Relationship between mass discharge rate and granular temperature of silo flow with variance of outlets

A lower granular temperature zone near the orifice in silos was found by speckle visibility spectroscopy technology.Surprisingly,the values of granular temperat...     

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.     

The experimental investigation of heat transport and water infiltration in granular packed bed due to supplied hot water from the top: Influence of supplied water flux, particle sizes and supplied water temperature

In the present study an experimental investigation of heat transport and water infiltration in granular packed bed (unsaturated porous media) due to supplied water flux is carried out. The study is focus on the one-dimensional flow in a vertical granular packed bed column assuming local thermal equilibrium between water and particles at any specific space. This experimental study described the dynamics of heat transport and water infiltration in various testing condition. Experimentally, the influences of particle sizes, supplied water flux and supplied water temperature on heat transport and water infiltration during unsaturated flow are clarified in details. The results showed that the granular packed bed with larger particle size results in faster infiltration rate and form a wider infiltration depth. Furthermore, the increase of the supplied water flux and supplied water temperature corresponds to faster infiltration rate, but the results not linearly related to the interference between the heat transport and hydrodynamics characteristics in granular packed bed.     

Penetration into a granular bed in the presence of upward gas flows

We present a numerical study on the penetration of spherical projectiles into a granular bed in the presence of upward gas flows. Due to the presence of interstitial fluid, the force chains between particles in the granular bed are weakened significantly, and this distinguishes the penetration behavior from that in the absence of fluid. An interesting phenomenon, namely granular jet, is observed during the penetration, and the mechanism for its formation and growth is attributed to the merging of granular vortices generated by the interaction between the intruder and primary particles. Moreover, both the final penetration depth and the maximum diameter of the crater are found to follow a power-law dependence with the impact velocity, and the maximum height reached by the granular jet tends to increase linearly as the impact velocity increases, agreeing well with the experimental results reported in the literature.     

煤仓内煤散料流动状态与力学行为影响因素

针对煤仓内煤散料流动问题及其力学行为,采用三维颗粒流模拟程序PFC3D建立了某型号煤仓与某种煤散料的离散元模型,简述了其力学模型与求解步骤,模拟分析了煤仓内煤散料卸料流动状态。通过分析水平向侧压力、颗粒速度场和接触力场,重点讨论了煤仓下部锥体内壁面摩擦系数、锥仓倾角和卸料口径等对煤散料颗粒流动状态和力学行为的影响。结果显示,深仓卸料流动为整体流动与中心流动混合状态,煤仓内壁摩擦系数、锥体倾角和卸料孔开口半径均对煤散料流动和水平侧压力有较大影响。     

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 different 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.     

Numerical Investigation and Feasibility Study of a PZT-driven Micro-valve Pulsed-jet Actuator

A micro-valve pulsed-jet vortex-generator driven by piezoelectric actuation was successfully modelled numerically to determine the feasibility of such a design. This includes: modelling the dynamic motion of a unimorph cantilever and the fluid-structure interaction occurring between the unimorph and the fluid flowing over such a structure; the unsteady developing channel flow that would occur through the outlet orifice was also modelled. The initial design was found to have several fundamental flaws that were shown to be easily remedied. The fluid-structure interaction was found to have a strong effect on the motion of the piezoelectric beam and therefore the performance of the pulsed-jet actuator. The response time of the actuator was found to be governed by the micro-valve opening rather than the time taken to establish the jet. However, the resistance of the pulsed-jet actuator was shown to be governed by the outlet orifice; it was an order of magnitude larger than the resistance of the micro-valve.     

Numerical simulation of fast granular flow facing obstacles on steep terrains

The interaction between fast shallow granular flow and obstacles on steep terrain is an important aspect of granular mechanics and defending against geological hazards. In this study, we used a depth-averaged model for granular flow facing obstacles on steep terrains in a bed-fitted coordinate system where the obstacle system is treated as a local bed deviation term. A second-order Riemann-free scheme is extended to compute the depth-averaged model with a wetting–drying technique, which is verified by several granular flow cases, such as aluminum bar collapse and granular flow runout on a steep slope. Numerical simulations were performed for the case of granular flow facing a (i) single hemispherical obstacle and (ii) system of three hemispherical obstacles to produce a dynamical process and deposit profile, and show good agreement with experimental results. Granular flow facing a single obstacle on a concave plane produces a detached shock wave that moves upstream and a tailing rapid transition zone that moves down, which will merge to form a new shock for deposition. Granular flows facing a three-hemisphere obstacle system produce a tailing rapid transition zone that moves downstream and a downstream wavy shock that results from the interaction of three bow shocks in front of each obstacle. The downstream wavy shock moves upstream and merges with the upstream transition zone to form a new curved shock, which later relaxes to a deposit owing to bed friction. These findings provide some supplemental understandings of flow structures of fast granular flow facing obstacles.     

Characteristics of R-123 two-phase flow through micro-scale short tube orifice for design of a small cooling system

We present a new discharge coefficient correction method for the orifice equation for R-123 two-phase flows. In this method, an evaporator is mounted after the orifice as a vapor refrigeration cycle, and the evaporator is used to measure the quality of downstream flow through the orifice. Quality is estimated from the measured temperature and pressure of the evaporator inlet and outlet, respectively, instead of by direct measurement of quality. The condition of upstream flow of the orifice is the liquid state at 3 bar and 60 °C. The liquid flow is changed to two-phase flow after passing through the orifice. Orifice diameters of 300, 350, 400, and 450 μm are used for the experiment, and the results are analyzed. Experiments are conducted for various conditions of flow rate between 20 and 70 ml/min and for cooling loads of 60, 80, and 100 W. The results show that the quality of flow downstream from the orifice can be calculated using the enthalpy difference between the inlet and outlet of the evaporator. An equation to determine the discharge coefficient is formulated as a function of quality. We expect that these results can be used to help design a small cooling system.     

Observations of “granular jump” in the pneumatic conveying system

This paper presents a preliminary study of a previously unreported phenomenon of the “gas driven granular jump”, observed in the gas–solids flow within the pneumatic conveying system. From the phenomenological point of view, it resembles the already known processes such as hydraulic jumps in shallow water or granular jumps in granular flows in chutes or avalanches (although it seems most appropriate to explain it by analogy to a propagating granular bore). Clearly, unlike in classical phenomena of this type, the flow itself is driven by the aerodynamic forces related to the gas flow and the behaviour of the front of the “jump” is modified significantly by their presence. A series of high-speed camera visualisations are presented, which focus on this unusual behaviour of the flow on the border-line between cluster and stratified flow regimes in a horizontal pipe. Some similarities are drawn between the observed phenomenon and the broader class of problems exhibiting transition between super- and sub-critical flows. The fluid dynamical aspects and possible mechanisms behind the new phenomenon are discussed and the results obtained are compared quantitatively with simple theoretical models.     

Dust distribution of solid and adhesive mixed dust in a granular bed filter

Granular bed filters can effectively filter adhesive dust in high-temperature flue gas. In this study, polyvinyl chloride (PVC) powder was used as adhesive dust, and the mixture of PVC and ash powder was used to simulate solid and adhesive mixed dust. The effects of gas temperature, velocity, and inlet adhesive dust mass content on dust distribution in granular bed (GBF) were discussed. Results show that the mixed dust mainly accumulates on the upper part of the granular bed, and the mass of the collected dust decreases exponentially from the upper layer to the bottom layer in the GBF. The adhesive dust content collected in each layer differs from that of the incoming dust, and their deviation varies approximately linearly along with the depth of the bed. The total dust distribution and adhesive dust content deviation are influenced by gas temperature and inlet adhesive dust content but independent of gas velocity. The correlations of dust distribution of solid and adhesive mixed dust are presented based on the experimental results.     

Primary wave transmission in systems of elastic rods with granular interfaces

We study analytically and numerically primary pulse transmission in one dimensional systems of identical linearly elastic non-dispersive rods separated by identical homogeneous granular layers composed of n beads. The beads interact elastically through a strongly (essentially) nonlinear Hertzian contact law. The main challenge in studying pulse transmission in such strongly nonlinear media is to analyze the ‘basic problem’, namely, the dynamical response of a single intermediate granular layer, confined from both ends by barely touching linear elastic rods subject to impulsive excitation of the left rod. The analysis of the basic problem is carried out under two basic assumptions; namely, of sufficiently small duration of the shock excitation applied to the first layer of the system, and of sufficiently small mass of each bead in the granular interface compared to the mass of each rod. In fact, the smallness of the mass of the bead defines the small parameter in the asymptotic analysis of this problem. Both assumptions are reasonable from the point of view of practical applications. In the analysis we focus only in primary pulse propagation, by neglecting secondary pulse reflections caused by wave scattering at each granular interface and considering only the transmission of the main (primary) pulse across the interface to the neighboring elastic rod. Two types of shock excitations are considered. The first corresponds to fixed time duration (but still much smaller compared to the characteristic time of pulse propagation through the length of each rod), whereas the second type corresponds to a pulse duration that depends on the small parameter of the problem. The influence of the number of beads of the granular interface on the primary wave transmission is studied, and it is shown that at granular interfaces with a relatively low number of beads fast time scale oscillations are excited with increasing amplitudes with increasing number of beads. For a larger number of beads, primary pulse transmission is by means of solitary wave trains resulting from the dispersion of the original shock pulse; in that case fast oscillations result due to interference phenomena caused by the scattering of the main pulse at the boundary of the interface. Considering a periodic system of rods we demonstrate significant reduction of the primary pulse when transmitted through a sequence of granular interfaces. This result highlights the efficacy of applying granular interfaces for passive shock mitigation in layered elastic media.     

Assessing continuum postulates in simulations of granular flow

Continuum mechanics relies on the fundamental notion of a mesoscopic volume “element” in which properties averaged over discrete particles obey deterministic relationships. Recent work on granular materials suggests that a continuum law may be inapplicable, revealing inhomogeneities at the particle level, such as force chains and slow cage breaking. Here, we analyze large-scale three-dimensional discrete-element method (DEM) simulations of different granular flows and show that an approximate “granular element” defined at the scale of observed dynamical correlations (roughly three to five particle diameters) has a reasonable continuum interpretation. By viewing all the simulations as an ensemble of granular elements which deform and move with the flow, we can track material evolution at a local level. Our results confirm some of the hypotheses of classical plasticity theory while contradicting others and suggest a subtle physical picture of granular failure, combining liquid-like dependence on deformation rate and solid-like dependence on strain. Our computational methods and results can be used to guide the development of more realistic continuum models, based on observed local relationships between average variables.     

Shock tube study of the dynamical behavior of granular materials

The purpose of the present study is to clarify both the compression phenomenon and the gas filtration effect that take place inside a granular medium when it is dynamically loaded by a shock wave. In order to measure the pore pressure and the total stress at different locations along the granular medium, pressure transducers were placed along the side-wall and at the end-wall of the shock tube test section, which was filled with the granular material. In order to elucidate the gas filtration effect, the results of two experiments with identical granular media but with and without filtration were compared. The gas filtration was eliminated by means of a thin plastic film, which was placed at the front edge of the granular medium. Based on this comparison quantitative information on the gas filtration and its role in the stress formation inside granular media of different material and length was obtained. Furthermore, curves of the dynamic compression and the Young moduli of the granular medium for the range of the operating conditions were reconstructed.     

Analysis of instantaneous dynamic states of vibrated granular materials

The behavior of granular materials subjected to continuous vertical vibrations is dependent on a variety of factors, including how energetically the containment vessel is shaken as well as particle properties. Motivation for the investigation reported here is based on phenomenon in which bulk solids attain an increase in density upon relaxation. The results of a detailed, discrete element study designed to examine the dynamic state of a granular material is presented, in which particles are represented as inelastic, frictional spheres. The phase in which the assembly finds itself immediately before vibrations are stopped is quantified by computing depth profiles of the translational energy ratio R in conjunction with profiles of solids fraction ν and granular temperature T. The use of particles that are more frictional tends to hinder or delay thermalization, while particle restitution coefficient plays a role when the flow is collision dominated. The structure before vibrations are applied plays an important role in determining the depth profiles and the phase pattern only at low accelerations. On the other hand, large accelerations can easily dislodge the poured configuration very quickly so that the initial condition is not major factor in the phase pattern.     

A numerical investigation into the behavior of ground-supported concrete silos filled with saturated solids

This paper introduces a finite-element solution for simulating the filling process of ground-supported concrete silos filled with saturated granular material. An elasto-plastic axisymmetric finite-element model is used to represent both the granular material and the concrete silo. The interaction between the two materials is modeled using interface elements to allow for relative movement. The filling process is idealized via a multi-stage numerical technique capable of representing both undrained and drained conditions for the granular material. The effects of the relative stiffness between the foundation and wall are examined, as are the boundary conditions at the top of the structure (the roof details).Depending on the drainage properties of the stored material, the effect of the filling process may be time-dependent. The excess pore water pressure resulting from the filling process may cause a substantial increase in the hoop stresses in the wall. The predicted internal forces may be influenced by the foundation rigidity, but not by the boundary condition at the top of the wall.The results of these analyses may be used to design experiments to evaluate existing silos, or to develop filling strategies to minimize loads on existing structures.     

Quasi-static simulations of compact polydisperse particle systems

The Discrete Element Method (DEM) was originally devised by Cundall and Strack (1979), as a technique to examine the micromechanics of granular media with the anticipation that this would lead to more physically reliable continuum theories to describe the quasi-static deformation of granular material such as sand. However, the methodology models the evolution of a system of particles as a dynamic process. Consequently there have been numerous publications of the application of DEM to an increasingly wider v...     

节流孔截面形状对静压干气密封稳态性能和压力波动特性影响

建立了不同形状节流孔出口截面静压干气密封的几何模型,采用湍流大涡模拟方法数值求解了小孔节流静压干气密封的瞬态流场和压力场,对比了节流孔出口开设圆角、倒角与经典小孔节流静压干气密封的开启力、泄漏率等稳态性能参数和节流孔出口附近的压力波动特性,以较大密封开启力和较小压力波动为目标,获得了节流孔出口倒角和圆角的优选值范围. 结果表明:相较于经典小孔节流静压干气密封,节流孔出口开设倒角或圆角能提高静压干气密封的开启力,显著降低节流孔出口压力波动,且当倒角为0.3~0.4 mm,圆角为0.1~0.3 mm时具有最佳效果.      

Translation and rotation of particles in different flow pattern areas of a silo

The present paper reports the results obtained for translational and rotational velocity profiles of spherical particles for the mixed flow in a conical silo. The discrete element method (DEM) based on Hertz-Mindlin (no slip) with RVD rolling friction contact model is used for simulations. Opposite correlations are found between translational and rotational velocities in different flow areas of the silo. In particular, the abrasion caused by rotation is dominant in the funnel flow area. In addition, increase of the mass flow rate of silo can effectively reduce the abrasion induced by rotation. This highlights that understanding of dynamic characteristics of particles is helpful for optimization of silos and reduction of granular material abrasion.