colonnes 2D et 3D dans les prochaines études. [source] Gas hold-up in bubble columns: Operation with concentrated slurries versus high viscosity liquidTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2000Rajamani Krishna Abstract The hydrodynamics of bubble columns with concentrated slurries of paraffin oil (density, ,L = 790 kg/m3; viscosity, ,L = 0.0029 Pa·s; surface tension, , = 0.028 N·m1) containing silica particles (mean particle diameter dp = 38 ,m) has been studied in columns of three different diameters, 0.1, 0.19 and 0.38 m. With increasing particle concentration, the total gas hold-up decreases significantly. This decrease is primarily caused by the destruction of the small bubble population. The hold-up of large bubbles is practically independent of the slurry concentration. The measured gas hold-up with the 36% v paraffin oil slurry shows remarkable agreement with the corresponding data obtained with Tellus oil (,L = 862 kg/m3; ,L = 0.075 Pa·s; , = 0.028 N·m,1) as the liquid phase. Dynamic gas disengagement experiments confirm that the gas dispersion in Tellus oil also consists predominantly of large bubbles. The large bubble hold-up is found to decrease significantly with increasing column diameter. A model is developed for estimation of the large bubble gas hold-up by introduction of an wake-acceleration factor into the Davies-Taylor-Collins relation (Collins, 1967), describing the influence of the column diameter on the rise velocity of an isolated spherical cap bubble. On a étudié dans des colonnes de trois diamètres différents, soient 0,1, 0,19 et 0,38 m, l'hydrodynamique de colonnes à bulles avec des suspensions concentrées d'huile de paraffine (masse volumique, ,L = 790 kg/m3; viscosité, ,L = 0,0029 Pa·s; tension de surface, , = 0,028 N·m,1) contenant des particules de silice (diamètre moyen des particules dp = 38 ,m). Lorsque la concentration de particules augmente, la rétention de gaz totale diminue considérablement. Cette diminution est principalement due à la destruction de la population de petites bulles. La rétention de grosses bulles est pratiquement indépendante de la concentration des suspensions. La rétention de gaz mesurée avec la suspension d'huile paraffine à 36% volumique concorde remarquablement bien avec les données correspondantes obtenues avec de l'huile de Tellus (,L = 86 kg/m,3; ,L = 0,075 Pa·s; , = 0,028 N·m,1) comme phase liquide. Des expériences de dégagement de gaz dynamiques confirment que la dispersion dans l'huile de Tellus se compose essentiellement de grosses bulles. On a trouvé que la rétention de grosses bulles diminuait de manière significative avec l'augmentation du diamètre de la colonne. On a mis au point un modèle pour l'estimation de la rétention de grosses bulles de gaz par l'introduction d'un facteur d'accélération dans le sillage dans la relation de Davies-Taylor-Collins (Collins, 1967), décrivant l'influence du diamètre de colonne sur la vitesse de montée d'une bulle à t,te sphérique isolée. [source] VOF-Simulation of the Lift Force for Single Bubbles in a Simple Shear FlowCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 9 2006D. Bothe Abstract Bubbles in shear flows experience a lift force, causing them to migrate sideways while they are rising. This lateral migration is investigated in numerical simulations, which are carried out with an extended version of the highly parallelized code FS3D, employing an advanced Volume-of-Fluid method. The movement of single bubbles in linear shear flows is simulated to obtain the magnitude of the lift force , expressed by the lift force coefficient CL , for various bubble diameters and material data. Simulation results are in good agreement with experiments for medium liquid phase viscosities. An investigation of the dynamic pressure on the bubble surface explains why large bubbles migrate in the opposite direction compared to small bubbles. [source] | ||||||||||