There is great potential for using computational fluid dynamics (CFD) as a tool in scale-up and design of bubble columns. Full-scale experimentation in bubble columns is expensive and CFD is an alternative approach to study bubble column hydrodynamics. However, CFD can be computationally intensive as a predictive tool for a full three-dimensional geometry. In this paper, a 0.2 m diameter semi-batch bubble column is numerically simulated and the results are compared to experimental measurements performed by Rampure et al. [1]. The objectives are to examine and determine an appropriate set of numerical parameters and to determine if two-dimensional simulations are able to accurately predict observed bubble phenomena so that the computational cost can be reduced. A two-fluid Eulerian-Eulerian model is employed to represent each phase as interpenetrating continua and the conservation equations for mass and momentum for each phase are ensemble-averaged. Time-averaged gas holdup is mainly examined due to its significant role in gasliquid mass transfer and to compare to available data. Numerical predictions are presented for gas holdup at various axial heights as a function of radial position for a superficial gas velocity of 0.1 m/s. The numerical predictions exhibit the axial development of the gas holdup profile phenomena; that is, the gas holdup at the center of the column increases with increasing axial height. The effects of grid resolution and convergence criteria on the numerical predictions are also demonstrated. However, FD ca n be computationally inten ive a a predict ive too l fo r a full th reedimensional geo metry. In th i paper, a 0.2 m diameter semibatch bubble column is numerica ll y imul ated and the res ults are compared to ex perimental meas urements performed by R ampure et al. [ !] . The objecti ve are to exa mine and determine an appropriate set of numerica l parameters and to determine if two-dimensional simul ations are abl e to accuratel y predtct observed bubble phenomena so that the computational cost can be reduced. A two-fluid Euleri an-Eul eri an model is employed to represent each phase as interpenetrating continua and the conservation equations fo r mass and momentum for each phase are ensemble-averaged. Time-averaged gas holdup ts mamly examined due to its significant role in gas-liquid mass transfer and to compare to available data. Numerical predictions are presented for gas holdup at various ax ial heights as a function of radial position for a superfi cial gas velocity of ~· 1 mls. The numerical predictions exhibit the axial evelopment of the gas holdup profile phenomena · that is the gas h ld ' '. 0 up at the center of the column increases with Increasing axial height. The effects of grid resolution and convergence .t . d en ena on the numerical predictions are also emonstrated.
The external loop airlift reactor (ELALR) is a modified bubble column reactor that is composed of two vertical columns that are interconnected with two horizontal tubes and is often preferred over traditional bubble column reactors because they can operate over a wider range of conditions. In the present work, the gas-liquid flow dynamics in an ELALR was simulated using an Eulerian-Eulerian ensemble-averaging method in two-dimensional (2D) and three-dimensional (3D) coordinate systems. The computational fluid dynamics (CFD) simulations were compared to experimental measurements from a 10.2 cm diameter ELALR for superficial gas velocities ranging from 1 cm/s to 20 cm/s. The effect of specifying a mean bubble diameter to represent the gas phase in the CFD modeling was investigated, and 2D and 3D simulations were found to be in good agreement with the experimental data. The ELALR flow regimes were compared for the reactor operating in bubble column, closed vent, and open vent modes, and the 2D simulations qualitatively predicted the behavior of bubble growth in the downcomer. However, it was found that 3D simulations were necessary to capture the physics of the ELALR for gas holdup, bulk density differences, and riser superficial liquid velocity. Disciplines Complex Fluids | Mechanical Engineering | Thermodynamics Comments
Inside of an effervescent atomizer gas is injected into a liquid cross-flow in order to produce a bubbly two-phase mixture. The presence of gas bubbles leads to enhanced liquid break-up as compared to simple pressure atomization of the liquid phase alone [1]. In the present work, the dynamic shapes and sizes of single air bubbles injected in liquid water cross flow of an effervescent atomizer’s mixing chamber are investigated numerically and experimentally. Particular focus is aimed on the convergent channel section just prior to the atomizer exit orifice where the bubble experiences a significant drop in pressure. Volume of fluid (VOF) modeling and simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS FLUENT and further provide information on the liquid velocities near the air bubble. A high-speed imaging system and digital image processing are used for capturing experimental data on this highly dynamic process. The numerical results are compared with experimental visualizations to better understand the physical interactions between the two phases approaching the atomizer exit.
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