162,320 research outputs found
Generalized Boltzmann kinetic theory for EMMS-based two-fluid model
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It has long been recognized that the solid particles in circulating fluidized bed risers are distributed heterogeneously in the form of clusters. In response to this fundamental phenomenon, an EMMS-based two-fluid model has been developed recently from the viewpoint of continuum mechanics, however, its microscopic foundation remains unknown. In this study, the statistical mechanics foundation of EMMS-based two-fluid model was presented using generalized Boltzmann kinetic theory. With respect to the gas phase, a new method was developed by considering the fluctuations at different scales simultaneously, with which we can for the first time derive the correct governing equations of gas phase via kinetic theory, in the sense that both the molecular stress and the Reynolds (or pseudo-Reynolds) stress can be obtained simultaneously, whereas all previous kinetic theory analyses failed to predict the appearance of Reynolds (or pseudo-Reynolds) stress in the momentum conservation equation of gas phase due to the assumption of uniform structure, although it is physically always existent no matter how small the Reynolds number is. In case of particle phase, the generalized Boltzmann equation considering the spatio-temporal variation of the volume, density and velocity of clusters was firstly derived, a set of macroscopic transport equations was then derived in different phase spaces. It was shown that the governing equations of dense phase in the EMMS-based two-fluid model derived from continuum mechanics viewpoint corresponds to the macroscopic transport equations at (r, t) space. Therefore, present study launches a solid microscopic foundation of EMMS-based two-fluid model. Finally, CFD simulations have been carried out to validate EMMS-based two-fluid model and to study the effect of gas phase pseudo-turbulence. (C) 2016 Elsevier Ltd. All rights reserved.</p
A simplified two-fluid model coupled with EMMS drag for gas-solid flows
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A simplified two-fluid model (STFM) combined with energy-minimization multi-scale (EMMS) drag was proposed for accurate and fast simulation of gas-solid flows. In the proposed approach, the solid phase viscosity is neglected, the solid phase pressure is calculated with an empirical formulation, and the interphase momentum transfer is modeled with EMMS drag, which takes the effects of meso-scale structures into consideration. Three typical fluidization cases, namely, a 2D circulating fluidized bed, a 3D lab-scale bubbling fluidized bed, and a 3D lab-scale full-loop circulating fluidized bed, were successfully simulated with this approach. The numerical results are compared with those of full two-fluid model (FTFM, i.e., the two-fluid model using the kinetic theory for granular flow to close solid phase stress term), as well as experimental data. Predictions of STFM coupled with EMMS drag are comparable with those of FTFM coupled with EMMS drag, and both agree well with experimental data. However, computational cost of STFM is significantly reduced compared with that of FTFM. It is suggested that drag model has a dominant effect on gas-solid simulation, and the effect of solid phase stress term seems to play a minor role, demonstrating the feasibility and practicality of STFM with EMMS drag for describing the hydrodynamics of heterogeneous gas-solid flows. (C) 2016 Elsevier B.V. All rights reserved.</p
A grid-independent EMMS/bubbling drag model for bubbling and turbulent fluidization
No full textThe EMMS/bubbling drag model takes the effects of meso-scale structures (i.e. bubbles) into modeling of drag coefficient and thus improves coarse-grid simulation of bubbling and turbulent fluidized beds. However, its dependence on grid size has not been fully investigated. In this article, we adopt a two-step scheme to extend the EMMS/bubbling model to the sub-grid level. Thus the heterogeneity index, HD, which accounts for the hydrodynamic disparity between homogeneous and heterogeneous fluidization, can be correlated as a function of both local voidage and slip velocity. Simulations over a periodic domain show the new drag model is less sensitive to grid size because of the additional dependence on local slip velocity. When applying the new drag model to simulations of realistic bubbling and turbulent fluidized beds, we find grid-independent results are easier to obtain for high-velocity turbulent fluidized bed cases. The simulation results indicate that the extended EMMS/bubbling drag model is a potential method for coarse-grid simulations of large-scale fluidized beds
The importance of vegetative seedling traits in distinguishing herbaceous legumes of differing impact in temperate natural ecosystems
No full textJason Emms, John G. Virtue, Christopher Preston and William D. Bellottihttp://www.weedinfo.com.au/bk_15awc.htm
Multi-scale simulation of gas solid fluidization based on EMMS- DPM
No full textThis presentation will discuss some efforts to improve the speed and accuracy of discrete particle method from physical models to computational methods.
For physical model, the multiscale method is used. At global scale, the particles are distributed according to global distribution predicted by the Energy Minimization Multi-Scale (EMMS) model, so that the computation domain can be decomposed non-uniformly for load balance. At grid scale, to improve accuracy, the structure dependent drag coefficient based on the EMMS is used. At particle scale, the coarse grained method is used. The size and solids concentration of the coarse-grained particles (CGP) are determined by the cluster properties which can be predicted by the EMMS model. The coefficient of restitution is modified according to the kinetic theory of granular flows (KTGF). The method thus established in called EMMS-DPM(Lu, Xu et al. 2014).
As for computation, using system shared memory, the CFD computation on CPU is fully overlapped with particle computation on GPU. Also, the computation program is coupled with parallel visualization and control program, forming an online interactive simulation platform(Ge, Lu et al. 2015).
This method is verified by the simulation of two different CFB risers and several orders of speedup can be achieved. A methanol to orifin (MTO) process is simulated for more than 6800s. We also simulated a CFB with 30kg 0.082mm particles in 3D full loop. Furthermore, the interactive simulation platform can also be used for education and training purpose since it allows virtual experiment on computers.
REFERENCES
1.Ge, W., L. Lu, S. Liu, J. Xu, F. Chen and J. Li (2015). Multiscale Discrete Supercomputing - A Game Changer for Process Simulation? Chemical Engineering & Technology 38(4): 575-584.
2.Lu, L., J. Xu, W. Ge, Y. Yue, X. Liu and J. Li (2014). EMMS-based discrete particle method (EMMS–DPM) for simulation of gas–solid flows. Chemical Engineering Science 120(0): 67-87
[Report to Chief J. E. Curry, by an unknown author #1]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
[Report to Chief J. E. Curry, by an unknown author #2]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
EMMS-based Eulerian simulation on the hydrodynamics of a bubbling fluidized bed with FCC particles
No full textAlthough great progress has been made in modeling the bubbling fluidization of Geldart B and D particles using standard Eulerian approach, recent studies have shown that suitable sub-grid scale models should be introduced to improve the simulation on the hydrodynamics of Geldart A particles. In this study, the flow structures inside a bubbling fluidized bed of KC particles are simulated in an Eulerian approach employing the energy minimization multi-scale (EMMS) model (Chemical Engineering Science, 2008. 63: 1553-1571) as the sub-grid scale model for effective inter-phase drag force, using an implicit cluster diameter expression. It was shown that the experimentally found axial and radial solid concentration profiles and radial particle velocity profiles can be well reproduced. (C) 2009 Elsevier B.V. All rights reserved
CFD study of exit effect of high-density CFB risers with EMMS-based two-fluid model
No full textIt has been widely recognized that the exit geometry of circulating fluidized bed (CFB) risers may have significant effects on the hydrodynamics of gas-solid flow. However, a systematical study of exit effect is still lack after extensive experimental studies, possibly due to the fact that systematical modification of exit geometry in experimental study is an expensive and time-consuming task. In this study, after further validation of the recently developed EMMS-based two-fluid model (Wang et al., 2012. Chem. Eng. Sci. 75, 349-358; Zhou et al., 2014. Chem. Eng. Sci. 107, 206-217), the model is used to systematically investigate the effect of exit geometry of high-density CFB risers, fully taking the advantage that bed geometry can be easily modified in computational fluid dynamics study. It is shown that (i) the type of exit has a significant effect on hydrodynamics, the use of abrupt exit results in an increased solids concentration, not only in the immediate vicinity of the exit but also for a considerable distance down to the riser; (ii) the cavity height of abrupt exit, the curvature diameter of smooth exit and the length of the horizontal tube connecting the exit and the primary cyclone only have a minor effect or have no influence on the bed hydrodynamics of the studied risers; (iii) more importantly, the decrease of the diameter of abrupt exit tube results in a remarkable increase of solids holdup, the symmetry of radial solids concentration near the exit can also be enhanced significantly. However, in case of smooth exit, the diameter of the exit tube has no effect on the bed hydrodynamics at all. All of those results are in agreement with conclusions obtained from previous experimental studies, thus offering further validation of the EMMS-based two-fluid model for modelling heterogeneous gas-solid flow. (C) 2015 Elsevier Ltd. All rights reserved
Simulation of gas/solid flow behaviors and choking for a CFB riser: The EMMS/CFD approach
No full textThis paper presents a drag model based on the Energy-Minimization Multi-Scale (EMMS) approach. Compared to the empirical drag correlations, the incorporation of this EMMS-based drag model with the two-fluid model can well capture the meso-scale heterogeneous structure and improve the simulation results of the gas/solid flow behaviors for a CFB riser with Geldart-A particles. The EMMS model itself is also adapted to predict the choking phenomena for a CFB riser, and the comparisons are carried out among this calculation, the experimental measurements, the empirical correlation presented by Xu et al.(1) and the simulation results from the combination of the EMMS and CFD approaches
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