A computational study was carried out on bubble dynamic behaviors and bubble size distributions in a pressurized lab-scale gas-solid fluidized bed of Geldart A particles.High-resolution 3-D numerical simulations were ...A computational study was carried out on bubble dynamic behaviors and bubble size distributions in a pressurized lab-scale gas-solid fluidized bed of Geldart A particles.High-resolution 3-D numerical simulations were performed using the two-fluid model based on the kinetic theory of granular flow.A finegrid,which is in the range of 3–4 particle diameters,was utilized in order to capture bubble structures explicitly without breaking down the continuum assumption for the solid phase.A novel bubble tracking scheme was developed in combination with a 3-D detection and tracking algorithm(MS3 DATA)and applied to detect the bubble statistics,such as bubble size,location in each time frame and relative position between two adjacent time frames,from numerical simulations.The spatial coordinates and corresponding void fraction data were sampled at 100 Hz for data analyzing.The bubble coalescence/break-up frequencies and the daughter bubble size distribution were evaluated by using the new bubble tracking algorithm.The results showed that the bubble size distributed non-uniformly over cross-sections in the bed.The equilibrium bubble diameter due to bubble break-up and coalescence dynamics can be obtained,and the bubble rise velocity follows Davidson’s correlation closely.Good agreements were obtained between the computed results and that predicted by using the bubble break-up model proposed in our previous work.The computational bubble tracking method showed the potential of analyzing bubble motions and the coalescence and break-up characteristics based on time series data sets of void fraction maps obtained numerically and experimentally.展开更多
The incipient condition of hang-up for three Geldart-D powders has been experimentally studied in a 21 m long standpipe hopper system. Experimental results show that the pressure gradient for hang-up to occur is indep...The incipient condition of hang-up for three Geldart-D powders has been experimentally studied in a 21 m long standpipe hopper system. Experimental results show that the pressure gradient for hang-up to occur is independent of the materials height in the hopper and the diameter of orifice and equals to (dpw/dl)s, which can be predicted by Eq. (7). While the corresponding gas velocity in the standpipe equals to the incipient fluidized velocity of particles at the high pressure and can be predicted by Kwauk's equation.展开更多
In gas fluidization processes involving different types of particles,the mixing or segregation behavior of the solid mixture is crucial to the overall outcome of the process.This study develops a model to predict the ...In gas fluidization processes involving different types of particles,the mixing or segregation behavior of the solid mixture is crucial to the overall outcome of the process.This study develops a model to predict the segregation directions of binary mixtures of Geldart B particles with density and size differences in bubbling fluidized beds.The proposed model was established by combining the particle segregation model,a previous particle segregation model,with a derived bed voidage equation of the bubbling fluidization based on the two-phase theory.The model was then analyzed with different function graphs of the model equations under various conditions.The results indicated that an increase in gas velocity or volume fraction of larger particles would strengthen size segregation,causing the larger and less dense components to descend.To validate the model,42 sets of data collected from 6 independent literature sources were compared with the predictions of the model.When the gas velocities were below 3.2 times the minimum gas velocity,the predictions were consistent with experimental results.This study has shed new light on the mechanisms of particle segregation in binary fluidized systems and provides a theoretical foundation for designing and manipulating gas-solid fluidized reactors.展开更多
Gas-solid Fluidized Bed Coal Beneficiator(GFBCB)process is a crucial dry coal beneficiation fluidization technology.The work employs the GFBCB process alongside a novel Geldart A^(-)dense medium,consisting of Geldart ...Gas-solid Fluidized Bed Coal Beneficiator(GFBCB)process is a crucial dry coal beneficiation fluidization technology.The work employs the GFBCB process alongside a novel Geldart A^(-)dense medium,consisting of Geldart A magnetite particles and Geldart C ultrafine coal,to separate small-size separated objects in the GFBCB.The effects of various operational conditions,including the volume fraction of ultrafine coal,the gas velocity,the separated objects size,and the separation time,were investigated on the GFBCB's separation performance.The results indicated that the probable error for 6∼3 mm separated objects could be controlled within 0.10 g/cm^(3).Compared to the traditional Geldart B/D dense medium,the Geldart A/A^(-)dense medium exhibited better size-dependent separation performance with an overall probable error 0.04∼0.12 g/cm^(3).Moreover,it achieved a similar separation accuracy to the Geldart B/D dense medium fluidized bed with different external energy for the small-size object beneficiation.The work furthermore validated a separation density prediction model based on theoretical derivation,available for both Geldart B/D dense medium and Geldart A/A^(-)dense medium at different operational conditions.展开更多
Fluidization technology has been used in CO_(2) capture processes, the successful design and operation of the heat exchangers involved in this process require much information on the bed-to-wall heat transfer of the s...Fluidization technology has been used in CO_(2) capture processes, the successful design and operation of the heat exchangers involved in this process require much information on the bed-to-wall heat transfer of the sorbent particles in fluidized states. In this study, the bed-to-wall heat transfer coefficient (h) of a solid amine sorbent was measured by a heat transfer probe in a large-scale circulating fluidized bed cold model unit, where full spectrum of fluidization regimes can be realized. The corresponding hydrodynamic signals were also studied by pressure sensors and optical fiber probes to further explain the newly discovered phenomenon. The results show that in a dense bed, due to the counterbalanced effect of time fraction of packet and packet renewal frequency, h of the Geldart B particle reaches a peak within the bubbling fluidized regime, and the radial distribution of h are opposite in bubbling and turbulent fluidized regimes. In a fast fluidization regime, gas convection becomes the dominant factor affecting h when the solids holdup is low enough. Correlations were provided or recommended to guide the design of heat exchangers in the fluidized bed CO_(2) capture processes.展开更多
Gas–solid separation fluidized bed is a typical method for coal separation without water utilization.Geldart A particles is also considered as the ideal dense medium to strengthen separation efficiency.Fluidization s...Gas–solid separation fluidized bed is a typical method for coal separation without water utilization.Geldart A particles is also considered as the ideal dense medium to strengthen separation efficiency.Fluidization stability reflects the bed pressure fluctuations and the distribution of bubble and emulsion phases,affecting the separation performance.And the main frequency of pressure fluctuations can directly reflect the degree of pressure fluctuations.Therefore,the detailed fluidization stability is analyzed combined the method of standard deviation of pressure fluctuations,power spectral density,etc.,for Geldart A particles.The results showed that maintaining an appropriate gas velocity resulted in an average bed pressure of around 2000 Pa.The main frequency is mainly concentrated around 1–1.5 Hz.Finally,a prediction model of the main frequency of pressure fluctuations is established,and the error can be controlled within±0.15.The investigation further proved the stable fluidization of Geldart A particles and provides a method for predicting the main frequency of pressure fluctuations in the gas–solid separation fluidized bed.展开更多
We have investigated the effect of cohesion and drag models on the bed hydrodynamics of Geldart A particles based on the two-fluid （TF） model. For a high gas velocity U0 = 0.03 m/s, we found a transition from the ho...We have investigated the effect of cohesion and drag models on the bed hydrodynamics of Geldart A particles based on the two-fluid （TF） model. For a high gas velocity U0 = 0.03 m/s, we found a transition from the homogeneous fluidization to bubbling fluidization with an increase of the coefficient C1, which is used to account for the contribution of cohesion to the excess compressibility. Thus cohesion can play a role in the bed expansion of Geldart A particles. Apart from cohesion, we have also investigated the influence of the drag models. When using the Wen and Yu drag correlation with an exponent n = 4.65, we find an under-prediction of the bed expansion at low gas velocities （U0 = 0.009 m/s）. When using a larger exponent （n = 9.6）, as reported in experimental studies of gas-fluidization, a much better agreement with the experimental bed expansion is obtained. These findings suggest that at low gas velocity, a scale-down of the commonly used drag model is required. On the other hand, a scale-up of the commonly used drag model is necessary at high gas velocity （U0 = 0.2 and 0.06 m/s）. We therefore conclude that scaling the drag force represent only an ad hoc way of repairing the deficiencies of the TF model, and that a far more detailed study is required into the origin of the failure of the TF model for simulating fluidized beds of fine powders.展开更多
The fiuidization behavior of Geldart A particles in a gas-solid micro-fluidized bed was investigated by Eulerian-Eulerian numerical simulation. The commonly used Gidaspow drag model was tested first. The simulation sh...The fiuidization behavior of Geldart A particles in a gas-solid micro-fluidized bed was investigated by Eulerian-Eulerian numerical simulation. The commonly used Gidaspow drag model was tested first. The simulation showed that the predicted minimum bubbling velocities were significantly lower than the experimental data even when an extremely fine grid size （of approximately one particle diameter） was used. The modified Gibilaro drag model was therefore tested next. The predicted minimum bubbling velocity and bed voidage were in reasonable agreement with the experimental data available in literature. The experimentally observed regime transition phenomena from bubbling to slugging were also reproduced successfully in the simulations. Parametric studies indicated that the solid-wall boundary conditions had a significant impact on the predicted gas and solid flow behavior.展开更多
Pressurized fluidized beds have been developed in quite a few industrial applications because of intensified heat and mass transfer and chemical reaction.The bubble behaviors under elevated pressure,strongly influenci...Pressurized fluidized beds have been developed in quite a few industrial applications because of intensified heat and mass transfer and chemical reaction.The bubble behaviors under elevated pressure,strongly influencing the fluidization and reaction conversion of the whole system,are of great research significance.In this work,the bubble behaviors of Geldart B particle in a pseudo two-dimensional(2D)pressurized fluidized bed were experimentally studied based on digital image analysis technique.The effects of pressure and fluidization gas velocity on the general bubble behaviors(i.e.,size,shape and spatial distribution)and the dynamic characteristics,such as the time-evolution of voidage distribution and local flow regimes,were comprehensively investigated.Results show that increasing pressure re-duces the stability of bubbles and facilitates gas passing through the emulsion phase,resulting in the"smoother"fluidization state with smaller bubbles and declined bubble fraction and standard deviation.The equivalent bubble diameter and bubble aspect ratio increase with the increasing gas velocity while decrease as pressure rises.The elevated pressure reduces bubbles extension in the vertical direction,prohibits the"short pass"of fluidization gas in large oblong bubbles/slugs and benefits the gas-solid interaction.The flow regimes variation with gas velocity is affected by the elevated pressure,and demonstrates different features in different local positions of the bed.展开更多
The fluidization state in the circulating fluidized bed(CFB)boiler is crucial to its stable and safe operation.However,up to now,the research field has not reached unanimity on whether the fluidization regime that the...The fluidization state in the circulating fluidized bed(CFB)boiler is crucial to its stable and safe operation.However,up to now,the research field has not reached unanimity on whether the fluidization regime that the upper furnace of the boiler operates in is the fast fluidization or pneumatic transport.To this end,this paper reviewed relevant research on the transition between the fast fluidization and pneumatic transport of Geldart group B particles,including the flow characteristics of the fast fluidization,the transition condition between the fast fluidization and pneumatic transport,the determination methods of the transport velocity utr and saturation carrying capacity G_(s)* and the influencing factors on these two parameters.Previous research findings can provide certain guidelines for the design and optimization of the CFB boiler,and result in plenty of prediction correlations for utr and G_(s)*.Nonetheless,owing to insufficient data available on Geldart group B particles,especially the ones obtained under high temperature or pressure conditions and in large-scale CFB apparatuses,the existing correlations are not well suited for the prediction of u_(tr) and G_(s)* of Geldart group B particles.Thus,further efforts are urgently demanded on the fast fluidization transition of Geldart group B particles.展开更多
We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve ...We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,展开更多
In this work, a new drag model for TFM simulation in gas-solid bubbling fluidized beds was proposed, and a set of equations was derived to determine the meso-scale structural parameters to calculate the drag character...In this work, a new drag model for TFM simulation in gas-solid bubbling fluidized beds was proposed, and a set of equations was derived to determine the meso-scale structural parameters to calculate the drag characteristics of Geldart-B particles under low gas velocities. In the new model, the meso-scale structure was characterized while accounting for the bubble and meso-scale structure effects on the drag coefficient. The Fluent software, incorporating the new drag model, was used to simulate the fluidization behavior. Experiments were performed in a Plexiglas cylindrical fluidized bed consisting of quartz sand as the solid phase and ambient air as the gas phase. Comparisons based on the solids hold-up inside the fluidized bed at different superficial gas velocities, were made between the 2D Cartesian simulations, and the experimental data, showing that the results of the new drag model reached much better agreement with exoerimental data than those of the Gidasoow dra~ model did.展开更多
基金supported by the National Natural Science Foundation of China(21908062)。
文摘A computational study was carried out on bubble dynamic behaviors and bubble size distributions in a pressurized lab-scale gas-solid fluidized bed of Geldart A particles.High-resolution 3-D numerical simulations were performed using the two-fluid model based on the kinetic theory of granular flow.A finegrid,which is in the range of 3–4 particle diameters,was utilized in order to capture bubble structures explicitly without breaking down the continuum assumption for the solid phase.A novel bubble tracking scheme was developed in combination with a 3-D detection and tracking algorithm(MS3 DATA)and applied to detect the bubble statistics,such as bubble size,location in each time frame and relative position between two adjacent time frames,from numerical simulations.The spatial coordinates and corresponding void fraction data were sampled at 100 Hz for data analyzing.The bubble coalescence/break-up frequencies and the daughter bubble size distribution were evaluated by using the new bubble tracking algorithm.The results showed that the bubble size distributed non-uniformly over cross-sections in the bed.The equilibrium bubble diameter due to bubble break-up and coalescence dynamics can be obtained,and the bubble rise velocity follows Davidson’s correlation closely.Good agreements were obtained between the computed results and that predicted by using the bubble break-up model proposed in our previous work.The computational bubble tracking method showed the potential of analyzing bubble motions and the coalescence and break-up characteristics based on time series data sets of void fraction maps obtained numerically and experimentally.
文摘The incipient condition of hang-up for three Geldart-D powders has been experimentally studied in a 21 m long standpipe hopper system. Experimental results show that the pressure gradient for hang-up to occur is independent of the materials height in the hopper and the diameter of orifice and equals to (dpw/dl)s, which can be predicted by Eq. (7). While the corresponding gas velocity in the standpipe equals to the incipient fluidized velocity of particles at the high pressure and can be predicted by Kwauk's equation.
基金the National Natural Science Foundation of China(grant No.52274275)the Graduate Research and Innovation Projects of Jiangsu Province(grant No.KYCX22_2640)the Graduate Innovation Program of China University of Mining and Technology(grant No.2022WLKXJ065).
文摘In gas fluidization processes involving different types of particles,the mixing or segregation behavior of the solid mixture is crucial to the overall outcome of the process.This study develops a model to predict the segregation directions of binary mixtures of Geldart B particles with density and size differences in bubbling fluidized beds.The proposed model was established by combining the particle segregation model,a previous particle segregation model,with a derived bed voidage equation of the bubbling fluidization based on the two-phase theory.The model was then analyzed with different function graphs of the model equations under various conditions.The results indicated that an increase in gas velocity or volume fraction of larger particles would strengthen size segregation,causing the larger and less dense components to descend.To validate the model,42 sets of data collected from 6 independent literature sources were compared with the predictions of the model.When the gas velocities were below 3.2 times the minimum gas velocity,the predictions were consistent with experimental results.This study has shed new light on the mechanisms of particle segregation in binary fluidized systems and provides a theoretical foundation for designing and manipulating gas-solid fluidized reactors.
基金National Natural Science Foundation of China(grant Nos.52220105008,52104276)China National Funds for Distinguished Young Scientists(grant No.52125403).
文摘Gas-solid Fluidized Bed Coal Beneficiator(GFBCB)process is a crucial dry coal beneficiation fluidization technology.The work employs the GFBCB process alongside a novel Geldart A^(-)dense medium,consisting of Geldart A magnetite particles and Geldart C ultrafine coal,to separate small-size separated objects in the GFBCB.The effects of various operational conditions,including the volume fraction of ultrafine coal,the gas velocity,the separated objects size,and the separation time,were investigated on the GFBCB's separation performance.The results indicated that the probable error for 6∼3 mm separated objects could be controlled within 0.10 g/cm^(3).Compared to the traditional Geldart B/D dense medium,the Geldart A/A^(-)dense medium exhibited better size-dependent separation performance with an overall probable error 0.04∼0.12 g/cm^(3).Moreover,it achieved a similar separation accuracy to the Geldart B/D dense medium fluidized bed with different external energy for the small-size object beneficiation.The work furthermore validated a separation density prediction model based on theoretical derivation,available for both Geldart B/D dense medium and Geldart A/A^(-)dense medium at different operational conditions.
基金support by the National Natural Science Foundation of China(grant No.21808245).
文摘Fluidization technology has been used in CO_(2) capture processes, the successful design and operation of the heat exchangers involved in this process require much information on the bed-to-wall heat transfer of the sorbent particles in fluidized states. In this study, the bed-to-wall heat transfer coefficient (h) of a solid amine sorbent was measured by a heat transfer probe in a large-scale circulating fluidized bed cold model unit, where full spectrum of fluidization regimes can be realized. The corresponding hydrodynamic signals were also studied by pressure sensors and optical fiber probes to further explain the newly discovered phenomenon. The results show that in a dense bed, due to the counterbalanced effect of time fraction of packet and packet renewal frequency, h of the Geldart B particle reaches a peak within the bubbling fluidized regime, and the radial distribution of h are opposite in bubbling and turbulent fluidized regimes. In a fast fluidization regime, gas convection becomes the dominant factor affecting h when the solids holdup is low enough. Correlations were provided or recommended to guide the design of heat exchangers in the fluidized bed CO_(2) capture processes.
基金National Natural Science Foundation of China(grant Nos.52220105008,52261135540)China National Funds for Distinguished Young Scientists(grant No.52125403)+1 种基金the Postgraduate Research&Practice Innovation Program of Jiangsu Province(grant No.SJCX23_1302)the Graduate Innovation Program of China University of Mining and Technology(grant No.2023WLJCRCZL081).
文摘Gas–solid separation fluidized bed is a typical method for coal separation without water utilization.Geldart A particles is also considered as the ideal dense medium to strengthen separation efficiency.Fluidization stability reflects the bed pressure fluctuations and the distribution of bubble and emulsion phases,affecting the separation performance.And the main frequency of pressure fluctuations can directly reflect the degree of pressure fluctuations.Therefore,the detailed fluidization stability is analyzed combined the method of standard deviation of pressure fluctuations,power spectral density,etc.,for Geldart A particles.The results showed that maintaining an appropriate gas velocity resulted in an average bed pressure of around 2000 Pa.The main frequency is mainly concentrated around 1–1.5 Hz.Finally,a prediction model of the main frequency of pressure fluctuations is established,and the error can be controlled within±0.15.The investigation further proved the stable fluidization of Geldart A particles and provides a method for predicting the main frequency of pressure fluctuations in the gas–solid separation fluidized bed.
文摘We have investigated the effect of cohesion and drag models on the bed hydrodynamics of Geldart A particles based on the two-fluid （TF） model. For a high gas velocity U0 = 0.03 m/s, we found a transition from the homogeneous fluidization to bubbling fluidization with an increase of the coefficient C1, which is used to account for the contribution of cohesion to the excess compressibility. Thus cohesion can play a role in the bed expansion of Geldart A particles. Apart from cohesion, we have also investigated the influence of the drag models. When using the Wen and Yu drag correlation with an exponent n = 4.65, we find an under-prediction of the bed expansion at low gas velocities （U0 = 0.009 m/s）. When using a larger exponent （n = 9.6）, as reported in experimental studies of gas-fluidization, a much better agreement with the experimental bed expansion is obtained. These findings suggest that at low gas velocity, a scale-down of the commonly used drag model is required. On the other hand, a scale-up of the commonly used drag model is necessary at high gas velocity （U0 = 0.2 and 0.06 m/s）. We therefore conclude that scaling the drag force represent only an ad hoc way of repairing the deficiencies of the TF model, and that a far more detailed study is required into the origin of the failure of the TF model for simulating fluidized beds of fine powders.
基金financial support from the Ministry of Science and Technology of China with Grant No.2011YQ12003909the ongoing support through the startup fund awarded to Xiaoxing Liu from the "Hundred Talents Program" of the Institute of Process Engineering,Chinese Academy of Sciences
文摘The fiuidization behavior of Geldart A particles in a gas-solid micro-fluidized bed was investigated by Eulerian-Eulerian numerical simulation. The commonly used Gidaspow drag model was tested first. The simulation showed that the predicted minimum bubbling velocities were significantly lower than the experimental data even when an extremely fine grid size （of approximately one particle diameter） was used. The modified Gibilaro drag model was therefore tested next. The predicted minimum bubbling velocity and bed voidage were in reasonable agreement with the experimental data available in literature. The experimentally observed regime transition phenomena from bubbling to slugging were also reproduced successfully in the simulations. Parametric studies indicated that the solid-wall boundary conditions had a significant impact on the predicted gas and solid flow behavior.
基金Financial support of this work by the National Natural Science Foundation of China(grant No.52106216)Fundamental Research Funds for the Central Universities(grant No.27RA2114005)are gratefully acknowledged.
文摘Pressurized fluidized beds have been developed in quite a few industrial applications because of intensified heat and mass transfer and chemical reaction.The bubble behaviors under elevated pressure,strongly influencing the fluidization and reaction conversion of the whole system,are of great research significance.In this work,the bubble behaviors of Geldart B particle in a pseudo two-dimensional(2D)pressurized fluidized bed were experimentally studied based on digital image analysis technique.The effects of pressure and fluidization gas velocity on the general bubble behaviors(i.e.,size,shape and spatial distribution)and the dynamic characteristics,such as the time-evolution of voidage distribution and local flow regimes,were comprehensively investigated.Results show that increasing pressure re-duces the stability of bubbles and facilitates gas passing through the emulsion phase,resulting in the"smoother"fluidization state with smaller bubbles and declined bubble fraction and standard deviation.The equivalent bubble diameter and bubble aspect ratio increase with the increasing gas velocity while decrease as pressure rises.The elevated pressure reduces bubbles extension in the vertical direction,prohibits the"short pass"of fluidization gas in large oblong bubbles/slugs and benefits the gas-solid interaction.The flow regimes variation with gas velocity is affected by the elevated pressure,and demonstrates different features in different local positions of the bed.
基金supported by the National Key Research Plan (2019YFE0102100)the Huaneng Group Science and Technology Research Project (HNKj20-H50)the C9 University Science and Technology Project (201903D421009).
文摘The fluidization state in the circulating fluidized bed(CFB)boiler is crucial to its stable and safe operation.However,up to now,the research field has not reached unanimity on whether the fluidization regime that the upper furnace of the boiler operates in is the fast fluidization or pneumatic transport.To this end,this paper reviewed relevant research on the transition between the fast fluidization and pneumatic transport of Geldart group B particles,including the flow characteristics of the fast fluidization,the transition condition between the fast fluidization and pneumatic transport,the determination methods of the transport velocity utr and saturation carrying capacity G_(s)* and the influencing factors on these two parameters.Previous research findings can provide certain guidelines for the design and optimization of the CFB boiler,and result in plenty of prediction correlations for utr and G_(s)*.Nonetheless,owing to insufficient data available on Geldart group B particles,especially the ones obtained under high temperature or pressure conditions and in large-scale CFB apparatuses,the existing correlations are not well suited for the prediction of u_(tr) and G_(s)* of Geldart group B particles.Thus,further efforts are urgently demanded on the fast fluidization transition of Geldart group B particles.
基金We acknowledge support from the National High Technology Research and Development Program of China （2012AA06A115）, National Natural Science Foundation of China （51476058, 91434120）, and Fundamental Research Funds for the Central Universities （2014MS13）.
文摘We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,
基金supports from the State Key Development Program for Basic Research of China(973 Program)under Grant Nos.2009CB219904,2013CB632603the National Science and Technology Support Program of Ministry of Science and Technology of the People's Republic of China(Grant No. 2012BAB14B03)
文摘In this work, a new drag model for TFM simulation in gas-solid bubbling fluidized beds was proposed, and a set of equations was derived to determine the meso-scale structural parameters to calculate the drag characteristics of Geldart-B particles under low gas velocities. In the new model, the meso-scale structure was characterized while accounting for the bubble and meso-scale structure effects on the drag coefficient. The Fluent software, incorporating the new drag model, was used to simulate the fluidization behavior. Experiments were performed in a Plexiglas cylindrical fluidized bed consisting of quartz sand as the solid phase and ambient air as the gas phase. Comparisons based on the solids hold-up inside the fluidized bed at different superficial gas velocities, were made between the 2D Cartesian simulations, and the experimental data, showing that the results of the new drag model reached much better agreement with exoerimental data than those of the Gidasoow dra~ model did.
