In order to better understand the mechanical properties of graded crushed rocks （GCRs） and to optimize the relevant design, a numerical test method based on the particle flow modeling technique PFC2D is developed fo...In order to better understand the mechanical properties of graded crushed rocks （GCRs） and to optimize the relevant design, a numerical test method based on the particle flow modeling technique PFC2D is developed for the California bearing ratio （CBR） test on GGRs. The effects of different testing conditions and micro-mechanical parameters used in the model on the CBR numerical results have been systematically studied. The reliability of the numerical technique is verified. The numerical results suggest that the influences of the loading rate and Poisson＇s ratio on the CBR numerical test results are not significant. As such, a loading rate of 1.0-3.0 mm/min, a piston diameter of 5 cm, a specimen height of 15 cm and a specimen diameter of 15 cm are adopted for the CBR numerical test. The numerical results reveal that the GBR values increase with the friction coefficient at the contact and shear modulus of the rocks, while the influence of Poisson＇s ratio on the GBR values is insignificant. The close agreement between the CBR numerical results and experimental results suggests that the numerical simulation of the CBR values is promising to help assess the mechanical properties of GGRs and to optimize the grading design. Be- sides, the numerical study can provide useful insights on the mesoscopic mechanism.展开更多
Tunnel excavation is a complicated loading-unloading-reloading process characterized by decreased radial stresses and increased axial stresses.An approach that considers only loading,is generally used in tunnel model ...Tunnel excavation is a complicated loading-unloading-reloading process characterized by decreased radial stresses and increased axial stresses.An approach that considers only loading,is generally used in tunnel model testing.However,this approach is incapable of characterizing the unloading effects induced by excavation on surrounding rocks and hence presents radial and tangential stress paths during the failure process that are different from the actual stress state of tunnels.This paper carried out a comparative analysis using laboratory model testing and particle flow code(PFC2D)-based numerical simulation,and shed light upon the crack propagation process and,microscopic stress and force chain variations during the loading-unloading process.The failure mode observed in the unloading model test is shear failure.The force chains are strongly correlated with the concrete fracture propagation.In addition,the change patterns of the radial and tangential stresses of surrounding rocks in the broken region,as well as the influence of the initial stress on failure loads are revealed.The surrounding soil of tunnel failure evolution as well as extent and shape of the damage zone during the excavation-induced unloading were also studied.展开更多
Combining with the indoor clogging tests of loose foundation in Tibet,the permeability clogging process of loose foundation was simulated based on particle flow method. Under the constant head of 2.2m, numerical micro...Combining with the indoor clogging tests of loose foundation in Tibet,the permeability clogging process of loose foundation was simulated based on particle flow method. Under the constant head of 2.2m, numerical micro-simulation was made in three cases,which was not adding any clogging materials,or adding the clogging materials with the diameter between 0.075-0.500 mm and 0.5-1 mm. The dynamic changes of fluid velocity,permeability coefficient,porosity and loss amount were recorded in the numerical simulation. The results have shown that fluid velocity and permeability coefficient decreased rapidly,when adding the clogging materials with the diameter between 0.5 mm and 1.0 mm. With seepage stability,fluid velocity value was very low. By using computer simulation in the two cases,we got that both cases induced clogging effect. Clogging effect was due to one of the interval of particles rather than one size,which could be seen in the phenomenon of the second case. To some extent,numerical method is useful in the study of clogging problems,which gave the same result obtained in laboratory test and simulation test. These data provided basis and reference for further study of clogging problems,and also provided a new method to study the micro-scale permeability clogging mechanism.展开更多
Vigorous particle collisions and mechanical processes occurring during high-velocity pneumatic con- veying often lead to particle degradation. The resulting particle size reduction and particle number increase will im...Vigorous particle collisions and mechanical processes occurring during high-velocity pneumatic con- veying often lead to particle degradation. The resulting particle size reduction and particle number increase will impact on the flow characteristics, and subsequently affect the electrostatic type of flow measurements. This study investigates this phenomenon using both experimental and numerical meth- ods. Particle degradation was induced experimentally by recursively conveying the fillite material within a pneumatic pipeline. The associated particle size reduction was monitored. Three electrostatic sensors were embedded along the pipeline to monitor the flow. The results indicated a decreasing trend in the electrostatic sensor outputs with decreasing particle size, which suggested the attenuation of the flow velocity fluctuation. This trend was more apparent at higher conveying velocities, which suggested that more severe particle degradation occurred under these conditions. Coupled computational fluid dynamics and discrete element methods （CFD-DEM） analysis was used to qualitatively validate these experimental results. The numerical results suggested that smaller particles exhibited lower flow velocity fluctua- tions, which was consistent with the observed experimental results. These findings provide important information for the accurate aoolication of electrostatic measurement devices in oneumatic conveyors.展开更多
Numerical modeling of a large scale circulating fiuidized bed （CFB） imposes many complexities and difficulties. Presence of a dense solid phase, a variety of spatial and time scales as well as complex model geometri...Numerical modeling of a large scale circulating fiuidized bed （CFB） imposes many complexities and difficulties. Presence of a dense solid phase, a variety of spatial and time scales as well as complex model geometries requires advanced numerical techniques. Moreover, the appropriate selection of a numerical model capable of solving granular flow, and geometrical model simplification can have a huge impact on the predicted flow field within the CFB boiler. In order to reduce the cost of the numerical simulations, the complex CFB boiler geometry is reduced to that of the combustion chamber. However, a question arises as to bow much one can simplify the geometrical model without losing accuracy of numerical simulations. To accurately predict the gas-solid and solid-solid mixing processes within subsequent sections of the CFB boiler （combustion chamber, solid separator, drain section）, a complete 3D geometrical model should be used. Nevertheless, because of the presence of various spatial and temporal scales within subsequent boiler sections, the complete model of the 3D CFB boiler is practically unrealizable in numerical simulations. To resolve the aforementioned problems, this paper describes a new approach that can be applied for complete boiler modeling. The proposed approach enables complex particle transport and gas flow problems within each of the boiler sections to be accurately resolved, It has been achieved by dividing the CFB boiler geometry into several submodels, where different numerical approaches can be used to resolve gas-solid transport. The interactions between computational domains were taken into account by connecting the inlets/outlets of each section using a set of user-defined functions implemented into the solution procedure. The proposed approach ensures stable and accurate solution within the separated boiler zones.展开更多
Liquid-solid binary fluidized beds are widely used in many industries. However, the flow behavior of such beds is not well understood due to the lack of accurate experimental and numerical data. In the current study, ...Liquid-solid binary fluidized beds are widely used in many industries. However, the flow behavior of such beds is not well understood due to the lack of accurate experimental and numerical data. In the current study, the behavior of monodisperse and binary liquid-solid fluidized beds of the same density but dif- ferent sizes is investigated using radioactive particle tracking （RPT） technique and a dense discrete phase model （DDPM）. Experiments and simulations are performed in monodisperse fluidized beds containing two different sizes of glass beads （0.6 and I mm） and a binary fluidized bed of the same particles for vari- ous bed compositions. The results show that both RPT and DDPM can predict the mixing and segregation pattern in liquid-solid binary fluidized beds. The mean velocity predictions of DDPM are in good agree- ment with the experimental findings for both monodisperse and binary fluidized beds. However, the axial root mean square velocity predictions are only reasonable for bigger particles. Particle-particle interac- tions are found to be critical for predicting the flow behavior of solids in liquid-solid binary fluidized beds.展开更多
基金supported by the Program for New Century Excellent Talents in University (NCET-08-0749)Fundamental Research Funds for the Central Universities (CHD2012JC054)
文摘In order to better understand the mechanical properties of graded crushed rocks （GCRs） and to optimize the relevant design, a numerical test method based on the particle flow modeling technique PFC2D is developed for the California bearing ratio （CBR） test on GGRs. The effects of different testing conditions and micro-mechanical parameters used in the model on the CBR numerical results have been systematically studied. The reliability of the numerical technique is verified. The numerical results suggest that the influences of the loading rate and Poisson＇s ratio on the CBR numerical test results are not significant. As such, a loading rate of 1.0-3.0 mm/min, a piston diameter of 5 cm, a specimen height of 15 cm and a specimen diameter of 15 cm are adopted for the CBR numerical test. The numerical results reveal that the GBR values increase with the friction coefficient at the contact and shear modulus of the rocks, while the influence of Poisson＇s ratio on the GBR values is insignificant. The close agreement between the CBR numerical results and experimental results suggests that the numerical simulation of the CBR values is promising to help assess the mechanical properties of GGRs and to optimize the grading design. Be- sides, the numerical study can provide useful insights on the mesoscopic mechanism.
基金the support by the National Natural Science Foundation of China(No.51608071)Natural Science Foundation of Chongqing,China(cstc2018jcyjAX0632)+1 种基金General Financial Grant from the China Postdoctoral Science Foundation(Grant No.2017M620414)the Special Funding for Post-doctoral Researchers in Chongqing(No.Xm2017007).
文摘Tunnel excavation is a complicated loading-unloading-reloading process characterized by decreased radial stresses and increased axial stresses.An approach that considers only loading,is generally used in tunnel model testing.However,this approach is incapable of characterizing the unloading effects induced by excavation on surrounding rocks and hence presents radial and tangential stress paths during the failure process that are different from the actual stress state of tunnels.This paper carried out a comparative analysis using laboratory model testing and particle flow code(PFC2D)-based numerical simulation,and shed light upon the crack propagation process and,microscopic stress and force chain variations during the loading-unloading process.The failure mode observed in the unloading model test is shear failure.The force chains are strongly correlated with the concrete fracture propagation.In addition,the change patterns of the radial and tangential stresses of surrounding rocks in the broken region,as well as the influence of the initial stress on failure loads are revealed.The surrounding soil of tunnel failure evolution as well as extent and shape of the damage zone during the excavation-induced unloading were also studied.
基金supported by the National Natural Science Foundation of P. R. China,clogging effects on affects and mechanism of the permeability of loose medium of altitude reservoirs (No. 41072197)
文摘Combining with the indoor clogging tests of loose foundation in Tibet,the permeability clogging process of loose foundation was simulated based on particle flow method. Under the constant head of 2.2m, numerical micro-simulation was made in three cases,which was not adding any clogging materials,or adding the clogging materials with the diameter between 0.075-0.500 mm and 0.5-1 mm. The dynamic changes of fluid velocity,permeability coefficient,porosity and loss amount were recorded in the numerical simulation. The results have shown that fluid velocity and permeability coefficient decreased rapidly,when adding the clogging materials with the diameter between 0.5 mm and 1.0 mm. With seepage stability,fluid velocity value was very low. By using computer simulation in the two cases,we got that both cases induced clogging effect. Clogging effect was due to one of the interval of particles rather than one size,which could be seen in the phenomenon of the second case. To some extent,numerical method is useful in the study of clogging problems,which gave the same result obtained in laboratory test and simulation test. These data provided basis and reference for further study of clogging problems,and also provided a new method to study the micro-scale permeability clogging mechanism.
文摘Vigorous particle collisions and mechanical processes occurring during high-velocity pneumatic con- veying often lead to particle degradation. The resulting particle size reduction and particle number increase will impact on the flow characteristics, and subsequently affect the electrostatic type of flow measurements. This study investigates this phenomenon using both experimental and numerical meth- ods. Particle degradation was induced experimentally by recursively conveying the fillite material within a pneumatic pipeline. The associated particle size reduction was monitored. Three electrostatic sensors were embedded along the pipeline to monitor the flow. The results indicated a decreasing trend in the electrostatic sensor outputs with decreasing particle size, which suggested the attenuation of the flow velocity fluctuation. This trend was more apparent at higher conveying velocities, which suggested that more severe particle degradation occurred under these conditions. Coupled computational fluid dynamics and discrete element methods （CFD-DEM） analysis was used to qualitatively validate these experimental results. The numerical results suggested that smaller particles exhibited lower flow velocity fluctua- tions, which was consistent with the observed experimental results. These findings provide important information for the accurate aoolication of electrostatic measurement devices in oneumatic conveyors.
基金This scientific work was supported by the National Center for Research and Development,within the confines of Research and Developm ent Strategic Program Advanced Technologies for Energy Generation Project No.2 Oxy-combustion technology for PC and FBC boilers with CO,capture.Agreement No.SP/E/2/66420/1 0.The support is gratefully acknow ledged.
文摘Numerical modeling of a large scale circulating fiuidized bed （CFB） imposes many complexities and difficulties. Presence of a dense solid phase, a variety of spatial and time scales as well as complex model geometries requires advanced numerical techniques. Moreover, the appropriate selection of a numerical model capable of solving granular flow, and geometrical model simplification can have a huge impact on the predicted flow field within the CFB boiler. In order to reduce the cost of the numerical simulations, the complex CFB boiler geometry is reduced to that of the combustion chamber. However, a question arises as to bow much one can simplify the geometrical model without losing accuracy of numerical simulations. To accurately predict the gas-solid and solid-solid mixing processes within subsequent sections of the CFB boiler （combustion chamber, solid separator, drain section）, a complete 3D geometrical model should be used. Nevertheless, because of the presence of various spatial and temporal scales within subsequent boiler sections, the complete model of the 3D CFB boiler is practically unrealizable in numerical simulations. To resolve the aforementioned problems, this paper describes a new approach that can be applied for complete boiler modeling. The proposed approach enables complex particle transport and gas flow problems within each of the boiler sections to be accurately resolved, It has been achieved by dividing the CFB boiler geometry into several submodels, where different numerical approaches can be used to resolve gas-solid transport. The interactions between computational domains were taken into account by connecting the inlets/outlets of each section using a set of user-defined functions implemented into the solution procedure. The proposed approach ensures stable and accurate solution within the separated boiler zones.
文摘Liquid-solid binary fluidized beds are widely used in many industries. However, the flow behavior of such beds is not well understood due to the lack of accurate experimental and numerical data. In the current study, the behavior of monodisperse and binary liquid-solid fluidized beds of the same density but dif- ferent sizes is investigated using radioactive particle tracking （RPT） technique and a dense discrete phase model （DDPM）. Experiments and simulations are performed in monodisperse fluidized beds containing two different sizes of glass beads （0.6 and I mm） and a binary fluidized bed of the same particles for vari- ous bed compositions. The results show that both RPT and DDPM can predict the mixing and segregation pattern in liquid-solid binary fluidized beds. The mean velocity predictions of DDPM are in good agree- ment with the experimental findings for both monodisperse and binary fluidized beds. However, the axial root mean square velocity predictions are only reasonable for bigger particles. Particle-particle interac- tions are found to be critical for predicting the flow behavior of solids in liquid-solid binary fluidized beds.
