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Junwu Wang - One of the best experts on this subject based on the ideXlab platform.

  • a unified emms based constitutive law for heterogeneous gas solid flow in cfb risers
    Chemical Engineering Science, 2020
    Co-Authors: Bidan Zhao, Junwu Wang
    Abstract:

    Abstract 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.

  • an emms Drag Model for coarse grid simulation of polydisperse gas solid flow in circulating fluidized bed risers
    Chemical Engineering Science, 2019
    Co-Authors: Quan Zhou, Junwu Wang
    Abstract:

    Abstract 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.

  • eulerian eulerian simulation of irregular particles in dense gas solid fluidized beds
    Powder Technology, 2015
    Co-Authors: Hu Zhao, Junwu Wang
    Abstract:

    As an important and essential physical property, particle shape affects fluidization characteristics of dense gas-solid fluidized beds greatly. However, numerical studies of irregular particles in gas-solid fluidization, especially using Eulerian-Eulerian Model, are relatively scarce, primarily due to the lack of an appropriate inter-phase Drag Model. To this end, a simple and effective Drag Model was proposed in this work to address the critical role of particle shape in determining the inter-phase Drag force. This Drag Model features the usage of Ganser correlation and Ergun correlation, respectively, in dilute and dense solid concentration conditions. Moreover, particle sphericity is introduced as a concise shape descriptor, and can be calibrated by experimentally measured minimum fluidization velocity and the corresponding voidage. To verify the established numerical Model, experimental study was also conducted in a lab-scale three-dimensional rectangular bed filled with irregular Geldart group B particles. Other two kinds of irregular bed materials from open literature were investigated as well to provide further validation. The predicted results using the proposed Drag Model are overall in relatively better quantitative agreement with the experimental data or available empirical correlations, in terms of both macro-scale and micro-scale behavior. In contrast, the numerical Model with the Drag force assuming perfectly spherical particles completely fails to reproduce the experimental hydrodynamics of the gas-solid fluidized bed. In addition, sensitivity analysis of the proposed Drag Model demonstrated its weak sensitivity to the minimum fluidization characteristics of particles. The sufficient comparison and analysis indicated that the proposed Drag Model together with the estimation method of particle sphericity is quite reasonable and convenient for Eulerian-Eulerian simulation of irregular particles behavior in lab-scale dense gas-solid fluidized beds. (C) 2015 Elsevier B.V. All rights reserved.

  • cfd study of mixing and segregation in cfb risers extension of emms Drag Model to binary gas solid flow
    Chemical Engineering Science, 2015
    Co-Authors: Quan Zhou, Junwu Wang
    Abstract:

    The Energy Minimization Multi-Scale (EMMS) Drag Model, using Sauter mean particle diameter to represent real particle size distribution, has proven to be effective in improving the accuracy of continuum Modeling of gas-solid flow. Nevertheless, mixing and segregation characteristics in circulating fluidized bed (CFB) risers are very important in many situations, which necessitates the explicit consideration of the effects of particle size distribution on the bed hydrodynamics. To this end, an attempt is made to extend the EMMS Drag Model to binary gas-solid system, where four input parameters that can be obtained from computational fluid dynamics (CFD) simulation, including two slip velocities between gas and each particle phase and two particle concentrations of each phase, are used to solve the proposed EMMS Drag Model. Heterogeneous indexes, which are used to modify the Drag correlation obtained from homogeneous fluidization, are then predicted and fed into multifluid Model (MFM) to predict the dynamical behavior of mixing and segregation of binary gas-solid flow in a CEO riser. The effects of different Drag force Models, kinetic theories and particle-particle Drag force Models are also systematically evaluated. It was shown that (i) MFM with the proposed EMMS Drag Model and the kinetic theory developed by Chao et al. (Chemical Engineering Science 2011,66: 3605-3616) is able to correctly predict the mixing and segregation pattern in the studied riser, while MFM with homogenous Drag forces and the simplified kinetic theory available in commercial software FLUENT completely fails; and (ii) with or without particle-particle Drag force has a substantial influence upon the particle behavior. (C) 2014 Elsevier Ltd. All rights reserved.

  • cfd simulation of solids residence time distribution in a cfb riser
    Chemical Engineering Science, 2014
    Co-Authors: Junwu Wang, Jinghai Li
    Abstract:

    Solids residence time distribution (RTD) in circulating fluidized bed risers is a critical parameter for evaluating reactor performances, however, it is still very difficult to be predicted via computational fluid dynamics (CFD) simulation due to the complexity of particle clustering phenomenon. This paper tries to establish an effective CFD Model to reasonably predict solids RTD of gas solids riser flows by means of properly addressing the paramount role of particle clusters in determining solids RTD. The gas solids hydrodynamic characteristics were solved by Eulerian-Eulerian Model, where an energy minimization multi scale (EMMS) Drag Model was applied to modify the gas solids Drag force to account for the influence of particle clusters. The motion of tracer particles was calculated using species transport equation, where the diffusion coefficient of particles, a vital parameter indicating particle diffusion capacity, was investigated thoroughly. The established CFD Model was validated against the available experimental data in the literature It was shown that axial profiles of solids volume fraction and radial profiles of solids mass flux can be well predicted with EMMS Drag Model, but not with homogeneous Drag Model. The proper prediction of bed hydrodynamics is also very crucial to the success of solids RTD simulation. On the other hand, the effect of the diffusion coefficient of particles, the magnitude of which can span a range from 10(-5) m(2)/s to 10 m(2)/s, is minor when compared with the convective transport mechanism, at least for the specific cases we studied. In addition, the importance of the sampling time resolution and tracer injection time for a RTD curve was addressed. The simulation results showed that a low time resolution often results in the loss of some micro-scale information, i.e. drastically smoothing the fluctuations of the RTD curve, and an inappropriate assessment of the tracer injection time can lead to a significant change of the RTD curve. (C) 2014 Elsevier Ltd. All rights reserved.

Qingshan Zhu - One of the best experts on this subject based on the ideXlab platform.

  • cfd simulations of tapered bubbling turbulent fluidized beds with without gas distributor based on the structure based Drag Model
    Chemical Engineering Science, 2019
    Co-Authors: Zheng Zou, Wenming Liu, Dong Yan, Zhaohui Xie, Qingshan Zhu
    Abstract:

    Abstract The hydrodynamic characteristics of tapered bubbling/turbulent fluidized beds with/without gas distributor are simulated based on the structure-based Drag Model. Besides the only parameter of gas voidage (eg) contained in the original Drag Model, the heterogeneous Drag index (Hd) for the tapered fluidized bed (TFB) also includes the parameter of gas velocity (ug) for the axial velocity gradient along bed height. Both the simulations of tapered bubbling and turbulent fluidized beds achieve more accurate predictions than the traditional Drag Models. There exist dilute center and dense annular regions with solids cycle flow structure in the TFB. The simulation also gives reasonable prediction for TFB without gas distributor, the variation of gas voidage profile reveals gas converging toward bed center with axial location rising.

  • Simulation of hydrodynamics in gas-solid bubbling fluidized bed with louver baffles in three dimensions
    Powder Technology, 2016
    Co-Authors: Shuai Yang, Wenming Liu, Xiaolin Lv, Hongzhong Li, Hu Zhao, Li Da-peng, Qingshan Zhu
    Abstract:

    In this study, the flow characteristics of Geldart A particles in the bubbling fluidized bed with louver baffles were simulated by CFD method for the first time. A new Drag Model for bubbling fluidized bed with louver baffles was developed after our early work [Yang et al. Chem. Eng. J. 259 (2015) 338-347]. In this Drag Model, the baffled fluidized bed was treated as a series connection of freely bubbling fluidized beds with the louver baffle as the gas distributor. The hydrodynamics of each freely bubbling fluidized bed were simulated using the structure-based Drag Model with the modification of the bubble diameter equation to adapt the change of bubble size after passing each louver baffle. Through analyzing bubble diameters measured by double optical fiber probe, it was found that the averaged diameter of bubbles regenerated from baffles was nearly equal to the distance between two adjacent vanes in the louver baffle. By this method, the bubbling fluidization can be correctly predicted and the simulation results showed good agreements with the experimental data of the radial and axial solid volume fraction distributions. In the simulation results, it can be seen that the variation of axial solid volume fraction across the louver baffle was fluctuant, which was different from other types of baffles. Meanwhile, the lower particle concentration at the position of louver baffle indicates the particles back-mixing inhibited by louver baffles. The guiding effect of louver baffles on the gas and solid flow can be captured in the gas and solid velocity profiles as well.

  • a new structural parameters Model based on Drag coefficient for simulation of circulating fluidized beds
    Powder Technology, 2015
    Co-Authors: Wenming Liu, Qingshan Zhu, Quanhong Zhu
    Abstract:

    This work presented a new scheme to establish structural parameters Model, and the Model was used to solve structural parameters based on the available structure-based Drag Model. By combining with the Eulerian two-fluid Model, the hydrodynamics of circulating fluidized beds (CFBs) was simulated. Different combinations of clusters properties, including the cluster voidage and diameter, were adopted to fit for Geldart A and B particles and to close the insufficient solving equations, respectively. The simulated solid mass flux, radial and axial voidage profiles were in agreement with the experimental data. The dilute-top/dense-bottom and the core-annular flow structure were also captured. Moreover the spatiotemporal fluctuation of clusters can be observed from those simulations. Simulation results showed the combination of the structural parameters Model with the available structure-based Drag Model can predict well the hydrodynamics for Geldart A and B particles in CFBs. (C) 2015 Elsevier B.V. All rights reserved.

Wei Wang - One of the best experts on this subject based on the ideXlab platform.

  • a grid independent emms bubbling Drag Model for bubbling and turbulent fluidization
    Chemical Engineering Journal, 2017
    Co-Authors: Hao Luo, Jingyuan Zhang, Wei Wang
    Abstract:

    Abstract 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.

  • mp pic simulation of cfb riser with emms based Drag Model
    Chemical Engineering Science, 2012
    Co-Authors: Feifei Song, Sofiane Benyahia, Wei Wang
    Abstract:

    MP-PIC (multi-phase particle in cell) method combined with the EMMS (energy minimization multiscale) Drag force Model was implemented with the open source program MFIX to simulate the gas-solid flows in CFB (circulating fluidized bed) risers. Calculated solid flux by the EMMS Drag agrees well with the experimental value; while the traditional homogeneous Drag over-predicts this value. EMMS Drag force Model can also predict the macro- and meso-scale structures. Quantitative comparison of the results by the EMMS Drag force Model and the experimental measurements show high accuracy of the Model. The effects of the number of particles per parcel and wall conditions on the simulation results have also been investigated in the paper. This work proved that MP-PIC combined with the EMMS Drag Model can successfully simulate the fluidized flows in CFB risers and it serves as a candidate to realize real-time simulation of industrial processes in the future. (C) 2012 Elsevier Ltd. All rights reserved.

  • simulation of heterogeneous structure in a circulating fluidized bed riser by combining the two fluid Model with the emms approach
    Industrial & Engineering Chemistry Research, 2004
    Co-Authors: Ning Yang, Wei Wang, Linna Wang
    Abstract:

    To consider the critical effect of mesoscale structure on the Drag coefficient, this paper presents a Drag Model based on the energy-minimization multiscale (EMMS) approach. The proposed structure parameters are obtained from the EMMS Model, and then the average Drag coefficient can be calculated from the structure parameters and further incorporated into the two-fluid Model to simulate the gas-solid flow in a circulating fluidized-bed riser. Simulation results indicate that the simulated flow structures are different for the EMMS-based Drag Model and the hybrid Model using the Wen and Yu correlation and the Ergun equation. The former shows its improvement in predicting the solids entrainment rate, the mesoscale heterogeneous structure involving clusters or strands, and the radial and axial voidage distributions. The simulation results support the idea that the average Drag coefficient is an important factor for the two-fluid Model and suggest that the EMMS approach could be used as a kind of multiscale closure law for Drag coefficient.

Hu Zhao - One of the best experts on this subject based on the ideXlab platform.

  • hydrodynamic and solids residence time distribution in a binary bubbling fluidized bed 3d computational study coupled with the structure based Drag Model
    Chemical Engineering Journal, 2017
    Co-Authors: Yunlong Zhao, Hu Zhao, Libo Zhang, Hongzhong Li
    Abstract:

    The simulation of bubbling fluidized beds (BFB) residence time distribution (RTD) based on the structure-based Drag Model are conducted for the single and binary gas-solid phases systems, a comparison of computed results with experimental data proves that our Model is applicable to both systems with better accuracy. The revised Drag coefficient (Hd) increases with decreasing the gas velocity or increasing the particle diameter. The increase of the feed rate could improve the solids flow pattern to be close to the plug flow, while increasing gas velocity or bed height would lead to a wider RTD. The particles in the binary mixture are in more diffusion-oriented movement so as to have less MRT (mean residence time) than that of the single system. The coarse particles with longer MRT are simulated to accumulate into the bed bottom with a slower vertical velocity.

  • numerical analysis of residence time distribution of solids in a bubbling fluidized bed based on the modified structure based Drag Model
    Particuology, 2017
    Co-Authors: Yunlong Zhao, Hongzhong Li, Hu Zhao, Yingbo Li
    Abstract:

    The residence time distribution (RTD) of solids and the fluidized structure of a bubbling fluidized bed were investigated numerically using computational fluid dynamics simulations coupled with the modified structure-based Drag Model. A general comparison of the simulated results with theoretical values shows reasonable agreement. As the mean residence time is increased, the RTD initial peak intensity decreases and the RTD curve tail extends farther. Numerous small peaks on the RTD curve are induced by the back-mixing and aggregation of particles, which attests to the non-uniform flow structure of the bubbling fluidized bed. The low value of t50 results in poor contact between phases, and the complete exit age of the overflow particles is much longer for back-mixed solids and those caught in dead regions. The formation of a gulf-stream flow and back-mixing for solids induces an even wider spread of RTD.

  • Simulation of hydrodynamics in gas-solid bubbling fluidized bed with louver baffles in three dimensions
    Powder Technology, 2016
    Co-Authors: Shuai Yang, Wenming Liu, Xiaolin Lv, Hongzhong Li, Hu Zhao, Li Da-peng, Qingshan Zhu
    Abstract:

    In this study, the flow characteristics of Geldart A particles in the bubbling fluidized bed with louver baffles were simulated by CFD method for the first time. A new Drag Model for bubbling fluidized bed with louver baffles was developed after our early work [Yang et al. Chem. Eng. J. 259 (2015) 338-347]. In this Drag Model, the baffled fluidized bed was treated as a series connection of freely bubbling fluidized beds with the louver baffle as the gas distributor. The hydrodynamics of each freely bubbling fluidized bed were simulated using the structure-based Drag Model with the modification of the bubble diameter equation to adapt the change of bubble size after passing each louver baffle. Through analyzing bubble diameters measured by double optical fiber probe, it was found that the averaged diameter of bubbles regenerated from baffles was nearly equal to the distance between two adjacent vanes in the louver baffle. By this method, the bubbling fluidization can be correctly predicted and the simulation results showed good agreements with the experimental data of the radial and axial solid volume fraction distributions. In the simulation results, it can be seen that the variation of axial solid volume fraction across the louver baffle was fluctuant, which was different from other types of baffles. Meanwhile, the lower particle concentration at the position of louver baffle indicates the particles back-mixing inhibited by louver baffles. The guiding effect of louver baffles on the gas and solid flow can be captured in the gas and solid velocity profiles as well.

  • eulerian eulerian simulation of irregular particles in dense gas solid fluidized beds
    Powder Technology, 2015
    Co-Authors: Hu Zhao, Junwu Wang
    Abstract:

    As an important and essential physical property, particle shape affects fluidization characteristics of dense gas-solid fluidized beds greatly. However, numerical studies of irregular particles in gas-solid fluidization, especially using Eulerian-Eulerian Model, are relatively scarce, primarily due to the lack of an appropriate inter-phase Drag Model. To this end, a simple and effective Drag Model was proposed in this work to address the critical role of particle shape in determining the inter-phase Drag force. This Drag Model features the usage of Ganser correlation and Ergun correlation, respectively, in dilute and dense solid concentration conditions. Moreover, particle sphericity is introduced as a concise shape descriptor, and can be calibrated by experimentally measured minimum fluidization velocity and the corresponding voidage. To verify the established numerical Model, experimental study was also conducted in a lab-scale three-dimensional rectangular bed filled with irregular Geldart group B particles. Other two kinds of irregular bed materials from open literature were investigated as well to provide further validation. The predicted results using the proposed Drag Model are overall in relatively better quantitative agreement with the experimental data or available empirical correlations, in terms of both macro-scale and micro-scale behavior. In contrast, the numerical Model with the Drag force assuming perfectly spherical particles completely fails to reproduce the experimental hydrodynamics of the gas-solid fluidized bed. In addition, sensitivity analysis of the proposed Drag Model demonstrated its weak sensitivity to the minimum fluidization characteristics of particles. The sufficient comparison and analysis indicated that the proposed Drag Model together with the estimation method of particle sphericity is quite reasonable and convenient for Eulerian-Eulerian simulation of irregular particles behavior in lab-scale dense gas-solid fluidized beds. (C) 2015 Elsevier B.V. All rights reserved.

Wenming Liu - One of the best experts on this subject based on the ideXlab platform.

  • cfd simulations of tapered bubbling turbulent fluidized beds with without gas distributor based on the structure based Drag Model
    Chemical Engineering Science, 2019
    Co-Authors: Zheng Zou, Wenming Liu, Dong Yan, Zhaohui Xie, Qingshan Zhu
    Abstract:

    Abstract The hydrodynamic characteristics of tapered bubbling/turbulent fluidized beds with/without gas distributor are simulated based on the structure-based Drag Model. Besides the only parameter of gas voidage (eg) contained in the original Drag Model, the heterogeneous Drag index (Hd) for the tapered fluidized bed (TFB) also includes the parameter of gas velocity (ug) for the axial velocity gradient along bed height. Both the simulations of tapered bubbling and turbulent fluidized beds achieve more accurate predictions than the traditional Drag Models. There exist dilute center and dense annular regions with solids cycle flow structure in the TFB. The simulation also gives reasonable prediction for TFB without gas distributor, the variation of gas voidage profile reveals gas converging toward bed center with axial location rising.

  • Simulation of hydrodynamics in gas-solid bubbling fluidized bed with louver baffles in three dimensions
    Powder Technology, 2016
    Co-Authors: Shuai Yang, Wenming Liu, Xiaolin Lv, Hongzhong Li, Hu Zhao, Li Da-peng, Qingshan Zhu
    Abstract:

    In this study, the flow characteristics of Geldart A particles in the bubbling fluidized bed with louver baffles were simulated by CFD method for the first time. A new Drag Model for bubbling fluidized bed with louver baffles was developed after our early work [Yang et al. Chem. Eng. J. 259 (2015) 338-347]. In this Drag Model, the baffled fluidized bed was treated as a series connection of freely bubbling fluidized beds with the louver baffle as the gas distributor. The hydrodynamics of each freely bubbling fluidized bed were simulated using the structure-based Drag Model with the modification of the bubble diameter equation to adapt the change of bubble size after passing each louver baffle. Through analyzing bubble diameters measured by double optical fiber probe, it was found that the averaged diameter of bubbles regenerated from baffles was nearly equal to the distance between two adjacent vanes in the louver baffle. By this method, the bubbling fluidization can be correctly predicted and the simulation results showed good agreements with the experimental data of the radial and axial solid volume fraction distributions. In the simulation results, it can be seen that the variation of axial solid volume fraction across the louver baffle was fluctuant, which was different from other types of baffles. Meanwhile, the lower particle concentration at the position of louver baffle indicates the particles back-mixing inhibited by louver baffles. The guiding effect of louver baffles on the gas and solid flow can be captured in the gas and solid velocity profiles as well.

  • a new structural parameters Model based on Drag coefficient for simulation of circulating fluidized beds
    Powder Technology, 2015
    Co-Authors: Wenming Liu, Qingshan Zhu, Quanhong Zhu
    Abstract:

    This work presented a new scheme to establish structural parameters Model, and the Model was used to solve structural parameters based on the available structure-based Drag Model. By combining with the Eulerian two-fluid Model, the hydrodynamics of circulating fluidized beds (CFBs) was simulated. Different combinations of clusters properties, including the cluster voidage and diameter, were adopted to fit for Geldart A and B particles and to close the insufficient solving equations, respectively. The simulated solid mass flux, radial and axial voidage profiles were in agreement with the experimental data. The dilute-top/dense-bottom and the core-annular flow structure were also captured. Moreover the spatiotemporal fluctuation of clusters can be observed from those simulations. Simulation results showed the combination of the structural parameters Model with the available structure-based Drag Model can predict well the hydrodynamics for Geldart A and B particles in CFBs. (C) 2015 Elsevier B.V. All rights reserved.