Particle Deposition Velocity Research Articles

Development of a three-dimensional computational slag flow model for coal combustion and gasification

Slagging and fouling are important phenomena associated with ash handling and discharge in coal combustion and gasification. Slagging may have strong impact on the heat transfer characteristics in the reactor, and may cause reliability and safety concerns. A novel, fully three-dimensional slag model has been developed for modeling the slag flow characteristics including char/ash deposition and molten slag flow on the reactor wall. The model couples two multiphase flow models, i.e., the Volume of Fluid (VOF) model and the Discrete Phase Model (DPM), to describe the gas phase, slag phase, and solid particle phase in slagging coal combustion. The model is implemented in the form of User Defined Functions (UDFs) in a three-dimensional Computational Fluid Dynamics (CFD) code, and applied for the simulation of slag flow in a full three-dimensional pilot scale coal slagging combustion facility. Including the particle dispersion model is necessary for accurate prediction of particle deposition rate near the wall. The simulation result shows that a thin molten slag layer up to 1–2mm is built up on the refractory wall in about 5h. The slag flows downward on the side wall, accumulates on the bottom and exits at the bottom end of the reactor, mainly driven by gravity. The average slag flow velocity is generally about 0.1mm/s. Experimental result in the test facility was compared with the simulation results, and the simulation results on ash particle deposition, slag thickness and flow velocity agree qualitatively well with the experimental observations.

Particle deposition study in a horizontal turbulent duct flow using optical microscopy and particle size spectrometry

A new technique based on optical microscopy and particle size spectrometry is here presented to investigate deposition of aerosol particles. Micron-sized monodisperse liquid particles were injected in a horizontal duct flow. The turbulent flow field was recorded using Particle Image Velocimetry. The airborne particle size and number concentration was determined by isokinetic sampling in combination with an Aerodynamic Particle Sizer (APS). The surface deposited particles were counted by means of optical microscopy, which was found to be a robust and simple recording method. The particle deposition velocity was directly determined out of the number of deposited particles detected by the microscope and the airborne particle number concentration recorded by the APS. The present results show good agreement with similar studies and prove the potential of optical microscopy for the detection of wall deposited particles in comparison to the more complex and expensive methods such as fluorescence based wet chemistry or neutron activation techniques.

Experimental Research and Modeling of Particle Deposition in Ventilation Ducts

The model to predict particle deposition velocity on rough walls in fully developed turbulent duct flows has been developed in previous studies. For particle deposition model boundary conditions, it is assumed that the concentration of the particle is zero on the surface and the resuspension velocity is constant. However, the resuspension velocity may not be constant with the increase in mass of the deposited dust. To analyze the behavior of resuspension in air flow, a set of experiments were designed and conducted. Results showed that there was a linear relationship between the mass of resuspension and deposited dust. Also, the particle deposition model was improved by adding a resuspension item to the equation, and the steady-state equation was developed into a time-varying equation. The analysis results are presented in this paper.

A simplified method for assessing particle deposition rate in aircraft cabins

Particle deposition in aircraft cabins is important for the exposure of passengers to particulate matter, as well as the airborne infectious diseases. In this study, a simplified method is proposed for initial and quick assessment of particle deposition rate in aircraft cabins. The method included: collecting the inclined angle, area, characteristic length, and freestream air velocity for each surface in a cabin; estimating the friction velocity based on the characteristic length and freestream air velocity; modeling the particle deposition velocity using the empirical equation we developed previously; and then calculating the particle deposition rate. The particle deposition rates for the fully-occupied, half-occupied, 1/4-occupied and empty first-class cabin of the MD-82 commercial airliner were estimated. The results show that the occupancy did not significantly influence the particle deposition rate of the cabin. Furthermore, the simplified human model can be used in the assessment with acceptable accuracy. Finally, the comparison results show that the particle deposition rate of aircraft cabins and indoor environments are quite similar.

Gaussian diffusion sphere model to predict deposition velocity onto wafers in laminar parallel airflow considering thermophoresis

The Gaussian Diffusion Sphere Model (GDSM) was developed and improved to predict the particle deposition velocity onto a flat plate exposed to parallel airflow by considering thermophoresis in addition to the Brownian diffusion and the gravitational settling of particles. The plate surface temperature was varied and considered to be either hotter or colder than the temperature of the parallel airflow. The GDSM was able to estimate the particle deposition velocity under the influence of thermophoresis not only correctly but also very quickly, compared to the numerical approach to calculate the deposition velocity by simulating thermo-flow and particle transport. As the next step, the particle deposition velocities onto both face-up and face-down surfaces of the 450 mm wafer exposed to the parallel airflow were predicted with the GDSM by varying the wafer temperature. It was anticipated that the schemes of heating the wafer and placing the critical surface inverted during the horizontal transport of the wafer could greatly reduce the particulate contamination of the wafer critical surface.

Particle deposition in turbulent duct flows

Particle deposition in fully developed turbulent square duct flows is simulated using large eddy simulation for Reynolds numbers, based on the bulk velocity and duct width, equal to 250k, 83k and 10,320. A particle equation of motion including Stokes drag, lift, buoyancy and gravitational forces is used for particle trajectory analysis. Results obtained for the fluid phase show good agreement with experimental data and the predictions of direct numerical simulations. Predictions for particles show that the secondary flow established in the duct cross-section plays an important role in the particle deposition process. Under the influence of this flow, high-inertia particles (particle Stokes number, St>12.38) tend to deposit close to the corners of the duct floor, while low-inertia particles (St<6.43) deposit near the floor centre. It is shown that the flow Reynolds number, particle size, drag force, shear-induced lift force and gravity all affect the particle deposition process. Particle deposition in the vertical direction increases with flow Reynolds number but simultaneously decreases in the horizontal direction. The particle deposition velocity is found to increase with both the particle size and the flow Reynolds number, with the tendency for deposition at the duct corners increasing with both variables. From a dynamic analysis, gravity most significantly affects particle deposition in the vertical direction, while in the horizontal direction the drag force dominates. The influence of the lift force increases with particle size, and its effect becomes significant as particles approach the duct floor; hence, it can act as another important factor causing particles to accumulate at the corners of the duct. Generally, and for all particle populations in the three flows considered, the particle deposition process can be described by the free-flight model.

The effects of simultaneous electrophoresis and thermophoresis on particulate contamination of an inverted EUVL photomask surface in parallel airflow

The combined influences of electrophoresis and thermophoresis on particle deposition on the inverted critical surface of a flat plate in parallel airflow were investigated by employing the statistical Lagrangian particle tracking approach in an effort to assess the degree of particulate contamination of EUVL photomasks during horizontal transport in cleanroom environments. The numerical method was validated through the comparison with the experimental data, found in the literature, about particle deposition velocity onto a wafer in vertical airflow with and without electrophoresis or thermophoresis. In addition, the validation of the present model was performed via the comparison with the theoretical prediction of particle deposition velocity onto a flat plate under no phoretic forces in parallel airflow. Then, the particle deposition velocity onto the face-down surface of a flat plate in parallel airflow was obtained by varying the temperature of the inverted critical surface in different strengths of uniform electric fields. Injected particles were assumed to be charged with -1 , 0, or +1 elementary unit of charge, in order to consider attractive or repulsive electric force. The degree of particulate contamination of the inverted critical surface was found to be significantly influenced by the combination of electrophoretic and thermophoretic effects.

Developing an Empirical Equation for Modeling Particle Deposition Velocity onto Inclined Surfaces in Indoor Environments

For the purpose of modeling indoor particle dispersion with an Eulerian drift flux model or analyzing indoor particle deposition onto various surfaces accurately, it may take considerable time to calculate the deposition velocity for each surface as numerical integration or calculation is usually needed. In this article, a modified three-layer model is presented to calculate indoor particle deposition velocities for surfaces with different inclinations and for different friction velocities. Then, 1020 cases, covering the common indoor scenarios, were modeled to obtain a database of indoor particle deposition velocities. Based on the results of the 1020 cases, an empirical equation was generated to determine indoor particle deposition velocities. The empirical equation was divided into four parts, named the Fine zone, Coarse zone, Zero zone, and Transition zone. In the Fine zone, the friction velocity decides the particle deposition velocity, while in the Coarse zone, the inclination angle of the surface i...

Influence of the Lagrangian Integral Time Scale Estimation in the Near Wall Region on Particle Deposition

In a high temperature pebble-bed reactor core where thousands of pebbles are amassed, the friction between the outer graphite layer of the fuel elements triggers the formation of carbonaceous dust. This dust is eventually conveyed by the cooling carrier phase and deposits in the primary circuit of the high temperature reactor. The numerical prediction of carbonaceous dust transport and deposition in turbulent flows is a key safety issue. Most particle tracking procedures make use of the Lagrangian integral time scale to reproduce the turbulent dispersion of the discrete phase. In the present Lagrangian particle tracking procedure, the effect of the Lagrangian integral time scale near the wall is thoroughly investigated. It is found that, in the linear sublayer, a value of the normalized wall normal component of the Lagrangian integral time scale lower that 4 delivers accurate particle deposition velocities. The value worked out here near the wall region is in accordance with Lagrangian integral time scales derived from recent direct numerical simulations.

Size-Dependent Dry Deposition of Airborne Polybrominated Diphenyl Ethers in Urban Guangzhou, China

Gaseous and size-segregated particulate PBDEs (specifically BDE-47, -99, -183, -207, and -209) in the air were measured in urban Guangzhou at 100 and 150 m above the ground in daytime and at night in August and December 2010, to assess dry deposition of these contaminants accurately with regards to influences of meteorological factors but without confounding surface effects. Particulate PBDEs were more abundant at night than in daytime, and slightly higher in winter than in summer, likely from varying meteorological conditions and atmospheric boundary layers. More than 60% of particulate-phase PBDEs was contained in particles with an aerodynamic diameter (D(p)) below 1.8 μm, indicating long-range transport potential. The average daily particle dry deposition fluxes of PBDEs in August ranged from 2.6 (BDE-47) to 88.6 (BDE-209) ng m(-2) d(-1), while those in winter ranged from 2.0 (BDE-47) to 122 (BDE-209) ng m(-2) d(-1). Deposition fluxes of all PBDE congeners were significantly higher in daytime than at night for both months, due to the effect of diurnal variability of meteorological factors. In addition, mean overall particle deposition velocities of individual BDE congeners ranged from 0.11 to 0.28 cm s(-1). These values were within a factor of 2 of assumed values previously used in southern China and the Laurentian Great Lakes, suggesting that such assumptions were reasonable for sites with similar particulate size distributions and PBDE sources. Dry deposition velocities of PBDEs were lower at night than those in the daytime, probably reflecting higher mechanical and thermal turbulence during daytime. Dry deposition of particulate-bound PBDEs is influenced by short-term temporal variability from meteorological factors, and also by particulate size fractions.

Dispersion and deposition mechanisms of particles suspended in a turbulent plane Couette flow

Inertial particle transfer in a turbulent plane Couette flow (C flow) was studied using Direct Numerical Simulation (DNS) of the flow combined with a Lagrangian particle tracking approach for particles with Stokes numbers (St) 5, 25 and 125. The particle concentration was assumed low enough, so that the simulations were done under one-way coupling condition.Our numerical simulations show that the particles suspended in a C flow (C particles) have velocities respectively smaller and larger than the carrier fluid velocities in the near-wall and core region. The C particles in comparison to the same sized particles suspended in a Poiseuille flow (P particles) with a same Reynolds number (Re), have larger fluctuating activities in the core, while the opposite was observed in the near-wall region. This fact is due to the Large Scale Structures (LSSs) existing in the core region of the C flows. The LSSs also strengthen the near-wall accumulation of C particles, so that these particles respect to the same sized P particles have more accumulation in the near-wall region. Moreover, the LSSs affect the deposition velocity of particles whereby among the P and C particles with smallest Stokes number (St=5), the C particles have larger deposition velocity while among the those ones with larger Stokes numbers (St=25,125) the opposite was the case.Finally, it was found that although the diffusional deposition mechanism for the C particles is less efficient than for the P ones, the number of deposited C particles during the time is greater than the number of deposited P particles.

Prediction of Particle Deposition in Rectangular Ventilation Ducts

The accurate prediction of particle deposition in ventilation ducts is an important step in estimating the exposure risk of building occupants to particulate matter. In this paper a mathematical model for predicting particle deposition in rectangular ventilation duct flow by incorporating existing theoretical and experimental results is presented. The influences of Brownian diffusion, turbulent diffusion and gravity on particle deposition are considered. Model equations are presented for calculating the deposition velocities on vertical surfaces and the floor and ceiling of ducts. Results of calculations are compared with the experimental results of Sippola (2002) and a good agreement of the deposition velocity of particles on the floor is shown. For the deposition velocity of particles on the vertical walls, the results obtained by this model have the same tendency as those observed by Sippola (2002). In the case of ceiling deposition, this model did not match the observation in some cases.

NUMERICAL STUDY OF THERMOPHORESIS ON AEROSOL PARTICLE DEPOSITION FROM HIEMENZ FLOW THROUGH POROUS MEDIUM ONTO A STRETCHING SURFACE

A theoretical study of aerosol particles responding to thermophoresis, involving the particle deposition rate onto a stretching permeable surface with internal heat source is proposed. The effects on particle transport mechanisms include Brownian diffusion, thermophoresis, porosity, stretching surface, heat source, and suction/injection velocity. The governing equations of continuity, momentum, energy, and particle concentration are transformed using similarity analysis, and the solutions are obtained through appropriate numerical schemes. The predicted results show that if the thermophoretic and stretching parameters increase, the deposition velocity increases for a cold surface. However, the deposition velocity decreases rapidly for a hot surface as the thermophoretic parameter increases. By way of the modeling analysis, the particle deposition velocities are calculated to control the particle mobility from the air.

Dry deposition velocity of total suspended particles and meteorological influence in four locations in Guangzhou, China

AbstractDry deposition velocity of total suspended particles (TSP) is an effective parameter that describes the speed of atmospheric particulate matter deposit to the natural surface. It is also an important indicator to the capacity of atmosphere self-depuration. However, the spatial and temporal variations in dry deposition velocity of TSP at different urban landscapes and the relationship between dry deposition velocity and the meteorological parameters are subject to large uncertainties. We concurrently investigated this relationship at four different landscapes of Guangzhou, from October to December of 2009. The result of the average dry deposition velocity is (1.49 ± 0.77), (1.44 ± 0.77), (1.13 ± 0.53) and (1.82 ± 0.82) cm/sec for urban commercial landscape, urban forest landscape, urban residential landscape and country landscape, respectively. This spatial variation can be explained by the difference of both particle size composition of TSP and meteorological parameters of sampling sites. Dry deposition velocity of TSP has a positive correlation with wind speed, and a negative correlation with temperature and relative humidity. Wind speed is the strongest factor that affects the magnitude of TSP dry deposition velocity, and the temperature is another considerable strong meteorological factor. We also find out that the relative humidity brings less impact, especially during the dry season. It is thus implied that the current global warming and urban heat island effect may lead to correlative changes in TSP dry deposition velocity, especially in the urban areas.

Calculation of Particle Deposition Velocity in the Lagrangian Frame

Calculation of Particle Deposition Velocity in the Lagrangian Frame

Thermophoresis particle deposition on unsteady two-dimensional forced convective heat and mass transfer flow along a wedge with variable viscosity and variable Prandtl number

In this paper, the effects of thermophoresis particle deposition on an unsteady two dimensional forced convective heat and mass transfer flow past a wedge taking into account the variation of fluid viscosity and fluid Prandtl number with temperature are studied. The local similarity equations are derived and solved numerically using Nachtsheim–Swigert shooting iteration technique along with the sixth order Runge–Kutta integration scheme. Comparisons with previously published work are performed, and the results are found to be in excellent agreement. Results for the non-dimensional velocity, temperature, concentration, Prandtl number and thermophoretic velocity are displayed graphically whereas thermophoretic deposition velocity is shown in the tabulated form for various values of the pertinent parameters. The obtained numerical results show that in modeling the thermal boundary-layer flow with a temperature-dependent viscosity, consideration of the Prandtl number as a constant within the boundary layer produces unrealistic results, and therefore, it must be treated as a variable rather than a constant within the boundary layer. The results also show that the thermophoretic particle deposition velocity decreases as the thermophoretic coefficient increases.

Size resolved particle emission rates from an evolving indoor aerosol system

Particle emission sources were characterized by evaluating aerosol data from an evolving aerosol with a mathematical model for indoor aerosols. The model incorporates ventilation, particle emission sources, coagulation, and deposition. The size-resolved particle emission rates were solved from the mathematical difference between measured and modeled concentration values. Particle sources were parameterized with log-normal particle emission rate distributions that were used for modeling input parameters. A series of particle emission tests in an emission chamber with a nanoparticle source, or both nanoparticle source and background particle source were conducted. The emission chamber particle deposition velocity was defined with a novel averaging method and with least square approximation. The size-resolved emission rates of the sources were defined by direct measurements and then solved with modelings from the emission chamber measurements. We developed iterative coagulation method which estimates particle coagulation in between the measurement time intervals. For two particle sources, a coupled particle general dynamic equation was developed. The study shows that the indoor aerosol model was capable of reconstructing the size-resolved particle emission rates of known sources, and to predict the sources influence on particle concentration levels with acceptable accuracy when the aerosol is fully mixed. We have shown that indoor aerosol modelings can be used for risk management of airborne particles by characterizing particle emitters and by estimating their influence on indoor concentrations.

A numerical study of bend-induced particle deposition in and behind duct bends.

This paper investigated the microparticle deposition and distribution due to the presence of duct bends by employing the Eulerian approach with Reynolds stress turbulent model and a Lagrangian trajectory method. The air velocity, particle velocity and particle deposition velocity were validated with available experimental data. Several particle deposition ratios were proposed to describe the particle accumulation due to bends. Particle deposition velocities in and behind bends were analyzed numerically. It is found that bend walls with surfaces of higher capture velocity tend to accumulate more contaminant particles as seen with an increased factor of 1.2 times on particle deposition velocity. Particle deposition reaches a maximum value near bend outlet, e.g. 15.2 times deposition ratio for particles of dp = 23 μm, and decay exponentially to a status of fully developed deposition in approximately 10D length. Compared to traditional consideration of sole deposition in bends, a new general concept of total deposition including that in bends and behind bends is proposed to better describe the particle deposition induced by bends since the enhancement deposition ratios behind bends compose 42–99% in the total ratios for particles of dp = 3–23 μm. Furthermore, models of fast power and exponential decay trend are demonstrated to uncover the relationship among enhancement factor of deposition velocity behind bend, dimensionless distance behind bends and particle Stokes number. The present study can contribute to the understanding and controlling of contaminant aerosol flow behavior in ducts, e.g. particle sampling, removal and associated epidemiologic study between particle and human health.

Dust deposition in ventilation and air-conditioning duct bend flows

Particles carried by airflows in ventilation and air-conditioning systems have adverse effects on the quality of air in buildings and hence the health of building occupants. Gaining insight on particle deposition onto ventilation and air-conditioning duct bends is important for controlling pollutant dispersion. Based on the Reynolds stress transport model (RSM), this paper has taken into account the effects of drag, lift force, gravity, inertia force, turbulent diffusions, particle size and air velocity on the dimensionless deposition velocity of particles in smooth duct bends using fully developed velocity profiles. At two different air velocities of 3.0 m/s and 7.0 m/s, the aforementioned effects were predicted by Reynolds-averaged Navier–Stokes (RANS)-Lagrangian simulation on square shaped duct bends with different ways of placement. Preliminary results suggest that gravity and inertia force enhance the dimensionless deposition as dimensionless relaxation time rises. Change tendency of the dimensionless particle deposition velocity on different surfaces of bend duct agrees well with previous studies. As air velocity and particle diameter increase, a significant increase of particle deposition coefficient in the duct bends is observed. Particle deposition to intrados can be intensified by the combined action of gravity and inertia force in different direction.

Statistical Lagrangian particle tracking approach to investigate the effect of thermophoresis on particle deposition onto a face-up flat surface in a parallel airflow

Thermophoresis can affect the particulate contamination of wafers and photomasks. Thermophoretic effect on particle deposition velocity in the cleanroom environment has been intensively investigated for the free-standing wafer situated perpendicular to the top-down airflow, but it has been examined by few studies for the wafers or photomasks in the parallel airflow. In this study, the particle deposition velocity onto a face-up flat surface under the influence of thermophoresis was numerically investigated, when the face-up flat surface was exposed to the parallel airflow. Statistical Lagrangian Particle Tracking (SLPT) model with the aid of commercial codes, i.e. FLUENT and DPM, was employed. The SLPT model was validated by comparing the numerically obtained particle deposition velocities with the theoretically predicted data, with and without considering the thermophoresis, and found to produce correct results. The effects of temperature difference (between the face-up flat surface and the ambient air), parallel airflow velocity, and particle density on the particle deposition velocity onto the face-up flat surface in the parallel airflow were investigated using the SLPT model, when the temperature of the face-up flat surface was either higher or lower than the ambient temperature.