Application of DEM modified with enlarged particle model to simulation of bead motion in a bead mill

We applied the discrete element method （DEM） of simulation modified by an enlarged particle model to simulate bead motion in a large bead mill. The stainless-steel bead mill has inner diameter of 102 mm and mill length of 198 mm. The bead diameter and filling ratio were fixed respectively at 0.5 mm and 85%. The agitator rotational speed was changed from 1863 to 3261 rpm. The bead motion was monitored experimentally using a high-speed video camera through a transparent mill body. For the simulation, enlarged particle sizes were set as 3-6 mm in diameter. With the DEM modified by the enlarged particle model, the motion of enlarged particles in a mill was simulated.The velocity data of the simulated enlarged particles were compared with those obtained in the experiment. The simulated velocity of the enlarged particles depends on the virtual frictional coefficient in the DEM model. The optimized value of the virtual frictional coefficient can be determined by considering the accumulated mean value. Results show that the velocity of the enlarged particles simulated increases with an increase in the optimum virtual frictional coefficient, but the simulated velocity agrees well with that determined experimentally by optimizing the virtual frictional coefficient in the simulation. The computing time in the simulation decreases with increased particle size.     

CFD–DEM simulation of the pneumatic conveying of fine particles through a horizontal slit

Fine particles play a significant role in many industrial processes. To study the dynamic behavior of fine particle and their deposition in rock fractures, the pneumatic conveying of fine particles (approximately 100 μm in diameter) through a small-scale horizontal slit (0.41 m × 0.025 m) was studied, which is useful for the sealing technology of underground gas drainage in coal mining production. The CFD–DEM method was adopted to model the gas-particle two-phase flow; the gas phase was treated as a continuum and modeled using computational fluid dynamics (CFD), particle motion and collisions were simulated using the DEM code. Then, the bulk movement of fine particles through a small-scale horizontal slit was explored numerically, and the flow patterns were further investigated by visual inspection. The simulation results indicated that stratified flow or dune flow can be observed at low gas velocities. For intermediate gas velocities, the flow patterns showed pulsation phenomena, and dune flow reappeared in the tail section. Moreover, periodic flow regimes with alternating thick and sparse stream structures were observed at a high gas velocity. The simulation results of the bulk movement of fine particles were in good agreement with the experimental findings, which were obtained by video-imaging experiments. Furthermore, the calculated pressure drop versus gas velocity profile was investigated and compared with relative experimental findings, and the results showed good agreement. Furthermore, the particle velocity vectors and voidage distribution were numerically simulated. Selected stimulation results are presented and provide a reference for the further study of fine particles.     

CFD-DEM simulation of spouting of corn-shaped particles

Three dimensionally coupled computational fluid dynamics (CFD) and discrete element method (DEM) were used to investigate the flow of corn-shaped particles in a cylindrical spouted bed with a conical base. The particle motion was modeled by the DEM, and the gas motion by the k-? two-equation turbulent model. A two-way coupling numerical iterative scheme was used to incorporate the effects of gas–particle interactions in terms of momentum exchange. The corn-shaped particles were constructed by a multi-sphere method. Drag force, contact force, Saffman lift force, Magnus lift force, and gravitational force acting on each individual particle were considered in establishing the mathematical modeling. Calculations were carried out in a cylindrical spouted bed with an inside diameter of 200 mm, a height of 700 mm, and a conical base of 60°. Comparison of simulations with experiments showed the availability of the multi-sphere method in simulating spouting action with corn-shaped particles, but it depended strongly on the number and the arrangement of the spherical elements. Gas–solid flow patterns, pressure drop, particle velocity and particle concentration at various spouting gas velocity were discussed. The results showed that particle velocity reaches a maximum at the axis and then decreases gradually along the radial direction in the whole bed. Particle concentration increases along the radial direction in the spout region but decreases in the fountain region, while it is nearly constant in the annulus region. Increasing spouting gas velocity leads to larger pressure drop, remarkably increased speed of particle moving upward or downward, but decreased particle concentration.     

Performance characteristics of a gas–liquid–solid airlift pump

Pump performance of a small airlift system for transporting solid particles is investigated experimentally. Six types of uprisers locally containing S-shaped portions are used to investigate the effect of local bends on the flow characteristics. The internal diameter of the upriser is 18 mm and its total height 3200 mm. Alumina particles of 3 mm diameter are used as solid particles. The relation between the discharged liquid flux and the flux of injected air are obtained, varying injected gas flux, particle flux, type of upriser, and submergence ratio. In addition, the particle motion in the upriser is investigated in detail by means of photographic observations. The physics of the phenomena is discussed from a practical viewpoint.     

Numerical evaluation of turbulence models for dense to dilute gas–solid flows in vertical conveyor

A two-fluid model (TFM) of multiphase flows based on the kinetic theory and small frictional limit boundary condition of granular flow was used to study the behavior of dense to dilute gas–solid flows in vertical pneumatic conveyor. An axisymmetric 2-dimensional, vertical pipe with 5.6 m length and 0.01 m internal diameter was chosen as the computation domain, same to that used for experimentation in the literature. The chosen particles are spherical, of diameter 1.91 mm and density 2500 kg/m³. Turbulence interaction between the gas and particle phases was investigated by Simonin's and Ahmadi's models and their numerical results were validated for dilute to dense conveying of particles. Flow regimes transition and pressure drop were predicted. Voidage and velocity profiles of each phase were calculated in radial direction at different lengths of the conveying pipe. It was found that the voidage has a minimum, and gas and solid velocities have maximum values along the center line of the conveying pipe and pressure drop has a minimum value in transition from dense slugging to dilute stable flow regime. Slug length and pressure fluctuation reduction were predicted with increasing gas velocity, too. It is shown that solid phase turbulence plays a significant role in numerical prediction of hydrodynamics of conveyor and the capability of particles turbulence models depends on tuning parameters of slip-wall boundary condition.     

Flow of mono-dispersed particles through horizontal bend

Pipeline slurry flow of mono-dispersed particles through horizontal bend is numerically simulated by implementing Eulerian two-phase model in FLUENT software. A hexagonal shape and Cooper type non-uniform three-dimensional grid is chosen to discretize the entire computational domain, and a control volume finite difference method is used to solve the governing equations. The modeling results are compared with the experimental data collected in 53.0 mm diameter horizontal bend with radius of 148.4 mm for concentration profiles and pressure drops. Experiments are performed on narrow-sized silica sand with mean diameter of 450 μm and for flow velocity up to 3.56 m/s (namely, 1.78, 2.67 and 3.56 m/s) and four efflux concentrations up to 16.28% (namely, 0%, 3.94%, 8.82% and 16.28%) by volume for each velocity. Eulerian model gives fairly accurate predictions for both the pressure drop and concentration profiles at all efflux concentrations and flow velocities.     

Experimental investigation of turbulence modulation in particle-laden coaxial jets by Phase Doppler Anemometry

The effect of solid particles on the flow characteristics of axisymmetric turbulent coaxial jets for two flow conditions was studied. Simultaneous measurements of size and velocity distributions of continuous and dispersed phases in a two-phase flow are presented using a Phase Doppler Anemometry (PDA) technique. Spherical glass particles with a particle diameter range from 102 to 212 μm were used in this two-phase flow, the experimental results indicate a significant influence of the solid particles and the Re on the flow characteristics. The data show that the gas phase has lower mean velocity in the near-injector region and a higher mean velocity at the developed region. Near the injector at low Reynolds number (Re = 2839) the presence of the particles dampens the gas-phase turbulence, while at higher Reynolds number (Re = 11 893) the gas-phase turbulence and the velocity fluctuation of particle-laden jets are increased. The particle velocity at higher Reynolds number (Re = 11 893) and is lower at lower Reynolds number (Re = 2839). The slip velocity between particles and gas phase existed over the flow domain was examined. More importantly, the present experiment results suggest that, consideration of the gas characteristic length scales is insufficient to predict gas-phase turbulence modulation in gas-particle flows.     

Effect of particle size distribution on pressure drop and concentration profile in pipeline flow of highly concentrated slurry

The experiments were conducted in 54.9 mm diameter horizontal pipe on two sizes of glass beads of which mean diameter and geometric standard deviation are 440 μm & 1.2 and 125 μm & 1.15, respectively, and a mixture of the two sizes in equal fraction by mass. Flow velocity was up to 5 m/s and overall concentration up to 50% by volume for each velocity. Pressure drop and concentration profiles were measured. The profiles were obtained traversing isokinetic sampling probes in the horizontal, 45° inclined and vertical planes including the pipe axis. Slurry samples of the mixture collected in the vertical plane were analyzed for concentration profiles of each particle batch constituting the mixture. It was found that the pressure drop is decreased for the mixture at high concentrations except 5 m/s and a distinct change of concentration profiles was observed for 440 μm particles indicating a sliding bed regime, while the profiles in the horizontal plane remains almost constant irrespective of flow velocity, overall concentration and slurry type.     

Discrete element method simulations of Mars Exploration Rover wheel performance

Mars Exploration Rovers (MERs) experienced mobility problems during traverses. Three-dimensional discrete element method (DEM) simulations of MER wheel mobility tests for wheel slips of i = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 0.99 were done to examine high wheel slip mobility to improve the ARTEMIS MER traverse planning tool. Simulations of wheel drawbar pull and sinkage MIT data for i ⩽ 0.5 were used to determine DEM particle packing density (0.62) and contact friction (0.8) to represent the simulant used in mobility tests. The DEM simulations are in good agreement with MIT data for i = 0.5 and 0.7, with reasonable but less agreement at lower wheel slip. Three mobility stages include low slip (i < 0.3) controlled by soil strength, intermediate slip (i ∼ 0.3–0.6) controlled by residual soil strength, and high slip (i > 0.6) controlled by residual soil strength and wheel sinkage depth. Equilibrium sinkage occurred for i < 0.9, but continuously increased for i = 0.99. Improved DEM simulation accuracy of low-slip mobility can be achieved using polyhedral particles, rather than tri-sphere particles, to represent soil. The DEM simulations of MER wheel mobility can improve ARTEMIS accuracy.     

An experimental study of circular sand–water wall jets

Laboratory experiments were carried out to study the effects of sand particles on circular sand–water wall jets. Mean and turbulence characteristics of sand particles in the sand–water wall jets were measured for different sand concentrations c_o ranging from 0.5% to 2.5%. Effects of sand particle size on the centerline sand velocity of the jets were evaluated for sand size ranging from 0.21 mm to 0.54 mm. Interesting results with the range of measurements are presented in this paper. It was found that the centerline sand velocity of the wall jets with larger particle size were 15% higher than the jets with smaller particle size. Concentration profiles in the vertical direction showed a peak value at x/d = 5 (where x is the longitudinal distance from the nozzle and d is the nozzle diameter) and the sand concentration decreased linearly for x/d > 5. Experimental results showed that the turbulence level enhanced from the nozzle to x/d = 10. For sand–water wall jets with a higher concentration (c_o = 1.5–2.5%), the turbulence intensity became smaller than the corresponding single-phase wall jets by 34% due to turbulent modulation. A modified logarithmic formulation was introduced to model the longitudinal turbulent intensity at the centerline and along the axis of the jet.     

Heat transfer and fluid dynamics of air–water two-phase flow in micro-channels

Heat transfer, pressure drop, and void fraction were simultaneously measured for upward heated air–water non-boiling two-phase flow in 0.51 mm ID tube to investigate thermo–hydro dynamic characteristics of two-phase flow in micro-channels. At low liquid superficial velocity j_l frictional pressure drop agreed with Mishima–Hibiki’s correlation, whereas agreed with Chisholm–Laird’s correlation at relatively high j_l. Void fraction was lower than the homogeneous model and conventional empirical correlations. To interpret the decrease of void fraction with decrease of tube diameter, a relation among the void fraction, pressure gradient and tube diameter was derived. Heat transfer coefficient fairly agreed with the data for 1.03 and 2.01 mm ID tubes when j_l was relatively high. But it became lower than that for larger diameter tubes when j_l was low. Analogy between heat transfer and frictional pressure drop was proved to hold roughly for the two-phase flow in micro-channel. But satisfactory relation was not obtained under the condition of low liquid superficial velocity.     

Dynamics of heavy particles in two swirling jets

We report flow visualisations and laser Doppler anemometry (LDA) velocity measurements in the near field of two swirling jets. The Reynolds number based on jet diameter and bulk velocity at the nozzle exit is 1.4 × 10⁵. In the first jet, a small recirculation region is formed around the jet axis, while, in the second, the streamwise velocity remains positive and overshoots near the jet centre. In both cases, flow visualisations show that the vortex core of the jets is depleted of seeding particles. By using time-averaged distributions of the streamwise and tangential velocities measured at the nozzle outlet, the dynamics of the particles is simulated, by integrating their simplified equations of motion. The particles trajectory thus computed agrees well with that observed in the flow visualisations. Although the turbulence intensity is substantially different in the core of the two jets, its effect on the seeding concentration is localised near the edge of the core.     

UDV measurements of single bubble rising in a liquid metal Galinstan with a transverse magnetic field

Liquid metal is an important type of energy transport carrier in nuclear reactors, such as in accelerator-driven sub-critical systems, fusion reactors and spallation neutron source devices. It is necessary to conduct research for bubbles rising in a liquid metal under different magnetic field intensities. The Perspex container is positioned concentrically inside a transverse magnetic field, which provides a homogeneous DC longitudinal magnetic field that passes through the fluid district. The coils are supplied with maximum field strength of 1.97 T. The equivalent diameter of the bubble is 3.1–5.6 mm. The Ultrasonic Doppler Velocimetry (UDV) method is used to evaluate the internal flow velocity of opaque liquid metals. Research shows that the influence of the Lorenz force on the bubble ascension velocity is not simply positive or negative. The magnetic field inhibits the ascension velocity of small bubbles with diameters of 3.1 mm and 3.4 mm. The terminal velocity for large bubbles with diameters of 4.57 mm, 5.15 mm and 5.6 mm is higher under a weak magnetic field than without a magnetic field. The positive effect happens under strong magnetic intensity. The target is to obtain the hydro-dynamical relationships between the terminal velocity, drag coefficient, the Eötvös number, Reynolds number, and Stuart number in a strong magnetic field using a multiple regression method to reveal that the mechanism of the induced current's restraining influence determines the ascension velocity of the bubble in viscous electric liquids with a strong magnetic field.     

Dynamics of disk-like particles in turbulent vertical channel flow

The dynamical behavior of inertial disk-like particles in turbulent vertical channel flow is investigated by an Eulerian–Lagrangian point-particle approach. Gravity effects on distribution, translation, rotation and orientation statistics of non-spherical particles modeled as oblate spheroids have been studied both in an upward and a downward flow and compared with results obtained in the absence of gravity. Altogether 12 different particle classes have been studied, with inertia and shape parameterized by means of Stokes number St and aspect ratio λ ≤ 1. The St = 5 disk-like particles distribute more evenly across the channel in upward than in downward flow. The gravity effect on the particle concentration diminishes with large inertia and the spheroid shape has only a modest influence. Although the gravity significantly affects the streamwise and wall-normal mean slip velocities with increasing inertia, the particle shape rarely has any impact on the translational motion, except for the mean wall-normal velocity. The fluctuations of the velocity of disk-like particles are mainly ascribed to inertia, whereas the gravity and shape only have marginal effects. The presence of gravity is moreover found to have a negligible effect on the particles’ orientation and rotation, in spite of the striking effect of λ on the orientation and rotation seen in the near-wall region. The tendency of the disks to align their symmetry axis orthogonal to the fluid vorticity in the channel center is stronger for particles with modest inertia. In the near-wall region, however, oblate spheroids preferentially align with the fluid vorticity for St >> 1. The observed behavior is believed to be caused by the influence of the gravity force on the turbophoresis; i.e. that inertial particles move towards low-turbulence regions.     

One-dimensional interfacial area transport of vertical upward bubbly flow in narrow rectangular channel

The design and safety analysis for miniature heat exchangers, the cooling system of high performance microelectronics, research nuclear reactors, fusion reactors and the cooling system of the spallation neutron source targets requires the knowledge of the gas–liquid two-phase flow in a narrow rectangular channel. In this study, flow measurements of vertical upward air–water flows in a narrow rectangular channel with the gap of 0.993 mm and the width of 40.0 mm were performed at seven axial locations by using the imaging processing technique. The local frictional pressure loss gradients were also measured by a differential pressure cell. In the experiment, the superficial liquid velocity and the void fraction ranged from 0.214 m/s to 2.08 m/s and from 3.92% to 42.6%, respectively. The developing two-phase flow was characterized by the significant axial changes of the local flow parameters due to the bubble coalescence and breakup in the tested flow conditions. The existing two-phase frictional multiplier correlations such as Chisholm, 1967, Mishima et al., 1993 and Lee and Lee (2001) were verified to give a good prediction for the measured two-phase frictional multiplier. The predictions of the drift-flux model with the rectangular channel distribution parameter correlation of Ishii (1977) and several existing drift velocity correlations of Ishii, 1977, Hibiki and Ishii, 2003 and Jones and Zuber (1979) agreed well with the measured void fractions and gas velocities. The interfacial area concentration (IAC) model of Hibiki and Ishii (2002) was modified by taking the channel width as the system length scale and the modified IAC model could predict the IAC and Sauter mean diameter acceptably.     

Graphite dust resuspension in an HTR-10 steam generator

Graphite dust has an important effect on the safety of high-temperature gas-cooled reactors (HTR). The flow field in the steam generator was studied by the computational fluid dynamics (CFD) method, with the results indicating that the friction velocity in the windward and the leeward of the heat transfer tubes is relatively low and is higher at the sides. Further analysis of the resuspension of graphite dust indicates that the resuspension fraction reaches nearly zero for particles with a diameter less than 1 μm, whereas it will increases as the helium velocity in the steam generator increases for particle size larger than 1 μm. Moreover, the resuspension fraction increases as the particle size increases. The results also indicate that resuspension of the particles with sizes larger than 1 μm exhibited obvious differences in different parts of the steam generator.     

Effects of various parameters on the attrition of bed material in a recirculating fluidized bed with a draft tube

We performed an experimental study to investigate the effects of various parameters on the attrition of bed material and its size distribution with increasing operation time in a recirculating fluidized bed (RCFB). The studied parameters included superficial velocity of fluidizing air, bed inventory, and spacing between the jet top and draft tube bottom (spacer height). The bed material was prepared from Indian Standard (IS) Grade I sand from sieves with a size range of 2.20–1.00 mm. Experiments were performed at ambient conditions, with the superficial air velocity ranging from 7.13–9.16 m/s, a bed inventory of 7–10 kg, spacing of 0.085 and 0.045 m between the jet top and draft tube bottom, and an operating time of 40 h. We investigated the influence of these parameters in terms of changes in the size distribution of particles, changes in the %-weight of particles of different size ranges, generation of particles with smaller diameters, the decrease of the downcomer bed height, variations in the coefficient of uniformity and coefficient of curvature, and material loss from entrainment of fines with increasing operation time. The mode of attrition was abrasion in all experiments. We found that with increasing operation time and other parameters (bed inventory, superficial air velocity, and spacer height) attrition of the bed material also increased. Generation and elutriation of fines were more pronounced at higher superficial air velocity, bed inventory, and spacer height.     

Experimental study of single bubble motion in a liquid metal column exposed to a DC magnetic field

The motion of single Argon bubbles rising in the eutectic alloy GaInSn under the influence of a DC longitudinal magnetic field (parallel to the direction of bubble motion) was examined. The magnetic field strength was varied up to 0.3 T corresponding to a magnetic interaction parameter N (which measures the ratio of electromagnetic forces to inertial forces) slightly greater than 1. The liquid metal was at rest in a cylindrical container. Bubble and liquid velocities were measured using ultrasound Doppler velocimetry (UDV). The measured bubble terminal velocity showed oscillations indicating a zigzag movement of ellipsoidal bubbles. For small bubbles (d_e ⩽ 4.6 mm) an increase of the drag coefficient with increasing magnetic interaction parameter N was observed, whereas for larger bubbles (d_e ⩾ 5.4 mm) the application of the magnetic field reduces the drag coefficient. The measurements revealed a distinct electromagnetic damping of the bubble induced liquid velocity leading to more rectilinear bubble trajectories when the magnetic field is applied. Moreover, significant modifications of the bubble wake structure were observed. Raising of the magnetic field strength caused an enlargement of the eddies in the wake. The Strouhal number decreases with increasing magnetic interaction parameter N.     

Magnetic resonance studies of jets in a gas–solid fluidised bed

Magnetic resonance imaging (MRI) has been used to study the behaviour of jets at the distributor of a 50 mm diameter fluidised bed of 0.5 mm diameter poppy seeds. Two perforated-plate distributors were examined, containing either 10 or 14 holes, each 1 mm diameter. Ultra-fast MR imaging was able to show the transient nature of the upper parts of the jets, where discrete bubbles are formed. Imaging in 3D showed that the central jets were the longest for flow rates below minimum fluidisation. Above minimum fluidisation, the outer jets, nearest the wall of the fluidised bed, arched inward towards the central axis. In this latter case, interpretation of the time-averaged 3D image required the use of ultra-fast MR imaging to identify the approximate height above the distributor at which discrete bubbles were formed. The apparently continuous void extending along the central axis above this height in the time-averaged 3D image was thus identified, using ultra-fast MR imaging, as representing the averaged paths of released bubbles. Time-averaged MR velocity mapping was also used to identify dead zones of stationary particles resting on the distributor between the jets. The dead zones could be observed when the superficial velocity of the gas approached minimum fluidisation, but they were smaller than those observed at lower gas superficial velocity. Comparable images of a single jet through 1.2 mm diameter poppy seeds from MRI and electrical capacitance volume tomography (ECVT) are also demonstrated.     

Numerical research on micro diesel spray characteristics under ultra-high injection pressure by Large Eddy Simulation (LES)

The Large Eddy Simulation model was introduced to study the micro spray characteristics under ultra-high injection pressure (>220 MPa). EFS8400 spray test platform was set up to verify the accuracy of the numerical model. The mechanisms of micro spray characteristics were studied intensively under different injection pressures (180 MPa, 240 MPa) and nozzle diameters (0.1 mm, 0.16 mm). The results indicated that the micro turbulence vortex structures can be captured, especially in the liquid spray core area. Large Eddy Simulation model combined with the small grid size of 0.25 mm show a huge advantage in studying the micro spray characteristics under ultra-high injection pressure; The turbulence vorticity and spray velocity for injection pressure of 240 MPa are more intensive than that of 180 MPa, and also the ultra-high injection pressure can contribute to strong turbulence disturbance between spray and surrounding air, which is helpful to improve the quality of spray; The spray velocity field extended wider for the diameter of 0.16 mm, and also the values of velocity in the spray center is higher than that of the diameter of 0.1 mm; The entrainment vortex appeared at the edge of the large velocity gradient between spray and surrounding air, and the higher velocity gradient for ultra-high injection pressure (240 MPa) between the spray and air is easier to increase the generation of entrainment vortex in the downstream of the spray, which can significantly increase the quality of spray and atomization.