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Surface flows generated by bubble plumes play an important role in many engineering applications. Hence, the flowfield's structure using air bubbles is the interest of many researchers. The purpose of this paper is to elucidate th...
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Surface flows generated by bubble plumes play an important role in many engineering applications. Hence, the flowfield's structure using air bubbles is the interest of many researchers. The purpose of this paper is to elucidate the relationship between the parameters of bubbly flow/bubble plume to describe the characteristics of bubble-generating surface flow. The flow depends on the gas flow rate Q g , the bubble size (the mean bubble diameter D), the void friction α, the bubble velocity v, the internal two-phase flow structure of the bubble plume, and the distance between the bubble generator and the free surface (water height in the experimental tank H). An experimental apparatus was designed in order to investigate bubble motion, to calculate the bubble parameters, and to find the links and interdependencies between them. From the data obtained by applying image processing of visualized images of bubble flow structure for the different sections of bubble regions, it is confirmed that the flow structure and bubble parameters are sensitively modulated by the gas flow rate, bubble velocity, bubble size, and the water height in the tank. It was found that when the gas volume flow rate increases the mean (average) bubble diameter, the bubble velocity along the bubble plume and the void friction increase. Furthermore, the bubble velocity increases as the water height in the experimental tank increases. Investigating bubbly flow characteristics and finding the relationship between the mentioned parameters support many applications for the future of industry and the environment. This will help designing the real system of surface flow technique generated by the bubble plume to enable it to control and collect surface-floating substances in naval systems, lakes, seas, rivers, and oceans, especially oil layers formed during large oil spill accidents. The surface flows generated by bubble plumes are considered key phenomena in various kinds of reactors, engineering processes, and industrial processes handling a free surface.
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The Eulerian–Eulerian two-fluid model (EE) is a powerful general model for multiphase flow computations. However, one limitation of the EE model is that it has no ability to estimate the local bubble sizes by itself. In this work...
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The Eulerian–Eulerian two-fluid model (EE) is a powerful general model for multiphase flow computations. However, one limitation of the EE model is that it has no ability to estimate the local bubble sizes by itself. In this work, we have combined the discrete phase model (DPM) to estimate the evolution of bubble sizes with the EE model. In the DPM, the change of bubble size distribution is estimated by coalescence, breakup, and volumetric expansion modeling of the bubbles. The time-varying bubble distribution is used to compute the local interface area between gas and liquid phase, which is then used to estimate the momentum interactions such as drag, lift, wall lubrication, and turbulent dispersion forces for the EE model. In this work, this newly developed hybrid model Eulerian–Eulerian discrete-phase model (EEDPM) is applied to compute an upward flowing bubbly flow in a vertical pipe and the results are compared with previous experimental work of Hibiki et al. (2001, “Axial Interfacial Area Transport of Vertical Bubbly Flows,” Int. J. Heat Mass Transfer, 44(10), pp. 1869–1888). The EEDPM model is able to reasonably predict the locally different bubble size distributions and the velocity and gas fraction fields. On the other hand, the standard EE model without the DPM shows good comparison with measurements only when the prescribed constant initial bubble size is accurate and does not change much. Parametric studies are implemented to understand the contributions of bubble interactions and volumetric expansion on the size change of bubbles quantitatively. The results show that coalescence is larger than other effects, and naturally increases in importance with increasing gas fraction.
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Bubble flow has received considerable attention in the last four decades and becomes a very important topic of research recently due to its large and wide range of applications value, and its effect on many processes and the effic...
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Bubble flow has received considerable attention in the last four decades and becomes a very important topic of research recently due to its large and wide range of applications value, and its effect on many processes and the efficiency of many devices. The motivation for studying bubble plumes is evident, from the fact that these plumes are encountered in a variety of engineering problems. In the past 10 years, the range of its application prompted scholars to do experiments and numerical research about this phenomenon. The motivation of the present work (part-II that is extended to part-I) is the dement to demonstrate, review and summarize the major finding of the previous research of the following points: 1) The techniques and the important models for the measurement of the dominated two-phase bubbly flow/bubble plume parameters such as gas flow rate bubble size, bubble velocity and void fraction which are considerably important and play an important role in operational safety, process control and reliability of continuum processes of many engineering applications. 2) Turbulent bubbly flow structure. 3) Some important applications especially on bubbly two-phase flow/bubble plume and its associated surface flow since it can contribute to improvements in various directions. The techniques of gas injection have been widely utilized in many engineering fields. The surface flows generated by bubble plumes are considered key phenomena in many kinds of processes in modern industries. It is utilized as an effective ways to control surface floating substances on lakes, oceans, as well as in various kinds of reactors and industrial processes handling a free surface.
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An important topic in fluid dynamics is multiphase flows. Multiphase flows can be found in numerous fields in engineering, e.g. aerospace, biomedical, chemical, electrical, environmental, mechanical, materials, nuclear and naval e...
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An important topic in fluid dynamics is multiphase flows. Multiphase flows can be found in numerous fields in engineering, e.g. aerospace, biomedical, chemical, electrical, environmental, mechanical, materials, nuclear and naval engineering. There is an enormous variation in applications, e.g. rocket engines, chemical reactors, contamination spreading, multiphase mixture transport, cavitation, sonoluminescence, ink-jet printing, particle transport in blood, crystallization, multiphase cooling, fluidized beds, drying of gases, air entrainment in oceans/rivers and anti-icing fluids. The number of papers on multiphase flow in the field of fluid dynamics is enormous and still growing. The diversity of flow types makes a general description almost impossible. This makes fundamental research necessary. Especially, controlled experiments are needed for a better physical understanding and as test cases for numerical and theoretical work. The purpose of this paper is the desire to demonstrate, review and summarize the major finding of the previous research of bubbly two-phase flow characteristics, structures, behaviors and flow patterns. Moreover, to elucidate some important models and techniques to measure the two-phase bubbly flow parameters such as bubble motion, flow regime, bubble shape which play a considerable role in many engineering applications.
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(1) Background. Bubbly flows are used in a wide variety of applications and require accurate modeling. In this paper, three modeling approaches are investigated using the geometrically simple configuration of a gas bubble strongly...
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(1) Background. Bubbly flows are used in a wide variety of applications and require accurate modeling. In this paper, three modeling approaches are investigated using the geometrically simple configuration of a gas bubble strongly oscillating in a bubbly medium. (2) Method of approach. A coupled Eulerian-Lagrangian, a multicomponent compressible, and an analytical approach are compared for different void fractions. (3) Results. While the homogeneous mixture models (analytical and multicomponent) compare well with each other, the Eulerian-Lagrangian model captures additional features and inhomogeneities. The discrete bubbles appear to introduce localized perturbations in the void fraction and the pressure distributions not captured by homogeneous mixture models. (4) Conclusions. The bubbly mixture impedes the growth and collapse of the primary bubble while wavy patterns in the velocity, pressure, and void fraction fields propagate in space and time.
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Prediction of the bubble size distribution in the wake of a ship is important to analyze its acoustic signature. To achieve CFD simulation of dynamic ships with moving control surfaces and rotating propellers in waves, a robust im...
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Prediction of the bubble size distribution in the wake of a ship is important to analyze its acoustic signature. To achieve CFD simulation of dynamic ships with moving control surfaces and rotating propellers in waves, a robust implementation is paramount. In this work a mass conserving multigroup discretization strategy of the Boltzmann transport equation for polydispersed bubbly flows is presented, as well as an analysis of available breakup and coalescence models. Modifications of the discrete equations for the fixed pivot method at the boundaries are introduced that guarantee exact bubble mass conservation. The role of the time stepping scheme in the conservation of mass and number of bubbles is discussed. Though the conservation properties of the discrete system of equations are satisfied provided they are solved exactly, in practice an iterative procedure must be used since the ODE's are non-linear. Three iterative schemes are proposed and they are analyzed in terms of robustness and efficiency. Breakup, coalescence and dissolution models are analyzed from the numerical point of view. Available models of breakup and coalescence are studied finding appropriate choices for ship applications. Other models are appropriate as well, but are more costly numerically. As appropriate for ship applications, an extension to the model of Prince and Blanch for salt water is proposed and analyzed. The final model is tested against experimental data and computations by other researchers, and convergence properties in bubble size discretization is studied. It is found that for salt water the final steady state is dependent on the initial condition since there is a range of sizes for which coalescence and breakup are both negligible.
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The bubble shape is fundamental for every aspect of modeling bubbly flows. The interface is usually highly deformable so that the bubble shape is in general dependent on the surrounding turbulent flow field. Since recent work on t...
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The bubble shape is fundamental for every aspect of modeling bubbly flows. The interface is usually highly deformable so that the bubble shape is in general dependent on the surrounding turbulent flow field. Since recent work on this topic addressed almost entirely single-bubbles rising in quiescent flow, the extent of such flow field dependencies as well as swarm effects is rather unknown. This study examines the effect on the bubble shape in bubbly flows with a liquid background flow that is generated by natural convection in the bubbly flow regime when flow properties, i.e. the gas flow rate, sparger setup, and column geometry, are changed by evaluating six different bubble column experiments. The results of this integral approach reveal that the bubble shape of small bubbles is distinctly influenced whereas the shape of large bubbles is unchanged. Averaged over all flow rates, we find that the size-dependent bubble shapes are quite similar for all six experiments. Further studies focusing on single local effects like the shear rate or wake effects are highly desirable to obtain a deeper understanding of the underlying processes; for this purpose, the given results can help to assess the most important effect and in which extend it should be studied. (C) 2017 Elsevier Inc. All rights reserved.
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Bubble generation plays a significant role for mass transfer. An experimental study was performed in a Venturi type bubble generator to evaluate the development of the bubble size distribution in the turbulence flow of an air-wate...
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Bubble generation plays a significant role for mass transfer. An experimental study was performed in a Venturi type bubble generator to evaluate the development of the bubble size distribution in the turbulence flow of an air-water system. As the air stream enters into the flowing water stream through small feedholes, it breaks into bubbles in the diverging section of the venturi tube. Based on the results of Kress's formulation on turbulence dissipation rate, the bubble size correlation with the Reynolds number and the surface tension coefficient was derived. For purpose of validation, the bubbly flow under various Re and gas volume ratios was measured by visualization technique. The experimental data proved that the volume averaged bubble diameter has a -1 power dependence on the Re, which agrees well with the theoretical derivation. Also, it is found that the bubble size is a linear function of the gas volume ratio and a revised formulation which includes the gas volume ratio effect was proposed.
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An experimental study has been carried out on bubble generation by means of air injection through an orifice submerged in water. An orifice, drilled in an hydrophobic horizontal plate and a radius a = 1 mm, have been used to inves...
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An experimental study has been carried out on bubble generation by means of air injection through an orifice submerged in water. An orifice, drilled in an hydrophobic horizontal plate and a radius a = 1 mm, have been used to investigate the effect of the flow rate working conditions on the bubble formation process; a wide range of volumetric gas flow rates (2.0 × 10≤Q≤1.8×10~44 mm~3/s) has been used, including different chamber volumes before the injection orifice. Two volumetric gas flow rates are apparent: the one into the experimental setup, which could be assumed as a constant, and a higher one into de bubble through the injection orifice; there is a first and significant time step with zero gas flow through the orifice. This drives to two different working conditions, named: constant volumetric gas flow rate, when both flow rates are equal, and non-constant volumetric gas flow rate, when both flow rates are different. First, a short experimental study, for constant volumetric gas flow rate, is presented; it represents a link between both working conditions. The main part of the work are devoted to non-constant volumetric gas flow rate and, it showed that the experimental data can be reduced approximately to a single bubble volume/flow rate relationship, as in the case of constant volumetric gas flow rate, if the properly scaled volumetric gas flow rate is used. Finally a simplified model, to estimate this proper volumetric gas flow rate, is presented and checked experimentally.
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A comprehensive database was obtained for stationary upward air-water flows in a vertical pipe with an inner diameter of 195.3 mm using the wire-mesh sensor technology. During the experiments the sensor was always mounted on the t...
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A comprehensive database was obtained for stationary upward air-water flows in a vertical pipe with an inner diameter of 195.3 mm using the wire-mesh sensor technology. During the experiments the sensor was always mounted on the top of the test section while the distance between gas injection and measuring plane was varied to up to 18 different L/D by using gas injection chambers at different vertical positions. The gas was injected via holes in the pipe wall. The pressure was kept at 0.25 MPa (absolute) at the location of the active gas injection while the temperature was constant at 30℃ ± 1 K. This procedure exactly represents the evolution of the flow along the pipe, as it would be observed for an injection at a constant height position and a shifting of the measurement plane. The experiments were done for 48 combinations of air and water superficial velocities varying from 0.04 m/s to 1.6 m/s for water and 0.0025 m/s to 3.2 m/s for air. From the raw data time-averaged data as: radial gas volume fraction profiles, bubble size distributions, radial volume fraction profiles decomposed according to the bubble size and the radial profiles of the gas velocity were calculated. Due to the combination of the new experimental procedure with the high spatial and temporal resolution of the wire-mesh sensor technology the data have new quality especially regarding their consistency in the evolution with increasing L/D. This closes a gap for data suitable for CFD code development and validation for two-phase flows, especially for models on bubble coalescence and break-up.
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