Variation Of Particle Density Research Articles

A Large Eddy Simulation Study on the Effect of Devolatilization Modelling and Char Combustion Mode Modelling on the Structure of a Large-Scale, Biomass and Coal Co-Fired Flame

This work focuses on the impact of the devolatilization and char combustion mode modelling on the structure of a large-scale, biomass and coal co-fired flame using large eddy simulations. The coal modelling framework previously developed for the simulation of combustion in large-scale facilities is extended for biomass capabilities. An iterative procedure is used to obtain devolatilization kinetics of coal and biomass for the test-case specific fuels and heating conditions. This is achieved by calibrating the model constants of two empirical models: the single first-order model and the distributed activation energy model. The reference data for calibration are devolatilization yields obtained with predictive coal and biomass multistep kinetic mechanisms. The variation of both particle density and diameter during char combustion is governed by the conversion mode, which is modelled using two approaches: the power law using a constant parameter that assumes a constant mode during char combustion and a constant-free model that considers a variable mode during combustion. Three numerical cases are considered: single first-order reaction with constant char combustion mode, distributed activation energy with constant char combustion mode, and single first-order reaction with variable char combustion mode. The numerical predictions from the large eddy simulations are compared with experimental results of a high co-firing rate large-scale laboratory flame of coal and biomass. Furthermore, results from single particle conversion under idealised conditions, isolating the effects of turbulence, are presented to assist the interpretation of the predictions obtained with large eddy simulations. The effects of the devolatilization and conversion mode modelling on the flame lift-off, flame length, and spatial distribution and radial profiles of O2 and CO2 are presented and discussed. Both the devolatilization and conversion mode modelling have a significant effect on the conversion of particles under idealised conditions. The large eddy simulations results show that the devolatilization model has a strong impact on the flame structure, but not on the flame lift-off. On the other hand, for the tested numerical conditions, the char combustion mode model has a marginal impact on the predicted results.

Development of a Portable Aerosol Collector and Spectrometer (PACS).

This article presents the development of a Portable Aerosol Collector and Spectrometer (PACS), an instrument designed to measure particle number, surface area, and mass concentrations continuously and time-weighted mass concentration by composition from 10 nm to 10 μm. The PACS consists of a six-stage particle size selector, a valve system, a water condensation particle counter to detect number concentrations, and a photometer to detect mass concentrations. The stages of the selector include three impactor and two diffusion stages, which resolve particles by size and collect particles for later chemical analysis. Particle penetration by size was measured through each stage to determine actual collection performance and account for particle losses. The data inversion algorithm uses an adaptive grid-search process with a constrained linear least-square solver to fit a tri-modal (ultrafine, fine, and coarse), log-normal distribution to the input data (number and mass concentration exiting each stage). The measured 50% cutoff diameter of each stage was similar to the design. The pressure drop of each stage was sufficiently low to permit its operation with portable air pumps. Sensitivity studies were conducted to explore the influence of unknown particle density (range from 500 to 3,000 kg/m3) and shape factor (range from 1.0 to 3.0) on algorithm output. Assuming standard density spheres, the aerosol size distributions fit well with a normalized mean bias of -4.9% to 3.5%, normalized mean error of 3.3% to 27.6%, and values of 0.90 to 1.00. The fitted number and mass concentration biases were within ±10% regardless of uncertainties in density and shape. However, fitted surface area concentrations were more likely to be underestimated/overestimated due to the variation in particle density and shape. The PACS represents a novel way to simultaneously assess airborne aerosol composition and concentration by number, surface area, and mass over a wide size range.

Floating potential of emitting surfaces in plasmas with respect to the space potential

The potential difference between a floating emitting surface and the plasma surrounding it has been described by several sheath models, including the space-charge-limited sheath, the electron sheath with high emission current, and the inverse sheath produced by charge-exchange ion trapping. Our measurements reveal that each of these models has its own regime of validity. We determine the potential of an emissive filament relative to the plasma potential, emphasizing variations in emitted current density and neutral particle density. The potential of a filament in a diffuse plasma is first shown to vanish, consistent with the electron sheath model and increasing electron emission. In a denser plasma with ample neutral pressure, the floating filament potential is positive, as predicted by a derived ion trapping condition. Lastly, the filament floated negatively in a third plasma, where flowing ions and electrons and nonnegligible electric fields may have disrupted ion trapping. Depending on the regime chosen, emitting surfaces can float positively or negatively with respect to the plasma potential.

Enhanced mobility of iron nanoparticles deposited onto a xenon-buffered silicon substrate

The growth of nanoparticles from the gas phase in combination with deposition under ultrahigh vacuum conditions enables one to obtain pure and mass-selected nanoparticles with variable particle density from a wide range of materials on virtually any solid support and is therefore of great interest for many applications. While the magnetic, chemical, and optical properties of such deposits are intensively investigated, less work has been undertaken so far to achieve ordered arrangements of nanoparticles deposited from a cluster beam. In this work we demonstrate a first step towards the control of nanoparticle spatial distribution for iron nanoparticles with a size of about 12 nm. We find that while deposition onto a bare Si substrate results in a random distribution of isolated nanoparticles, the introduction and subsequent desorption of a Xe buffer layer induces mobility and agglomeration of nanoparticles on the Si surface. The agglomerates are extended and exhibit a height equal to about 15 nanoparticles. The vertical growth of the agglomerates is assigned to the presence of a magnetic field which was applied perpendicular to the substrate surface. We suggest that by tuning the magnetic field and deposition conditions, the spatial arrangement of magnetic nanoparticles could be controlled.

Chemical Composition-Microstructure-Dielectric Constant Relations of Mg-Doped Calcium Titanate Synthesized by Solid State Reaction Technique

It has been generally accepted that doping of dielectric materials could significantly contribute to compositional and microstructural evolution, which consequently lead to alteration in dielectric properties. In this study, the effects of adding magnesium (Mg) at 5,10 and 20at% on the chemical composition, microstructure and dielectric constant of calcium titanate (CaTiO3) synthesized by solid state reaction was assessed. Chemical composition analysis using an X-ray diffraction technique CaTiO3 doped with 5 at% Mg has been found to contain a single phase whereas samples doped with 10 and 20 at% Mg both exhibited apparent secondary phase (MgO). Microstructural examination however, revealed that no significant variation in particle size, grain size and density were evident among the samples of different Mg contents. Average dielectric constants obtained from the entire samples ranged from 245.9 to 387.6 (at 1 MHz) and the sample with the highest dielectric constant was that doped with 5 at% Mg. Enhancement of dielectric constant in the samples with the lowest level of Mg doping has been attributed largely to the homogeneity of its chemical composition.

One-particle density matrix of trapped one-dimensional impenetrable bosons from conformal invariance

The one-particle density matrix of the one-dimensional Tonks-Girardeau gas with inhomogeneous density profile is calculated, thanks to a recent observation that relates this system to a two-dimensional conformal field theory in curved space. The result is asymptotically exact in the limit of large particle density and small density variation, and holds for arbitrary trapping potentials. In the particular case of a harmonic trap, we recover a formula obtained by Forrester et al. from a different method.

Investigating effect of pulsed flow on hydrodynamics of gas-solid fluidized bed using two-fluid model simulation and experiment

Gas-solid flow behavior in a pulsed fluidized bed was studied experimentally and numerically. Eulerian-Eulerian two-fluid model approach in conjunction with the kinetic theory of granular flow was used for simulation of dense gas-solid flow. The effect of pulsating flow with frequency range of 1 to 10Hz for particle sizes of Geldart B and A/B group with variation of particle density was investigated. Model simulation results were compared reasonably well with researcher's findings and the experimental data. Time-averaged local pressure drop and solid volume fraction were obtained and the effect of various pulsation frequencies on the bed expansion ratio, solid volume fraction, voidage and solid axial velocity was discussed. By increasing the pulsation frequency up to 10Hz, results show reduction in the bubble size, bed expansion ratio and pressure fluctuation. Time-averaged axial velocity and voidage have a flatter profile with increasing pulsation frequency.

The use of unburned propellant powder for shooting-distance determination. Part I: Infrared luminescence

Unburned propellant powder particles in gunshot residue (GSR) were detected at near infrared by optical excitation in the visible wavelength range. A series of ammunition (different brands and different manufacturers) was analyzed concerning the luminescence of their propellant.Shooting target samples with different shooting distances were produced on standard textile tissue and analyzed with this optical infrared inspection. The number of luminescent GSR particles per area was measured and curves with particle density vs. shooting distance were drawn.The method was applied on three ammunition types with different particle morphology shot with a pistol and one ammunition type shot with a revolver. The shooting series performed with the revolver showed a large particle density variation within the samples of identical shooting distances. In this case, the ratio of the amount of particles within the area around the bullet hole and within a ring with a defined distance from the bullet hole was calculated. These data resulted in measures with much lower standard deviations, which is a prove that the distribution pattern depends on the shooting distance and not on the amount of GSR particles.It has been shown, that imaging of target tissue with the aid of infrared luminescence is an easy, fast, reproducible and non-destructive method for shooting-distance determination.

Acoustic backscatter by suspended cohesive sediments: Field observations, Seine Estuary, France

Observations of suspended sediment size and concentration, flow and acoustic backscatter intensity collected on the Seine Estuary (France) are used to study the acoustic response in cohesive-sediment dominated environments. Estimates of suspended sediment concentration based on optical backscatter sensors and water samples are used to calibrate the acoustic backscatter intensity. The vertical structure of suspended sediment concentration is then estimated from acoustic backscatter information. To our knowledge, this is the first field application of the recently proposed model of acoustic scattering by flocculating suspensions based on the variation of particle density (floc-scattering model). The estimates of sediment concentration reproduce well the observations under different tidal (neap/spring) conditions, confirming the applicability of the new model in the field when detailed particle size measurements are available. When particle size measurements are not available, using estimated floc sizes based on the turbulence intensities may provide reasonable SSC profiles. During spring tide events (associated with strong currents, small flocs and large concentrations), the performances of the new floc-scattering model and the previous models given for solid particle-scattering are comparable. The floc-scattering model increases the quality of the SSC estimates especially during low-energy conditions characterized with larger flocs.

Nanoparticle stochastic motion in the inertial regime and hydrodynamic interactions close to a cylindrical wall.

We have carried out direct numerical simulations (DNS) of the fluctuating Navier-Stokes equation together with the particle equations governing the motion of a nanosized particle or nanoparticle (NP) in a cylindrical tube. The effects of the confining boundary, its curvature, particle size, and particle density variations have all been investigated. To reveal how the nature of the temporal correlations (hydrodynamic memory) in the inertial regime is altered by the full hydrodynamic interaction due to the confining boundaries, we have employed the Arbitrary Lagrangian-Eulerian (ALE) method to determine the dynamical relaxation of a spherical NP located at various positions in the medium over a wide span of time scales compared to the fluid viscous relaxation time τv = a2/v, where a is the spherical particle radius and v is the kinematic viscosity. The results show that, as compared to the behavior of a particle in regions away from the confining boundary, the velocity autocorrelation function (VACF) for a particle in the lubrication layer initially decays exponentially with a Stokes drag enhanced by a factor that is proportional to the ratio of the particle radius to the gap thickness between the particle and the wall. Independent of the particle location, beyond time scales greater than a2/v, the decay is always algebraic followed by a second exponential decay (attributed to the wall curvature) that is associated with a second time scale D2/v, where D is the vessel diameter.

Multi-fluid Modeling Biomass Fast Pyrolysis in the Fluidized-Bed Reactor Including Particle Shrinkage Effects

The fast pyrolysis of biomass in a bubbling fluidized-bed reactor was simulated with the multi-fluid model combing the variable particle density and diameter model based on the mass conservation at the particle scale. Different particle shrinkage effects on the reactor performance were investigated through changing the apparent density of char species. The detailed distributions of particle density and diameter in the reactor and entrained out of the system were revealed. The results demonstrate that the reactor performance, including the particle density and diameter distribution, entrainment behavior, biochar composition and yield, and biomass conversion, is dramatically affected by the particle shrinkage effects. The average particle density increases, while the average particle diameter decreases, with the increase of the char density, which means more intense shrinkage. A weaker shrinkage effect leads to stronger entrainment behavior, larger biochar yield and mass fraction of biomass in biochar, and ...

Multi-fluid model with variable particle density and diameter based on mass conservation at the particle scale

The multi-fluid model (MFM) has been widely used in the simulation of gas–solid fluidized bed reactor, in which the density and diameter of particles normally change due to the heterogeneous reactions. However, the particle diameter always keeps constant although the particle density varies along with reaction in the previous MFM, which may disobey the law of mass conservation at the particle scale. Therefore, in order to overcome the limitation of constant diameter in the previous MFM, the biomass pyrolysis reaction was taken as an example to develop the variable particle density and diameter model based on the mass conservation at the particle scale, which was successfully incorporated into MFM to simulate the biomass pyrolysis process in a fluidized bed reactor. The results indicate that the segregation/mixing and entrainment behavior are significantly affected by the variation model of particle density and diameter and the species density in the solid phase.

Radial mixing and segregation of granular bed bi-dispersed both in particle size and density within horizontal rotating drum

Taking simultaneous variations in both particle volume and density into account, the radial mixing and segregation of binary granular bed in a rotating drum half loaded were investigated by a 3D discrete element method. Then, based on the competition theory of condensation and percolation, radial segregation due to differences in particle volume and/or density was analyzed. The results show that if either percolation effect induced by volume difference or condensation effect induced by density difference dominates in the active layer of moving bed, separation will occur. Controlling the volume ratio or density ratio of the two types of particles can achieve an equilibrium state between percolation and condensation, and then homogenous mixture can be obtained. When the percolation balances with the condensation, the relationship between volume ratio and density ratio presents nearly a power function. Scaling up a rotating drum will not affect the mixing degree of the granular bed so long as the volume ratio and density ratio are predefined.

Simulating the pitch sensitivity of twisted nematics of patchy rods

Stiff, elongated biomolecules such as filamentous viruses, DNA or cellulose nanocrystals are known to form liquid crystals often exhibiting a helical supramolecular organization. Little is known about the microscopic origin, size and handedness of the helical pitch in these, so-called cholesteric phases. Experimental observations in chiral lyotropics suggest that long-ranged chiral forces of electrostatic origin acting between the mesogens are responsible for such organization. Using large-scale computer simulation we study the sensitivity of the pitch imparted by soft microscopic helices and confirm that the helical sense is sensitive to a change of packing fraction, magnitude of the molecular pitch and amplitude of the chiral interactions. In particular, we find evidence that the cholesteric helix sense may change spontaneously upon variation of particle density, at fixed molecular chirality. These pitch inversions have been reported in recent theoretical studies but simulation evidence remains elusive. We rationalize these sudden changes in the supramolecular helical symmetry on the basis of detailed measurements of the mean-torque generated by the twisting of the helices. The simulation methodology employed does not require confining the twisted nematic in a slab geometry and allows for a simultaneous measurement of the pitch and the twist elastic constant. We find that the twist elastic constant increases almost linearly with density suggesting that twisted nematic shows no signs of anomalous stiffening due to pre-smectic fluctuations at higher packing fraction.

Computational study of particle temperature in a bubbling spout fluidized bed with hot gas injection

CFD–DEM simulations of a hot gas jet issuing into a pseudo 2D gas–solid fluidized bed of glass particles are reported. A novel CFD–DEM model accounting for fluid–particle heat transfer has been developed and used for this particular research. In this work the background gas is maintained at minimum fluidization velocity and room temperature (300K) while a hot gas jet (500K) is injected from the center of a pseudo 2D bed. Three different particle sizes have been studied (1mm, 2mm and 3mm). The hot gas jet causes continuous bubble formation and propagation in the bed thus creating a circulation pattern of particles. Using the detailed simulation data various kinds of analysis are presented on the particle temperature statistics like standard deviation and distribution profiles of time-averaged particle density, volume flux and temperature variation about the mean. Further, a tracer particle analysis is presented that shows the variation of particle temperatures about the mean for different particle sizes.

Density assessment method of chemical components in urban submicron aerosol particles

During the analysis of aerosol particle density variation in urban environment it was noticed that a Quadrupole Aerosol Mass Spectrometer (Q-AMS) and a Scanning Mobility Particle Sizer (SMPS) indicated the particle diameter differently. Measurements presented in this paper were conducted in Šiauliai town from 15 March to 27 March 2012. Size distribution and concentration spectra for non-refractory submicron aerosol particles (PM1) of NH4+, NO3–, SO42–and organics were obtained by using Q-AMS and SMPS. The organic particles with a diameter of up to 100 nm were selected for the analysis due to the considerable influence of traffic emission on the sampling site. The major formation of particles in the range of up to 100 nm occurs from pollutants of traffic emissions. It was observed that the density of organic aerosol particles in the range up to 100 nm was relatively too high if compared with the density sum of all components in the range above 100 nm (less than 1 g cm–3). This observation was opposite to the result that was expected. The latter results imply incorrectness for direct interpretation of the data obtained from Q-AMS and SMPS. Therefore, the method for evaluation of particle diameters provided by Q-AMS and SMPS is discussed in this paper.

A Modified SPH Model for Simulating Water Surface Waves

As a flexible Lagrangian particle method, smoothed particle hydrodynamics (SPH) can easily capture large interface deformation, breaking, merging and splashing, which is quite suitable for the simulation of water surface waves. The classic weakly compressible smoothed particle hydrodynamics (WCSPH) algorithm for incompressible fluid flow usually needs small time steps to ensure numerical stability, which makes it computational time consuming. The major reason is that the pressure is linked with the density deviation which should be zero everywhere in incompressible fluid flows. Generally, the equation of state for a weakly compressible fluid is adopted to simulate incompressible fluid. To enforce the incompressibility, the stiffness of fluid is set to be large enough to keep the density deviation less than an allowable error. That will lead to a problem that a tiny error of computed density will cause a large pressure error and further numerical instability. In order to obtain rational pressure distribution and good computational efficiency as well, a new algorithm is proposed considering that flow field is globally incompressible from the view of spatial averaged flow field and weakly compressible locally due to the computational error from the view of particles. The basic conception of the proposed modified SPH is to split the pressure into a global part and local fluctuating part. The global part of pressure or the spatial averaged pressure on the resolution of background mesh is obtained by solving the pressure Poisson equation based on the velocity divergence free condition. The local fluctuating pressure is related to the local particle density variation by solving the local pressure Poisson equation, which is applied to maintain particles’ spacing in relative equilibrium. Based on this idea, formulas for the spatial averaged pressure and local fluctuating pressure are derived. The spatial averaged pressure is used as a primary estimation of the pressure gradient force and then the local fluctuating pressure is added as a correction of local density deviation. The proposed algorithm is verified by simulating the classic dam broken problem and then applied to simulate water wave breaking. A comparison of the particle configurations with previous studies at different times is shown. Good agreement between the present results and the previous results can be found. A discussion on computational efficiency is also carried out. The result shows that the proposed algorithm has better stability and can greatly reduce the computational time cost.

Computational Modeling of Biomass Thermochemical Conversion in Fluidized Beds: Particle Density Variation and Size Distribution

The design and scale-up of fluidized-bed reactors is an important step to commercialize viable conversion pathways (such as fast pyrolysis) for biomass into hydrocarbon intermediates and fuels that lead to “drop-in” replacements for jet fuel, diesel, gasoline, and other petroleum-based products. Detailed information about the particle size distribution (PSD) and particle density evolution throughout the fluidized-bed reactor can play a critical role in determining in situ catalyst selectivity, intermediate components, and reactor performance. This work presents an Euler–Euler computational fluid dynamics (CFD) model applied to biomass thermochemical conversion for use in fluidized-bed reactor simulations. The complex chemical and physical processes of particle devolatilization and their interaction with the reacting gas environment are described within a multifluid framework based on the kinetic theory of granular flows. The direct quadrature method of moments is used to describe the biomass PSD. Continuo...

Is the continuous matter creation cosmology an alternative to ΛCDM?

The matter creation cosmology is revisited, including the evolution of baryons and dark matter particles. The creation process affects only dark matter and not baryons. The dynamics of the ΛCDM model can be reproduced only if two conditions are satisfied: 1) the entropy density production rate and the particle density variation rate are equal and 2) the (negative) pressure associated to the creation process is constant. However, the matter creation model predicts a present dark matter-to-baryon ratio much larger than that observed in massive X-ray clusters of galaxies, representing a potential difficulty for the model. In the linear regime, a fully relativistic treatment indicates that baryons are not affected by the creation process but this is not the case for dark matter. Both components evolve together at early phases but lately the dark matter density contrast decreases since the background tends to a constant value. This behaviour produces a negative growth factor, in disagreement with observations, being a further problem for this cosmology.

Reprint of: Multi-fluid CFD modeling of biomass gasification in polydisperse fluidized-bed gasifiers

This work presents an Eulerian computational fluid dynamics (CFD) model of biomass gasification for use in fluidized-bed gasifier (FBG) simulations. The physical and chemical processes of particle gasification and their interaction with the reactive gas flow are modeled within a multi-fluid framework derived from kinetic theory of granular flows. The transport equations of continuous solid phases and species mass fraction of CO, CO2, CH4, H2, H2O, O2, N2, tar, char, and ash are coupled with chemical kinetic models, which describe moisture vaporization, particle devolatilization, homogeneous volatile reaction, heterogeneous char oxidation and char gasification. Continuously variable particle density due to volatilization of lighter components and chemical reactions is implemented to account for the evolution of reacting particles' physical properties. A time-splitting method is employed for decoupling the chemical source from convection. A time-step adaption and restart procedure is implemented to provide the solution stability for strong chemical reaction and to achieve numerical solution efficiency for energy reactor simulations in the time-splitting approach. The chemical source terms based on chemical kinetics are implemented with first-order accuracy in the finite-volume framework. The CFD model is used to simulate wood gasification using air as a fluidization agent in a lab-scale FBG. The simulation results provide detailed information on the dynamic particle processes, char elutriation, and gas composition at the reactor outlet. Operating conditions of different air/biomass mass flow ratio, reactor temperature, and biomass moisture content are simulated and analyzed as to their influence on gas composition and product yields at the gasifier outlet.