Home About us Contact
|
Home About us Contact | ||
|
|
||
Tracer Particles (tracer + particle)
Selected AbstractsThe linkage between velocity patterns and sediment entrainment in a forced-pool and riffle unitEARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2009D. M. Thompson Abstract A field-based project was initiated in order to characterize velocities and sediment entrainment in a forced-pool and riffle sequence. Three-dimensional velocities and turbulence intensities were measured with an acoustic Doppler velocimeter at 222 different points at three similar flows that averaged approximately 4·35 m3 s,1 within a large pool,riffle unit on North Saint Vrain Creek, Colorado. Sediment-sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location. Tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow, which reached a maximum level of 14·8 m3 s,1. Velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools. Patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle. Flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing, and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply. Jet flow, flow separation, vortex scour and turbulence generation enhance scour in the centre of pools. Ultimately, multiple processes appear to play some role in maintenance of this forced pool and the associated riffle. Copyright © 2008 John Wiley & Sons, Ltd. [source] Single-particle motion and heat transfer in fluidized bedsAICHE JOURNAL, Issue 12 2006Yee Sun Wong Abstract Fluidized beds are particularly favored as chemical reactors because of their ability to exchange heat through immersed heat-exchange surfaces. However, little is known about how the heat-exchange process works on a single-particle level. The most commonly applied theory of fluidized bed heat exchange is that developed by Mickley and Fairbanks in the 1950s,the so-called packet model. The work described in this article is an attempt to understand the process of heat transfer by solids convection, using positron emission particle tracking to follow the trajectory of a single tracer particle in the bed. In particular, the residence time of particles in the vicinity of the surface is determined here for the first time. Using these data, the observed heat-transfer variations are interpreted mechanistically. © 2006 American Institute of Chemical Engineers AIChE J, 2006 [source] Multimodal Flow Visualization and Optimization of Pneumatic Blood Pump for Sorbent Hemodialysis SystemARTIFICIAL ORGANS, Issue 4 2009Fangjun Shu Abstract:, Renal Solutions Allient Sorbent Hemodialysis System utilizes a two-chambered pneumatic pump (Pulsar Blood Pump, Renal Solutions, Inc., Warrendale, PA, USA) to avoid limitations associated with peristaltic pumping systems. Single-needle access is enabled by counter-pulsing the two pump chambers, thereby obviating compliance chambers or blood reservoirs. Each chamber propels 20 cc per pulse of 3 s (dual access) or 6 s (single access) duration, corresponding to a peak Reynolds number of approximately 8000 (based on inlet velocity and chamber diameter). A multimodal series of flow visualization studies (tracer particle, dye washout, and dye erosion) was conducted on a sequence of pump designs with varying port locations and diaphragms to improve the geometry with respect to risk of thrombogenesis. Experiments were conducted in a simplified flow loop using occluders to simulate flow resistance induced by tubing and dialyzer. Tracer visualization revealed flow patterns and qualitatively indicated turbulence intensity. Dye washout identified dwell volume and areas of flow stagnation for each design. Dye erosion results indicated the effectiveness and homogeneity of surface washing. Compared to a centered inlet which resulted in a fluid jet that produced two counter-rotating vortices, a tangential inlet introduced a single vortex, and kept the flow laminar. It also provided better surface washing on the pump inner surface. However, a tangential outlet did not present as much benefit as expected. On the contrary, it created a sharp defection to the flow when transiting from filling to ejection. [source] The linkage between velocity patterns and sediment entrainment in a forced-pool and riffle unitEARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2009D. M. Thompson Abstract A field-based project was initiated in order to characterize velocities and sediment entrainment in a forced-pool and riffle sequence. Three-dimensional velocities and turbulence intensities were measured with an acoustic Doppler velocimeter at 222 different points at three similar flows that averaged approximately 4·35 m3 s,1 within a large pool,riffle unit on North Saint Vrain Creek, Colorado. Sediment-sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location. Tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow, which reached a maximum level of 14·8 m3 s,1. Velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools. Patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle. Flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing, and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply. Jet flow, flow separation, vortex scour and turbulence generation enhance scour in the centre of pools. Ultimately, multiple processes appear to play some role in maintenance of this forced pool and the associated riffle. Copyright © 2008 John Wiley & Sons, Ltd. [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] Measuring velocity distributions of viscous fluids using positron emission particle tracking (PEPT)AICHE JOURNAL, Issue 7 2004S. Bakalis Abstract Positron emission particle tracking (PEPT) can be used to trace the path of a radioactive particle within opaque fluids in pilot-scale equipment; the method can track particles through several centimeters of metal. PEPT has been successfully used to follow isokinetic tracers in viscous fluids and thus to measure velocity distributions under both isothermal and nonisothermal conditions in pipe flow. The accuracy of the method decreased as the measured velocities increased; the faster the particle traveled, the less accurate its detection. For velocities of up to 0.5 m/s the accuracy of the method was acceptable. Agreement between experimentally measured and theoretical velocity distributions was very good, for a range of fluids and process conditions. As tracer particles are used, there were problems ensuring that all parts of the measurement volume were sampled. This is possible to overcome to an extent by adjusting particle size; 600-,m tracers did not pass within 1 mm from the tube wall, whereas 240-,m particles passed much closer to the boundaries of the flow. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1606,1613, 2004 [source] Chemical enrichment of the intracluster medium by FR II radio sourcesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007D. Heath ABSTRACT We present 2D axisymmetric hydrodynamic simulations investigating the long-term effect of Fanaroff,Riley type II radio galaxies on the metal distribution of the surrounding intracluster medium (ICM). A light jet is injected into a cooling flow atmosphere for 10,30 Myr. We then follow the subsequent evolution for 3 Gyr on a spherical grid spanning 3 Mpc in radius. A series of passive tracer particles were placed in an annulus about the cluster core to simulate metal carrying clouds in order to calculate the metallicity (Z) as a function of time and radial distance from the cluster centre. The jet has a significant effect on the ICM over the entire 3-Gyr period. By the end of the simulations, the jets produced metallicities of ,10 per cent of the initial metallicity of the cluster core throughout much of the cluster. The jets transport the metals not only in mixing regions, but also through upwelling ICM behind the jet, enriching the cluster over both long and short distances. [source] Positron Emission Tomography Applied to Fluidization EngineeringTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2005Chutima Dechsiri Abstract The movement of particles in a laboratory fluidized bed has been studied using Positron Emission Tomography (PET). With this non-invasive technique both pulses of various shapes and single tracer particles were followed in 3-D. The equipment and materials used made it possible to label actual bed particles as tracer. This paper briefly describes the data analysis, some of the results and compares them with a stochastic model. The results confirm the basic assumptions of the model qualitatively but not quantitatively. Further analysis of the results indicates that the system exhibits gulf-streaming, a feature which is not yet accounted for in the model, but is common in fluidized beds in practice. On a étudié le mouvement des particules dans un lit fluidisé de laboratoire par la tomographie à émission de positrons (PET). Cette technique non intrusive a permis de suivre en 3-D des impulsions de formes diverses aussi bien que des particules traçantes uniques en 3-D. L'équipement et les matériaux utilisés ont permis d'employer des particules de lit réelles comme traceurs. On décrit brièvement dans cet article l'analyse des données et certains des résultats, puis une comparaison est effectuée avec un modèle stochastique. Les résultats confirment les hypothèses de base du modèle de façon qualitative mais non quantitative. Une analyse plus approfondie des résultats indique que le système montre un phénomène de « gulf-streaming », une caractéristique pas encore prise en compte dans le modèle mais qui est courante dans les lits fluidisés réels. [source] Measurement of Solid Circulation Patterns in Liquid,Solid and Gas,Liquid,Solid Fluidized BedsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2003Shahzad Barghi Abstract Solid circulation patterns were determined by measuring collisions between tracer particles and cylindrical probes in liquid,solid and gas,liquid,solid fluidized beds. Special probes were used to obtain two- and three-dimensional views of particle motion. Circulation patterns for 3 and 5 mm glass beads were studied. Mixing cells, which had been formed at low liquid velocities, grew in size and eventually merged as the liquid velocity increased. The flow patterns of smaller particles having the same density as bed particles and particles lighter than bed particles (graphite particles) with the same size were also measured. On a déterminé des profils de circulation des solides en mesurant les collisions entre des traceurs particulaires et des sondes cylindriques dans des lits fluidisés liquide, solide et gaz,liquide,solide. Des sondes spéciales ont été employées pour obtenir des images bidimensionnelle et tridimensionnelle du déplacement des particules. Les profils de circulation pour des billes de verre de 3 mm et 5 mm ont été étudiés. Les cellules de mélange, qui se forment à de faibles vitesses de liquide, augmentent en taille et finissent par fusionner à mesure que la vitesse du liquide augmente. On a également mesuré les profils d'écoulement des particules plus petites ayant la même masse volumique que les particules de lit et des particules plus légères que les particules de lit (particules de graphite) ayant la même taille. [source] | ||||||||||