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Particle Velocity (particle + velocity)
Selected AbstractsSimultaneous Measurement of Particle Size and Particle Velocity by the Spatial Filtering Technique,PARTICLE & PARTICLE SYSTEMS CHARACTERIZATION, Issue 6 2002Dieter Petrak Abstract The objective of this study was to compare the measuring results of a fiber-optical probe based on a modified spatial filtering technique with given size distributions of different test powders and also with particle velocity values of laser Doppler measurements. Fiber-optical spatial filtering velocimetry was modified by fiber-optical spot scanning in order to determine simultaneously the size and the velocity of particles. The fiber-optical probe system can be used as an in-line measuring device for sizing of particles in different technical applications. Spherical test particles were narrow-sized glass beads in the range 30,100,,m and irregularly shaped test particles were limestone particles in the range 10,600,,m. Particles were dispersed by a brush disperser and the measurements were carried out at a fixed position in a free particle-laden air stream. Owing to the measurement of chord lengths and to the influence of diffraction and divergent angle, the probe results show differences from the given test particle sizes. Owing to the particle-probe collisions, the mean velocity determined by the probe is smaller than the laser Doppler mean velocity. [source] Soil creep and convex-upward velocity profiles: theoretical and experimental investigation of disturbance-driven sediment transport on hillslopesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 13 2004Joshua J. Roering Abstract The movement of unconsolidated materials near the Earth's surface is often driven by disturbances that occur at a range of spatial and temporal scales. The nature of these disturbances ranges from highly variable, such as tree turnover, to periodic and predictable, such as frost heave or creep. To explore the effect of probabilistic disturbances on surface processes, we formulated a granular creep model with analogy to rate process theory (RPT) used for chemical reactions. According to the theory, individual particles must be energized to a height greater than adjacent particles in order for grain dilation and transport to occur. The height of neighbouring particles (which is akin to activation energy in chemical reactions) varies with slope angle such that energy barriers get smaller in the downslope direction as slopes steepen. When slopes approach the friction-limited angle of repose, the height of energy barriers approaches zero and grains ,ow in the absence of disturbance. An exponential function is used to describe the probability distribution of particle excitation height although alternative distributions are possible. We tested model predictions of granular dynamics in an experimental sandpile. In the sandpile, acoustic energy serves as the disturbance agent such that grains dilate and shear in response. Particle velocities are controlled by the frequency of energy pulses that result in grain displacement. Using tracer particles, we observed a convex-upward velocity pro,le near the surface of the sandpile, consistent with predictions of our RPT-based velocity model. In addition, we depth-integrated the velocity model to predict how ,ux rates vary with inclination of the sandpile and observed non-linear ,ux,gradient curves consistent with model predictions. By varying the acoustic energy level in the experimental sandpile, we documented changes in the rate of grain movement; similar changes in modelled velocities were achieved by varying the exponent of the particle excitation probability distribution. The general agreement between observed and modelled granular behaviour in our simple laboratory sandpile supports the utility of RPT-based methods for modelling transport processes (e.g. soil creep, frost heave, and till deformation), thus enabling us to account for the probabilistic nature of disturbances that liberate sediment in natural landscapes. Copyright © 2004 John Wiley & Sons, Ltd. [source] The effect of mixer properties and fill level on granular flow in a bladed mixerAICHE JOURNAL, Issue 2 2010Brenda Remy Abstract The discrete element method was used to study the effect of mixer properties and fill level on the granular flow of monodisperse, cohesionless spheres in a bladed mixer. For fill levels just covering the span of the blades, a three-dimensional (3-D) recirculation zone develops in front of the blades, which promotes vertical and radial mixing. Increasing fill level reduces the size of the recirculation zone, decreases bed dilation and hinders particle diffusivities. However, above a critical fill level, the behavior of the particles within the span of the blade is found to be invariant of fill level. At low-fill levels, the pressure within the particle bed varies linearly with bed height and can be approximated by hydrostatics. At higher fill levels, a constant pressure region develops within the span of the blades due to the angled pitch of the blades. Cylinder wall friction is shown to significantly influence granular behavior in bladed mixers. At low-wall friction, the 3-D recirculation zone observed for high-wall friction conditions does not develop. High-wall friction leads to an increase in convective and diffusive particle mixing. Shear stresses are shown to be a function of wall friction. Blade position along the vertical axis is shown to influence flow patterns, granular temperature and stress. The effect of increasing the mixer diameter at a constant particle diameter was also studied. When the mixer diameter is larger than a critical size such that wall effects are minimized, the observed granular behavior follows simple scaling relations. Particle velocities and diffusivities scale linearly with mixer size and blade speed. Normal and shear stress profiles are found to scale linearly with the total weight of the particle bed. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Grain transport mechanics in shallow overland flowECOHYDROLOGY, Issue 3 2009S.N. Prasad Abstract A physical model based on continuum multiphase flow is described to represent saltating transport of grains in shallow overland flows. The two-phase continuum flow of water and sediment considers coupled StVenant-type equations. The interactive cumulative effect of grains is incorporated by a dispersive stress term. The mean fluid thrust on the particle in the saltation layer of grains is expressed in terms of a slip velocity. The continuum model leads to the unexpected, but an interesting result is that particle velocity increases with the solid concentration. This increase predicts monotonic behaviour leading to overestimates of particle velocity at higher sediment concentration. To improve the predictions, grain dynamic equations, which incorporate bed collision, are analysed. The analysis leads to an improved model for predicting saltation height. Incorporation of the results in the continuum model yields a velocity-concentration relationship that is consistent with experimental observations for increasing concentration. Laboratory flume experiments explore the evaluation of various parameters from the measured particle velocities by photonic probes. Copyright © 2009 John Wiley & Sons, Ltd. [source] Investigation of nonuniformity in a liquid,solid fluidized bed with identical parallel channelsAICHE JOURNAL, Issue 1 2010Long Fan Abstract Previous work has demonstrated that multiphase flow through identical parallel channels and multiple cyclones can give rise to significant nonuniformity among the flow paths. This article presents results from a study where the distribution of voidage and flux through parallel channels in liquid,solid fluidized beds is investigated. Experiments and computational fluid dynamics simulations were performed with 1.2 mm glass beads fluidized by water where a cross baffle divided a 191 mm diameter column into four identical parallel channels. Voidages were measured by optical fiber probes. Simulations from a three-dimensional unsteady-state Eulerian,Eulerian model based on FLUENT software showed good agreement with the experimental results. Despite the symmetrical geometry of the system, the average voidage and particle velocities in one channel differed somewhat from those in the others. Increasing the superficial liquid velocity could increase voidage greatly and affect the degree of nonuniformity in the four channels. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Simulation and experiments of mixing and segregation in a tote blenderAICHE JOURNAL, Issue 3 2005O. S. Sudah Abstract Experimental and computational investigation of mixing and segregation of granular material in a tote blender was carried out. The discrete element method (DEM) was used to simulate flow of spherical, free-flowing particles where the results of the computations were compared to blending. Computational results are compared to blending experiments of monodisperse and bidisperse systems using spherical glass beads in a 1:1 scale. Although some discrepancies were observed, DEM simulations illustrated good agreement with experimentally measured mixing and segregation rates for different fill levels and loading conditions. The effects of blender geometry on particle velocities and flow patterns were examined using DEM. The presence of a hopper and bin section, as well as the axial offset proved to introduce greater axial mixing rates that would be expected from pure dispersion. Vibrated experiments showed better agreement than not-vibrated experiments, indicating that modeling of friction forces needs to be further improved to enhance the accuracy of DEM methods. © 2005 American Institute of Chemical Engineers AIChE J, 51: 836,844, 2005 [source] Gas-solids flow behavior: CFB riser vs. downerAICHE JOURNAL, Issue 9 2001H. Zhang Comparisons are made in a circulating fluidized-bed riser/downer system between a 15.1 m high, 0.10 m ID riser and a 9.3 m high, 0.10 m ID downer, based on the measurements of the radial distributions of the local solids holdups and local particle velocities along the two columns. Although the core-annulus flow structures exist in both the riser and downer, the radial flow structure in the downer differs largely from that in the riser. The radial distributions of solids holdup and particle velocity in the downer are much more uniform than those in the riser, thus ensuring the low back mixing and the narrow particle residence time distribution in the downer. The axial flow structure in the downer is also more uniform than that in the riser. Due to the high particle acceleration and the high particle velocity in the downer, the overall solids holdup is significantly lower than that in the riser. The microflow structure in the downer, characterized by the low intermittency indices, is also more uniform than that in the riser. These key properties of the downer make it a very promising candidate for industrial applications where short reaction times and high product selectivity are required. [source] Measurements of local flow structures of conical spouted beds by optical fibre probesTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2009Zhiguo Wang Abstract A new calibration set-up was designed and assembled, and a comprehensive sensitivity analysis was conducted to investigate factors that may affect the calibration of the effective distance of an optical fibre particle velocity probe. It is shown that the glass window has the most significant impact on the calibration result. The optical fibre probe was then applied to measure local particle velocities and solid fractions inside a half and a full circular conical spouted bed. It is found that the overall particle velocity profiles, as well as the shapes of the spout and fountain are quite similar in both columns. Un nouveau réglage d'étalonnage a été conçu et assemblé, et une analyse de sensibilité complète a été effectuée afin d'investiguer les facteurs risquant d'avoir un impact sur l'étalonnage de la distance réelle d'une sonde à fibres optiques de la vitesse des particules. Il a été démontré que la fenêtre de vitre a l'impact le plus important sur les résultats de l'étalonnage. La sonde à fibres optiques a par la suite été utilisée pour mesurer les vitesses locales des particules et des fractions de solide à l'intérieur d'un demi et d'un plein lit jaillissant conique circulaire. Il est démontré que l'ensemble des profils de vitesse des particules, ainsi que la forme du bec et de la fontaine sont très semblables dans les deux colonnes. [source] Decreasing the Sampling Time Interval in Radioactive Particle TrackingTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2003Navid Mostoufi Abstract The study of the movement of solids in multiphase reactors using radioactive particle tracking is currently limited to fairly modest particle velocities because of count-rate limitations of the detection system. In this work, this restriction was overcome by increasing the activity of the radioactive tracer, by decreasing the sampling time interval and by modifying the particle tracking software to recognize which detectors were saturated and to use only the data from the remaining unsaturated detectors. Higher tracer activity resulted in lower standard deviation of the calculated tracer coordinates. L'étude du mouvement des solides par traçage avec des particules radioactives dans un réacteur polyphasique est actuellement limitée à des vitesses de particules relativement modestes à cause des limites de comptage du système de détection. Dans ce travail, on contourne cette restriction en accroissant l'activité du traceur radioactif, en diminuant l'intervalle de temps d'échantillonnage et en modifiant le logiciel de traçage des particules de façon à reconnaître les détecteurs qui sont saturés et à utiliser uniquement les données venant des détecteurs non saturés restants. Une plus grande activité du traceur donne un écart type plus petit pour les coordonnées de traceur calculées. [source] Radial nonuniformity index (RNI) in fluidized beds and other multiphase flow systemsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2001Jing-Xu (Jesse) Zhu Abstract The radial nonuniformity index (RNI) is a new non-dimensional index recently devised to quantify the extent of radial variations of flow parameters in fluidized beds and other multiphase flow systems. The index, defined as the ratio of the standard deviation of the given flow parameter in the radial direction to the maximum practically possible standard deviation of that particular parameter, is shown to be an excellent measure of the radial flow structure. This index allows the use of a single value to summarize the radial variation of a given flow parameter. The higher the value of the RNI the less uniform is the flow, and vice versa. Using this technique, radial distributions of local solids concentration and particle velocities from different circulating fluidized bed systems (gas-solid and liquid-solid, upflow and downflow) were examined. It has been found that the RNI can be confidently related to the flow conditions in the circulating fluidized beds, and more insight understanding has been achieved. L'indice de non-uniformité radiale (RNI) est un nouvel indice adimensionnel introduit récemment pour quantifier l'étendue des variations radiales des paramètres d'écoulement dans les lits fluidisés et autres systèmes d'écoulement multiphasiques. On montre que cet indice, défini comme le rapport entre I'écart type du paramètre d'écoulement donné dans la direction radiale et I'écart type maximum pratiquement possible de ce paramètre particulier, est une excellente mesure de la structure d'écoulement radial. Il permet I'utilisation d'une valeur unique pour décrire la variation radiale d'un paramètre d'écoulement donné. Plus la valeur de RNI est élevée, moins I'écoulement est uniforme, et vice-versa. À I'aide de cette technique, on a examiné les distributions radiales des concentrations de solides locales et des vitesses de particules pour différents systèmes de lits fluidisés circulants (gaz-solide et liquide-solide, ascendant et descendant). On a trouvé que le RNI pouvait ,tre relié en toute confiance aux conditions d'écoulement dans les lits fluidisés circulants, ce qui permet de mieux comprendre ce problème. [source] Quantum-Statistical Equation-of-State Models of Dense Plasmas: High-Pressure Hugoniot Shock AdiabatsCONTRIBUTIONS TO PLASMA PHYSICS, Issue 6 2007J. C. Pain Abstract We present a detailed comparison of two self-consistent equation-of-state models which differ from their electronic contribution: the atom in a spherical cell and the atom in a jellium of charges. It is shown that both models are well suited for the calculation of Hugoniot shock adiabats in the high pressure range (1 Mbar-10 Gbar), and that the atom-in-a-jellium model provides a better treatment of pressure ionization. Comparisons with experimental data are also presented. Shell effects on shock adiabats are reviewed in the light of these models. They lead to additional features not only in the variations of pressure versus density, but also in the variations of shock velocity versus particle velocity. Moreover, such effects are found to be responsible for enhancement of the electronic specific heat. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Grain transport mechanics in shallow overland flowECOHYDROLOGY, Issue 3 2009S.N. Prasad Abstract A physical model based on continuum multiphase flow is described to represent saltating transport of grains in shallow overland flows. The two-phase continuum flow of water and sediment considers coupled StVenant-type equations. The interactive cumulative effect of grains is incorporated by a dispersive stress term. The mean fluid thrust on the particle in the saltation layer of grains is expressed in terms of a slip velocity. The continuum model leads to the unexpected, but an interesting result is that particle velocity increases with the solid concentration. This increase predicts monotonic behaviour leading to overestimates of particle velocity at higher sediment concentration. To improve the predictions, grain dynamic equations, which incorporate bed collision, are analysed. The analysis leads to an improved model for predicting saltation height. Incorporation of the results in the continuum model yields a velocity-concentration relationship that is consistent with experimental observations for increasing concentration. Laboratory flume experiments explore the evaluation of various parameters from the measured particle velocities by photonic probes. Copyright © 2009 John Wiley & Sons, Ltd. [source] Analysis of electrokinetic transport of a spherical particle in a microchannelELECTROPHORESIS, Issue 4 2007Harikrishnan N. Unni Abstract Electrokinetically driven microfluidic devices that are used for biological cell/particle manipulation (e.g., cell sorting, separation) involve electrokinetic transport of these particles in microchannels whose dimension is comparable with particles' size. This paper presents an analytical study on electrokinetic transport of a charged spherical particle in a charged parallel-plate microchannel. Under the thin electric double-layer assumption, solutions in closed-form solutions for the particle velocity and disturbed electrical and fluid velocity fields are obtained for plane-symmetric (along the channel centerline) and asymmetric (off the channel centerline) motions of a sphere in a parallel-plate microchannel. The effects of relative particle size and eccentricity (i.e., off the centerline distance) on a particle's translational and rotational velocities are analyzed. [source] Considerations of the discontinuous deformation analysis on wave propagation problemsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 12 2009Jiong Gu Abstract In rock engineering, the damage criteria of the rock mass under dynamic loads are generally governed by the threshold values of wave amplitudes, such as the peak particle velocity and the peak particle acceleration. Therefore, the prediction of wave attenuation across fractured rock mass is important on assessing the stability and damage of rock mass under dynamic loads. This paper aims to investigate the applications of the discontinuous deformation analysis (DDA) for modeling wave propagation problems in rock mass. Parametric studies are carried out to obtain an insight into the influencing factors on the accuracy of wave propagations, in terms of the block size, the boundary condition and the incident wave frequency. The reflected and transmitted waves from the interface between two materials are also numerically simulated. To study the tensile failure induced by the reflected wave, the spalling phenomena are modeled under various loading frequencies. The numerical results show that the DDA is capable of modeling the wave propagation in jointed rock mass with a good accuracy. Copyright © 2009 John Wiley & Sons, Ltd. [source] Wave propagation in nonlinear one-dimensional soil modelINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2009J. Ahn Abstract The objective of the research conducted by the authors is to explore the feasibility of determining reliable in situ values of shear modulus as a function of strain. In this paper the meaning of the material stiffness obtained from impact and harmonic excitation tests on a surface slab is discussed. A one-dimensional discrete model with the nonlinear material stiffness is used for this purpose. When a static load is applied followed by an impact excitation, if the amplitude of the impact is very small, the measured wave velocity using the cross-correlation indicates the wave velocity calculated from the tangent modulus corresponding to the state of stress caused by the applied static load. The duration of the impact affects the magnitude of the displacement and the particle velocity but has very little effect on the estimation of the wave velocity for the magnitudes considered herein. When a harmonic excitation is applied, the cross-correlation of the time histories at different depths estimates a wave velocity close to the one calculated from the secant modulus in the stress,strain loop under steady-state condition. Copyright © 2008 John Wiley & Sons, Ltd. [source] Source signature and elastic waves in a half-space under a sustainable line-concentrated impulsive normal forceINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2002Moche Ziv Abstract First, the response of an ideal elastic half-space to a line-concentrated impulsive normal load applied to its surface is obtained by a computational method based on the theory of characteristics in conjunction with kinematical relations derived across surfaces of strong discontinuities. Then, the geometry is determined of the obtained waves and the source signature,the latter is the imprint of the spatiotemporal configuration of the excitation source in the resultant response. Behind the dilatational precursor wave, there exists a pencil of three plane waves extending from the vertex at the impingement point of the precursor wave on the stress-free surface of the half-space to three points located on the other two boundaries of the solution domain. These four wave-arresting points (end points) of the three plane waves constitute the source signature. One wave is an inhibitor front in the behaviour of the normal stress components and the particle velocity, while in the behaviour of the shear stress component, it is a surface-axis wave. The second is a surface wave in the behaviour of the horizontal components of the dependent variables, while the third is an inhibitor wave in the behaviour of the shear stress component. An inhibitor wave is so named, since beyond it, the material motion is dying or becomes uniform. A surface-axis wave is so named, since upon its arrival, like a surface wave, the dependent variable in question features an extreme value, but unlike a surface wave, it exists in the entire depth of the solution domain. It is evident from this work that Saint-Venant's principle for wave propagation problems cannot be formulated; therefore, the above results are a consequence of the particular model proposed here for the line-concentrated normal load. Copyright © 2002 John Wiley & Sons, Ltd. [source] Note on non-orthogonality of local curvilinear co-ordinates in a three-dimensional boundary element methodINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2005C. Fochesato Abstract We give a more general derivation of the particle velocity and acceleration used in the numerical wave model of Grilli et al. (Int. J. Numer. Meth. Fluids 2001; 35:829,867), by expressing these quantities in a local orthogonal co-ordinate system. Computations of solitary waves propagating and breaking over a sloping bottom show that the new formulation gives better results than the former one in the latest stages of overturning. Nevertheless, both formulations are found to be as suitable for the simulation of non-overturning waves. Results on wave profiles as well as on surface and internal kinematics are presented. Copyright © 2004 John Wiley & Sons, Ltd. [source] Experimental and numerical research for fluidization behaviors in a gas,solid acoustic fluidized bedAICHE JOURNAL, Issue 7 2010Changqing Cao Abstract The effects of sound assistance on fluidization behaviors were systematically investigated in a gas,solid acoustic fluidized bed. A model modified from Syamlal,O'Brien drag model was established. The original solid momentum equation was developed and an acoustic model was also proposed. The radial particle volume fraction, axial root-mean-square of bed pressure drop, granular temperature, and particle velocity in gas,solid acoustic fluidized bed were simulated using computational fluid dynamics (CFD) code Fluent 6.2. The results showed that radial particle volume fraction increased using modified drag model compared with that using the original one. Radial particle volume fraction was revealed as a parabolic concentration profile. Axial particle volume fraction decreased with the increasing bed height. The granular temperature increased with increasing sound pressure level. It showed that simulation values using CFD code Fluent 6.2 were in agreement with the experimental data. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Gas-solids flow behavior: CFB riser vs. downerAICHE JOURNAL, Issue 9 2001H. Zhang Comparisons are made in a circulating fluidized-bed riser/downer system between a 15.1 m high, 0.10 m ID riser and a 9.3 m high, 0.10 m ID downer, based on the measurements of the radial distributions of the local solids holdups and local particle velocities along the two columns. Although the core-annulus flow structures exist in both the riser and downer, the radial flow structure in the downer differs largely from that in the riser. The radial distributions of solids holdup and particle velocity in the downer are much more uniform than those in the riser, thus ensuring the low back mixing and the narrow particle residence time distribution in the downer. The axial flow structure in the downer is also more uniform than that in the riser. Due to the high particle acceleration and the high particle velocity in the downer, the overall solids holdup is significantly lower than that in the riser. The microflow structure in the downer, characterized by the low intermittency indices, is also more uniform than that in the riser. These key properties of the downer make it a very promising candidate for industrial applications where short reaction times and high product selectivity are required. [source] Hydrodynamic modelling of cometary particles captured in aerogel and the Earth's atmosphereMETEORITICS & PLANETARY SCIENCE, Issue 10 2009S. G. Coulson We present a simplified model for the formation of the tracks formed in aerogel by hypervelocity impacts of cometary material. Using a hydrodynamic approach to model this class of problem overcomes some of the errors associated with previous semi-analytical models for track formation (Coulson 2009). The hydrodynamic models developed allow the particle velocity, temperature and pressure to be calculated as a function of track length within aerogel. A qualative description of how this model can be extended to the formation of bulbous cavities using the Chapman-Jouquet theory is provided. [source] Direct Measurements of Instantaneous Solid Flux in a CFB Riser using a Novel Multifunctional Optical Fiber ProbeCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 4 2009S. Ye Abstract With a novel optical fiber probe that can measure instantaneous local particle velocity and solid concentration simultaneously, extensive experiments were conducted to study transient flow structures in a 15.1-m long circulating fluidized bed (CFB) riser of 100,mm in diameter. This study analyzed the radial and axial distributions of solid concentration, particle velocity, and their variations with nine operating conditions and at six axial levels. Instantaneous local solid concentration and particle velocity were found to be well correlated at most of the radial positions. The detailed time evolution, axial and radial distribution of instantaneous solid flux, and the variation of solid flux with operating conditions were also investigated. The radial solid flux profile showed a flat shape with a maximum at near wall area under most operating conditions. The instantaneous solid flux was found to have a strong fluctuation at a radial position of r/R,=,0.8 ~ 0.9. [source] | ||||||||||||||||||||||