Particle Concentration Fluctuations Research Articles

Experimental study on the concentration fluctuation of dust particles with different sizes during a dust storm

The particle size distribution (PSD) and concentration fluctuation were analyzed during different stages of a dust event. Our results show that dust smaller than 0.5 μm were dominant during the development stage; dust less than 0.5 μm decreased and dust from 0.5 μm to 10 μm increased in the high and low concentration stage; dust from 0.5 μm to 10 μm decreased and dust less than 0.5 μm increased in the dissipation stage. PSD of the first and fourth stages shows the unimodal distribution, but that of the other stages shows the distribution of 4 peaks. For large scale fluctuation with a period larger than 300 s, the concentration fluctuation of particles with size less than 0.4 μm and size larger than 0.4 μm has a high correlation. For small scale fluctuation with period less than 300 s, some particle size segments have high correlation at different stages, while some particle size segments have poor correlation at different stages. At the same stage, the size of the benchmark particle that is highly correlated in different particle size segments fluctuates with increasing particle size. In addition, the concentration fluctuation in 0.01276 Hz ∼ 0.02084 Hz and 0.00285 Hz ∼ 0.01096 Hz has significant contributions.

The effect of turbulent motions on particle spatial distribution in high-Reynolds-number particle-laden flows

The spatial relationship between turbulent and particle concentration structures is investigated based on the turbulent velocity and particle concentration data obtained synchronously at the Qingtu Lake Observation Array site. In addition to the observation of particle concentration structures that contain not only large-scale coherence but also significant energy in the high-Reynolds-number atmospheric surface layer (ASL), the scale of turbulent motions that have the most significant coherence with particle concentration is found to follow a 1/2 power scaling law with the local height and ASL thickness. Moreover, large-scale turbulent velocity fluctuations have a significant amplitude modulation effect on particle concentration fluctuations, but the modulating influence is different for small dust particles and large saltating particles. Based on the interphase amplitude modulation, there exists a particle–turbulence structure phase difference that varies with height, which further makes the structure inclination angle of the particle concentration larger than that of the turbulence. In this scenario, a conceptual model reflecting the relationship between the two is proposed, and a quantitative formulation is further derived and found to be in good agreement with the experimental results. These findings and the proposed model contribute insights into particle–turbulence interactions, thereby providing theoretical support for a unified model of turbulence dynamics and particle kinematics.

Characterizing inertial particle clustering with 1D signals

Preferential concentration in turbulent flows laden with inertial particles still presents many open fundamental physics questions. These gaps hinder the reconciliation of different experimental and numerical studies into a coherent quantitative view, which is needed to enable accurate high resolution modeling. We examine the influence of the dimensionality of the measurement technique (1D line, 2D planes or 3D volumes) on the characterization of cluster properties/preferential concentration, and proposes an approach to disentangle the cluster-characterizing results from random contributions that could contaminate the cluster statistical analysis. We studied this effect by projecting 2D and 3D data snapshots containing inertial particle clusters onto a 1D axis. The objective was to simulate 1D sensors (widely used experimentally: hot wire, fiber optics probes, LDV/PDA, etc) with different sensing lengths. These projected records were analysed by means of one-dimensional Voronoï tessellations. For the experimental parameters explored in this study, our results retrieved average clustering properties, when the measuring window is equal or larger than the Kolmogorov length scale (η), and smaller than of the integral length scale of the turbulence . These observations are consistent with previous research using 2D and 3D datasets taken under similar experimental conditions. The raw probability density function (PDF) of the Voronoï length in 1D does not provide error-free information on the clusters size or local concentration. This measurement bias is corrected based on the histograms of the number of particles within each cluster. Importantly, the analysis of these histograms explains the well-known clusters size PDFs power law reported in previous numerical and experimental research using 2D and 3D datasets. This power-law behavior, however, spanning less than a decade, is not robust in 1D. To address this, we use conditioned statistics (on the number of particles inside each cluster) to discern between turbulence-driven clustering and random particle concentration fluctuations. This secondary analysis complements standard cluster identification algorithms that rely on Voronoï tessellations.

Particle anti-accumulation design at impeller suction of deep-sea mining pump and evaluation by CFD-DEM simulation

deep-sea mining pumps work in the mining lifting system, transport slurry containing coarse particles. The impeller suction was the most dangerous place to clog in slurry transportation, because of its highest concentration over the whole pump. The axially mounted splitter plates were proposed to improve the convey condition in the impeller suction, numerical pump models with three lengths (50, 100, 150 mm) of splitter plates were established. The improvement comparison of particle transport characteristics and hydraulic performance of the deep-sea mining pump was evaluated by CFD-DEM simulation. Through the analysis and comparison of the particle distribution, concentration fluctuations, flow field, head loss and inlet velocity, etc. The 100 mm splitter plates were considered to be the best scheme among the three. 100 mm splitter plates boosted the hydraulic head between the pump inlet and the outlet of the first stage pump by 4.08% under rated condition, reduced the local slurry increasing concentration from 27.44% to 5.93%, reduced the slurry concentration fluctuation amplitude from 12.72% to 6.78%, shielded the reverse flows between adjacent runners, limited and corrected most of the pre-rotation flow, solved the particle accumulation problem in impeller suction area, improved the safety of deep-sea mining pump.

Impact of ventilation methods on indoor particle concentrations in a daycare center.

The objective of this study was to quantify the influence of ventilation methods on children's exposure to indoor particles in a daycare center located in an urban area. The ventilation methods applied to the center were monitored for 1 year. It appears that indoor PM10 and PM2.5 concentrations of the center were basically determined by outdoor conditions. The fluctuations in outdoor particle concentration also affected the ventilation behavior during class. The windows and doors of the classroom were frequently closed during both class hours and nights when the outdoor particle concentrations were at high levels. Statistically significant differences in the I/O ratios were found among the ventilation methods. The PM10 I/O ratio with the closed windows was significantly higher (p < 0.01) than that with the open windows, and when the mechanical fans were operated, the I/O ratio dramatically decreased (p < 0.01). The I/O ratio of PM2.5 showed a similar trend to that of PM10 except for the mechanical fan operation. The filters rated lower than MERV 11 appear to be insufficient to remove submicron particles from the mechanically supplied outdoor air when the PM2.5 concentrations are high, such as during the heating period.

Effect of drum structure on particle mixing behavior based on DEM method

The optimization of the drum structure is beneficial to improve the particle motion and mixing in rotary drums. In this work, two kinds of drum structures, Lacy cylinder drum (LC) and Lacy-lifters cylinder drum (LLC), are developed on the basic of cylinder drum to enhance the heat transfer area. The particle motion and mixing process are simulated by DEM method. Based on the grid independence and model validation, the contact number between particles and wall, particle velocity profile, thickness of active layer, particle exchange coefficient, particle concentration profile and mixing index are demonstrated. The influences of the drum structure and the operation parameters are further evaluated. The results show that the contact number between particles and wall is improved in LC and LLC compared to cylinder drum. The particle velocity in LC is higher than that in cylinder drum at high rotating speed, and the particle velocity of the particle falling region is significantly improved in LLC. Compared to cylinder drum and LC, the thickness of active layer in LLC is smaller, while the local particle mixing quality is proved to be the best in the active region. In addition, the particle exchange coefficients between static region and active region in the three drums are compared and LLC is found tending to weaken the particle flow. Besides, the fluctuations of particle concentration in the active region, static region, and boundary region are weakened in LLC, and the equilibrium state is reached earlier. In addition, the overall particle mixing performance in cylinder drum, LC and LLC is analyzed. The particle mixing performance in cylinder drum is the worst, while the difference in mixing quality of LC and LLC depends on the operation conditions.

Diffusion of Gold Nanoparticles in Inverse Opals Probed by Heterodyne Dynamic Light Scattering

The diffusive behavior of nanoparticles inside porous materials is attracting a lot of interest in the context of understanding, modeling, and optimization of many technical processes. A very powerful technique for characterizing the diffusive behavior of particles in free media is dynamic light scattering (DLS). The applicability of the method in porous media is considered, however, to be rather difficult due to the presence of multiple sources of scattering. In contrast to most of the previous approaches, the DLS method was applied without ensuring matching refractive indices of solvent and porous matrix in the present study. To test the capabilities of the method, the diffusion of spherical gold nanoparticles within the interconnected, periodic nanopores of inverse opals was analyzed. Despite the complexity of this system, which involves many interfaces and different refractive indices, a clear signal related to the motion of particles inside the porous media was obtained. As expected, the diffusive process inside the porous sample slowed down compared to the particle diffusion in free media. The obtained effective diffusion coefficients were found to be wave vector-dependent. They increased linearly with increasing spatial extension of the probed particle concentration fluctuations. On average, the slowing-down factor measured in this work agrees within combined uncertainties with literature data.

RANS-based predictions of dense solid–liquid suspensions in turbulent stirred tanks

This work is aimed at investigating the impact of different meso-scale models and constitutive equations for the RANS-based two-fluid model simulations of a turbulent solid–liquid stirred vessel with high solids loading. The model assessment is preceded with a grid convergence study, which confirms the variability of the discretization requirements depending on the observed variable. The simulation results demonstrate that for the investigated system the high solids loading contribution modelled by the kinetic theory of granular flows is negligible, both in incomplete and complete suspension conditions. Instead, the particle concentration fluctuations contribution included in the momentum equations dramatically affect the predictions, particularly in incomplete suspension conditions. The evaluation of the models is completed by the comparison of the predicted solids concentration profiles with experimental data measured by Electrical Resistance Tomography. The computational strategy for achieving realistic predictions of the solid distribution both in complete and incomplete suspension conditions is outlined.

Investigation of the Collection Efficiency of a Wet Electrostatic Precipitator at a Municipal Solid Waste-Fueled Combined Heat and Power Plant Using Various Measuring Methods

This article reports results from measurements of mainly submicrometer particles at the inlet and outlet of a newly designed industrial wet electrostatic precipitator (WESP) in a combined heat and power plant fueled with municipal solid waste. The measurements were carried out with dual electric low-pressure impactors in parallel at the precipitator inlet and outlet. In addition, measurements were carried out with traditional total dust filters, low-pressure impactors, a scanning mobility particle sizer, and an aerodynamic particle sizer. The measurements aimed to characterize the aerosol particles and measure the efficiency of the WESP with special attention to fine and ultrafine particles. In general, the WESP performance and response to varying conditions was found to be in line with predictions made for the design. The WESP featured a cooled collector surface, but based on the limited results, no conclusion could be drawn regarding any possible improvement from collector cooling. The characterization of the aerosol particulate matter was challenging because of fast fluctuations in particle concentration. Methodological considerations are pointed out, mainly regarding the SMPS and ELPI measuring systems.

The role of clusters on heat transfer in sedimenting gas-solid flows

Abstract In this study, Eulerian-Lagrangian simulations of unbounded sedimenting gas-solid flows are conducted to quantify the effect of two-phase flow interactions on thermal transport. Cold particles are initially randomly distributed in a hot gas and settle under gravity for a wide range of volume fractions and Prandtl numbers. Two-way coupling spontaneously generates clusters of particles with a characteristic length scale much larger than the particle diameter, where fluctuations in particle concentration generate and sustain the carrier-phase turbulence. Simulations reveal that the gas phase is cooled relatively fast in the vicinity of clusters while hot spots persist in regions void of particles. This non-homogeneity was found to delay the overall heat transfer by as much as a factor of four compared to systems with a homogeneous particle distribution. To identify the mechanisms responsible for these variations, the spatially-averaged temperature equation is derived for statistically homogeneous (zero-dimensional) time-dependent particle-laden flows. It is found that the local fluid volume fraction-temperature covariance, referred to here as the fluid temperature seen by the particles, is the key term that accounts for the effect of clusters on heat transfer. Next, a framework for studying thermally fully-developed flows is presented, and statistics at steady state are reported. Consistent with previous work, local variations in the interphase heat transfer coefficient are found to vary by several orders of magnitude. Finally, a statistical modeling approach based on the concept of presumed-shape probability distribution functions is proposed.

Extending the Lifetime of Resonant Atmospheric Particulate Mass Sensors With Solvent Rinses

The cleaning of a collection-based sensor extends its lifetime and reduces its effective cost. Existing cleaning regimes for silicon-based devices typically require access to large laboratory equipment. A simple cleaning method based on solvent rinses is presented here for the application of microresonator atmospheric particulate mass sensors. The suggested approach is intended for scenarios with limited access to laboratory equipment. Two piezoelectric resonator topologies (in-plane bulk mode and out-of-plane flexural) collected particles via impaction for 1 hr before rinsing. The solvent rinses reset the resonant frequency and quality factor of each resonator to within 0.4% and 10% of their original values, respectively. Subsequent mass collections were largely repeatable despite fluctuations in particle concentration and deposition location. The presented method provides a straightforward but effective cleaning method for soluble particulate removal. A physical cleaning method is required after substantial insoluble particle adsorption.

Turbulent particle flux measurement with pulse-coherent Doppler sonar

The accuracy of velocity estimates from pulse-coherent acoustic Doppler systems is related to the magnitude, R, of the ensemble-averaged complex pulse-pair correlation. The closer R is to unity, the more accurate the estimate. The accuracy of particle concentration estimates is also related to the value of R but, in contrast, high accuracy requires R to be less than unity, i.e., the amplitudes from individual pulses in an ensemble should be independent, in order to beat down Rayleigh statistics associated with configuration noise. Consequently, when estimating the turbulent component of the particle flux, i.e., the product of the particle concentration and velocity fluctuations, a tradeoff arises between these conflicting requirements for velocity and concentration accuracy. This tradeoff is investigated through a statistical model of sound scattering from particles embedded in idealized turbulence, and laboratory experiments in which particle velocities in turbulent flow are measured both acoustically, via pulse-coherent sonar, and optically, using particle imaging velocimetry.

On fluid–particle dynamics in fully developed cluster-induced turbulence

At sufficient mass loading and in the presence of a mean body force (e.g. gravity), an initially random distribution of particles may organize into dense clusters as a result of momentum coupling with the carrier phase. In statistically stationary flows, fluctuations in particle concentration can generate and sustain fluid-phase turbulence, which we refer to as cluster-induced turbulence (CIT). This work aims to explore such flows in order to better understand the fundamental modelling aspects related to multiphase turbulence, including the mechanisms responsible for generating volume-fraction fluctuations, how energy is transferred between the phases, and how the cluster size distribution scales with various flow parameters. To this end, a complete description of the two-phase flow is presented in terms of the exact Reynolds-average (RA) equations, and the relevant unclosed terms that are retained in the context of homogeneous gravity-driven flows are investigated numerically. An Eulerian–Lagrangian computational strategy is used to simulate fully developed CIT for a range of Reynolds numbers, where the production of fluid-phase kinetic energy results entirely from momentum coupling with finite-size inertial particles. The adaptive filtering technique recently introduced in our previous work (Capecelatroet al.,J. Fluid Mech., vol. 747, 2014, R2) is used to evaluate the Lagrangian data as Eulerian fields that are consistent with the terms appearing in the RA equations. Results from gravity-driven CIT show that momentum coupling between the two phases leads to significant differences from the behaviour observed in very dilute systems with one-way coupling. In particular, entrainment of the fluid phase by clusters results in an increased mean particle velocity that generates a drag production term for fluid-phase turbulent kinetic energy that is highly anisotropic. Moreover, owing to the compressibility of the particle phase, the uncorrelated components of the particle-phase velocity statistics are highly non-Gaussian, as opposed to systems with one-way coupling, where, in the homogeneous limit, all of the velocity statistics are nearly Gaussian. We also observe that the particle pressure tensor is highly anisotropic, and thus additional transport equations for the separate contributions to the pressure tensor (as opposed to a single transport equation for the granular temperature) are necessary in formulating a predictive multiphase turbulence model.

Effect of the instantaneous turbulent flow structures on the particle distribution near the wall of a channel

We analyzed databases obtained from direct numerical simulations of the turbulent flow in a plane channel at low Reynolds numbers. Particles with different inertia were tracked using the dilute phase approximation. The main objective of this study is the determination of the fluctuations of concentration produced by two types of instantaneous near-wall flow structures. The first type of structure produces extreme negative fluctuations of the wall shear stress (τw′≪0) and the second type of structures are associated with large positive fluctuations (τw′≫0). A conditional sampling technique was used to educe the topologies of these two types of flow structures and the corresponding conditional averaged particle concentration fluctuations. The contribution of each type of flow event to the wall shear stress history is about 10% but their contribution to the particle velocity component perpendicular to the wall, in the near-wall region, is about 50% for the flow structures associated to events with τw′≪0 and about 40% for those with τw′≫0, indicating that the role of these structures is significant in the particle fluxes from and towards the wall. The flow structures consist in two parallel counterrotating streamwise vortices that, in the case of τw′≪0, create a flow ejection from the wall between them and in the case of τw′≫0, produce a sweep towards the wall. These events produce important fluctuations of the particle concentration near the wall, with intensities up to 200% for the flow structure associated with τw′≪0 and for the largest Stokes number considered (St=25).

Effect of Flow Characteristics on Ultrafine Particle Emissions from Range Hoods

In order to understand the physical mechanisms of the production of nanometer-sized particulate generated from cooking oils, the ventilation of kitchen hoods was studied by determining the particle concentration, particle size distribution, particle dimensions, and hood's flow characteristics under several cooking scenarios. This research varied the temperature of the frying operation on one cooking operation, with three kinds of commercial cooking oils including soybean oil, olive oil, and sunflower oil. The variations of particle concentration and size distributions with the elevated cooking oil temperatures were presented. The particle concentration increases as a function of temperature. For oil temperatures ranging between 180°C and 210°C, a 5°C increase in temperature increased the number concentration of ultrafine particles by 20-50%. The maximum concentration of ultrafine particles was found to be approximately 6 × 10(6) particles per cm(3) at 260°C. Flow visualization techniques and particle distribution measurement were performed for two types of hood designs, a wall-mounted range hood and an island hood, at a suction flow rate of 15 m(3) min(-1). The flow visualization results showed that different configurations of kitchen hoods induce different aerodynamic characteristics. By comparing the results of flow visualizations and nanoparticle measurements, it was found that the areas with large-scale turbulent vortices are more prone to dispersion of ultrafine particle leakage because of the complex interaction between the shear layers and the suction movement that results from turbulent dispersion. We conclude that the evolution of ultrafine particle concentration fluctuations is strongly affected by the location of the hood, which can alter the aerodynamic features. We suggest that there is a correlation between flow characteristics and amount of contaminant leakage. This provides a comprehensive strategy to evaluate the effectiveness of kitchen hoods in capturing cooking oil fumes, which is based on an assessment of the entire hood face exposure instead of on breathing-zone sampling alone.

Experimental attempt to detect stimulated “diffusion” scattering by particles in liquid

The change in the shape of the correlation function of the intensity of light scattered by particles in liquid, i.e., the appearance of a cosine component, with increasing excitation radiation intensity, is observed. The amplitude of this component increases with the intensity of exciting radiation. A quadratic component appears in the dependence of the scattering intensity on the exciting radiation intensity. These results point to the appearance of stimulated “diffusion” scattering by particle concentration fluctuations.

Wavelet analysis of the particle distribution in a down-flow fluidized bed based on electrical capacitance tomography images

In the present paper, the extraction of dominant particle concentration fluctuation at various time–space frequency levels from reconstructed electrical capacitance tomography (ECT) images by means of the discrete wavelet transform has been proposed. Using this concept, three-dimensional images (time and two-dimensional space) obtained by an ECT system are decomposed into different time and space levels to characterize the intrinsic features of the particle distribution in a down-flow fluidized bed with a newly designed distributor using three-dimensional discrete wavelet multiresolution. As a result, the dominant time and space particle distribution characteristics for a specific frequency level can be visualized. On the basis of the wavelet analysis results, the best operation condition, under which the most stable and uniform particle distribution can be obtained, was clarified. This provides a good reference for the optimization of a downer reactor to qualify the refinery process in the petroleum industry.

Numerical investigation on gas–particle flows in horizontal channel under the reduced gravity environments

A unified second-order-moment gas–particle two-phase turbulent model incorporated with kinetic theory of granular flows (USM-θ) is developed to study the particle dispersion behavior of dense gas–particle flows in horizontal channel with 6.96 μm wall roughness and with earth, lunar and microgravity environments, respectively. Anisotropy of gas and particle two-phase stresses and the interaction between two-phase stresses are fully considered by constructing two-phase Reynolds stress model and the transport equation of two-phase stress correlation. The flow behavior of particles in a horizontal channel of Kussin and Sommerfeld [12] experiments is numerically simulated. Results show that the reduced gravity conditions affect the particle concentration distribution, particle velocity and fluctuation velocity, particle temperature, axial–axial fluctuation velocity correlation of gas and particle and particle collision frequency. Under microgravity conditions, particle temperature and collision frequency are much less than those of earth and lunar gravity. Compared with earth gravity, anisotropic of two-phase flow and sedimentation are weaker.

Variations in retention efficiency of bivalves to different concentrations and organic content of suspended particles

Retention efficiencies (RE) of scallop (Chlamys farreri), oyster (Crassostrea gigas) and mussel (Mytilus edulis) in a flow-through system were measured to understand the short-term response to various particle and organic matter concentrations. By comparing the RE of C. farreri with that of C. gigas and M. edulis, we gained further knowledge on the feeding physiological characteristics of C. farreri and ascertained the possible cause of high summer mortalities of this species. The experimental feeding conditions included natural differences in the abundance and composition of suspended seston, as well as conditions in which seston abundance and composition were manipulated by adding natural silt or cultured microalgae. The results show that in natural sea water, the minimum particle size for maximal retention in M. edulis, C. gigas and C. farreri was approximately 4, 6, and 8 μm, respectively; the RE of 2-μm (equivalent spherical diameter) particles was 17%, 19%, and 8%, respectively; and the relative standardized RE was 58%, 49%, and 18%, respectively. In C. gigas and M. edulis, the minimal particle size for maximal retention did not change with food quality (organic content). C. farreri was more sensitive to fluctuations in particle concentration and organic content. With particle concentration increase, the minimal particle size for maximal retention in scallop shifted to large particles and the RE for 2-μm particle decreased from 8% in natural seawater to 1.6%–6% in silt-enriched groups. With organic content increase, the minimal particle size for maximal retention shifted from 8 to 5 μm in natural seawater.Variation in RE of C. farreri with food conditions and the relationship between lower RE and smaller particle size may hamper C. farreri from food taking due to the decrease in the size distribution of phytoplankton in Sungo Bay.

Nanoparticle measurements used to detect an engine oil control ring failure

A cyclic variation in total particle number concentration was observed while making routine exhaust emission measurements. Many dilution and engine operating conditions were examined and, by sequentially shutting down individual cylinders, the problem was traced to cylinder 2. The engine was disassembled and piston 2's oil control ring was found to be fractured. Replacement of the ring eliminated the particle concentration fluctuation. This paper presents the results of experimental measurements made to determine the cause of the irregular emissions.