Particle Mixing Rates Research Articles

Insight into hydrodynamics and heat transfer characteristics in a gas–solid spout fluidized bed from a multiple zone view

In this work, the hydrodynamics and heat transfer characteristics within a gas–solid spout fluidized bed were investigated from a multiple-zone view by employing the coupled computational fluid dynamic and discrete element method. Results show that both the axial and radial particle mixing rates are accelerated with the axial particle mixing rate much faster than the radial one when increasing the spouting gas velocity. Besides, the particle-scale and the multiple-zone heat transfer analyses show that the annulus zone makes the major contribution to inter-particle conduction with the particle–particle conduction and the particle–fluid-particle conduction heat fluxes taking up about 64 % and 73 %, followed by the fountain zone taking up about 28 % and 22 %, respectively. As for the particle–fluid convective heat transfer, the spouting and annulus zones make almost the comparative contributions being about 38 %, followed by the fountain zone being less than 24 %. Moreover, the full-bed time-averaged heat transfer analysis shows that the particle–fluid convective heat transfer plays a dominant role in the heat transfer process in the spout fluidized bed with the particle–particle conductive and the particle–fluid-particle conductive heat fluxes being about 0.1 % and 1 % of the particle–fluid convective heat flux. The results of this study can provide an efficient theoretical reference for the gas–solid flow-mixing-heat transfer processes optimization and intensification of spout fluidized beds.

Comparison Between Fluid Simulation With Test Particles and Hybrid Simulation for the Kelvin‐Helmholtz Instability

AbstractA quantitative investigation of plasma transport rate via the Kelvin‐Helmholtz (KH) instability can improve our understanding of solar‐wind‐magnetosphere coupling processes. Simulation studies provide a broad range of transport rates by using different measurements based on different initial conditions and under different plasma descriptions, which makes cross literature comparison difficult. In this study, the KH instability under similar initial and boundary conditions (i.e., applicable to the Earth's magnetopause environment) is simulated by Hall magnetohydrodynamics with test particles and hybrid simulations. Both simulations give similar particle mixing rates. However, plasma is mainly transported through a few big magnetic islands caused by KH‐driven reconnection in the fluid simulation, while magnetic islands in the hybrid simulation are small and patchy. Anisotropic temperature can be generated in the nonlinear stage of the KH instability, in which specific entropy and magnetic moment are not conserved. This can have an important consequence on the development of secondary processes within the KH instability as temperature asymmetry can provide free energy for wave growth. Thus, the double‐adiabatic theory is not applicable and a more sophisticated equation of state is desired to resolve mesoscale process (e.g., KH instability) for a better understanding of the multi‐scale coupling process.

Variability in Particle Mixing Rates in Sediments with Polymetallic Nodules in the Equatorial Eastern Pacific as Determined from Measurements of Excess 210Pb

Radionuclide activities of 210Pb and 226Ra were measured to determine bioturbation coefficients (Db) in seven sediment cores from the Korean licensed block for polymetallic nodules in the Clarion–Clipperton Fracture Zone. Variability in Db is considered in the context of the sedimentological, geochemical, and geotechnical properties of the sediments. Db values in the studied cores were estimated using a steady-state diffusion model and varied over a wide range from 1.1 to 293 cm2/yr with corresponding mixing depths (L) of 26 to 144 cm. When excepting for spurious results obtained from cores where diffusive mixing does not apply, Db values range from 1.1 to 9.0 cm2/yr with corresponding mixing depths (L) of 26 to 63 cm. Such wide variability in Db and L values is exceptional in sites with water depths of ∼5000 m and is attributed in this study to an uneven distribution of sediment layers with different shear strengths and total organic carbon (TOC) contents, caused by erosion events. The studied cores can be grouped into two categories based on lithologic associations: layers with high maximum shear strength (MSS) and low TOC content, showing a narrow range of Db values (1.1–9.0 cm2/yr); and layers with low MSS and high TOC content, yielding much higher Db values of over 30 cm2/yr. The distribution of different lithologies, and the resultant spatial variability in MSS and labile organic matter content, controls the presence and maximum burrowing depth of infauna by affecting their mobility and the availability of food. This study provides a unique case showing that shear strength, which relates to the degree of sediment consolidation, might be an important factor in controlling rates of bioturbation and sediment mixing depths.

Particle mixing rates using the two-fluid model

In this work, a new methodology is introduced to calculate the solids mixing rate in dense gas-fluidized beds using the two-fluid model. The implementation of this methodology into an existing two-fluid model code was carefully verified. The solids phase continuity equation was satisfied using our method, and the sensitivity of the computational results to the time step, computational cell size, and discretization scheme was investigated to determine the optimal simulation settings. Using these simulation settings, the degree of solids mixing was observed to rapidly (exponentially) increase with increasing operating pressure and linearly decrease with increasing bed diameter. Our novel methodology can be applied to analyze mixing processes in large lab-scale beds as an alternative to existing time-consuming simulation techniques such as computational fluid dynamics combined with the discrete element model.

Determination of sedimentation, diffusion, and mixing rates in coastal sediments of the eastern Red Sea via natural and anthropogenic fallout radionuclides

The Red Sea is a unique ecosystem with high biodiversity in one of the warmest regions of the world. In the last five decades, Red Sea coastal development has rapidly increased. Sediments from continental margins are delivered to depths by advection and diffusion-like processes which are difficult to quantify yet provide invaluable data to researchers. Beryllium-7, lead-210 and ceseium-137 were analyzed from sediment cores from the near-coast Red Sea near Jeddah, Saudi Arabia. The results of this work are the first estimates of diffusion, mixing, and sedimentation rates of the Red Sea coastal sediments. Maximum chemical diffusion and particle mixing rates range from 69.1 to 380cm−2y−1 and 2.54 to 6.80cm−2y−1, respectively. Sedimentation rate is constrained to approximately 0.6cm/yr via multiple methods. These data provide baselines for tracking changes in various environmental problems including erosion, marine benthic ecosystem silting, and particle-bound contaminant delivery to the seafloor.

Development of a multi-fluid model for poly-disperse dense gas–solid fluidised beds, Part II: Segregation in binary particle mixtures

Particle mixing and segregation rates in a bi-disperse freely bubbling fluidised bed have been studied with a new multi-fluid model (MFM) based on the kinetic theory of granular flow for multi-component systems (see Part I). The MFM simulation results have been compared with digital image analysis experiments obtained by Goldschmidt et al. [2003. Digital image analysis of bed expansion in dense gas-fluidised beds. Powder Technology 138, 135–159] for bi-disperse mixtures of glass beads. In strong contrast to MFMs previously described in the literature, that strongly overestimate the segregation rates, the new MFM seems to underestimate the segregation rates at longer times. This underprediction of the segregation rate is probably related to the neglect of frictional stresses associated with long-term multiple-particle contacts resulting in an overestimation of the mobility of the emulsion phase, which is corroborated by discrete particle simulations without friction between the particles and the particles and the wall. The level of the granular temperature of the segregating system, as computed with the new MFM, compares reasonably well with the granular temperatures found in the DPM simulation.

Simultaneous study of particle reworking, irrigation transport and reaction rates in sediment bioturbated by the polychaetes Heteromastus and Marenzelleria

The present study employed simultaneously methods to investigate particle and solute transport and reaction rates in sandy sediments inhabited by two worms (2500 m − 2 ) with different feeding modes. Heteromastus filiformis is a head-down deposit-feeder and the main activities exerted by this worm are transport of particles as faecal pellets from subsurface to surface sediments and burrow ventilation. Marenzelleria viridis is a surface deposit-feeder that actively searches for food by burrowing near the sediment surface, producing a network of ventilated galleries in this zone. M. viridis exhibited 1.5 to 2.2 times higher particle mixing rates ( D b = 3.3 to 4 × 10 − 3 cm − 2 d − 1 ) compared to H. filiformis. In M. viridis treatments, continuous advection (eddy diffusion) was the major factor influencing solute transport resulting in apparent diffusion rates ( D a = 2.2 cm − 2 d − 1 ), which were 3 times higher than molecular diffusion within the sediment. In H. filiformis inhabited sediments, the transport of solutes was discontinuous and driven by a surprisingly high nonlocal exchange ( α = 1.1–1.3 d − 1 ), emphasizing its strong irrigation effects. Accordingly, the enhancement of solute fluxes was more pronounced for H. filiformis compared to M. viridis. Depth integrated TCO 2 production derived from diagenetic modelling, which takes into account three reaction zones, is in good agreement with rates obtained from measured fluxes, indicating the applicability of both approaches to get reliable rates. However, the reaction rates showed that the presence of animals had a modest effect on microbial carbon oxidation. The results proved that transport conditions are deeply related to feeding modes. Exchange of solutes was the most important transport process by H. filiformis, while M. viridis affected both mixing and solute transport.

Modern organic carbon burial fluxes, recent sedimentation rates, and particle mixing rates from the upper continental slope near Cape Hatteras, North Carolina (USA)

Abstract The prominent band of organic-rich surface sediment (2–3% organic carbon) that straddles the upper continental slope near Cape Hatteras suggests that this region may be an important depocenter for phytodetritus exported from the highly productive shelf waters of the Mid-Atlantic Bight (MAB). To understand better the role of modern (post- 1955 ) carbon burial in continental slope sediments, we quantified rates of recent (defined as 2 -yr timescale) sediment accumulation, particle mixing, and organic carbon burial for 12 cores from 10 upper slope stations (212–1004 m) in the proximity of Cape Hatteras. Recent sedimentation and mixing rates were evaluated using diagenetic models calibrated by coupling two modern particle tracers with distinct input functions: excess 210 Pb and fallout Pu. These two radionuclides provide independent information that can be used to deconvolve the effects of sediment accumulation and mixing. Recent sedimentation rates range from 10 to 425 cm kyr −1 (mean=106 cm kyr −1 ) and biodiffusion coefficients for the deep mixing layer (9–21 cm) range from 0.3 to 5.2 cm 2 yr −1 (mean=1.3 cm 2 yr −1 ). Total carbon burial for this region based on recent sedimentation rates (1.1 mol C m −2 yr −1 ) is considerably higher than that reported for the more northern sections of the MAB. Although the narrowing shelf and convergence of southward flowing shelf waters and the north-flowing Gulf Stream promote carbon export and deposition off Cape Hatteras, we estimate that modern carbon burial in upper slope sediments in this region accounts for much less than 1% of the shelf production.

Infaunal density, biomass and bioturbation in the sediments of the Arctic Ocean

Little is known about the benthic communities of the Arctic Ocean's slope and abyssal plains. Here we report on benthic data collected from box cores along a transect from Alaska to the Barents Abyssal Plain during the Arctic Ocean Section of 1994. We determined: (1) density and biomass of the polychaetes, foraminifera and total infauna; (2) concentrations of potential sources of food (pigment concentration and percent organic carbon) in the sediments; (3) surficial particle mixing depths and rates using downcore 210Pb profiles; and (4) surficial porewater irrigation using NaBr as an inert tracer. Metazoan density and biomass vary by almost three orders of magnitude from the shelf to the deep basins (e.g. 47 403 individuals m−2 on the Chukchi Shelf to 95 individuals m−2 in the Barents Abyssal Plain). Water depth is the primary determinant of infaunal density, explaining 39% of the total variability. Potential food concentration varies by almost two orders of magnitude during the late summer season (e.g. the phaeopigment concentration integrated to 10 cm varies from 36.16 mg m−2 on the Chukchi Shelf to 0.94 mg m−2 in the Siberia Abyssal Plain) but is not significantly correlated with density or biomass of the metazoa. Most stations show evidence of particle mixing, with mixing limited to ≤3 cm below the sediment-water interface, and enhanced pore water irrigation occurs at seven of the nine stations examined. Particle mixing depths may be related to metazoan biomass, while enhanced pore water irrigation (beyond what is expected from diffusion alone) appears to be related to total phaeopigment concentration. The data presented here indicate that Arctic benthic ecosystems are quite variable, but all stations sampled contained infauna and most stations had indications of active processing of the sediment by the associated infauna.

Particle Mixing Rates of Freshwater Bivalves: Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae)

Sediment mixing by freshwater suspension feeding bivalves Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae) was studied by adding illite clay particles with adsorbed I37Cs as a submillimeter thick layer to the surface of silt clay sediments contained in clear rectangular cells maintained in a temperature regulated aquarium. A Nal gamma detector scanned the sediment column in each cell at 0.2 cm intervals five times over 22 days and recorded changes in I37Cs activity over time with depth in cells containing 3 A. grandis, 4 S. striatinum, and a control cell containing no bivalves. Sediment mixing by these organisms was diffusional. The diffusion coefficient in the control cell was 0.02 cm2/yr, consistent with molecular diffusion of137Cs tracer. Whole cell biodiffusion coefficients (Db)for A. grandis and S. striatinum were 0.81–2.11 cm2/yr and 0.53 cm2/yr, respectively. Adjusting to equal population densities, the 11-27× higher sediment mixing rate of A. grandis was likely due primarily to its larger size. When Dbfor similar sized organisms was compared, S. striatinum was found to mix sediments at about the same rate as the marine bivalve Nucula proxima but at a 5× lower rate than the freshwater amphipod Diporeia sp. A. grandis mixes sediments 5–14× more slowly than the similar sized conveyor belt deposit feeding marine bivalve, Yoldia limatula.. While deposit feeding organisms are the dominant sediment mixers in the Great Lakes, suspension feeding bivalves can be locally significant.

Particle mixing rates in sediments of the northeast tropical Atlantic: Evidence from 210Pb xs, 137Cs, 228Th xs and 234Th xs downcore distributions

Particle mixing rates were estimated from distribution of 137Cs and excess activities of 210Pb, 228Th and 234Th measured in nine sediment cores taken from the northeast tropical Atlantic, i.e. at the three sites of the French EUMELI (EUtrophic, MEsotrophic and oLIgotrophic) programme. At the oligotrophic site, activities of 137Cs and excess 210Pb decrease smoothly with depth. On the other hand, at the mesotrophic and eutrophic sites, 210Pb xs and 137Cs profiles suggest a mixing process with three different regimes, with subsurface sediments being mixed more rapidly than those at the surface. Bioturbation rates were found to be slowest at the oligotrophic site, intermediate at the mesotrophic site, and fastest at the eutrophic site. These mixing intensities appear closely linked to benthic fauna abundance, organic carbon flux to the sediments, and primary production in overlying surface waters. The use of a multitracer approach for studying sediment particle reworking has allowed bioturbation rates to be better constrained and an assessment of whether radionuclide distributions in surface sediments are in steady state. For only one core from the mesotrophic site were surface sediments not in steady state, perhaps because of the occurrence of a benthic animal (i.e., a xenophyophore) at the surface of this core.

Particle motion and mixing in a rotary kiln

The motion and mixing of solid materials within a rotary kiln was studied experimentally by visual observationof tagged particles in a laboratory kiln. Particle mixing rates were evaluated by the time required to achieve complete or randomized mixing of the kiln bed. Experiments were conducted to investigate the effects of particle type, kiln loading, and wall friction on mixing rates in the bed with the bed in either slipping mode or rolling mode. In slipping mode, the bed behaved as a relatively solid mass with little mixing of particles when compared to operation in rolling mode. In rolling mode the following effects were noted: Increasing kiln loading increased the time required to achieve complete mixing; e.g. in one experiment, a 500% increase in kiln loading resulted in a 67% increase in the time to achieve mixing. Particle type was shown to have an effect on mixing time and an empirical relationship was developed to predict kiln mixing times based on the ratio of the coefficient of particle/particle friction to the coefficient of particle/wall friction.

Seasonal cycles of particle and solute transport processes in nearshore sediments: [formula omitted] and [formula omitted] disequilibrium at a site in Buzzards Bay, MA

Radiochemical measurements leading to the estimation of particle mixing rates and irrigation rates in the sediments of a site in Buzzards Bay, Massachusetts, have been made over two nearly full seasonal cycles. Both particle mixing and sediment irrigation show significant seasonal variability at this site; the variability is consistent with biologically driven transport mechanisms. 234Th 238U disequilibrium measurements were used to estimate particle mixing rates: based on three cores taken from December through March and four taken from June through October, a cold-season average mixing rate of 5.6 cm 2/yr and a warm-season average rate of 17 cm 2/yr were found. Excess 234Th inventories in the upper 5 cm of the sediments also varied seasonally, with a cold-season average value of 1.8 dpm/cm 2, significantly less than both the warm-season value of 3.4 dpm/cm 2 and the predicted average value for Buzzards Bay (≈3 dpm/cm 2). 222Rn 226Ra disequilibrium in sediment pore waters was used to estimate sediment irrigation. Based on a total of seven cores, three seasonal groupings of results were made: (1) December through March, when 222Rn deficits could be explained by vertical molecular diffusion alone, (2) early summer (June), when irrigation was important to at least 20 cm, the maximum depth of the sampled region, and (3) early fall, when irrigation was important to about 10 cm. A nonlocal exchange/vertical molecular diffusion model ( Emerson et al., 1984) was used to obtain quantitative estimates of irrigation rates; when the exchange parameter was allowed to vary exponentially with depth below the sediment-water interface, model Rn deficit profiles fit measured profiles quite well.

Fine scale distributions of porosity and particulate excess 210Pb, organic carbon and CaCO 3 in surface sediments of the deep equatorial Pacific

Sediment samples were recovered from the central equatorial Pacific Ocean, sectioned at 1-mm intervals, and analyzed for porosity, organic carbon, excess 210Pb and CaCO 3. Steep porosity gradients were measured in the upper 1 cm of the sediment column with extremely high values observed near the sediment surface. Similarly, particulate organic carbon contents are highest at the sediment surface, decrease sharply in the upper 1 cm, and are relatively constant between 1 and 5 cm. CaCO 3 values, on the other hand, are lowest at the sediment surface and increase to a constant value below 5–10 mm depth. At the carbonate ooze sites, excess 210Pb is present throughout the upper 5 cm of the sediments suggesting relatively rapid particle mixing rates. However, extremely high excess 210Pb activities (> 100 dpm/g) are observed at the sediment surface with sharp gradients present in the upper 1 cm which would suggest slow rates of mixing. This apparent contradiction along with the major features of the CaCO 3 and particulate organic carbon profiles can be explained by a particle-selective feeding mechanism in which organic carbon, excess 210Pb-enriched particles are preferentially maintained at the sediment surface via ingestion and defecation by benthic organisms.

Cold flow mixing rates in confined, recirculating coaxial jets with angular secondary injection

AbstractThe mixing of gases and particles in confined, coaxial jets with angular secondary steam injection into suddenly expanded mixing chambers was studied. The effects of secondary stream injection angle, mixing chamber diameter and length, primary jet solids loadings, particle size, and secondary stream velocity on particle and gas mixing rates were determined. Gas mixing was much faster than particle mixing. Angular secondary injection significantly increased the initial gas and particle mixing rates but had little effect on the overall length of duct needed for complete mixing. The effects on initial gas mixing rate of combined angular secondary injection and increased secondary velocity were additive.

Particle mixing rates in sediments of the eastern equatorial Pacific: Evidence from 210Pb, 239,240Pu and 137Cs distributions at MANOP sites

Particle mixing rates ( D B ) calculated from excess 210Pb gradients in sediments of the east equatorial Pacific range from 0.04 to 0.5 cm 2/y, with variation of a factor of 3–4 at a single site. Diffusion of the 236Ra daughter 222Rn may affect 210Pb distributions under conditions of slow mixing and low 210Pb flux to the seafloor, as shown by a siliceous ooze-clay core which contained the fallout radionuclides 239,240Pu and 137Cs but no excess 210Pb (relative to 226Ra). There is no clear relationship between 210Pbderived mixing rates and sediment type, accumulation rate or organic carbon flux to the sediments. Comparison of 210Pb mixing rates with those calculated from 239,240Pu and 137Cs distributions reveals better agreement for a pulse input of the fallout radionuclides ( D B = 0.03−0.4 cm 2/ y) than for continuous input at a constant rate ( D B = 0.1−1.6 cm 2/ y), although the Pu and 137Cs data are better fit by the latter model. The agreement may be fortuitous because 239,240Pu and 137Cs appear significantly deeper than 210Pb in at least one core. Tracer separation could be caused by particle size-selective mixing by the benthic fauna or by chemical mobilization. If the fallout radionuclides are scavenged from surface waters by large, organic-rich particles such as fecal pellets, their release and migration may result from decomposition of the carrier in surface sediments. Either a relatively unreactive form of Pu (e.g. oxidized Pu) has been released by this process or a one-dimensional model is inadequate to explain its observed penetration into the sediments. Activity ratios of 239,240Pu 137Cs in the sediments decrease with increasing north latitude, and the trend reflects higher fluxes of 239,240Pu near the weapons test site at Christmas Island (2°N). The 239,240Pu 137Cs ratios and fluxes to the sediment (assuming constant input) at the siliceous ooze-red clay site are consistent with published sediment trap data from a nearby site. Thus if fallout radionuclide fluxes to the sea floor were higher in the past, both 239,240Pu and 137Cs have been released from sinking particles.

The influence of bioturbation on physical, chemical and biological parameters in aquatic environments: A review

The role of benthic macroinvertebrates in sediment processing is of importance for investigators attempting to describe the dynamics of a wide range of materials in aquatic environments. Benthic macroinvertebrates have been shown to blur sediment stratigraphy, alter particle size and pore spaces, and influence shear strength of the sediment and turbidity of overlying waters. The macroinfauna can increase oxygen and E h penetration into the sediment and shift pH levels in actively worked substrates. These changes have been correlated with trace element redistributions among physicochemical forms and the flux of N, P, PCBs, Cu, Fe, I, Hg, Mn, Si, Zn and radioisotopes to the water column. There is general agreement that bioturbation can stimulate microbial activity, although the data remain inconclusive. Realistic estimates of sediment particle mixing rates are required in order to estimate the impact of bioturbation on core dating techniques, material cycling studies and contaminant remobilisation from polluted spoils.

Particle mixing rates in deep-sea sediments determined from excess 210Pb and 32Si profiles

Particle mixing rates in deep-sea sediments determined from excess 210Pb and 32Si profiles

Particle mixing rates in deep-sea sediments determined from excess 210Pb and 32Si profiles

Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210 Pb and 32 Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments 210 Pb mixing coefficients (0.04–0.16 cm 2 /y) are in reasonable agreement with the 32 Si mixing coefficient (0.2 or 0.4 cm 2 /y, depending on 32 Si half-life). In an equatorial Pacific sediment core, however, the 210 Pb mixing coefficient (0.22 cm 2 /y) is 3–7 times greater than the 32 Si mixing coefficient (0.03 or 0.07 cm 2 /y). The difference in 210 Pb and 32 Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210 Pb activity relative to coarser particles (large radiolaria) containing the 32 Si activity, or (3) the supply of 222 Rn from the bottom of manganese nodules which increases the measured excess 210 Pb activity (relative to 226 Ra) at depth and artificially increases the 210 Pb mixing coefficient. Based on 32 Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32 Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.

Cold flow mixing rates with recirculation for pulverized coal reactors

AbstractThe mixing of particles and gases in confined, coaxial jets in the presence of recirculation was studied. Particle and gas mixing rates were measured for conditions simulating processes involving pulverized coal combustion and entrained gasification. The effects of mixing chamber diameter, gas velocity, particle size, and particle loading level on mixing rates were determined from particle mass flux and gas tracer measurements. In every case, the gas mixed much faster than did the particles. Introduction of an expanded mixing chamber of larger diameter (with recirculation zone) significantly increased both gas and particle mixing rates over that for a smaller mixing chamber without recirculation. However, this observed increase was not strongly dependent upon the diameter of the larger mixing chamber. Increasing secondary velocity also increased mixing rates significantly.