Radial Distribution Of Gas Holdup Research Articles

Investigation of a GL-EMMS gradual drag model by comparative simulations of bubble columns

The mesoscale model for gas–liquid flow based on the energy minimization multi-scale method, which is denoted by the GL-EMMS model, demonstrates its capability of predicting the regime transition in bubble columns. Since the dominant mechanisms and stability conditions are important for this mesoscale model, it is desirable to reveal more details through the computational fluid dynamics (CFD) work. This work proposes a gradual drag correlation based on the GL-EMMS model, and then incorporates it into the Euler–Euler (E–E) and Euler–Lagrange (E–L) frameworks respectively. Cases with different gas distributors are simulated via the opensource platform of OpenFOAM. A simplified meshing treatment of the inlet boundary is adopted and has proven effective to reducing the computing load. The E–E simulation predicts the total gas holdup and the radial distribution of liquid velocity well, but obvious deviation is observed in the radial distribution of gas holdup, especially at lower superficial gas velocities for the single-orifice aeration. Nonetheless, this shortage can be overcome saliently by the E–L method. These verified simulations enable further possible iterative investigation between the GL-EMMS model and the CFD work, and also inspire some meaningful simulating strategy for bubble columns.

Effects of bubble coalescence and breakup models on the simulation of bubble columns

This work investigates the effects of coalescence and breakup models on the radial distribution of gas holdup and local bubble size distribution in a laboratory scale bubble column operated at 0.11 m/s, 0.14 m/s, 0.19 m/s and 0.23 m/s. The results quantitatively show the influences of bubble coalescence due to various mechanisms, coalescence efficiency calculated by different models, modification coefficients for coalescence rates, critical energy required for breakup and bubble breakage caused by large scale eddies and viscous shear on numerical simulations. Furthermore, qualitative analysis is performed to analyze the effects of these factors. The performance of various models is evaluated (e.g. Han’s breakup model and Das’s coalescence model). Some optimized combinations of coalescence and breakup models that can be used to accurately predict the local gas holdup and bubble size distribution in laboratory scale bubble columns operated at heterogeneous regime are proposed.

CFD simulation of gas-liquid-solid flow in slurry bubble columns with EMMS drag model

The drag models reflecting the complex phase interactions are of critical importance in the CFD simulation of gas-liquid or gas-liquid-solid flows. In this study, we demonstrate that the Dual-Bubble-Size (DBS) drag model based on the Energy-Minimization Multi-Scale (EMMS) concept, is capable of effectively predicting the distribution of gas holdup in slurry bubble columns. For solid-free or low solid loading systems, a thorough comparison of the DBS-Global and other drag models indicates that, without any fitting parameters, the DBS-Global drag model prediction on the radial distribution of gas holdup and solid hydrodynamics is in conformity with experiments over a wide range of superficial gas velocities. The three-fluid and two-fluid models are also compared in this work, indicating that the three-fluid model in combination with the DBS-Global drag model gives the best prediction. Previous experiments show that the effects of solid presence on hydrodynamics become pronounced at high solid loading conditions. The prediction of the original DBS-Global drag model deviates from the experiments at high solid loading conditions, and the solid effects need to be taken into account. This work then proposes a modified drag model for simulating three-phase systems at high solid concentration. The new model is capable of accurately predicting the local gas holdup for the systems of solid loading of 25% or 40%. Further fundamental study of the solid effect on the change of flow structure and dominant mechanisms in gas-liquid flow is required.

The use of electrical resistance tomography for the characterization of gas holdup inside a bubble column bioreactor containing activated sludge

Gas holdup is a determinant factor regarding the efficiency of the activated sludge processes. Despite the numerous studies on the characterization of gas holdup in two or three phase systems, lack of experimental data in systems including actual cells as the solid phase has been emphasized in the literature. This study is a response to the need for an appropriate knowledge on the gas holdup behavior in the real activated sludge systems. The capabilities of electrical resistance tomography in performing local and global measurements in a non-intrusive manner made it possible to examine the influence of MLSS concentration and aeration intensity on the overall as well as spatial distribution of gas holdup within the activated sludge bioreactor. In order to cover all activated sludge processes, a wide range of MLSS concentration (0.712–15.86g/L) was employed, and the effect of MLSS on rheological characteristics of the corresponding mixed liquor was measured and modeled. The results of the present study revealed an initially increasing followed by decreasing variation of overall gas holdup with increasing MLSS concentration which is reported in this paper for the first time. Radial distribution of gas holdup was also modeled and the variations of central, wall, and cross sectional average gas holdups with axial location, MLSS, and air velocity were specified.

Parameters measurement of hydrodynamics and CFD simulation in multi‐stage bubble columns

The effect of superficial gas velocity and structural parameters of plate (perforated plate and tongue plate) on the hydrodynamics parameters of column are studied in a multi‐stage bubble column, with 2000 mm height and 282 mm diameter of the multi‐stage bubble column, by using the electrical resistance tomography (ERT) technique and computational fluid dynamics (CFD). According to the results, ERT and CFD techniques can be well applied to the measurement and estimation of gas–liquid two‐phase processes. The gas holdups, the radial distribution of gas holdup and gas cap height strongly depend on the operating condition and tray structure. By increasing the superficial gas velocity, the gas holdup in the multi‐stage bubble column increases. With a decrease of the open area ratio, the hole diameter and tongue angle, the gas holdups and gas cap height below the perforated plate increase. The addition of the perforated plate enhances the flow behaviour of gas–liquid in bubble column. The CFD can then aid the prediction of gas cap height and gas holdup in a multi‐stage bubble column.

Multiphase hydrodynamics and distribution characteristics in a monolith bed measured by optical fiber probe

The optical fiber probe has been for the first time applied to investigate the hydrodynamics and gas‐phase distribution at high gas/liquid ratios in a two‐phase flow monolith bed with 0.048 m diameter and 400 cpsi. Local hydrodynamic parameters including gas holdup, bubble frequency, bubble velocity, and bubble length in single channels were measured by 16 inserted single‐point optical fiber probes within the bed under a nozzle as the liquid distributor. The following findings are reported. (1) The optical fiber probe can be used as an efficient and convenient technique for measuring local hydrodynamic parameters inside the channels of a monolith bed; (2) within the range of high gas/liquid ratios under which experiments were conducted, churn flow regime occurred. In this regime, the monolith bed radial distribution of gas holdup, bubble frequency, bubble velocity, and bubble length is nonuniform in nature. © 2013 American Institute of Chemical Engineers AIChE J 60: 740–748, 2014

Numerical Simulation of Bubble Column Flows in Churn-Turbulent Regime: Comparison of Bubble Size Models

Numerical simulations of cylindrical bubble column operating in the churn-turbulent regime have been simulated using Euler–Euler approach incorporated with the RNG k–ε model for liquid turbulence. Single-size bubble model, double-size bubble model, and the multiple size group model (MUSIG), including the homogeneous and inhomogeneous discrete methods, are employed in the simulations. Mass conserved formulations of breakup and coalescence rates were used in the computation of bubble size distributions. The Schiller–Naumann drag force was used in the single-size model, and the Ishii–Zuber drag force was used for the MUSIG simulations. For the double-size bubble model, an empirical drag formulation was adapted. The simulation results of time-averaged axial velocity and gas holdup obtained with the three models were compared with reported experimental data in the literature. The results showed that only MUSIG models with lift force can reproduce the measured radial distribution of gas holdup in the fully developed flow regime and that the inhomogeneous MUSIG model performs a little better than other models in the prediction of axial liquid velocity. The RNG k–ε model was used in all simulations, and the results confirmed that this version of k–ε model did yield relatively high turbulence dissipation rates and high bubble breakup rates and, thus, resulted in a rational bubble size distribution. The ad hoc manipulation of the breakup rates was avoided. The simulation results indicated profound mutual effects of drag force, mean bubble sizes, and turbulence characteristics. An increase in drag force yielded a decrease in the relative velocity between phases, the later could result in decreases in k and ε. A large Sauter diameter results from a low bubble breakup rate which was directly connected to the dissipation rates of turbulence. The change of Sauter diameter, in turn, influenced the drag force.

Multi-scale analysis of gas–liquid interaction and CFD simulation of gas–liquid flow in bubble columns

The ratio of effective drag coefficient to bubble diameter is of critical importance for CFD simulation of gas–liquid flow in bubble columns. In this study, a novel model is proposed to calculate the ratio on the basis of the Dual-Bubble-Size (DBS) model. The motivation of the study is that a stability condition reflecting the compromise between different dominant mechanisms can serve for a closure in addition to mass and momentum conservative constraints, and the interphase momentum transfer should be related to different paths of energy dissipation. With the DBS model, we can first offer a physical interpretation on macro-scale regime transition via the shift of global minimum point of micro-scale energy dissipation from one potential trough to the other. Then the proposed drag model is integrated into a CFD simulation. Prior to this integration, we investigate the respective effects of bubble diameter and correction factor and found that the effect of bubble diameter is limited, whereas the correction factor due to the bubble swarm effect is eminent and appropriate correction factor has to be selected for different correlations of standard drag efficient to be in accord with experiments. By contrast, the DBS drag model can well predict the radial gas holdup distribution, the total gas holdup as well as the two-phase flow field without the need to adjust model parameters, showing its great potential and advantage in understanding the complex nature of multi-scale structure of gas–liquid flow in bubble columns.

Gas-Liquid flow characterization in bubble columns with various gas-liquid using electrical resistance tomography

Electrical resistance tomography (ERT) is an advanced and new detecting technique that can measure and monitor the parameters of two-phase flow on line, such as gas-liquid bubble column. It is fit for the industrial process where the conductible medium serves as the disperse phase to present the key bubble flow characteristics in multi-phase medium. Radial variation of the gas holdup and mean holdups are investigated in a 0.160 m i. d. bubble column using ERT with two axial locations (Plane 1 and Plane 2). In all the experiments, air was used as the gas phase, tap water as liquid phase, and a series of experiments were done by adding KCl, ethanol, oil sodium, and glycerol to change liquid conductivity, liquid surface tension and viscosity. The superficial gas velocity was varied from 0.02 to 0.2 m/s. The effect of conductivity, surface tension, viscosity on the mean holdups and radial gas holdup distribution is discussed. The results showed that the gas holdup decrease with the increase of surface tension and increase with the increase of viscosity. Meanwhile, the settings of initial liquid conductivity slightly influence the gas holdup values, and the experimental data increases with the increase of the initial setting values in the same conditions.

Measurement of gas holdup profiles in a gas liquid cocurrent bubble column using electrical resistance tomography

Radial variation of the gas holdup and mean holdups were investigated in a 0.160 m i.d. bubble column using electrical resistance tomography with two axial locations (Plane 1 and Plane 2). In all the experiments tap water was used the liquid phase and air was the gas phase. Superficial gas velocity was varied from 0.02 to 0.25 m/s, and superficial liquid velocity varied from 0 to 0.011 m/s. The effect of liquid velocity on the mean holdups and radial gas holdup distribution was discussed. The experimental results showed the liquid velocity slightly influence the mean holdup and radial hold-ups distribution in the operating condition, and the liquid flow can improve the transition gas velocity for the homogeneous regime to heterogeneous regime. Meanwhile the mean gas holdups as a function of gas velocity were derived from using differential pressure method and electrical resistance tomography method. The agreement between results obtained by these two methods is generally very good in the homogeneous regime. But in the transition regime and heterogeneous regime, results with ERT are slightly larger than one with the differential pressure method. According to the experimental results, a correlation for the centreline holdup is obtained.

Overview of Multiphase Flow Phenomena in Moving Time‐Averaged Space

AbstractAn overview of multiphase flow phenomena is described on the basis of three relations; a relation between an interaction force and time‐averaged physical quantities, a relation between an interaction force and the surrounding flow field, and a relation between time‐averaged physical quantities and multiphase flow. The three relations used to theoretically derive the parabolic radial distribution of gas holdup for recirculating turbulent flow in a bubble column are in good agreement with experimental data. General applicability of the three relations for a variety of multiphase flows is also discussed.

Force Balance Controlling Radial Gas Holdup Distribution for the Recirculating Turbulent Flow in Bubble Columns

Radial distribution of gas holdup for recirculating turbulent flow in a bubble column is theoretically studied on the basis of time-averaged Navier-Stokes equations for both gas and liquid phases. The basic equations are simplified for the recirculating turbulent flow, and the radial distribution of gas holdup is successfully shown to be parabolic, which is experimentally well-known. On the way of derivation, it is shown that the radial velocity fluctuation of the liquid phase was mainly caused by two adjacent bubbles, and that the radial drug force acting on a bubble is proportional to the Reynolds stress of the gas–liquid multiphase.

Flow Characterization of a Three‐Phase Circulating Fluidized Bed Using an Ultrasonic Technique

AbstractThe axial and radial distributions of solid and gas holdups were investigated in an air‐water‐glass bead circulating fluidized bed (GLSCFB) using a new ultrasonic technique, with a new method based on signal fluctuations. The cross‐sectional averaged gas and solid holdups measured at two axial positions appear to be similar at all studied operating conditions. The radial non‐uniformity decreases with increasing liquid velocity but does not change with an increasing solid circulating rate. The radial distribution of gas holdup was more uniform for 1.3 mm beads than for 433 µm glass beads.

Flow Regimes and Radial Gas Holdup Distribution in Three-Phase Magnetic Fluidized Beds

This study examined the fluidization behavior of gas−liquid−solid magnetic fluidized beds to provide prerequisite knowledge for reactor design. Air, water, and iron shots (0.19, 0.39, and 0.93 mm in diameter) were used as gas, liquid, and solid phases, respectively. The radial gas holdup distribution and overall gas holdup in three-phase magnetic fluidized beds were measured. Flow regimes of three-phase magnetic fluidized beds with different particle sizes were also discussed. The radial gas holdup distribution was measured by using an optical fiber probe; the change of radial gas holdup distribution under different flow regimes was also studied.

Scale-up effects on the time-averaged and dynamic behavior in bubble column reactors

Scale-up effects on the dynamic and time-averaged behavior are investigated in bubble columns with diameters of 200, 400 and 800 mm , respectively. Using a hot-wire probe, the local instantaneous heat transfer rates and the local time-averaged gas holdups were measured at the different locations of the columns over the gas velocity range from 20 to 90 mm/ s . The dyanamic behavior of local heat transfer was characterized by frequency domain and state-space analysis in terms of power spectrum, correlation dimension and Kolmogorov entropy. The power spectra of heat transfer rate fluctuations were found to be insensitive to the bubble column size. The state-space analysis indicates that heat transfer rate exhibits a chaotic behavior with the correlation dimension varying from 2.0 to 3.0. No significant effect of column size on the correlation dimension was observed, whereas the Kolmogorov entropies are apparently dependent on the column diameter. With the increase in column diameter, Kolmogorov entropy decreases significantly and exhibits rather flat radial distribution in the large-scale column. With respect to the time-averaged behavior of local gas holdup, the effects of column size on the radial distribution of gas holdup was examined. It was found that the 800 mm column exhibited a relatively uniform distribution of gas holdup. The dependence of both dynamic and time-averaged behavior on the column scale is considered to be closely related to the different macroscopic flow structure, which is a function of the scale of the column.

Gas holdup distributions in large-diameter bubble columns measured by computed tomography

Using the computed tomography (CT) and computer automated radioactive particle tracking (CARPT) facilities at the Chemical Reaction Engineering Laboratory (CREL), time-averaged gas holdup distributions and liquid recirculation velocities were measured in a 44 cm diameter bubble column for air–water and air–drakeoil systems at 2, 5, and 10 cm/s superficial gas velocities, which cover bubbly, transition and churn-turbulent flow regimes, respectively. Gas holdup was found to increase only slightly with the increase in axial distance from the distributor, but increased significantly with the increase in superficial gas velocity, as expected. A lower gas holdup was observed in the air–drakeoil system than in the air–water system. This could be predominantly attributed to the formation of large bubbles in the former case due to the higher viscosity of drakeoil (approximately 0.03 Pas (=30 cp)). At high superficial gas velocities, the time-averaged cross-sectional gas holdup distributions were almost symmetric for both air–water and air–drakeoil systems. However, at 2 cm/s superficial gas velocity, an asymmetry in the holdup distribution was observed, which manifested itself in an asymmetric liquid recirculation pattern. At all gas velocities, the radial gas holdup distribution for the air–water system was steeper than that for the air–drakeoil system, yielding steeper radial liquid velocity profiles. Comparison of the gas holdup obtained in the 44 cm diameter column and that obtained in a 10 cm diameter column is discussed.

Bubble and liquid flow characteristics in a wood’s metal bath stirred by bottom helium gas injection

A model study was carried out to elucidate bubble and liquid flow characteristics in the reactor of metals refining processes stirred by gas injection. Wood’s metal with a melting temperature of 70 °C was used as the model of molten metal. Helium gas was injected into the bath through a centered single-hole bottom nozzle to form a vertical bubbling jet along the centerline of the bath. The bubble characteristics specified by gas holdup, bubble frequency, and so on were measured using a two-needle electroresistivity probe, and the liquid flow characteristics, such as the axial and radial mean velocity components, were measured with a magnet probe. In the axial region far from the nozzle exit, where the disintegration of rising bubbles takes place and the radial distribution of gas holdup follows a Gaussian distribution, the axial mean velocity and turbulence components of liquid flow in the vertical direction are predicted approximately by empirical correlations derived originally for a water-air system, although the physical properties of the two systems are significantly different from each other. Under these same conditions, those turbulent parameters in high-temperature metals refining processes should thus be accurately predicted by the same empirical correlations.

Behavior of gas jet and plume in liquid metal.

Measurement of the distribution of gas holdup in nitrogen injection into mercury was made by means of the electroresistivity probe technique with the aid of high speed data processing system. On the basis of the measurement near an orifice or a nozzle, two regimes of gas flow are distinguished; bubbling and jetting. In the jetting regime, the gas jet issuing from the tuyere into mercury becomes very thin and rises rapidly. The radial distribution of gas holdup expands with increasing vertical distance due to entrainment of the surrounding liquid. The vertical distance at which transition of flow behavior from jet to plume occurs is about 30-40mm above the tuyere under the present experimental conditions. The effect of bath depth on the gas holdup is discussed.The upward flow of gas-liquid mixture in the plume zone is analyzed on the basis of macroscopic mass and momentum balances. The calculated results of plume radius and gas holdup are compared with the experimental results. It is presumed that the circulating flow in the bath affects the dispersion of nitrogen.