Bubble-based Drag Research Articles

Numerical simulation of polydisperse particle segregation based on a bubble-based drag model

This work is devoted to the development of a novel polydisperse drag model based on particle type for bubbling fluidized beds. Traditional homogeneous drag models do not take into account the heterogeneous mesoscale structures, while this model is a mesoscale-structure-dependent drag model. The model combined with Energy Minimum Multiscale (EMMS) theory has been successfully applied to the simulation of polydisperse particle segregation. Four particle size distribution (PSD) systems (including a binary system with different sizes, a binary system with different sizes and densities, a normal PSD and a lognormal PSD) were selected to validate the model under various superficial gas velocities. The PSD was decomposed into several types of solids, each governed by its own set of hydrodynamic equations. The solution algorithm was able to find the heterogeneous index for each solid, which was dependent on both slip velocity and solid concentration. Experiments were subsequently conducted under identical conditions to compare the experimental data with the simulated data. The average relative error between the simulation data and the experimental data about the mean particle size distribution was less than 5%. These results indicated that the proposed model outperformed other drag models in accurately predicting particle segregation behavior.

A hybrid drag model for studying hydrodynamics in a 2D gas-solid tapered fluidized bed

The present work proposes a hybrid drag model by combining homogeneous Ergun correlation and heterogeneous bubble-based drag formulation. The predictions of bubble size, bubble fraction, bed expansion ratio, particulate fluidized area fraction, and solid volume fraction in a 2D gas-solid tapered fluidized bed using the hybrid, Gidaspow, and Energy Minimization Multi-Scale (EMMS) drag models have been compared with the experimental results. The hybrid drag model predicts these parameters closer to the experimental findings. A heterogeneous flow structure has been observed in simulations similar to experiments for the tapered fluidized bed. The bed shows larger solid volume fraction values near its periphery compared to its center. The solid particles show a central-upward and peripheral-downward flow pattern in the tapered fluidized bed.

A bubble-based drag model of rough sphere for the simulation of bubbling fluidized bed

A bubble-based drag model of rough sphere for the simulation of bubbling fluidized bed

A clouded bubble-based drag model for the simulations of bubbling fluidized beds

• A clouded bubble-based drag model is developed for bubbling fluidization. • The non-uniformity of drag force caused by bubble cloud is considered. • The feasibility of the clouded bubble-based drag model is validated. • Application of the modified drag model for Geldart A/B and A particle. A clouded bubble-based (CBB) drag model is developed in view of gas flow patterns around the clouded bubble. The bubble cloud is taken as the inter-phase between the bubble phase and emulsion phase based on the muti-scale resolution of the heterogeneous system. Moreover, the non-uniform distributions of drag force in the bubble crown and wake as well as the bubble sides are considered. The structure parameters are obtained by solving a series of equations including the mass conservation equations and empirical or semi-empirical correlations. The heterogeneous drag model is implemented into the two-fluid model (TFM) for 2D & 3D simulations of bubbling fluidized beds with Geldart A/B and A particles. The results demonstrate that the simulated results using the CBB drag model agree better with the experimental results compared to the homogeneous drag model and the structure-based drag model without the bubble cloud effect.

CFD-DEM simulation of Small-Scale Challenge Problem 1 with EMMS bubble-based structure-dependent drag coefficient

In this study, the energy minimization multi-scale (EMMS)/Bubbling model is coupled with the computational fluid dynamics/discrete element method (CFD-DEM) model via a structure-dependent drag coefficient to simulate the National Energy Technology Laboratory (NETL) small-scale challenge problem using the open-source multiphase flow code MFIX. The numerical predictions are compared against particle velocity measurements obtained from high-speed particle image velocimetry (HSPIV) and differential pressure measurements. The drag-reduction effect of the EMMS bubble-based drag coefficient is observed to significantly improve predictions of the horizontal particle velocity and granular temperature when compared to several other drag coefficients tested; however, the vertical particle velocity and pressure fluctuation characteristic predictions are degraded. The drag-reduction effect is characterized by a reduction in the sizes of slugs or voids, as identified through spectral decomposition of the pressure fluctuations. Overall, this study shows great promise in employing drag coefficients, developed via multi-scale approaches (such as the EMMS paradigm), in CFD-DEM models.

Numerical investigation of binary particle mixing in gas-solid fluidized bed with a bubble-based drag EMMS model

Heterogeneous flow structure of bubbling deeply affects gas–solid momentum transfer in a binary gas–solid fluidized bed. This work presented a binary particle bubble-based Energy Minimum Multi-scale (EMMS) model, and an assumption that bubble-emulation drag force acting on solid 1 and solid 2 depending on each solid volume ratio in the emulation phase was applied to simplify the force balance of the binary particle-phase. The bubble-based EMMS drag was incorporated in the Eulerian multi-fluid model to predict the mixing behaviors of two binary particle systems. The simulation results agree well with the experimental observations in terms of binary solid mixing, bed expansion, and bubble diameter. Compared with the prediction results by the Gidaspow drag model, the jetsam solid fraction and bubble size predicted by the present drag model is in more agreement with the measured results, which indicate the EMMS drag model is an alternative choice for modeling binary gas–solid bubbling system.

Numerical studies of mass transfer performance in fluidized beds of binary mixture

The steam absorption process in a fluidized bed of binary mixture is investigated to study the mass transfer performance of active particles surrounding inert particles by means of CFD approaches. A bubble-based drag model is incorporated to the multi-fluid model to account for the bubble effect on mixing and segregation behaviors of binary mixture in a fluidized bed system. The predictions using different drag models are compared with experimental results. The effect of mixing and segregation behaviors of inert particles and active particles on mass transfer performance is evaluated. The sensitivity of mass transfer process to operating conditions is also analyzed. The results reveal that the bubble-based drag model can give a better prediction with measured data. The enhancement of mixing degree of inert particles and active particles can promote the mass transfer performance.

Assessment of a bubble-based bi-disperse drag model for the simulation of a bubbling fluidized bed with a binary mixture

In the simulation of a bubbling fluidized bed, it is necessary to consider the bubble effect on interphase drag force. In this work, the bubble-based drag model is extended to a binary mixture system. A bi-disperse gas-solid drag model considering the bubble effect is developed and implemented into the multi-fluid model. Three simulations of a bubbling fluidized bed with a binary mixture are carried out. The mixing and segregation performance of binary mixture is investigated. Meanwhile, the sensitivity analysis of drag model to local structure parameters is conducted. The results reveal that the bi-disperse drag model does not only depend on the voidage, but also on the solid component. With consideration of bubble effects, the predicted bed expansion is reduced and the model can obtain a better agreement with experimental results.

Multi-scale CFD modeling of gas-solid bubbling fluidization accounting for sub-grid information

An improved bubble-based heterogeneous drag model was formulated in which the original bubble-based EMMS drag model at macroscale bed level was extended to microscale sub-grid level for accurate coarse-grid two-fluid modeling (TFM) of Geldart A powder bubbling fluidization. At both the macroscale and microscale levels, the physically heterogeneous structures were modeled by three pseudo phases, i.e., bubble phase, emulsion phase, and inter-phase, where conservation equations and a stability condition were applied at multi-scales present in a bubbling fluidized bed. This new improved bubble-based drag model utilizes a two-step macro-to-micro scheme, where the multi-scale drag coefficient is related to phase volume fractions, velocities, and accelerations within each coarse grid. 2D and 3D modeling results show that TFM combined with the improved drag model can accurately reproduce the heterogeneous structures in a laboratory-scale Geldart A powder bubbling fluidized bed. At relatively coarse grid, the predicted axial and radial distributions of solid concentration are in very good agreement with experimental measurements. This shows that our proposed new bubble-based drag model for TFM simulations is promising for simulation of large-scale Geldart A powder bubbling fluidization.

Triple Phase Simulation of Bubbling Fluidized Beds of Geldart “A” Particles with a Bubble-Based Drag Model

A triple-phase numerical model was developed for the simulations of gas-solid bubbling fluidized beds with Geldart “A” particles. In the model, the gas-solids suspension was modeled as a primary gas phase presented in the emulsion, a secondary gas phase presented in the bubble, and a third phase consisting of solids. A bubble-based drag model was derived taking into account of the heterogeneous flow structures of the emulsion phase. The KTGF model was used to describe the solid phase stresses. Simulations were conducted for two different scales bubbling fluidized beds of Group “A” particles. Mesh independences were confirmed up to a radial grid size of 200 times particle diameter, and the sensitivity of the transitional results on the time-steps was investigated. The predicted bed expansion and the voidage distribution, axial and radial solids volume fraction profiles, and the solids velocity vectors have been validated against literature data.

A Bubble-Based Drag Model at the Local-Grid Level for Eulerian Simulation of Bubbling Fluidized Beds

A bubble-based drag model at the local-grid level is proposed to simulate gas-solid flows in bubbling fluidized beds of Geldart A particles. In this model, five balance equations are derived from the mass and the momentum conservation. This set of equations along with necessary correlations for bubble diameter and voidage of emulsion phase is solved to obtain seven local structural parameters (uge,upe,εe,δb,ub,db, andab) which describe heterogeneous flows of bubbling fluidized beds. The modified drag coefficient obtained from the above-mentioned structural parameters is then incorporated into the two-fluid model to simulate the hydrodynamics of Geldart A particles in a lab-scale bubbling fluidized bed. The comparison between experimental and simulation results for the axial and radial solids concentration profiles is promising.

CFD Simulation of a Bubbling Fluidized Bed Gasifier Using a Bubble-Based Drag Model

Wood gasification in a fluidized bed reactor is regards as a promising technique to efficiently exploit the energy from biomass, which has attracted great attention. To further understand complex fluid mechanics and thermal behaviors of wood during the gasification process, a multiphase reactive model is presented, and a computational fluid dynamics (CFD) simulation of wood gasification in a bubbling fluidized bed gasifier is implemented. To account for the impact of bubble-structure on the gas–solid interaction, a revised bubble-based energy minimization multiscale (EMMS) drag model is developed and incorporated into a two-fluid model. Flow behaviors and reactive characteristics in a bubbling fluidized bed gasifier are analyzed. Profiles of temperature and concentration of gas and solid phase are also obtained. By comparisons of the outlet gas species concentrations, the prediction by a revised bubble-based EMMS drag model agrees better with experimental results than that by the conventional drag model.

Frictional drag reduction by wavy advection of deformable bubbles

Bubbles can reduce frictional drag in wall turbulence, and its effect is expected to use for ships and pipelines to save their power consumptions. A number of basic experiments have been carried out to date for finding out the best condition for enhancing the drag reduction. One issue that remains at present is the difference of the performance between steady and unsteady status in terms of bubble concentration. All the experiments in the past deal with the steady effect, i.e., the drag reduction is evaluated as a function of mean void fraction or given gas flow rate of continuous injection. Despite to this, the actual phenomena highly depend on local interaction between two phases upon unsteady manner. We focus on this point and elucidate the influence of time-fluctuating void fraction on the total response to the drag reduction. This view is in fact important to estimate the persistency of the bubble-based drag reduction in the flow direction since bubbles formulate wavy advection during their migration.Our experiments are designed to measure the above-mentioned effect from laminar, transitional, and turbulent flows in a horizontal channel. For avoiding the contamination effect that worsens the reproducibility of the experiment, Silicone oil is used as carrier fluid. The oil also simulates the high Weber number bubble condition because of low surface tension. The unsteady interaction between the wavy advection of bubbles and the local skin friction, a synchronized system is constructed to connect the high-speed camera with the shear transducer, which can evaluate the interaction at 1000 fps. From the results, we confirm that the drag reduction is provided at Re>3000 in the turbulent flow regime, and also the total drag reduction is enhanced by the presence of the waves.