Effects Of Mesoscale Structures Research Articles

Effect of mesoscale structures on solid phase stress in gas–solid flows

Solid phase stress plays an important role in hydrodynamic modeling and simulations of gas–solid flows. However, heterogeneous gas–solid flows are typically regarded as homogeneous systems. Their solid phase stresses are usually calculated by the classical kinetic theory of granular flow (KTGF) without considering mesoscale structures, which is a major source of inaccuracies. This work investigates the effect of mesoscale structures in gas–solid flows via a dilute-dense two-phase partition, and proposes a phase-specific model to predict solid phase stress. Subsequently, the sensitive of threshold for phase partition is discussed, and a specific threshold is utilized in the proposed model to investigate the effect of mesoscale structures on solid phase stress with large-scale particle-resolved direct numerical simulation (PR-DNS) data. Numerical results show that classical KTGF underestimates the solid phase stress due to the ignorance of these structures.

Industrial practice and CFD investigation of a multi-chamber fluidized bed reactor for MnO2 ore reduction

The multi-chamber fluidized bed reactor (MCFBR) is beneficial to the higher solids conversion rate, but the complex configuration needs special methodology for the rational design and smooth operation. In this study, an actual plant data of MCFBR for the reduction of MnO2 ore were analyzed, which indicated that the recovery rate of Mn was confirmed to be kept above 95% normally. To investigate the multi-chamber fluidized bed hydrodynamics and reaction characteristics, the 3D computational fluid dynamics (CFD) numeration coupled with structure-based transfer model and multi-chamber calculation method was established. The effect of mesoscale structure on the prediction of multi-chamber reaction process was evaluated as well. The simulation results showed that the model was able to calculate the gas–solid reaction behavior more accurately than the traditional model without taking mesoscale structure into account. The computed tanks-in-series number of 2.0 is less than the actual chamber number of 4.0 for the wide gap between the vertical baffle and the side wall. The quantitative effects of partition configuration, fluidized structure, and operating condition on the solids conversion rate were estimated as well.

Numerical investigation for the suitable choice of bubble diameter correlation for EMMS/bubbling drag model

Mesoscale bubbles exist inherently in bubbling fluidized beds and hence should be considered in the constitutive modeling of the drag force. The energy minimization multiscale bubbling (EMMS/bubbling) drag model takes the effects of mesoscale structures ( i.e ., bubbles) into the modeling of drag coefficient and thus improves the coarse-grid simulation of bubbling and turbulent fluidized beds. However, its dependence on the bubble diameter correlation has not been thoroughly investigated. The hydrodynamic disparity between homogeneous and heterogeneous fluidization is accounted for by the heterogeneity index, H d , which can be affected by choice of bubble diameter correlation. How this choice of bubble diameter correlation influences the model prediction calls for further fundamental research. This article incorporated seven different bubble diameter correlations into EMMS/bubbling drag model and studied their effects on H d . The performance of these correlations has been compared with the correlation used previously by EMMS/bubbling drag model. We found that some of the correlations predicted lower H d by order of a magnitude than the correlation used by the original EMMS/bubbling drag. Based on such analysis, we proposed a modification in the EMMS drag model for bubbling and turbulent fluidized beds. A computational fluid dynamics (CFD) simulation using two-fluid model with the modified EMMS/bubbling drag model was performed for two bubbling and one turbulent fluidized beds. Voidage distribution, time averaged solid concentration and axial solid concentration profiles were studied and compared with the previous version of the EMMS/bubbling drag model and experimental data. We found that the right choice of bubble diameter correlations can significantly improve the results for CFD simulations.

Effect of meso-scale structures and hyper-viscoelastic mechanics on the nonlinear tensile stability and hysteresis of woven materials

Woven textiles are not only a craft and industrial product but also a thousand-year-old crystallization of human technology. However, the highly sought after mechanical behavior of fabric, generally undergoing large structural distortion along with material deformation even under small stress, is still not clearly understood despite a growing interest in emerging applications, such as flexible electron devices, biomedicine and other engineering fields. Herein, a numerical methodology was introduced to strengthen the comprehensive understanding of the synergy effect of material mechanics and mesostructures of woven materials. A hyper-viscoelastic constitutive model for yarn materials was proposed, and a meso-scale geometry model captured from a resin-cured woven fabric was used, down to micron-sized weaving structures, to investigate the uniaxial loading and unloading process based on finite element (FE) method. The tensile and hysteresis mechanics was identified based on the validated FE model and parameter study of friction effect and fabric structures. The nonlinear tensile and recovery behaviors were reasonably represented by the developed models and the synergistic effect of inner yarn friction and viscoelasticity on the hysteresis was proved. This study can provide an effective method to analyze and predict the nonlinear tensile and hysteresis behavior of woven fabric, laying down the way to textile-based strain sensing materials by enhancing our design and tuning capabilities of the dimension stability of woven materials under tension.

A unified EMMS-based constitutive law for heterogeneous gas-solid flow in CFB risers

Energy minimization multi-scale (EMMS) drag model has been demonstrated to be very successful in quantifying the effect of mesoscale structure on the effective interphase drag force of heterogeneous gas-solid flow in circulating fluidized bed (CFB) risers, however, a corresponding model for the effective particle phase stress is not yet available. To this end, an EMMS drag model was extended to predict the granular temperature of particles inside the dilute phase and the dense phase as well as the granular temperature of clusters. A model for the effective particle phase stress was then developed based on the concept of multiscale analysis in EMMS model. It was shown that the experimentally measured granular temperature, granular pressure and granular viscosity as a function of mean solid concentration can be predicted reasonably well. The agreement demonstrates its effectiveness in the quantification of the effect of mesoscale structure on the particle phase stress, thus establishing a unified EMMS-based model for the constitutive law of heterogeneous gas-solid flow in CFB risers.

Numerical Investigation of Nickel–Copper Oxygen Carriers in Chemical-Looping Combustion Process with Zero Emission of CO and H2

The chemical-looping combustion (CLC) technique with an intrinsic feature for carbon capture is playing a critical role in the industrial process planning. As the key technology in CLC, the behaviors of oxygen carriers with different weight ratios of NiO/CuO are investigated using the two-fluid model in this work. The kinetic theory of granular flow is employed for the description of particle motion, while the EMMS model is used to clarify the effect of mesoscale structure on gas–solid interaction in the fuel reactor. With the coexistence of Ni and Cu in the oxygen carriers, a zero emission of H2 and CO is observed at the outlet of the fuel reactor, while the intermediate products of H2 and CO arise in the system with Ni-based oxygen carriers. The complete conversion of CH4 is observed in the fuel reactor with various components of Ni–Cu oxygen carriers. The reactivity of CuO and NiO with gas fuel is analyzed, and the equilibrium state in the CLC system with different weight ratios of NiO/CuO is computed ...

An EMMS drag model for coarse grid simulation of polydisperse gas–solid flow in circulating fluidized bed risers

Polydisperse gas–solid systems are more common in both industry and the natural world than their monodisperse counterparts. This paper aims to investigate the hydrodynamics of polydisperse gas–solid flow systems by extending the energy minimization multiscale (EMMS) drag model. The continuous particle size is discretized into several characteristic sizes, by which the polydisperse particles are classified into several discrete groups. A polydisperse EMMS drag model is then developed by treating these particle groups as solid phases. Finally, the proposed drag model is coupled with the multifluid model to simulate the hydrodynamics of polydisperse gas–solid flow in CFB risers. Extensive simulations are conducted to validate the effectiveness of the polydisperse EMMS drag model and investigate the effects of gas–particle drag models, kinetic theories, and particle–particle drag models on the simulation results. It was found that (i) the polydisperse EMMS drag model can predict the concentration and particle size distribution of polydisperse gas–solid flows better than the traditional drag model; (ii) the particle–particle drag force has an important influence on the mixing and segregation of different particles whereas the particulate phase stress has only a very minor effect, especially when the superficial gas velocity is low. The second finding highlights the need for a suitable particle–particle drag model that considers the effects of mesoscale structures, which will be the topic of our future study.

CFD simulations of a full-loop CFB reactor using coarse-grained Eulerian–Lagrangian dense discrete phase model: Effects of modeling parameters

The hydrodynamics of a 3D full-loop circulating fluidized bed (CFB) reactor were investigated using a coarse-grained dense discrete phase model (DDPM). The effects of changes in key modeling parameters such as the drag force, particle number per parcel, and particle–particle/particle–wall restitution coefficients were thoroughly investigated. Previous experimental data were used as a benchmark for validating the numerical results. In terms of the drag force, the simulation results show that the effect of mesoscale structures on drag force must be considered in the DDPM approach. Regarding the number of particles per parcel, the predictions indicate parcel-independent behavior at all of the coarse-graining ratios tested (dcl/dp = 55, 95, and 125), which is further proof that parcel-independent results can be achieved when the parcel diameter is in the size range of clusters. The effect of energy dissipation from particle–particle (epp) collisions shows that better results are generated when epp is in the non-ideal range (0.1–0.9) than for the ideal case (epp = 1.0). However, the particle–wall restitution coefficient (epw) has little effect on hydrodynamic behavior in either the ideal or non-ideal collision range. In addition, the effect of the particle size distribution on the full-loop hydrodynamics was investigated. The predicted results are similar to those from mean particle diameter simulations, but the distributions for different particles in the CFB are different. This would certainly affect the heat and mass transfer and reactions, and will be further investigated in our ongoing simulations of combustion in CFBs.

An effective three-marker drag model via sub-grid modeling for turbulent fluidization

The effect of meso-scale structures on hydrodynamic predictions is not considered in the classically uniform drag models when the coarse grid is used. To address this issue, this study tries to develop an effective three-marker drag correlation via straightforward sub-grid modeling, which accounts for a parabolic spatial concentration distribution within a computational grid. The reliability and accuracy of the developed model is then assessed in detail. How the uniform drag inputs affect the derived sub-grid correction is quantified for the first time. Besides, a comprehensive comparison between several typical sub-grid models and present work is implemented. Results reveal a systematic dependence of our drag modification on the concentration gradient as an additional marker. Coarse-grid hydrodynamic validation shows that the developed model yields a fairly improved agreement with experiments under various operating conditions in a 3D turbulent fluidized bed. Furthermore, results demonstrate that the present model using different uniform drag inputs still can exhibit satisfactory performance. The developed model is able to resolve the heterogeneous flow behavior both cheaply and adequately, which is potentially applied for industrial reactor design and optimization.

Behavior of meso-scale heterogeneous concrete under uniaxial tensile and compressive loadings

Meso-scale modeling approach is helpful for understanding the mechanism of both global nonlinear behavior and local failure pattern of concrete materials and structures. As a typical multiphase composite material, concrete is composed of randomly-distributed coarse and fine aggregates, mortar, interfacial transition zone (ITZ) together with initial pores or defects. The meso-scale mechanical behavior of concrete usually exhibits nonlinear and stochastic characteristics. In this paper, the influence of both distribution and geometrical shape of aggregates, the existence of ITZ, and meshing approach on macro stress-strain relationship, damage evolution and the final failure pattern of concrete are studied systematically. The results indicate that the effect of meso-scale structure of concrete on the nonlinear and stochastic characteristics of macro stress-strain curves and failure patterns is different when the concrete specimen is under tension and compression loadings. Comparatively, the geometrical parameters of coarse and fine aggregates have limited effect on the macroscopic tensile strength, but obvious effect on the post-tension segment and the compressive macro stress-strain curve. Moreover, the damage evolution and failure pattern of concrete under uniaxial tensile and compression are sensitive to the meshing approach. The existence of ITZ affects the tensile and compressive strength of concrete significantly. The statistic characteristics of macro stress-strain curves under both tension and compression loadings from the multi-scale simulation is compared with that from experimental studies and the rationality of the meso-scale simulation approach is validated.

Development and validation of an enhanced filtered drag model for simulating gas-solid fluidization of Geldart A particles in all flow regimes

Coarse-grid two-fluid simulation of gas-solid fluidized bed reactors based on the kinetic theory of granular flow exhibits a significant dependence on drag models, especially for Geldart A particles. Many drag models are available in the literature, which have been reported to work for different systems. This study focused on the evaluation of an enhanced filtered drag model along with other different drag models derived from different methods for three-dimensional two-fluid model simulations of gas-solid fluidized beds of Geldart A particles covering a broad range of fluidization regimes, including bubbling fluidization, turbulent fluidization, fast fluidization, and dilute phase transport regimes. Eight drag models were selected, which included five heterogeneous drag models and three homogeneous drag models. Comparison with the available experimental data demonstrates the need for modification of homogeneous drag models to account for the effect of mesoscale structures (i.e., bubbles and clusters). The enhanced filtered drag model and energy-minimization multi-scale (EMMS) drag models were found to achieve superior predictions in all fluidization regimes, while the other drag models were only capable of predicting certain fluidization regimes. The results of this work provide a guideline for choosing appropriate drag models for simulating Geldart A particles and suggestions on developing more reliable and general drag models applicable in all flow regimes.

Effects of mesoscale structures on the distribution of cephalopod paralarvae in the Gulf of California and adjacent Pacific

Vertical distribution of the cephalopod paralarvae was investigated in relation to a system of two cyclonic and three anticyclonic eddies in the southern Gulf of California and a front in the adjacent Pacific Ocean. Results showed that the preferential habitat for the Sthenoteuthis oualaniensis – Dosidicus gigas “SD-complex” in both regions was the oxygenated surface mixed layer and the thermocline. The highest abundances occurred in of one of the anticyclonic eddies and a frontal zone, which are convergent structures. Enoploteuthid and Pyroteuthid paralarvae both displayed their highest abundances in the thermocline. Pyroteuthids dominated in the cyclonic eddy whereas Enoploteuthidae were less evident in the eddy system. Pyroteuthids were observed on the western (California Current) side of the frontal zone, and Enoploteuthids on its eastern (Gulf of California) side. The octopods and the complex of Ommastrephes-Eucleoteuthis-Hyaloteuthis paralarvae were present below the thermocline. Both groups had a scarce presence in the eddy system and high abundance near the frontal zone. The octopods abounded on the eastern side in association with the low dissolved oxygen concentrations (< 44µmolkg−1) of Subtropical-Subsurface Water; the complex on the western front side was immersed in California Current Water. It may be concluded that the spawning and early stages of development of these cephalopod groups are associated with particular mesoscale structures of the water masses. For example, the “SD complex” inhabits the surface water masses, preferentially in convergence zones generated by mesoscale activity.

A grid-independent EMMS/bubbling drag model for bubbling and turbulent fluidization

The 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.

A novel approach to investigate the effect of meso-scale yarn structure on the in-plane mechanical response of woven monofilament textiles by numerical modeling of experiments

ABSTRACTThe mechanical response of textiles depends upon various parameters including materials, yarn structure, and the architecture of the textile. These factors play an important role in the nonlinear mechanical response. In this work, a novel numerical modeling approach is proposed to better capture the effect of the meso-scale structure on the textile's mechanical response. Monofilament textiles exhibiting meso-scale irregularities due to the manufacturing are studied. A novel numerical method is developed to study these effects on the in-plane mechanical response. The in-plane experiments validated the proposed modeling approach. The results show the importance of the meso-scale structure induced by manufacturing.

Discrete simulation of granular and particle-fluid flows: from fundamental study to engineering application

AbstractMultiphase chemical reactors with characteristic multiscale structures are intrinsically discrete at the elemental scale. However, due to the lack of multiscale models and the limitation of computational capability, such reactors are traditionally treated as continua through straightforward averaging in engineering simulations or as completely discrete systems in theoretical studies. The continuum approach is advantageous in terms of the scale and speed of computation but does not always give good predictions, which is, in many cases, the strength of the discrete approach. On the other hand, however, the discrete approach is too computationally expensive for engineering applications. Developments in computing science and technologies and encouraging progress in multiscale modeling have enabled discrete simulations to be extended to engineering systems and represent a promising approach to virtual process engineering (VPE, or virtual reality in process engineering). In this review, we analyze this emerging trend and emphasize that multiscale discrete simulations (MSDS), that is, considering multiscale structures in discrete simulation through rational coarse-graining and coupling between discrete and continuum methods with the effect of mesoscale structures accounted in both cases, is an effective way forward, which can be complementary to the continuum approach that is being improved by multiscale modeling also. For this purpose, our review is not meant to be a complete summary to the literature on discrete simulation, but rather a demonstration of its feasibility for engineering applications. We therefore discuss the enabling methods and technologies for MSDS, taking granular and particle-fluid flows as typical examples in chemical engineering. We cover the spectrum of modeling, numerical methods, algorithms, software implementation and even hardware-software codesign. The structural consistency among these aspects is shown to be the pivot for the success of MSDS. We conclude that with these developments, MSDS could soon become, among others, a mainstream simulation approach in chemical engineering which enables VPE.

A simplified two-fluid model coupled with EMMS drag for gas-solid flows

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.

Unification of EMMS and TFM: structure-dependent analysis of mass, momentum and energy conservation

The two-fluid model (TFM) has been widely applied in simulation of various multiphase flow systems. In particular, for fine-particle circulating fluidization, the drag force plays a critical role whereas the classic drag models based on empirical correlations of homogeneous fluidization are found inadequate. Therefore, various approaches have been proposed in recent years to account for the effects of meso-scale structure on the drag force, in which the energy-minimization multi-scale model (EMMS) has received rapidly growing applications.However, the relationship between the TFM and EMMS has not been clarified to enable their combination. To solve this problem, we present a structure-dependent analysis of mass, momentum and energy conservation equations. This analysis is rooted in the structure-dependent multi-fluid model (SFM), which details the composition of drag forces and energy consumptions and their relationships with consideration of meso-structures. With assumption of homogeneous structures, it reduces to the TFM equations; for steady-state systems with structures, it restores the force balance equations, equal pressure drop relation and energy relationship of the EMMS. In future, the scale dependence of this analysis deserves more efforts to understand the applicability of the EMMS stability condition on different scales.

Experimental and theoretical investigation of mass transfer in a circulating fluidized bed

Catalytic oxidation of carbon monoxide over a Pt catalyst was employed as a model reaction to investigate the effect of meso-scale structure on gas-solid mass transfer in a circulating fluidized bed (CFB). Both experimental and theoretical analyses were performed to determine the conditions under which the reaction process was dominated by mass transfer. The experimental works involved the measurements of the axial distribution of carbon monoxide concentration, whereas the theoretical analyses were on the use of an Multi-Scale Mass Transfer (MSMT) model with considering effects of clusters on gas-solid transport phenomena in the CFB. The results of MSMT model show a good agreement with experimental data, suggesting that the particle clusters play an important role in the gas-solid momentum transfer and mass transfer in CFBs.

“Generalized Fluidization” Revisited

The generalized fluidization diagram was drawn by Kwauk [Scientia Sinica, 1963, 12 (4), 587–612] and Kwauk [Science Press, Beijing: 1992] for fluidization under steady-state motion. In this article, we try to extend the work of Kwauk and redraw the diagram by taking into account the effects of mesoscale structures on drag force closure. The generalized choking is then defined on the new diagram, which is characterized by the bistable, coexisting states in both concurrent and countercurrent flows. The theoretical predictions are tested against experimental data, showing fair agreement especially for countercurrent gas-up flows and concurrent-down flows.

Evaluation of drag models for cocurrent and countercurrent gas–solid flows

Gas–solid systems, whether cocurrent or countercurrent, manifest meso-scale heterogeneity to some extent. However, most of the previous modelling efforts, including the study of the effects of meso-scale structures as well as relevant drag closures, are mainly focused on cocurrent up riser flows, leaving cocurrent down and countercurrent systems less explored. In this work, by extending drift flux analysis and experimental validation for cocurrent down and countercurrent flow systems, we show that the most commonly used homogeneous drag models may be successful in a certain range of operating conditions, especially with fine-grid resolution. Fine-grid resolution is indeed helpful for reliable simulation of coexisting states, by predicting concave slip velocity curves. By comparison, the energy minimization multiscale (EMMS) drag model allows the use of much coarser grid resolution and better prediction over wider range of operating conditions. Its predictability of the overall slip velocity as well as the drift flux is nearly grid independent over the entire range of voidage. This fact leads us to the obvious advantage of saving computational time.