Radial mixing and segregation of granular bed bi-dispersed both in particle size and density within horizontal rotating drum

Taking simultaneous variations in both particle volume and density into account, the radial mixing and segregation of binary granular bed in a rotating drum half loaded were investigated by a 3D discrete element method. Then, based on the competition theory of condensation and percolation, radial segregation due to differences in particle volume and/or density was analyzed. The results show that if either percolation effect induced by volume difference or condensation effect induced by density difference dominates in the active layer of moving bed, separation will occur. Controlling the volume ratio or density ratio of the two types of particles can achieve an equilibrium state between percolation and condensation, and then homogenous mixture can be obtained. When the percolation balances with the condensation, the relationship between volume ratioand density ratiopresents nearly a power function. Scaling up a rotating drum will not affect the mixing degree of the granular bed so long as the volume ratio and density ratio are predefined.

Performance of rotating drum biofilter for volatile organic compound removal at high organic loading rates

Uneven distribution of volatile organic compounds （VOCs） and biomass, and excess biomass accumulation in some biofilters hinder the application of biofiltration technology. An innovative multilayer rotating drum biofilter （RDB） was developed to correct these problems. The RDB was operated at an empty bed contact time （EBCT） of 30 s and a rotational rate of 1.0 r/min. Diethyl ether was chosen as the model VOC. Performance of the RDB was evaluated at organic loading rates of 32,1, 64.2, 128, and 256 g ether/（m^3·h） （16.06 g ether/（m^3·h） ≈ 1.0 kg chemical oxygen demand （COD）/（m^3·d））. The EBCT and organic loading rates were recorded on the basis of the medium volume. Results show that the ether removal efficiency decreased with an increased VOC loading rate. Ether removal efficiencies exceeding 99% were achieved without biomass control even at a high VOC loading rate of 128 g ether/（m^3·h）. However, when the VOC loading rate was increased to 256 g ether/（m^3·h）, the average removal efficiency dropped to 43%. Nutrient limitation possibly contributed to the drop in ether removal efficiency. High biomass accumulation rate was also observed in the medium at the two higher ether loading rates, and removal of the excess biomass in the media was necessary to maintain stable performance. This work showed that the RDB is effective in the removal of diethyl ether from waste gas streams even at high organic loading rates. The results might help establish criteria for designing and operating RDBs.

Effect of moving baffle on average velocity and mixing of binary particles in rotating drums

Adding a moving baffle to the drum is a new way to enhance the motion and mixing of particles in rotating drums.To obtain its influence on binary particles,horizontal rotating drums provided with a moving baffle were investigated by discrete element method(DEM).AtΩ=15 r/min,increasing the length of moving baffle can increase the fluctuation amplitude of average particle velocity.AtΩ=60 r/min,the influence of the moving baffle on the average velocity fluctuation tends to be more random.At both rotational speeds,the moving baffle causes the average particle velocity to fluctuate more sharply.The moving baffle can enhance particle mixing.AtΩ=15 r/min,the moving baffle with length ofδ=1/3 can best enhance particle mixing.However,atΩ=60 r/min,only the moving baffle with a specific length(δ=1/4)can enhance mixing.This basic research has a positive reference value for the application of the moving baffle in industry.

Investigation of solids segregation of binary mixtures in a rotating drum at various Froude numbers

To investigate the effect of the Froude number(Fr)on solid segregation in a rotating drum,a two dimensional mathematical modelling on solids behaviour in horizontally oriented rotating drums operated in rolling,cascading and cataracting modes has been carried out by using Euler-Euler multi-fluid model in Fluent6.2 environment.Small particles and big particles are used in the work as binary mixtures to investigate segregation characteristics.The effect of Froude number(rotating velocity)on the flow field is investigated.It is found that the model captures the main features of solids motion and segregation in the drum and numerical results agree well with limited experimental data for solid velocity.

Particle segregation mechanism of S–D system in a rotating drum

Complex segregation occurs in a binary particle system with differing particle sizes and densities,particularly when the larger particles are heavier(S–D system,i.e.,size minus density system).Predicting the segregation pattern driven by multiple mechanisms simultaneously is often challenging.This study explores the segregation mechanisms in a quasi-2D circular drum containing a S–D system,realizing a transition between the S-core and Core-and-band patterns by adjusting the drum rotation speed.During the transition of the segregation pattern,only the S-core pattern chiefly driven by the percolation mechanism is initially observed.As the rotation speed increases,the buoyancy mechanism and particle diffusion gradually strengthen,jointly driving the formation of the Core-and-band pattern.A dimensionless strength ratio,λ=H/h,where H and h respectively represent the diffusion and buoyancy strengths at length scales,is introduced to elucidate this transition.The Core-and-band pattern emerges whenλreached 1.4.

Multi-level coarse-grained discrete element method modeling of cylinder particle flow in a rotating drum

The coarse-grained discrete element method(DEM)is probably a feasible option for simulating an actual drum-type biomass boiler,which contains over 10 million cylinder particles.A multi-level study was conducted based on particle and coarse-grained level data to evaluate the adequacy of the coarse-grained approach in terms of geometrical characteristics,kinematic features,and dynamic properties.Two scaling laws for contact parameters were used and compared during the simulations.The results show that the coarse-grained approach can accurately predict the positions of the free surface and active-passive interface,the mixing index,and the orientation properties.Deviations in the velocity fields may occur due to the worse flowability of coarse-grained particles near the free surface.The efficiency is significantly improved by the coarse-grained model compared with the corresponding original case(the same DEM code without a coarse-grained model was used for the original simulations).

The effect of side walls on particles mixing in rotating drums

In this paper, we study the effects of the presence and shape of side walls and of the overall length of rotating cylindrical drums on the mixing of particles with differing sizes by application of the discrete element method (DEM). By varying the semi-axis of the spheroidally shaped side walls and the length of the overall drum, we observe the formation of circulation patterns near the side walls. Although there is a vast amount of literature studying mixing regimes in rotating drums, little is known about the effect of the side walls of the drum on particle mixing. The results of our study demonstrate that introducing curved side walls induces a strong circulation pattern near these side walls, but has, paradoxically, a negative impact on mixing and actually promotes segregation. The cause for this segregation is the difference in velocity of differently sized particles near the curved side walls. Large particles accumulate at the curved side walls, whereas small particles move away from the curved side walls. When the length of the drum is increased, the overall effect of the side walls is decreased, although it does remain observable, even in very large drums.

Prediction of segregation characterization based on granular velocity and concentration in rotating drum

In a binary granular system composed of two types of particles with different granule sizes and the same density,particle sorting occurs easily during the flow process.The segregation pattern structure is mainly affected by the granular velocity and granular concentration in the flow layer.This paper reports on the experimental velocity and concentration measurement results for spherical particles in a quasi-two-dimensional rotating drum.The relationship between the granular velocity along the depth direction of the flow layer and granular concentration was established to characterize structures with different degrees of segregation.The corresponding relationships between the granular velocity and concentration and the segregation pattern were further analyzed to improve the theoretical models of segregation(convection-diffusion model and continuous flow model)and provide a reference for granular segregation control in the production process.

Computational fluid dynamics simulations of aerosol behavior in a high-speed (Heubach) rotating drum dustiness tester

Potential exposure from hazardous dust may be assessed by evaluating the dustiness of the powders being handled.Dustiness is the tendency of a powder to aerosolize with a given input of energy.Previously we used computational fluid dynamics(CFD)to numerically investigate the flow inside the European Standard(EN15051)rotating drum dustiness tester during its operation.The present work extends those CFD studies to the widely used Heubach rotating drum.Air flow characteristics are investigated within the Abe-Kondoh-Nagano k-epsilon turbulence model;the aerosol is incorporated via a Euler-Lagrangian multiphase approach.The air flow inside these drums consists of a well-defined axial jet penetrating relatively quiescent air.The spreading of the Heubach jet results in a fraction of the jet recirculating as back-flow along the drum walls;at high rotation rates,the axial jet becomes unstable.This flow behavior qualitatively differs from the stable EN15051 flow pattern.The aerodynamic instability promotes efficient mixing within the Heubach drum,resulting in higher particle capture efficiencies for particle sizes d<80μm.

Granular avalanche statistics in rotating drum with varied particle roughness

We experimentally investigate the avalanche statistics of dry granular materials in a slowly rotating drum for five types of beads with varied surface roughness.For all beads,two distinct angles,i.e.,repose angleθr and maximal angleθm,can be clearly defined,and the avalanche size distributions P(δθ)are Gaussian-like.θr,θm,and the span in P(δθ)are all positively correlated with bead surface roughness.This observation thus contrasts with a power-law P(δθ)predicted by self-organized criticality,but is reminiscent of a first-order phase transition.We speculate that both the inertia effect and the velocity-weakening mechanism during an avalanche process can enhance the first-order features,which are however absent in plasticity of sheared amorphous solids.We also discuss the dependence betweenθr andθm for various particles,as well as the correlation between starting and stopping angles for an individual avalanche.

2D DEM simulation of particle mixing in rotating drum: A parametric study

Mixing behaviors of equal-sized glass beads in a rotating drum were investigated by both DEM simula- tions and experiments. The experiments indicated that higher rotation speed can significantly enhance mixing. The particle profiles predicted by 2D DEM simulation were compared with the experimental results from a quasi-2D drum, showing inconsistency due to reduction of contacts in the single-layer 2D simulation which makes the driving friction weaker than that in the quasi-2D test, better results could be reached by specifying a higher frictional coefficient between the particles and the cylinder wall. In order to explore the influences of physical properties （density, size or friction） on mixing behavior, numerical 2D simulations were carried out systematically, in which one examined specific property being examined was exaggerated while the others were kept the same as that in the control group. The DEM simulations reveal that particle density and size are the dominating factors affecting mixing behaviors, while the effect of frictional coefficient is less significant. However, segregation due to any of the factors can be diminished by specifying a proper particle size distribution （multi-size with lower size ratio）. 2009 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Numerical investigation of granular flow similarity in rotating drums

The theory of flow similarity has not been well established for granular flows, in contrast to the case for conventional fluids, owing to a lack of reliable and general constitutive laws for their continuum descrip- tion. A rigorous investigation of the similarity of velocity fields in different granular systems would he valuable to theoretical studies. However, experimental measurements face technological and physical problems. Numerical simulations that employ the discrete element method （DEM） may be an alterna- tive to experiments by providing similar results, where quantitative analysis could be implemented with virtually no limitation. In this study, the similarity of velocity fields is investigated for the rolling regime of rotating drums by conducting simulations based on the DEM and using graphics processing units. For a constant Froude number, it is found that the particle-to-drum size ratio plays a dominant role in the determination of the velocity field, while the velocity field is much more sensitive to some material properties than to others. The implications of these findings are discussed in terms of establishing theoretical similarity laws for granular flows.

Numerical and experimental study of radial segregation of bi-disperse particles in a quasi-two-dimensional horizontal rotating drum

Radial segregation easily occurs in a horizontal rotating drum partially filled with particles of different properties under various operational conditions.DEM(discrete element method)simulations and experiments were combined together to investigate the segregation of bi-disperse particles of the same density but unequal sizes in a quasi-two-dimensional horizontal rotating drum.A linear spring-dashpot model was adopted in simulations.An easy and effective image analysis was conducted for the segregation/mixing of particles of different sizes.By comparing the repose angles,degrees of segregation,and particular phenomenon("sun pattern"and reverse segregation)in simulations under different operating conditions with those in experiments,the discrete-element model is verified.The effects of rotational speed and volume ratio on radial segregation are also considered systematically.From an analysis of the results of experiments and simulations,the degree of segregation generally decreases with increasing rotational speed,whereas the volume ratio shows different influences on segregation in different flow regimes.Moreover,the mechanism underlying the reverse segregation in the cataracting regime has been clarified as well.

Influence of the number of flights on the dilute phase ratio in flighted rotating drums by PTV measurements and DEM simulations

Particle distribution in the cross-section of the flighted rotating drum(FRD)is critical to the analysis of heat and mass transfer between gas and solids.In this work,the particle tracking velocimetry(PTV)method is applied to study the influence of the number of flights on the particle motion in FRDs.The drum,installed with 1,4,8,or 12 rectangular flights,is filled with plastic balls to 15%and operated at various rotating speeds ranging from 10 rpm to 30 rpm.The results show that the number of flights has different effects on the holdup ratio and cascading rate of single flight and active flights.With 8 and 12 flights,the FRD produces a larger and more stable particle ratio of the dilute phase.Moreover,DEM simulations agree with PTV measurements,whereas literature models show significant deviations.

Discrete element method study of shear-driven granular segregation in a slowly rotating horizontal drum

Segregation and mixing of granular materials are complex processes and are not fully understood. Motivated by industrial need, we performed a simulation using the discrete element method to study size segregation of a binary mixture of granular particles in a horizontal rotating drum. Particles of two dif- ferent sizes were poured into the drum until it was 50% full. Shear-driven segregation was induced by rotating the side-plates of the drum in the opposite direction to that of the cylindrical wall. We found that radial segregation diminished in these systems but did not completely vanish. In an ordinary rotating drum, a radial core of smaller particles is formed in the center of the drum, surrounded by larger revolving particles. In our system, however, the smaller particles were found to migrate toward the side-plates. The shear from anti-spinning side-plates reduces the voidage and increases the bulk density. As such, smaller particles in the mixer tend to move to denser regions. We varied the shear by changing the coefficient of friction on the side-plates to study the influence of shear rate on this migration. We also compared the extent of radial segregation with stationary side-plates and with side-plates moving in different angular directions.

Effect of drum structure on particle mixing behavior based on DEM method

The optimization of the drum structure is beneficial to improve the particle motion and mixing in rotary drums.In this work,two kinds of drum structures,Lacy cylinder drum(LC)and Lacy-lifters cylinder drum(LLC),are developed on the basic of cylinder drum to enhance the heat transfer area.The particle motion and mixing process are simulated by DEM method.Based on the grid independence and model validation,the contact number between particles and wall,particle velocity profile,thickness of active layer,particle exchange coefficient,particle concentration profile and mixing index are demonstrated.The influences of the drum structure and the operation parameters are further evaluated.The results show that the contact number between particles and wall is improved in LC and LLC compared to cylinder drum.The particle velocity in LC is higher than that in cylinder drum at high rotating speed,and the particle velocity of the particle falling region is significantly improved in LLC.Compared to cylinder drum and LC,the thickness of active layer in LLC is smaller,while the local particle mixing quality is proved to be the best in the active region.In addition,the particle exchange coefficients between static region and active region in the three drums are compared and LLC is found tending to weaken the particle flow.Besides,the fluctuations of particle concentration in the active region,static region,and boundary region are weakened in LLC,and the equilibrium state is reached earlier.In addition,the overall particle mixing performance in cylinder drum,LC and LLC is analyzed.The particle mixing performance in cylinder drum is the worst,while the difference in mixing quality of LC and LLC depends on the operation conditions.

Mixing and heat transfer of binary mixtures of monodispersed spherical particles with different densities and thermal diffusivities

Mixing and heat transfer among particles in rotating drums are widely applied in numerous industrial processes.Generally,mixing and heat transfer occur simultaneously.Consequently,the interrelationship between mixing and heat transfer must be investigated for industrial applications.In this study,the radial mixing and heat transfer of spherical granular materials(3 mm)with various properties in batch rotating drums are investigated numerically using the discrete element method.The evolution of mixing and heat transfer characteristics with rotation speed is analyzed from the perspective of time and number of revolutions,respectively.The results indicate that,depending on the physical parameters and thermal properties of particles,the mixing quality does not always accurately reflect the heat transfer effect.In binary granular beds of monodispersed spherical particles with different densities and thermal diffusivities,the main heat transfer mechanism is related to the ratio of the thermal conductivity of the particles to that of the fluid.

STUDY ON THE ANGLE OF REPOSE OF NONUNIFORM SEDIMENT

A conception of exposure degree is used in this article to account for the hiding and exposure mechanism of nonuniform sediment transport. The force arms of particles on the bed slope are assumed to be a function of the exposure degree, which is stochastically related to the size and gradation of bed materials. Based on this conception, the formula for the angle of repose of nonuniform sediment is developed. The angle of repose of nonuniform sediment is experimentally investigated in rotation drum at the rolling motion of sediment particles. The nonuniform sediment is gotten from the mixture of two kinds of uniform sediment with different weight ratios and two experimental plans that one for changing the weight ratio of two kinds of uniform sediment from 1：9 to 9：1 and the other changing the uniform sediment size and weight ratio are used. The effect of the rotation speed and the filling degree on the angle of repose is investigated, too. Finally, the formula has been tested against a wide range of laboratory data, the prediction by the newly proposed formulas is very good.