CFD Simulation on the Hydrodynamics in Gas-Liquid Airlift Reactor

2017 ◽  
Vol 12 (4) ◽  
Author(s):  
Shi Yan Liew ◽  
Jolius Gimbun
Keyword(s):  
Cfd Simulation ◽  
Predictive Accuracy ◽  
Liquid Velocity ◽  
Fluid Model ◽  
Gas Holdup ◽  
Airlift Reactor ◽  
Turbulent Dispersion ◽  
Dispersion Force ◽  
Phase System ◽  
Closure Model

AbstractTwo-fluid model approach to simulate gas-liquid airlift reactors is widely implemented but have yet to reach a consensus on the closure model to account the gas-liquid interphase forces. Proper selection of a closure model is required in order to accurately capture the hydrodynamics in the complex of the two-phase system. Our work concerns the evaluation of the interfacial forces models (i. e. drag, lift and turbulent dispersion force) and their effects on local gas holdup and liquid velocity. A transient three-dimensional airlift reactor simulation was carried out using computational fluid dynamics by implementing the dispersed standardk-εturbulence model. Four drag models governed by spherical bubble, bubble deformation and Rayleigh-Taylor were being evaluated in our work. The significance on the inclusion of the lift model on predictive accuracy on the flow field was also studied as well. Whereas, two turbulent dispersion force models were selected to evaluate on their performance in improving the predictive accuracy of the local hydrodynamics. Results showed that the drag governed by Rayleigh-Taylor which accounts the bubble swarm effect had better predictions on the gas holdup in the downcomer and improved predictions in radial gas holdup. The inclusion of the lift model improved local gas holdup predictions at higher heights of the reactor and shifted the bubble plume towards the centre region of the riser. Meanwhile, the turbulent dispersion models improved the overall results of predicted local gas holdup with closer agreement obtained when the drift velocity model was considered in the simulation. The axial liquid velocity was well predicted for all cases. The consideration of the drag, lift and turbulent dispersion forces resulted in a closer agreement with experimental data.

Prediction of Parameters Distribution of Upward Boiling Two-Phase Flow With Two-Fluid Models

2002 ◽  
Author(s):  
Wei Yao ◽  
Christophe Morel
Keyword(s):  
Void Fraction ◽  
Fluid Model ◽  
Constitutive Relations ◽  
Turbulent Dispersion ◽  
Dispersion Force ◽  
Liquid Temperature ◽  
Two Phase ◽  
Radial Profiles ◽  
Two Fluid Model ◽  
Two Fluid

In this paper, a multidimensional two-fluid model with additional turbulence k–ε equations is used to predict the two-phase parameters distribution in freon R12 boiling flow. The 3D module of the CATHARE code is used for numerical calculation. The DEBORA experiment has been chosen to evaluate our models. The radial profiles of the outlet parameters were measured by means of an optical probe. The comparison of the radial profiles of void fraction, liquid temperature, gas velocity and volumetric interfacial area at the end of the heated section shows that the multidimensional two-fluid model with proper constitutive relations can yield reasonably predicted results in boiling conditions. Sensitivity tests show that the turbulent dispersion force, which involves the void fraction gradient, plays an important role in determining the void fraction distribution; and the turbulence eddy viscosity is a significant factor to influence the liquid temperature distribution.

Model Development of Turbulent Dispersion Force for Advanced Two-Fluid Model in Consideration of Bubble-Liquid Phase Interactions

2010 ◽  
Author(s):  
Hideaki Hosoi ◽  
Hiroyuki Yoshida
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Constitutive Equations ◽  
Fluid Model ◽  
Turbulent Dispersion ◽  
Dispersion Force ◽  
Phase Flow ◽  
Two Phase ◽  
Two Fluid Model ◽  
Two Fluid

Two-fluid model can simulate two-phase flow by computational cost less than detailed two-phase simulation method such as interface tracking method. Therefore, two-fluid model is useful for thermal hydraulic analysis in large-scale domain such as rod bundles. However, since two-fluid model include a lot of constitutive equations, applicability of these constitutive equations must be verified by use of experimental results, and the two-fluid model has problems the result of analyses depends on accuracy of constitutive equations. To solve these problems, an advanced two-fluid model has been developed in Japan Atomic Energy Agency. In the model, an interface tracking method is combined with the two-fluid model to predict large interface structure behavior accurately. Liquid clusters and bubbles larger than a computational cell are calculated using the interface tracking method, and those smaller than a cell are simulated by the two-fluid model. Constitutive equations to evaluate the effect of small bubbles or droplets on two-phase flow required in the advanced two-fluid model as same as a conventional two-fluid model. However, dependency of small bubbles and droplets on two-phase flow characteristic is relatively small, and the experimental results to verify the equations are not required much. The turbulent dispersion force term is one of the most important constitutive equations for the advanced two-fluid model. The turbulent dispersion force term has been modeled by many researchers for the conventional two-fluid model. However, the existing models include effects of large bubbles and deformation of bubbles implicitly, these models are not applicable to the advanced two-fluid model. In this study, we develop the new model for turbulent dispersion force term. In this model, effect of large bubbles and deformation of bubbles are neglected. The liquid phase turbulent kinetic energy and bubble-induced turbulent kinetic energy are considered to evaluate driving force in the turbulent diffusion of small bubbles. The bubble-induced turbulent kinetic energy is given by the function of bubble diameter and local relative velocity, and the liquid phase turbulent kinetic energy is similar to the single phase flow case. Furthermore, we considered energy transfer from the bubble-induced kinetic energy to the liquid phase turbulent kinetic energy. To verify the developed model, the advanced two-fluid model and the model for turbulent dispersion term were incorporated to the 3-dimensional two-fluid model code ACE-3D, and comparisons between the results of analyses and air-water two-phase flow experiments in 200 mm diameter vertical pipe were performed.

scholarly journals The Effect of a Modified Downcomer on the Hydrodynamics in an External Loop Airlift Reactor

2006 ◽  
Author(s):  
Samuel T. Jones ◽  
Theodore J. Heindel
Keyword(s):  
Liquid Velocity ◽  
Bubble Column ◽  
Gas Holdup ◽  
Area Ratio ◽  
Airlift Reactor ◽  
Open Area ◽  
Gas Velocity ◽  
External Loop ◽  
Superficial Gas Velocity ◽  
Superficial Liquid Velocity

Gas holdup and superficial liquid velocity in the downcomer and riser are studied for an external loop airlift reactor with an area ratio of 1:16. Two downcomer configurations are investigated consisting of the downcomer open or closed to the atmosphere. Experiments for these two configurations are carried out over a range of superficial gas velocities from UG = 0.5 to 20 cm/s using three aeration plates with open area ratios of 0.62, 0.99 and 2.22%. These results are compared to a bubble column operated with similar operating conditions. Experimental results show that the gas holdup in the riser does not vary significantly with a change in the downcomer configuration or bubble column operation, while a considerable variation is observed in the downcomer gas holdup. Gas holdup in both the riser and downcomer are found to increase with increasing superficial gas velocity. Test results also show that the maximum gas holdup for the three aerator plates is similar, but the gas holdup trends are different. The superficial liquid velocity is found to vary considerably for the two downcomer configurations. However, for both cases the superficial liquid velocity is a function of the superficial gas velocity and/or the flow condition in the downcomer. These observed variations are independent of the aerator plate open area ratio. When the downcomer vent is open to the atmosphere, the superficial liquid velocity is initially observed to increase with increasing superficial gas velocity until the onset of choking occurs in the downcomer. Increasing the superficial gas velocity beyond the onset of choking increases the effect of choking and decreases the superficial liquid velocity. Once maximum choking is reached, the superficial liquid velocity becomes independent of the superficial gas velocity. When the downcomer vent is closed to the atmosphere, the superficial liquid velocity is initially observed to decrease with increasing superficial gas velocity as choking in the downcomer is immediately present. Once maximum choking occurs, the superficial liquid velocity once again becomes independent of the superficial gas velocity.

scholarly journals Hydrodynamics of a self-agitated draft tube airlift reactor

2014 ◽  
Vol 20 (1) ◽  
pp. 59-69 ◽  
Author(s):  
Miodrag Tekic ◽  
Ivana Sijacki ◽  
Milenko Tokic ◽  
Predrag Kojic ◽  
Dragan Petrovic ◽  
...  
Keyword(s):  
Draft Tube ◽  
Liquid Velocity ◽  
Mixing Time ◽  
Gas Holdup ◽  
Airlift Reactor ◽  
Hydrodynamic Characteristics ◽  
Energy Losses ◽  
Liquid Circulation ◽  
Bubble Breakup ◽  
Constructed Self

The main hydrodynamic characteristics of a novel-constructed, self-agitated draft tube airlift reactor (DT-ALR) were investigated. Ten impellers, driven only by the means of gas throughput and induced liquid circulation, were inserted in the draft tube. The insertion of impellers caused bubble breakup and reduction of both mean bubble size and coalescence, even under the conditions of high gas throughputs. Although the impellers induced energy losses, the resistance to the flow was relatively lower due to their rotation, unlike the internals used in other research reported in the literature. In comparison to the conventional configuration of a DT-ALR, it was found that the presence of impellers led to significant changes in hydrodynamics: riser gas holdup and mixing time increased, while overall gas holdup and liquid velocity in the downcomer decreased.

Computational fluid dynamics of rectangular external loop airlift reactor

2020 ◽  
Vol 18 (5-6) ◽  
Author(s):  
Shivanand M. Teli ◽  
Channamallikarjun S. Mathpati
Keyword(s):  
Large Scale ◽  
Lift Coefficient ◽  
Volume Ratio ◽  
Reynolds Stress Model ◽  
Airlift Reactor ◽  
Experimental Results ◽  
Turbulent Dispersion ◽  
Drag Forces ◽  
External Loop ◽  
Turbulent Models

AbstractThe novel design of a rectangular external loop airlift reactor is at present the most used large-scale reactor for microalgae culture. It has a unique future for a large surface to volume ratio for exposure of light radiation for photosynthesis reaction. The 3D simulations have been performed in rectangular EL-ALR. The Eulerian–Eulerian approach has been used with a dispersed gas phase for different turbulent models. The performance and applicability of different turbulent model’s i.e., K-epsilon standard, K-epsilon realizable, K-omega, and Reynolds stress model are used and compared with experimental results. All drag forces and non-drag forces (turbulent dispersion, virtual mass, and lift coefficient) are included in the model. The experimental values of overall gas hold-up and average liquid circulation velocity have been compared with simulation and literature results. It is seemed to give good agreements. For the different elevations in the downcomer section, liquid axial velocity, turbulent kinetic energy, and turbulent eddy dissipation experimental have been compared with different turbulent models. The K-epsilon Realizable model gives better prediction with experimental results.

CFD Simulation of Particle Transport and Dispersion in Indoor Environment by Human Walking

2012 ◽  
Author(s):  
Iman Goldasteh ◽  
Goodarz Ahmadi ◽  
Andrea Ferro
Keyword(s):  
Particulate Matter ◽  
High Speed ◽  
Cfd Simulation ◽  
Three Dimensional ◽  
Study Data ◽  
Turbulent Dispersion ◽  
Experimental Chamber ◽  
Indoor Environments ◽  
Three Dimensional Model ◽  
Particle Resuspension

Particle resuspension is an important source of particulate matter in indoor environments that significantly affects the indoor air quality and could potentially have adverse effect on human health. Earlier efforts to investigate indoor particle resuspension hypothesized that high speed airflow generated at the floor level during the gate cycle is the main cause of particle resuspension. The resuspended particles are then assumed to be dispersed by the airflow in the room, which is impacted by both the ventilation and the occupant movement, leading to increased PM concentration. In this study, a three dimensional model of a room was developed using FLUENT™ CFD package. A RANS approach with the RNG k-ε turbulence model was used for simulating the airflow field in the room for different ventilation conditions. The trajectories of resuspended particulate matter were computed with a Lagrangian method by solving the equations of particle motion. The effect of turbulent dispersion was included with the use of the eddy lifetime model. The resuspension of particles due to gait cycle was estimated and included in the computational model. The dispersion and transport of particles resuspended from flooring as well as particle re-deposition on flooring and walls were simulated. Particle concentrations in the room generated by the resuspension process were evaluated and the results were compared with experimental chamber study data as well as simplified model predictions, and good agreement was found.

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