Particle Contact Time Research Articles

CFD‐DEM simulation of wall sheeting and particles charge in fluidized beds

AbstractThe effect of the electrostatic charge of particles was investigated in fluidized beds with conductive and nonconductive walls. For this purpose, changes in the wall surface charge density and particle contact time near the wall were examined. Computational fluid dynamics/discrete element method (CFD‐DEM) was utilized to simulate the accumulation of particle and wall‐induced electrostatic charges in a quasi‐two‐dimensional fluidized bed. Particles and walls were initially considered neutral and gained charge with time. The simulations showed that aggregated particles are formed due to the presence of the opposite electrostatic charges. Agglomerates are produced by joining the aggregated particles together. Agglomerates have a layered‐like structure and are made of positive and negative particles. Accumulation of these agglomerates and aggregated particles on the wall forms wall sheeting, which has a layered‐like structure. In other words, particles do not accumulate on the wall individually. A significant accumulation of particles near the wall was seen in the presence of electrostatic charges compared with the neutral bed. Also, the contact time of particles on the wall in the conductive case was more prolonged than in the nonconductive case. The results indicate that the CFD‐DEM approach is a helpful technique for predicting particles behaviour in the presence of the electrostatic charge.

Scaling law of contact time for particles settling in a quiescent fluid

Numerical simulations are conducted for the gravity–driven settling of two particles in a viscous quiescent fluid. A method is proposed to determine the scaling law of inter–particle contact time. The method is simple and general. The resulting scaling law explains the observations of literature and predicts the influences of particle–fluid density ratio, fluid viscosity, friction, rolling friction, and adhesion force.

Numerical Study of Heat Transfer in Gravity-Driven Particle Flow around Tubes with Different Shapes

In the present paper, the heat transfer of gravity-driven dense particle flow around five different shapes of tubes is numerically studied using discrete element method (DEM). The velocity vector, particle contact number, particle contact time and heat transfer coefficient of particle flow at different particle zones around the tube are carefully analyzed. The results show that the effect of tube shape on the particle flow at both upstream and downstream regions of different tubes are remarkable. A particle stagnation zone and particle cavity zone are formed at the upstream and downstream regions of all the tubes. Both the stagnation and cavity zones for the circular tube are the largest, and they are the smallest for the elliptical tube. As the particle outlet velocity (vout) changes from 0.5 mm/s to 8 mm/s at dp = 1.72 mm/s, when compared with the circular tube, the heat transfer coefficient of particle flow for the elliptical tube and flat elliptical tube can increase by 20.3% and 15.0% on average, respectively. The proper design of the downstream shape of the tube can improve the overall heat transfer performance more efficiently. The heat transfer coefficient will increase as particle diameter decreases.

Cessation of a dense granular flow down an inclined plane

The cessation of a dense granular flow down an inclined plane upon decrease in the angle of inclination is studied using particle-based simulations for the linear and Hertzian particle contact models for ordered and disordered flows. The nature of the flow is examined by progressively decreasing the angle of inclination by fractions of a degree, with the objective of examining the range of angles for which the hard-particle model can be used to describe the flow and the nature of the flow dynamics very close to cessation where the hard-particle approximation fails. For a disordered flow, when the angle inclination exceeds the angle for flow cessation by about 0.5 degrees for the linear contact model and about 1 degrees for the Hertzian model, the flow is well described by Bagnold rheology, and the Bagnold coefficients are independent of layer height and the particle stiffness, implying that the flow dynamics is well described by the hard-particle approximation. When the angle of inclination exceeds the angle for flow cessation by less than 0.5 degrees for the linear contact model and 1 degrees for the Hertzian contact model, the flow transitions into a layered state consisting of a faster shearing zone of height about 30 particle diameters atop a bottom slowly shearing zone. There are sinusoidal oscillations in the velocity of the center of mass of the flow, and the period of these oscillations is proportional to the characteristic time for particle interactions, indicating that the particle contact time does affect the dynamics of the layered flow. The flow evolution is qualitatively different for an ordered flow. In this case, there is an abrupt transition from a Bagnold flow to a plug flow with sliding at the base when the angle of inclination is decreased by 0.05 degrees. There is no discernible intermediate flow regime where the particle contact time becomes relevant. We also examine the deceleration of the flow when the angle of inclination is decreased from a flowing state to a final angle below the cessation angle. The initial decrease in the flow velocity is exponential for both contact models and for all final angles of inclination. This is followed by a more rapid decrease to the static state. The time constant for the initial decrease is significantly higher for an ordered flow in comparison to a disordered flow. The time constant is independent of the contact model and particle stiffness, and increases with height proportional to h(3/2), as expected for the hard-particle model.

Hydrodynamics and heat transfer around a horizontal tube immersed in a Geldart b bubbling fluidized bed

In dense gas–solid fluidized beds the heat transfer to immersed objects is strongly coupled to the hydrodynamic behavior. The objective of this study is to experimentally and numerically assess the heat transfer coefficient around a horizontal tube in a Geldart B bubbling fluidized bed and derive a numerical-correlative approach for predicting the angular dependent heat transfer coefficient. The considered system consists of corundum as the solid bed material and air as the fluidization gas, entering the cylindrical geometry through a Tuyere nozzle distributor. Experimental data are obtained from a pilot scale test-rig with different tubular heat transfer probes and evaluated in a comprehensive uncertainty analysis. The resulting magnitude and angle dependent variations of the heat transfer coefficient at different superficial gas velocities are compared to three dimensional numerical simulations. The applied CFD model of the fluidized bed treats both gas and powder as Eulerian phases. The size distribution of the particles is described by two granular phases with corresponding mean diameters and a sphericity factor to account for their non-spherical shape. The fluid–solid interactions in this Multi Fluid Model are considered by incorporating the Kinetic Theory of Granular Flow and a sphericity-adapted drag model. The hydrodynamics at the tube surface, e.g. solid volume fraction, gas and particle velocities, are used for a novel numerical-correlative calculation of the angle dependent heat transfer coefficient between the bed material and the immersed tube. Special focus is set on depicting the particle contact time at the tube surface appropriately. The numerical results show the correct tendency of an increasing heat transfer coefficient with rising gas velocity and are partially in good agreement with the experimental observations.

On stress relaxation timescales for dense binary particulate systems

Abstract We study contact stress relaxation timescales, especially the temporal correlation involved in dense binary particulate systems, which offers insight into the intriguing relationship between the contact stresses and the contact time of particle interactions under non-equilibrium state. The contact time (also referred to as contact age) of a pair of particles is defined by the duration between current time and the instant when the contact was formed. The interspecies inter-particles contact stresses are derived from Liouville's theorem. We apply particle dynamics methods (e.g. molecular dynamics, discrete element method) to simulate 3D dense binary particulate systems with periodic boundary conditions. External perturbation is exerted on the system to balance the dissipation of energy due to the viscoelastic collisions. The contact stresses, Reynolds stresses, and the probability density function of the contact time of particles are predicted at different volume fraction of particles. The obtained stress-strain rate data are used to examine the constitutive relation of macroscopic materials. The study targets the impact of the short-term and the long-term contact/collision on the contact stress relaxation. The simulation results reveal distinct effects of the short-term and the long-term contact/collision on the contact stresses, which have been treated by only an averaged expression of particle interactions in discrete element methods before.

The performance of pressure assisted casting process to improve the mechanical properties of Al-Si-Mg alloys matrix reinforced with coated B4C particles

Stringent requirements of material quality in automotive and aerospace industries have necessitated the development of lightweight aluminum alloy matrix composites. In this study, the Al–Si alloy matrix-reinforced B4C particles are fabricated by squeeze casting process. The inherent fast heat transfer of the squeeze casting process permits only very short liquid metal–particle contact time with a greatly reduced risk of particle–matrix interaction. This makes squeeze casting process ideal for the manufacture of B4C-reinforced Al matrix composites. TiB2 was coated on the surface of the B4C particles to have higher wettability with aluminum. Experimental results show strong adhesion at the particle/matrix interface which improves load transfer, increases the yield strength and stiffness, and delays the onsets of particle/matrix decohesion.

Improvements on a gravity-driven low-rate particle feeder

A gravity-driven particle feeder has been modified to achieve sustained operation at steady rates. Particle reservoirs and rod for controlling the nozzle opening are completely redesigned. Particle attrition and rod wobbling are the two main contributors to the feed instabilities. They, in turn, are affected by the height of the particle bed, particle contact time with the moving rod, strength of the magnetic field, and the weight, shape, and position of the rod in the magnetic field. A secondary reservoir minimizes the residence time of particles in the main reservoir. Its shape, orientation, and connection with the main reservoir have profound influences on the feeding stabilities. Tests have been conducted with particles of different types, sizes, and feed rates; results showed good long-term and short-term stabilities.

Statistics of particle interactions in dense granular material under uniaxial compression

Stress evolution in a dense granular material is closely related to interactions of contacting particles. We investigate statistics related to particle interactions and the relationship between the averaged local relative motion and the macroscopic motion. The validity of the Voigt and Reuss assumptions is examined, and extensions to these assumptions are proposed. Effects of history in the dense granular material are investigated. Statistical samples used in this paper are obtained using three-dimensional numerical simulations of dense granular media under uniaxial cyclical compression. The results show that stresses arise mostly from normal forces between particles, and direct contributions from frictional tangential forces between particles are small. Tangential friction, however, significantly increases the particle contact time, and thus reduces the rate of contact breakage. The contact breakage rate is demonstrated to be a stress relaxation rate. Therefore, stress increases significantly with friction between particles as a result of prolonged relaxation time.

Experimental investigation of particle contact time at the wall of gas fluidized beds

The contact time of particles at the walls of gas fluidized beds has been studied using a radioactive particle tracking technique to monitor the position of a radioactive tracer. The solids used were sand or FCC particles fluidized by air at room temperature and atmospheric pressure at various superficial velocities, covering both bubbling and turbulent regimes of fluidization. Based on the analysis of tracer positions, the motion of individual particles near the walls of the fluidized bed was studied. The contact time, contact distance and contact frequency of the particles at the wall were evaluated from these experimental data. It was found that in a bed of sand particles, the mean wall contact time of the fluidized bed of sand particles decreases by increasing the gas velocity in the bubbling and increases in the turbulent fluidization. In other words, the particle–wall contact time is minimum at the onset of turbulent fluidization in the bed of sand particles. However, the mean wall contact time is almost constant in both regimes of fluidization in the bed of FCC particles. All the existing models in the literature predict a decreasing contact time when the gas velocity in the bed is increased. It was also shown that the contact distance increases monotonously by increasing the gas velocity in the bed of sand particles, while it is almost constant for the bed of FCC particles. Contact frequency has a trend similar to that of the contact time for both sand and FCC particles.

Mechanistic study of defluidization by numerical simulation

Mechanisms of defluidization are investigated by discrete element method simulation. The particles used are 0.2, 0.5, 0.8, 1.2 and 1.5 mm in diameter and 2650 kg/m 3 in density. An artificial interparticle force is imposed on the particles in order to reproduce defluidization. The global bed behaviour is related to the mobility of individual particles, enabling a quantitative description of the degree of defluidization to be obtained. The effect of gas velocity on the degree of defluidization is analysed. New results of collision dynamics, such as particle contact time and particle speed, are obtained for conditions where interparticle cohesive force cannot be neglected. A new criterion for defluidization is developed based on analysis of normal forces acting on the particles.

Chlorination of dibenzofuran and dibenzo- p-dioxin vapor by copper (II) chloride

Dibenzofuran (DF) is formed from phenol and benzene in combustion gas exhaust streams prior to particle collection equipment. Subsequent chlorination at lower temperatures on particle surfaces is a potential source of chlorinated dibenzofuran (CDF). Gas streams containing 8% O 2 and approximately 0.1% DF vapor were passed through particle beds containing copper (II) chloride (0.5% Cu, mass) at temperatures ranging from 200 to 400 °C to investigate the potential for CDF formation during particle collection. Experiment duration was sufficient to provide an excess amount of DF (DF/Cu=3). The efficiency of DF chlorination by CuCl 2 and the distribution of CDF products were measured, with effects of temperature, gas velocity, and experiment duration assessed. Results of a more limited investigation of dibenzo- p-dioxin (DD) chlorination by CuCl 2 to form chlorinated DD (CDD) products are also presented. The efficiency of DF/DD chlorination by CuCl 2 was high, both in terms of CuCl 2 utilization and DF/DD conversion. Total yields of Cl on CDF/CDD products of up to 0.5 mole Cl per mole CuCl 2 were observed between 200 and 300 °C; this suggests that nearly 100% CuCl 2 was utilized, assuming a conversion of two moles of CuCl 2 to CuCl per mole Cl added to DD/DF. In a short duration experiment (DF/Cu=0.3), nearly 100% DF adsorption and conversion to CDF was achieved. The degree of CDF chlorination was strongly dependent on gas velocity. At high gas velocity, corresponding to a gas–particle contact time of 0.3 s, mono-CDF (MCDF) yield was largest, with yields decreasing with increasing CDF chlorination. At low gas velocity, corresponding to a gas–particle contact time of 5 s, octa-CDF yield was largest. DF/DD chlorination was strongly favored at lateral sites, with the predominant CDF/CDD isomers within each homologue group those containing Cl substituents at only the 2,3,7,8 positions. At the higher temperatures and lower gas velocities studied, however, broader isomer distributions, particularly of the less CDD/CDF products, were observed, likely due to preferential destruction of the 2,3,7,8 congeners.

Dioxin and furan formation on CuCl 2 from chlorinated phenols with one Ortho chlorine

The formation of chlorinated dibenzo-p-dioxins (CDDs) and dibenzofurans (CDFs) on CuCl2 particlesfrom four phenols with one ortho chlorine was studied in a flow reactor over the temperature range 300 to 450°C and contrasted with gas-phase results between 500 and 700°C. Heated nitrogen gas streams containing 7/8% oxygen, 1.7% benzene vapor, and equal amounts of 2-chlorophenol 2,3-dichlorophenol, 2,3,5-trichlorophenol, and 2,3,4,5-tetrachlorophenol vapor (450 ppmv, each) were passed through a particle bed containing 0.5% (mass) CuCl2: the nominal gas particle contact time was 0.3 s. CDD/F product yields of greater than 1% phenol conversion were observed between 350 and 425°C. Unlike gas-phase CDD/F formation, formation on CuCl2 favored CDDs over CDFs, and CDDs were formed predominantly with loss of one chlorine atom. While total CDD/F yields varied significantly with temperature, CDD/F isomer distributions did not. Thus, isomer patterns may provide a fingerprint for CDD/F formation from phenol precursors. Based on results of experiments with single phenol precursors, phenol precursors were assigned to all CDD/F products 2,3,4-Trichlorophenol was found to have the greatest propensity to form CDDs, whereas 2,3-dichlorophenol and 2,3,4,5-tetrachlorophenol were found to have the greatest propensity to form CDFs. Similar to gas-phase CDD/F formation, these results on CuCl2 suggest that chlorine substitution at phenol meta sites favors CDF formation, whereas chlorine substitution at ortho and para sites favors CDD formation.