Energy-minimization Multi-scale Drag Research Articles

A CFD-PBM-EMMS integrated model applicable for heterogeneous gas-solid flow

Accurate prediction of the structure-dependent interphase drag coefficient in a computational cell is of importance for the simulation of heterogeneous gas-solid flow. However, the cluster diameter in the cell was usually correlated with macroscopic hydrodynamic parameters or simply approximated as a constant value, though there is a cluster size distribution (CSD) at the sub-grid scale. Based on the energy minimization multiscale (EMMS) model and the population balance model (PBM), a CFD-PBM-EMMS integrated model was proposed to account for the effect of CSD on the interphase drag within the computational grid in this article, in which the PBM was used to describe the spatio-temporal evolution of CSD by introducing an EMMS-based cluster growth rate model. The CFD-PBM-EMMS model was validated by the experiments in a pilot-scale circulating fluidized bed riser of Geldart A and/or B particles. Comparing with the two-fluid model using the EMMS drag or homogeneous drag law, the CFD-PBM-EMMS integrated model shows the best agreement with the experiments under various operating conditions. Much effort is being devoted to incorporating the coalescence and breakage kinetics of clusters to the PBM further to improve the simulation accuracy of the CFD-PBM-EMMS integrated model, especially for dense gas-solid flow.

Impacts of solid stress model on MP-PIC simulation of a CFB riser with EMMS drag

The cluster-based EMMS (energy minimization multi-scale) drag model in combination with MP-PIC (multi-phase particle in cell) method can successfully simulate the fluidization behavior in CFBs (circulating fluidized beds). The solid normal stress model is very important to MP-PIC simulations, but still lacks thoroughly investigation, especially in CFB riser simulation. In this work, the effects of parameters from the solid normal stress model have been investigated by using 2D simulations of a laboratory-scale CFB riser with MFIX (Multiphase Flow with Interphase eXchanges) open-source code. The results show that the role of the solid stress parameters is more significant when the solid approaches close packing state. Based on the comparison between simulation results and experimental data and the elapsed simulation time, optimal values for the solid stress model parameters were obtained. The thickness of the third direction for 2D simulations has also been investigated. Increase of the thickness entails additional computational costs with little effect on the accuracy and the recommended value of thickness is the diameter of one single particle.

Structure-dependent multi-fluid model for mass transfer and reactions in gas–solid fluidized beds

A fluidized bed reactor embraces complex coupling among flow, heat/mass transfer and reaction kinetics over a wide range of spatial-temporal scales, on which the meso-scale structure in forms of bubble or cluster plays the critical role. The traditional two-fluid model (TFM) is based on local equilibrium assumption and neglects the effects of sub-grid meso-scale structures, thus, it is not suitable for simulating dense, heterogeneous gas–solid riser flow. To be consistent with the meso-scale characteristics, we proposed a structure-dependent multi-fluid model (SFM). It reduces to the conventional two-fluid model (TFM) if local equilibrium or homogeneity is assumed within each grid, and reverts to the energy-minimization multi-scale (EMMS) model if it is used to describe global behavior of a fluidized bed. The multiscale CFD based on SFM and EMMS drag has been successfully applied in simulating gas–solid fluidized bed flows in recent years. In this work, we will extend such approach to the realm of reactive flows with mass transfer and reactions. The heterogeneity indexes for reaction and mass transfer are thereby defined, which may facilitate TFM-based reactive simulations with structure-dependent corrections. Two experiments of ozone decomposition in circulating fluidized beds are simulated to validate this approach with comparison to data.

The simulation and experimental validation on gas-solid two phase flow in the riser of a dense fluidized bed

Gas-solid flow in dense CFB (circulating fluidized bed)) riser under the operating condition, superficial gas 15.5 m/s and solid flux 140 kg/m2s using Geldart B particles (sand) was investigated by experiments and CFD (computational fluid dynamics) simulation. The overall and local flow characteristics are determined using the axial pressure profiles and solid concentration profiles. The cold experimental results indicate that the axial solid concentration distribution contains a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. The typical core-annulus structure and the back-mixing phenomenon near the wall of the riser can be observed. In addition, owing to the key role of the drag force of gas-solid phase, a revised drag force coefficient, based on the EMMS (energy-minimization multi-scale) model which can depict the heterogeneous character of gas-solid two phase flow, was proposed and coupled into the CFD control equations. In order to find an appropriate drag force model for the simulation of dense CFB riser, not only the revised drag force model but some other kinds of drag force model were used in the CFD. The flow structure, solid concentration, clusters phenomenon, fluctuation of two phases and axial pressure drop were analyzed. By comparing the experiment with the simulation, the results predicted by the EMMS drag model showed a better agreement with the experimental axial average pressure drop and apparent solid volume fraction, which proves that the revised drag force based on the EMMS model is an appropriate model for the dense CFB simulation.