Effect of source location on particle dispersion in displacement ventilation rooms

A proposed computer model for predicting aerosol particle dispersion in indoor spaces was validated with experimental data found in the literature, and is then used to study the effect of the area and point source locations on particle dispersion in displacement ventilation （DV） rooms. The results show that aerosol source location has a strong impact on the spatial distribution and removal rate of indoor particles. Particle removal performance depends strongly on ventilation efficiency and particle deposition rate on indoor surfaces. Important consideration for both relative ventilation efficiency and deposition rate consists of the position of the aerosol source relative to the main airflow pattern and the occupied zone.

PDA MEASUREMENTS OF TWO-PHASE FLOW STRUCTURE AND PARTICLE DISPERSION FOR A PARTICLE-LADEN JET IN CROSSFLOW

The two-phase flow structure and particle dispersion for a dilute particle-laden jet in crossflow （JICF） were experimentally investigated by means of Phase Doppler Anemometry （PDA） measurement. The two-phase flow experiments were conducted for different flow conditions and solid particle parameters, including the ratio of the jet velocity to crossflow velocity, the particle size and mass loading. The experimental results indicate that the fine particles with the size of 70 micron and the mass loading of 0.05% have a minor influence on the mean and fluctuation velocity fields of the two-phase JICF. However, the fine particle transport by the two-phase JICF is dominantly and preferentially affected by the shear layer vortices and exhibits a somewhat enhanced dispersion as compared to the fluid. For the coarse particles with the particle size ranging from 300 micron to 700 micron and the mass loading less than 0.16%, the effect of the particle parameters on the fluid phase is associated with both the anisotropic properties of the flow field and the trajectory deviation of the settling particles from the fluid. Compared to the single-phase JICF, the two-phase JICF laden with the coarse particles is recognized to possess more pronounced mean velocity alteration and turbulence modulation of the fluid phase in the presence of the particles with the larger particle size and higher mass loading.

The dispersion of particles in turbulent semi-circular duct flows

The flow field in a semi-circular duct is simulated by Large Eddy Simulation(LES)and its particle field is simulated by Lagrange particle tracking method.Reynolds number Reb(based on bulk velocity and hydraulic diameter)is 80,000 and Ret(based on friction velocity and hydraulic diameter)is 3528.Particle diameter dpis chosen as 10,50,100,500 mm corresponding to St as 0.10,2.43,9.72,243.05.The results show that the intensity of the secondary flow near the ceiling is less than that near the floor because the ceiling is curved and able to inhibit the secondary flow.It is found that the difference between the semicircular duct and the square duct is that the secondary flow in a corner of the semi-circular duct is not symmetrical along the diagonal although they have the same generation mechanism.Regarding the particles,small particles(dp≤10 mm)are found to uniformly distribute in the duct,while large particles(dp≥50 mm)preferentially distribute in the corner and floor center.The maximum particles(dp=500mm)fall on the floor quickly and their dispersion mainly depends on the secondary flow near the floor.Particle deposition in the corner depends on particle size due to the effect of secondary flow and gravity.The effect of lift force on particles becomes more significant for 50 and 100 mm particles in comparison with other smaller particles.In the end,the effect of secondary flow is found to be more significant to dominate particle behavior than that of flow fluctuation.

Effects of supply air temperature and inlet location on particle dispersion in displacement ventilation rooms

The effects of supply temperature and vertical location of inlet air on particle dispersion in a displacement ventilated （DV） room were numerically modeled with validation by experimental data from the literature. The results indicate that the temperature and vertical location of inlet supply air did not greatly affect the air distribution in the upper parts of a DV room, but could significantly influence the airflow pattern in the lower parts of the room, thus affecting the indoor air quality with contaminant sources located at the lower level, such as particles from working activities in an office. The numerical results also show that the inlet location would slightly influence the relative ventilation efficiency for the same air supply volume, but particle concentration in the breathing zone would be slightly lower with a low horizontal wall slot than a rectangular diffuser. Comparison of the results for two different supply temperatures in a DV room shows that, although lower supply temperature means less incoming air volume, since the indoor flow is mainly driven by buoyancy, lower supply temperature air could more efficiently remove passive sources （such as particles released from work activities in an office）. However, in the breathing zone it gives higher concentration as compared to higher supply air temperature. To obtain good indoor air quality, low supply air temperature should be avoided because concentration in the breathing zone has a stronger and more direct impact on human health.

RESEARCH ON THE PARTICLE DISPERSION IN THE PARTICULATE TWO-PHASE ROUND JET

In this paper, the three_dimensional vo rtex filament method was used to simulate the evolution of vortex structures in the axisymmetric round jet. The results agree well with the ones given by Chung and Troutt. Then one_coupling model was employed to calculate the particle motio n based on the computed flows. The results show that the particle motion is affe cted by flows obviously at the case of particle number St1 and negligibly at St1 ,particles distribute around the vortex structures uniformly at St ～1 . When perturbations with wavenumber 5 are introduced to vortex rings, part icles disperse wider along radial direction, which conforms to the experimental results. The degree of particle dispersion is in the direct ratio to the amplitu de of perturbation. The conclusions given in the paper are useful to the practic e usage.

Predicting indoor particle dispersion under dynamic ventilation modes with high-order Markov chain model

Mechanical and natural ventilations are effective measures to remove indoor airborne contaminants,thereby creating improved indoor air quality(IAQ).Among various simulation techniques,Markov chain model is a relatively new and efficient method in predicting indoor airborne pollutants.The existing Markov chain model(for indoor airborne pollutants)is basically assumed as first-order,which however is difficult to deal with airborne particles with non-negligible inertial.In this study,a novel weight-factor-based high-order(second-order and third-order)Markov chain model is developed to simulate particle dispersion and deposition indoors under fixed and dynamic ventilation modes.Flow fields under various ventilation modes are solved by computational fluid dynamics(CFD)tools in advance,and then the basic first-order Markov chain model is implemented and validated by both simulation results and experimental data from literature.Furthermore,different groups of weight factors are tested to estimate appropriate weight factors for both second-order and third-order Markov chain models.Finally,the calculation process is properly designed and controlled,so that the proposed high-order(second-order)Markov chain model can be used to perform particle-phase simulation under consecutively changed ventilation modes.Results indicate that the proposed second-order model does well in predicting particle dispersion and deposition under fixed ventilation mode as well as consecutively changed ventilation modes.Compared with traditional first-order Markov chain model,the proposed high-order model performs with more reasonable accuracy but without significant computing cost increment.The most suitable weight factors of the simulation case in this study are found to be(λ_(1)=0.7,λ_(2)=0.3,λ_(3)=0)for second-order Markov chain model,and(λ_(1)=0.8,λ_(2)=0.1,λ_(3)=0.1)for third-order Markov chain model in terms of reducing errors in particle deposition and escape prediction.With the improvements of the efficiency of state transfer matrix construction and flow field data acquisition/processing,the proposed high-order Markov chain model is expected to provide an alternative choice for fast prediction of indoor airborne particulate(as well as gaseous)pollutants under transient flows.

INFLUENCE OF LARGE-SCALE VORTICAL STRUCTURES ON THE PARTICLE DISPERSION IN A PLANE MIXING LAYER

The present study considers the developing mixing layer that is formed bymerging of two free streams initially separated by a splitter plate. To investigate the influence ofthe vortical structures on the particle dispersion, numerical simulation was conducted when thevelocity ratio, defined as R = U_∞ - U_(-∞)/U_∞ + U_(-∞), is 0. 5. Large-Eddy Simulation (LES)was employed to understand the effect of large-scale vortical structures originated by theKelvin-Helmholtz instability on the partical dispersion. The flyash with the particle sizes 10, 50,100, 150, and 200um respectively were loaded at the origin of the two-dimensional mixing layer. Itis confirmed that the particle dispersion depends strongly on the motion of large-scale vorticalstructures. The particle dispersion is visualized numerically by following the particle trajectoriesin the mixing layer undergoing pairing interaction.

Preparation of Fe_3O_4/PS Magnetic Particles by Dispersion Polymerization

Fe_3O_4/PS magnetic particles with core/shell structure has been prepared in the presence of Fe3O4 magnetic fluid in ethanol/water mixture.Magnetic particles with diameter size range from 5. 54 t0 187. 32 μm were obtained by different reaction conditions.Some parameters such as ethanol, PEG and monomer which affect particle size diameter and size distribution are discussed briefly in this paper.

THE SIZE EFFECT ON THE DISPERSION OF A PARTICLE IN A HOMOGENEOUS ISOTROPIC TURBULENCE

The mechanism of the response motion of a suspended particle to turbulent motion of its surrounding fluid is different according to si:e of turbulent eddies. The particle is dragged by the viscous force of large eddies, and meanwhile driven randomly by small eddies. Based on this understanding, the dispersion of a particle with finite size in a homogeneous isotropic turbulence is calculated in this study. Results show that there are two competing effects: when enhanced by the inertia of a particle, the long-term particle diffusivity is reduced by the finite size of the particle.

Effect of Particle Number Density on Wave Dispersion in a Two-Dimensional Yukawa System

Effect of the particle number density on the dispersion properties of longitudinal and transverse lattice waves in a two-dimensional Yukawa charged-dust system is investigated using molecular dynamics simulation. The dispersion relations for the waves are obtained. It is found that the frequencies of both the longitudinal and transverse dust waves increase with the density and when the density is sufficiently high a cutoff region appears at the short wavelength. With the increase of the particle number density, the common frequency tends to increase, and the sound speed of the longitudinal wave also increases, but that of the transverse wave remains low.

NUMERICAL STUDY OF PARTICLE DISTRIBUTION IN WAKE OF LIQUID-PARTICLE FLOWS PAST A CIRCULAR CYLINDER USING DISCRETE VORTEX METHOD

Particle-laden water flows past a circular cylinder were numerically investigated. The discrete vortex method （DVM） was employed to evaluate the unsteady water flow fields and a Lagrangian approach was applied for tracking individual solid particles. A dispersion function was defined to represent the dispersion scale of the particle. The wake vortex patterns, the distributions and the time series of dispersion functions of particles with different Stokes numbers were obtained. Numerical results show that the particle distribution in the wake of the circular cylinder is closely related to the particle＇s Stokes number and the structure of wake vortices： （1） the intermediate sized particles with Stokes numbers, St, of 0.25, 1.0 and 4.0 can not enter the vortex cores and concen- trate near the peripheries of the vortex structures, （2） in the circular cylinder wake, the dispersion intensity of particles decreases as St is increased from 0.25 to 4.0.

Analysis of Equivalent Oxygen DifFusivity of Particle Dispersed Composites

This paper presents a new method to determine the equivalent oxygen diffusivities of particle dispersed composites. This method can be used to design FGM thermal barrier systems with the function of oxygen barrier. A qualitative explanation of the oxidation of nickel with the increment of zirconia contents in the composite samples can be accepted by this method. The values of equivalent oxygen diffusivities obtained with this method are in excellent agreement with those from the EMT method for the composites with ZrO2 particle dispersed phase when the volume fractions of dispersed phase are lower than 25%.

Development of re-crosslinkable dispersed particle gels for conformance improvement in extremely high-temperature reservoirs

Micro-scale and nano-scale dispersed gel particles(DPG)are capable of deep migration in oil reservoirs due to their deformability,viscoelasticity,and suitable particle size.Therefore,it has been widely studied and applied in reservoir conformance control in recent years.However,for highly permeable channels,their plugging performance is still limited.In addition,conventional in situ cross-linked polymer gels(ISCPGs)have fast gelation time under extremely high-temperature conditions,which often causes problems such as difficulty in pumping.Therefore,a re-cross linkable dispersed particle gel(RDPG)system applied for conformance control in highly permeable channels of extremely high-temperature petroleum reservoirs was investigated.The particle size distribution,gelation time,gel strength,injection performance,and perfo rmance strength in po rous media were investigated using a laser particle size meter,the Sydansk bottle test method,rheometer,and core displacement experiments,respectively.Results show that the RDPG suspension can be stable for more than 6 months at room temperature with storage modulus G’much lower than 10 Pa.It can pass through the pore throat by elastic deformation effect and does not cause strong blockage.Moreover,it can undergo re-crosslinking reaction at 150℃to form a strong bulk gel.The gel strength G’of re-crosslinked RDPG can be as high as 69.3 Pa,which meets the strength requirement of conformance control.The RDPG suspension has the properties of easy injection,and it also has strong plugging,and high-temperature resistance after re-crosslinked in the core,which can be a very promising material for conformance improvement in extremely high-temperature reservoirs.

Probing the effect of Young's modulus on the plugging performance of micro-nano-scale dispersed particle gels

The effect of mechanical strength of the dispersed particle gel(DPG)on its macro plugging performance is significant,however,little study has been reported.In this paper,DPG particles with different mechanical strengths were obtained by mechanical shearing of bulk gels prepared with different formula.Young’s moduli of DPG particles on the micro and nano scales were measured by atomic force microscope for the first time.The mapping relationship among the formula of bulk gel,the Young’s moduli of the DPG particles and the final plugging performance were established.The results showed that when the Young’s moduli of the DPG particles increased from 82 to 328 Pa,the plugging rate increased significantly from 91.46%to 97.10%due to the distinctly enhanced stacking density and strength at this range.While when the Young’s moduli of the DPG particles surpassed 328 Pa,the further increase of plugging rate with the Young’s moduli of the DPG particles became insignificant.These results indicated that the improvement of plugging rate was more efficient by adjusting the Young’s moduli of the DPG particles within certain ranges,providing guidance for improving the macroscopic application properties of DPG systems in reservoir heterogeneity regulation.

EFFECT OF DISPERSED PHASE PARTICLES ON ELECTRICAL CONDUCTION OF PEO-NaSCN

Fast ionic conductors are one kind of solid state material with ionic conductivity as high as that of melten salts or liquid electrolytes.Ionic conductivity is one of the important parameters for characterizing a fast ionic conductor.For a long time materialists and chemists have made great efforts in search of new fast ionic conductors with high ionic conductivity.In view of structure,they have synthesised silver and copper fast ionic conductors with so called open structures.But it is not so successful for searching more applicable alkaline fast ionic conductors.Since polymer has flexibility for making thin film,it concentrates attention on the polymer-alkaline salt complex.Fenton et al.have first reported poly(ethylene oxide) (PEO)-alkaline salt complex.Later on Armard et al.have investigated the electrical property of PEO-NaSCN.

A numerical study on dispersion of particles from the surface of a circular cylinder placed in a gas flow using discrete vortex method

The dispersion of particles emitted from the surface of a circular cylinder placed in a gas flow at the Reynolds number of 200 000 is numerically investigated using the discrete vortex method coupled with a Lagrangian approach for solid particle tracking. The wake vortex patterns, the temporal-spatial distributions and trajectories as well as the dispersion functions for particles with various Stokes numbers（St） ranging from 0.001 to 1.0 are obtained. The numerical results reveal that：（1） Solid particles on the cylinder surface are picked up and then transported away from the cylinder by the wake vortex flow.（2） Solid particles emitted from the cylinder surface always follow the vortices in the cylinder wake, and the response of particles to wake vortices is directly related to their Stokes numbers（particles with St= 0.001, 0.0038, 0.01 can distribute both in the vortex core and around the vortex periphery, whereas those with St= 0.1, 1.0 can not enter the vortex core and congregate mainly around the vortex periphery）.（3） The particles move in rolling state in the wake region, and the dispersion intensity of particles in the lateral direction decreases remarkably as the Stokes number of particles is increased from 0.001 to 1.0.

Space–time correlations in turbulent flow: A review

This paper reviews some of the principal uses, over almost seven decades, of correlations, in both Eulerian and Lagrangian frames of reference, of properties of turbulent flows at variable spatial locations and variable time in- stants. Commonly called space-time correlations, they have been fundamental to theories and models of turbulence as well as for the analyses of experimental and direct numerical simulation turbulence data.

Particles dispersion on fluid-liquid interfaces

This paper is concerned with the dispersion of particles on the fluid-liquid interface. In a previous study we have shown that when small particles, e.g., flour, pollen, glass beads, etc., contact an air-liquid interface, they disperse rapidly as if they were in an explosion. The rapid dispersion is due to the fact that the capillary force pulls particles into the interface causing them to accelerate to a large velocity. In this paper we show that motion of particles normal to the interface is inertia dominated; they oscillate vertically about their equilibrium position before coming to rest under viscous drag. This vertical motion of a particle causes a radially-outward lateral （secondary） flow on the interface that causes nearby particles to move away. The dispersion on a liquid-liquid interface, which is the primary focus of this study, was relatively weaker than on an air-liquid interface, and occurred over a longer period of time. When falling through an upper liquid the particles have a slower velocity than when falling through air because the liquid has a greater viscosity. Another difference for the liquid-liquid interface is that the separation of particles begins in the upper liquid before the particles reach the interface. The rate of dispersion depended on the size of the particles, the densities of the particle and liquids, the viscosities of the liquids involved, and the contact angle. For small particles, partial pinning and hysteresis of the three-phase contact line on the surface of the particle during adsorption on liquid-liquid interfaces was also important. The frequency of oscillation of particles about their floating equilibrium increased with decreasing particle size on both air-water and liquid-liquid interfaces, and the time to reach equilibrium decreased with decreasing particle size. These results are in agreement with our analysis.

MORPHOLOGY EVOLUTION IN PTFE AS A FUNCTION OF MELT TIME AND TEMPERATURE——Ⅰ.HIGH MOLECULAR WEIGHT SINGLE-AND MULTI-MOLECULE FOLDED CHAIN SINGLE CRYSTALS AND BAND STRUCTURES

The effect of sintering dispersed dispersion and nano-emulsion particles of high molecular weightpolytetrafluoroethylene(PTFE)on a substrate as a function of“melt”time and temperature is described.Folded chain singlecrystals parallel to the substrate and as ribbons on-edge(with double striations),as well as bands,are produced for longersintering times;particle merger and diffusion of individual molecules,crystallizing as folded chain,single(or few)molecule,single crystals when“trapped”on the substrate by cooling occur for shorter sintering times.It is suggested the observedstructures develop with sintering time,in a mesomorphic melt.The structure of the nascent particles is also discussed.

Jet dispersion and deposition of charged particles in confined chambers

Dispersion and surface deposition of charged particles by gas-solids jets in confined chambers are constantly encountered in many industrial applications such as in electrostatic precipitation and dry powder coating processes. Understanding and control of flow patterns and trajectories of charged particles are important to the optimal design and operation of such devices. In this study, modeling of flow fields and particle trajectories of dilute gas-solid two-phase flows with charged particles in confined chambers is performed. The dilute gas-solid two-phase flows are simulated by use of a hybrid Eulerian-Lagrangian approach with the one-way coupling between the gaseous phase and particle phase. The space charge distribution is included as a source term in equations of motion or Lagrangian equation of charged particles, which in turn depends on the particle trajectories that determine the space charge distribution. Our modeling predictions suggested that the electrostatic charge plays a significant role in particle radial dispersion. Effect of voltage has limited influence on particle trajectories however it can have a big impact on the residence time. Cone angle has a significant effect on the structure of flow field. For cone with a larger cone angle （typically over 15°）, there will be a flow separation along the side wall near the flow entrance region. By comparing with the conical chamber, the cylindrical chamber has a big vortex and three smaller vortexes in the lower part of the chamber, which would complicate the particle dispersion with or without the coupling of charging.