The prediction of flow distribution in flow manifolds is important for the design of heat exchangers. The pressure drop along the flow in the header is the most influential factor in flow distribution. Various continu...The prediction of flow distribution in flow manifolds is important for the design of heat exchangers. The pressure drop along the flow in the header is the most influential factor in flow distribution. Various continuous models available in literature have failed to satisfactorily predict the pressure distribution in the headers of the flow manifolds. In this article, a discrete model matching the real physical phenomena has been proposed, to predict the pressure distribution in headers. An experimental evaluation of relevant flow characteristic parameters has been carried out to support the discrete model calculations. The validity of the theoretical discrete model has been performed with experimental results, under specific conditions. Refined experimental probes, for pressure heads with ultrasonic measuring devices, have been used to obtain accurate results. The experimental results fully substantiate the soundness of the theoretical prediction. In addition, the advantage of the ability to accommodate local disturbances in the discrete model has been pointed out. The effect of some local disturbances may be substantial. As a result of the analysis presented in this article, improved designs of flow manifolds in heat exchangers can be realized, to assure operation safety under severe operating conditions.展开更多
Granular material discharge from a flat-bottomed silo has been simulated by using continuum modeling and a three-dimensional discrete-element method （DEM）. The predictive abilities of three commonly used frictional ...Granular material discharge from a flat-bottomed silo has been simulated by using continuum modeling and a three-dimensional discrete-element method （DEM）. The predictive abilities of three commonly used frictional viscosity models （Schaeffer, S-S, and μ（I）） were evaluated by comparing them with the DEM data. The funnel-flow pattern （type C） and the semi-mass-flow pattern （type B） that was predicted by DEM simulations can be represented when the Schaeffer orμ（I） model is used, whereas the S-S model gives a consistent type-B flow pattern. All three models over-estimate the discharge rate compared with the DEM. The profiles of the solids volume fraction and the vertical velocity above the outlet show that the larger discharge rates given by the Schaeffer and μ（I） model result from an over-estimation of volume fraction, whereas the deviation in the S-S model stems from the failure to predict a solid vertical velocity and a volume fraction.展开更多
The phenomenon of the soil plug usually rising inside the suction foundations during suction penetration was quantitatively described and predicted. The formation process of the soil plug was simulated and calculated ...The phenomenon of the soil plug usually rising inside the suction foundations during suction penetration was quantitatively described and predicted. The formation process of the soil plug was simulated and calculated by DEM （discrete element method） model. The seepage flow, the self-weight of soil, the friction on the chamber wall as well as the suction inside the chamber are considered as the main external forces in the process. The results are compared with a set of laboratory model tests performed by using three soil types （sand, silty clay and clay） in the Bohai Sea area. The heights of soil plug from numerical estimations are lower than those from model test results, mainly because the suction pressure and friction resistance are applied in an ideal way under the numerical simulation.展开更多
The controlled and homogeneous flow of dry granular powders through hoppers is essential for applications,namely,packaging of food grains,fertilizers and additive manufacturing processes such as directed energy deposi...The controlled and homogeneous flow of dry granular powders through hoppers is essential for applications,namely,packaging of food grains,fertilizers and additive manufacturing processes such as directed energy deposition for better product quality.One of the major issues encountered in the granular flows through hoppers is flow stagnation due to the well-known arching phenomenon.Vibration-assisted granular flow through hoppers is one of the mechanisms used for better mass flow control.In this work,the influence of external mechanical vibration on the powder flow is investigated experimentally and using discrete element simulations.First,the mass flow rate through the hopper increases with an increase in vibration amplitude and then decreases,signifying the existence of an optimal amplitude of vibration.The DEM simulations explained the underlying mechanisms for the existence of an optimal amplitude of vibration corresponding to the maximum mass flow rate.A range of vibration amplitudes from 0 mm to 3.5 mm is used to study the flow behaviour;the maximum flow of around 33 g/s to 35 g/s is observed for 0.75 mm to 1.25 mm vibration amplitude for the hopper-particle combination studied in this work.The work also reports the influence of vibration frequency,hopper,and particle dimension on the flow characteristics.The research facilitates the effective use of mechanical vibration to enhance powder flow that can further be extended to non-spherical and multi-material particles.展开更多
This paper analyses three popular methods simulating granular flow at different time and length scales： discrete element method （DEM）, averaging method and viscous, elastic-plastic continuum model. The theoretical ...This paper analyses three popular methods simulating granular flow at different time and length scales： discrete element method （DEM）, averaging method and viscous, elastic-plastic continuum model. The theoretical models of these methods and their applications to hopper flows are discussed. It is shown that DEM is an effective method to study the fundamentals of granular flow at a particle or microscopic scale. By use of the continuum approach, granular flow can also be described at a continuum or macroscopic scale. Macroscopic quantities such as velocity and stress can be obtained by use of such computational method as FEM. However, this approach depends on the constitutive relationship of materials and ignores the effect of microscopic structure of granular flow. The combined approach of DEM and averaging method can overcome this problem. The approach takes into account the discrete nature of granular materials and does not require any global assumption and thus allows a better understanding of the fundamental mechanisms of granular flow. However, it is difficult to adapt this approach to process modelling because of the limited number of particles which can be handled with the present computational capacity, and the difficulty in handling non-spherical particles. Further work is needed to develoo an aoorooriate aooroach to overcome these problems.展开更多
Grains are widely present in industrial productions and processing,and are stored in silos.In the silo,auxiliary structures are added to achieve efficient production.However,little effort has been devoted to the influ...Grains are widely present in industrial productions and processing,and are stored in silos.In the silo,auxiliary structures are added to achieve efficient production.However,little effort has been devoted to the influence of the internal structure of the silo on the granular flow.In this work,a silo with a central decompression tube is studied through experimental measurements and discrete element methods.Then,the influences of the central decompression tube on the flow behavior of grains and wall pressure are analyzed.Results show that the grains are in mass flow in the silo without a central decompression tube,while the grains are in funnel flow in the silo with a central decompression tube.Moreover,regardless of whether there is a central decompression tube in the silo,the maximum pressure appears at the top of the conical silo.In the lower part of the silo,the wall pressure of the silo with a central decompression tube is lower than that of the silo without a central decompression tube.Therefore,a silo with a central decompression tube is more conducive to grain storage and discharge than a silo without a central decompression tube.展开更多
The differential evolution(DE)algorithm was deployed to calibrate microparameters of the DEM cohesive granular material.4 macroparameters,namely,uniaxial compressive strength,direct tensile strength,Young’s modulus a...The differential evolution(DE)algorithm was deployed to calibrate microparameters of the DEM cohesive granular material.4 macroparameters,namely,uniaxial compressive strength,direct tensile strength,Young’s modulus and Poisson’s ratio,can be calibrated to high accuracy.The best calibration accuracy could reach the sum of relative errors RE_(sum)<0.1%.Most calibrations can be achieved with RE_(sum)<5%within hours or RE_(sum)<1%within 2 days.Based on the calibrated results,microparameters uniqueness analysis was carried out to reveal the correlation between microparameters and the macroscopic mechanical behaviour of material:(1)microparameters effective modulus,tensile strength and normal-to-shear stiffness ratio control the elastic behaviour and stable crack growth,(2)microparameters cohesion and friction angles present a negative linear correlation that controls the axial strain and lateral strain prior to the peak stress,and(3)microparameters friction coefficient controls shear crack friction and slip mainly refers to the unstable crack behaviour.Consideration of more macroparameters to regulate the material mechanical behaviour that is dominated by shear crack and slip motion is highlighted for future study.The DE calibration method is expected to serve as an alternative method to calibrate the DEM cohesive granular material to its peak strength.展开更多
文摘The prediction of flow distribution in flow manifolds is important for the design of heat exchangers. The pressure drop along the flow in the header is the most influential factor in flow distribution. Various continuous models available in literature have failed to satisfactorily predict the pressure distribution in the headers of the flow manifolds. In this article, a discrete model matching the real physical phenomena has been proposed, to predict the pressure distribution in headers. An experimental evaluation of relevant flow characteristic parameters has been carried out to support the discrete model calculations. The validity of the theoretical discrete model has been performed with experimental results, under specific conditions. Refined experimental probes, for pressure heads with ultrasonic measuring devices, have been used to obtain accurate results. The experimental results fully substantiate the soundness of the theoretical prediction. In addition, the advantage of the ability to accommodate local disturbances in the discrete model has been pointed out. The effect of some local disturbances may be substantial. As a result of the analysis presented in this article, improved designs of flow manifolds in heat exchangers can be realized, to assure operation safety under severe operating conditions.
基金This work is supported financially by the National Natural Science Foundation of China, grant No. 21576265. Xiaoxing Liu acknowledges the financial support from the Hundred Talents Pro- gram of Chinese Academy of Sciences.
文摘Granular material discharge from a flat-bottomed silo has been simulated by using continuum modeling and a three-dimensional discrete-element method （DEM）. The predictive abilities of three commonly used frictional viscosity models （Schaeffer, S-S, and μ（I）） were evaluated by comparing them with the DEM data. The funnel-flow pattern （type C） and the semi-mass-flow pattern （type B） that was predicted by DEM simulations can be represented when the Schaeffer orμ（I） model is used, whereas the S-S model gives a consistent type-B flow pattern. All three models over-estimate the discharge rate compared with the DEM. The profiles of the solids volume fraction and the vertical velocity above the outlet show that the larger discharge rates given by the Schaeffer and μ（I） model result from an over-estimation of volume fraction, whereas the deviation in the S-S model stems from the failure to predict a solid vertical velocity and a volume fraction.
基金SUPPORTED BY NATIONAL NATURAL SCIENCE FOUNDATION OF CHINA ( NO. 50079016).
文摘The phenomenon of the soil plug usually rising inside the suction foundations during suction penetration was quantitatively described and predicted. The formation process of the soil plug was simulated and calculated by DEM （discrete element method） model. The seepage flow, the self-weight of soil, the friction on the chamber wall as well as the suction inside the chamber are considered as the main external forces in the process. The results are compared with a set of laboratory model tests performed by using three soil types （sand, silty clay and clay） in the Bohai Sea area. The heights of soil plug from numerical estimations are lower than those from model test results, mainly because the suction pressure and friction resistance are applied in an ideal way under the numerical simulation.
文摘The controlled and homogeneous flow of dry granular powders through hoppers is essential for applications,namely,packaging of food grains,fertilizers and additive manufacturing processes such as directed energy deposition for better product quality.One of the major issues encountered in the granular flows through hoppers is flow stagnation due to the well-known arching phenomenon.Vibration-assisted granular flow through hoppers is one of the mechanisms used for better mass flow control.In this work,the influence of external mechanical vibration on the powder flow is investigated experimentally and using discrete element simulations.First,the mass flow rate through the hopper increases with an increase in vibration amplitude and then decreases,signifying the existence of an optimal amplitude of vibration.The DEM simulations explained the underlying mechanisms for the existence of an optimal amplitude of vibration corresponding to the maximum mass flow rate.A range of vibration amplitudes from 0 mm to 3.5 mm is used to study the flow behaviour;the maximum flow of around 33 g/s to 35 g/s is observed for 0.75 mm to 1.25 mm vibration amplitude for the hopper-particle combination studied in this work.The work also reports the influence of vibration frequency,hopper,and particle dimension on the flow characteristics.The research facilitates the effective use of mechanical vibration to enhance powder flow that can further be extended to non-spherical and multi-material particles.
文摘This paper analyses three popular methods simulating granular flow at different time and length scales： discrete element method （DEM）, averaging method and viscous, elastic-plastic continuum model. The theoretical models of these methods and their applications to hopper flows are discussed. It is shown that DEM is an effective method to study the fundamentals of granular flow at a particle or microscopic scale. By use of the continuum approach, granular flow can also be described at a continuum or macroscopic scale. Macroscopic quantities such as velocity and stress can be obtained by use of such computational method as FEM. However, this approach depends on the constitutive relationship of materials and ignores the effect of microscopic structure of granular flow. The combined approach of DEM and averaging method can overcome this problem. The approach takes into account the discrete nature of granular materials and does not require any global assumption and thus allows a better understanding of the fundamental mechanisms of granular flow. However, it is difficult to adapt this approach to process modelling because of the limited number of particles which can be handled with the present computational capacity, and the difficulty in handling non-spherical particles. Further work is needed to develoo an aoorooriate aooroach to overcome these problems.
基金We would like to acknowledge the finical support by the Key Laboratory of Agro-Products Postharvest Handling,Ministry of Agriculture support(Grant No.KLAPPH2-2017-04).
文摘Grains are widely present in industrial productions and processing,and are stored in silos.In the silo,auxiliary structures are added to achieve efficient production.However,little effort has been devoted to the influence of the internal structure of the silo on the granular flow.In this work,a silo with a central decompression tube is studied through experimental measurements and discrete element methods.Then,the influences of the central decompression tube on the flow behavior of grains and wall pressure are analyzed.Results show that the grains are in mass flow in the silo without a central decompression tube,while the grains are in funnel flow in the silo with a central decompression tube.Moreover,regardless of whether there is a central decompression tube in the silo,the maximum pressure appears at the top of the conical silo.In the lower part of the silo,the wall pressure of the silo with a central decompression tube is lower than that of the silo without a central decompression tube.Therefore,a silo with a central decompression tube is more conducive to grain storage and discharge than a silo without a central decompression tube.
文摘The differential evolution(DE)algorithm was deployed to calibrate microparameters of the DEM cohesive granular material.4 macroparameters,namely,uniaxial compressive strength,direct tensile strength,Young’s modulus and Poisson’s ratio,can be calibrated to high accuracy.The best calibration accuracy could reach the sum of relative errors RE_(sum)<0.1%.Most calibrations can be achieved with RE_(sum)<5%within hours or RE_(sum)<1%within 2 days.Based on the calibrated results,microparameters uniqueness analysis was carried out to reveal the correlation between microparameters and the macroscopic mechanical behaviour of material:(1)microparameters effective modulus,tensile strength and normal-to-shear stiffness ratio control the elastic behaviour and stable crack growth,(2)microparameters cohesion and friction angles present a negative linear correlation that controls the axial strain and lateral strain prior to the peak stress,and(3)microparameters friction coefficient controls shear crack friction and slip mainly refers to the unstable crack behaviour.Consideration of more macroparameters to regulate the material mechanical behaviour that is dominated by shear crack and slip motion is highlighted for future study.The DE calibration method is expected to serve as an alternative method to calibrate the DEM cohesive granular material to its peak strength.
