Particle Deposition Velocity Research Articles

Studies on steam condensation and particle diffusiophoresis in a heat exhanger tube

In this paper, studies on steam condensation and aerosol behaviour in a heat exchanger are presented. The heat exchanger is a model of one single tube of a Passive Containment Condenser (PCC) that is used in the European Simplified Boiling Water Reactor (ESBWR). A hot carrier gas containing nitrogen and steam with Ag and CsOH particles flows through the heat exchanger. The walls of the heat exchanger are cooled with a water jacket, thus causing steam condensation and diffusiophoretic particle deposition. The amount of condensed steam is measured, as well as the temperatures, particle mass concentrations and size distributions before and after the heat exchanger. The experiments are done with different proportions of steam and nitrogen in the carrier gas. Heat and mass transfer in the system are modelled with well known engineering correlations, producing results that agree nicely with the experimental results. The diffusiophoretic particle deposition velocity is shown to be proportional to the steam condensation rate, as expected.

The role of sea‐salt particles as cloud condensation nuclei over the remote oceans

AbstractIf recent measurements of the fluxes of sea‐salt particles over the oceans as a function of wind speed are extrapolated to include the smallest particles detected over the oceans and in laboratory experiments on bursting bubbles, the flux F of particles having radii between r0 = 0.1 μm and r μm at 80% relative humidity may be represented by F = 2U3.5(e−0.2r0) – e−0.2r) m−2) s−1, where U is the near‐surface wind speed in m s−1. The flux of particles of r ≤ 5 μm in winds of 16 m s−1 is 20 000 m−2s−1. The corresponding local rate of production by air bubbles bursting in the foam patches of breaking waves that occupy, on average, only 5% of the ocean surface at this wind speed is 40 cm−2s−1. The average concentration of particles over the sea area producing whitecaps is: where vr is the deposition velocity for particles of radius r. In winds of 16 m s−1 the total concentration of sea‐salt particles is calculated to be 40 cm−3. These, and the fluxes and concentrations calculated for other wind speeds, are consistent with measurements made at sea and in the laboratory by many scientists during the last 50 years.Recently O'Dowd et al. have reported on sea‐salt particles of dry radius as small as 0.03 μm, with radius 0.06 μm at 80% relative humidity and mass 5 × 10−16 g. If the above equation is extended to include such small particles, it predicts a total concentration of 60 cm−3 in winds of 17 m s−1, fairly close to the reported 70 cm−3.In general these computed and observed concentrations of sea‐salt particles are adequate to account for observed droplet concentrations in clouds over remote oceans uncontaminated by continental aerosols. However, on rather rare occasions droplet concentrations in maritime cumulus exceed 100 cm−3. Observations are cited to suggest that, in such cases, sea‐salt nuclei may be augmented by biogenic particles originating at the sea surface and by sulphate/sulphuric acid nuclei produced by the absorption of derivatives of dimethyl sulphide in cloud droplets.Locations such as Hawaii and Tasmania are occasionally contaminated by continental or local pollution which may account for the rare reports of droplet populations being as high as 400 cm−3.

Strategies for including vegetation compartments in multimedia models

The incentives for including vegetation compartments in multimedia Level I, II and III fugacity calculations are discussed and equations and parameters for undertaking the calculations suggested. Model outputs with and without vegetation compartments are compared for 12 non-ionic organic chemicals with a wide variety of physical-chemical properties. Inclusion of vegetation compartments is shown to have a significant effect on two classes of chemicals: (1) those that are taken up by atmospheric deposition and (2) those that are taken up by transpiration through the plant roots. It is suggested that uptake from the atmosphere is important for chemicals with log K OA greater than 6 and a log K AW of greater than −6. Plant uptake by transpiration is important for chemicals with log K OW less than 2.5 and a log K AW of less than −1. At log K OA>9 atmospheric uptake is dominated by particle-bound deposition and the importance of partitioning to vegetation is largely dependent on the relative magnitude of the particle deposition velocities to soil and vegetation. These property ranges can be used to determine if a chemical will significantly partition to vegetation. If the chemical falls outside the property ranges of the two classes it will probably be unnecessary to include vegetation in models for assessing environmental fate. The amount of chemical predicted to partition to vegetation compartments in the model is shown to be highly sensitive to certain model assumptions. Further experimental research is recommended to obtain more reliable equations describing equilibrium partitioning and uptake/depuration kinetics.

Penetration of Particles into Buildings and Associated Physical Factors. Part I: Model Development and Computer Simulations

The PM2.5 standard proposed by the U.S. Environmental Protection Agency (EPA) has stimulated research on the relationships between particulate matter concentrations and the exposures and subsequent health responses of sensitive subpopulations, such as the elderly. Since individuals in these subpopulations may spend more than 90% of their time indoors, understanding the relationship between outdoor particle concentrations and those found in indoor microenvironments is critical. This research resulted in a time-dependent indoor air quality model incorporating all potential particle sources and loss mechanisms. Monte Carlo simulations of the model identified the mechanisms, such as particle loss during penetration through the building envelope, that modify the outdoor particle size distribution during transport into the interior of a building, calculated indoor-to-outdoor (I/O) concentration ratios, and estimated penetration factors as a function of particle size. Indoor particle generation and transport of outdoor particles through the HVAC system were the most important contributors to the indoor concentration in residential and commercial buildings, respectively. The most significant removal mechanisms included ventilation through and particle removal by the HVAC filter if an HVAC system was present, or particle deposition on indoor surfaces if an HVAC system was not present. The modeled I/O concentration ratios varied between 0.05 and 0.5, depending on particle size and type of ventilation system, and agreed well with published experimental results. Penetration factors less than unity were calculated for particles with aerodynamic diameters larger than 0.2 θ m if the air exchange rate and steady-state I/O concentration ratio were correlated during the simulations. An additional correlation between the air exchange rate and particle deposition velocity is required if penetration factors less than unity are to be modeled for particles with aerodynamic diameters smaller than 0.2 θ m. These results

Atmospheric deposition of nitrogen and sulphur compounds in the Czech Republic.

Estimates of dry and wet deposition of nitrogen and sulphur compounds in the Czech Republic for the years 1994 and 1998 are presented. Deposition has been estimated from monitored and modeled concentrations in the atmosphere and in precipitation, where the most important acidifying compounds are sulphur diOxide, nitrogen Oxides, ammonia, and their reaction products. Measured atmospheric concentrations of SO2, NOx, NH3, and aerosol particles (SO4, NO3, and NH4), along with measured concentrations of SO4, NO3, and NH4 in precipitation, weighted by precipitation amounts, were interpolated with Kriging technique on a 10- x 10-km grid covering the whole Czech Republic. Wet deposition was derived from concentration values for SO4, NO3, and NH4 in precipitation and from precipitation amounts. Dry deposition was derived from concentrations of gaseous components and aerosol in the air, and from their deposition velocities. A multiple resistance model was used for calculation of SO2, NOx, and NH3 deposition velocities. Deposition velocities of particles were parameterized. It was estimated that the annual average deposition of SOx in the Czech Republic decreased from 1384 to 1027 mol H ha a between 1994 and 1998. The annual average NOy deposition was estimated to be 972 and 919 mol H a in 1994 and 1998, respectively. The annual average NHx deposition was estimated to be 887 mol H a and 779 mol H a in 1994 and 1998, respectively. It was estimated that the annual average of the total potential acid deposition decreased from 3243 to 2725 mol H a between 1994 and 1998. Sulphur compounds (SOx) contributed about 38%, Oxidized nitrogen species (NOy) 34%, and reduced nitrogen species (NHx) 28% to the total potential acid deposition in 1998. The wet deposition contributed 42% to the total potential acid deposition in 1998.

Penetration of Particles into Buildings and Associated Physical Factors. Part I: Model Development and Computer Simulations

The PM2.5 standard proposed by the U.S. Environmental Protection Agency (EPA) has stimulated research on the relationships between particulate matter concentrations and the exposures and subsequent health responses of sensitive subpopulations, such as the elderly. Since individuals in these subpopulations may spend more than 90% of their time indoors, understanding the relationship between outdoor particle concentrations and those found in indoor microenvironments is critical. This research resulted in a time-dependent indoor air quality model incorporating all potential particle sources and loss mechanisms. Monte Carlo simulations of the model identified the mechanisms, such as particle loss during penetration through the building envelope, that modify the outdoor particle size distribution during transport into the interior of a building, calculated indoor-to-outdoor (I/O) concentration ratios, and estimated penetration factors as a function of particle size. Indoor particle generation and transport of outdoor particles through the HVAC system were the most important contributors to the indoor concentration in residential and commercial buildings, respectively. The most significant removal mechanisms included ventilation through and particle removal by the HVAC filter if an HVAC system was present, or particle deposition on indoor surfaces if an HVAC system was not present. The modeled I/O concentration ratios varied between 0.05 and 0.5, depending on particle size and type of ventilation system, and agreed well with published experimental results. Penetration factors less than unity were calculated for particles with aerodynamic diameters larger than 0.2 θ m if the air exchange rate and steady-state I/O concentration ratio were correlated during the simulations. An additional correlation between the air exchange rate and particle deposition velocity is required if penetration factors less than unity are to be modeled for particles with aerodynamic diameters smaller than 0.2 θ m. These results

Development of an Atmospheric Particle Dry Deposition Model

A model to predict the atmospheric dry deposition velocities of particles has been developed that is similar to a model developed for the prediction of particle deposition velocities in vertical pipes. The model correlates the particle deposition velocity (Vd ) with Stokes settling velocity (V st), friction velocity (V*), dimensionless inertial deposition velocity (V + di ), and dimensionless Brownian diffusion deposition velocity (V + dd ). V + di

Fine Particle Dry Deposition onto a Cropland — a Trial to Estimate Deposition Velocity

The dry deposition velocity of fine particles is usually on the order of several tenths of cm s−1, and accordingly, the concentration gradient of particles above the surface is very small. This makes the application of the gradient method to the determination of the particle deposition velocity very difficult. This study is a trial to find a way around this difficulty. The idea is to evaluate the thickness of the molecular diffusion layer, zb by simultaneously measuring the concentration gradient of gaseous species such as SO2 ; zb is obtained by combining the turbulent and molecular transport equations for the gas. Then the particle deposition velocity is estimated on the basis of zb and the Brownian diffusion coefficient with an assumption that zb is equal to the thickness of the Brownian diffusion layer. By applying this method to the data collected in a field observation, the velocity of deposition onto a cropland was estimated for submicron particles.

Using Meteorological Model Output as a Surrogate for On-Site Observations to Predict Deposition Velocity

The National Oceanic and Atmospheric Administration's Multi-Layer Model (NOAA-MLM) is used by several operational dry deposition networks for estimating the deposition velocity of O3, SO2, HNO3, and particles. The NOAA-MLM requires hourly values of meteorological variables. Since collection of on-site meteorology can be expensive, a study was performed to compare NOAA-MLM predicted deposition velocitiesusing modeled meteorological data in lieu of on-site meteorological data. NOAA-MLM was run for three sites in the Clean Air Status and Trends Network using on-site data as well as the output of two mesoscale meteorological models, Eta and MM5. The differences between the deposition velocities predictedusing the mesoscale models and those predicted using the on-sitemeteorological measurements ranged from –0.001 to 0.106 cm s-1 and were within the model error determined in NOAA-MLM evaluation studies. This research shows that the NOAA-MLM is particularly sensitive to differences in atmospheric turbulence,soil moisture budget, and canopy wetness.

On the Formulation of Lagrangian Stochastic Models for Heavy-Particle Trajectories

The modeling approach of B. L. Sawford and F. H. Guest (“8th Symposium of Turbulence and Diffusion; San Diego, CA,” pp. 96–99. Am. Meteorol. Soc., Boston, MA, 1990) is extended to encompass the formulation of Lagrangian stochastic models for fluid velocities along heavy-particle trajectories in inhomogeneous turbulent flows. The approach ensures consistency with prescribed Eulerian fluid velocity statistics. Models are formulated and then used in conjuction with the equations of motion for heavy particles to simulate the trajectories of heavy particles in vertical turbulent pipe flow. Model predictions for particle-velocity statistics, particle deposition velocities, and mean particle concentrations are shown to be in good agreement with experimental results. In contrast with “eddy-interaction” models but in accord with the results of direct numerical simulations, the models predict a buildup of mean particle concentration within the viscous sublayer at y+≈ 0.2. It is suggested that Lagrangian stochastic models for fluid-particle motions provide a good description of fluid velocities along the trajectories of heavy particles, when Lagrangian timescales are appropriately modified.

A sublayer model for deposition of nano- and micro-particles in turbulent flows

Deposition of aerosol particles from 10 nm to 50 μm in turbulent duct flows is studied. The sublayer model for the turbulent deposition process is extended to cover the effects of gravity, Brownian, and lift forces. The model is based on the detailed analyses of particle trajectories in turbulence coherent structures near a wall. The Stokes drag, the Saffman lift, and the Brownian excitation are included in the particle equation of motion. Limiting trajectories for various conditions are evaluated, and the deposition velocity for a range of particle relaxation times and flow conditions are presented. In particular, a new method for evaluating the deposition velocity of Brownian particles based on the limiting trajectory concept is presented. Effects of flow shear velocity on the deposition rates of different size particles are also studied.

Spatial variations and chronologies of aliphatic hydrocarbons in Lake Michigan sediments.

Four sediment cores were collected in fine-grained depositional areas of the southern basin of Lake Michigan. Spatial variations of aliphatic hydrocarbons in surficial sediments were consistent with a lakeward movement of riverine sediments in a series of resuspension-settling cycles in which an unresolved complex mixture (UCM) of hydrocarbons associated with dense sediments is deposited in nearshore areas, fine-grained sediments of terrestrial origin accumulate in the deep basin, and planktonic hydrocarbons are depleted by microbial degradation during transport to the deep basin. The rate of accumulation of the UCM (a marker of petroleum residues) in deep basin sediments has increased by more than an order of magnitude since 1880, from 60 microg m(-2) x a(-1) to approximately 960 microg m(-2) x a(-1) in 1980. Crude estimates of the atmospheric loading of the UCM (1100 microg m(-2) x a(-1)) indicate that accumulations in deep-basin sediments might be supported by atmospheric deposition. Agreement was poor between the atmospheric flux of the terrestrial n-alkanes (sigmaC25, C27, C29, C31) to the deep basin (3200 microg m(-2) x a(-1)) and the sediment accumulation rate (660 microg m(-2) x a(-1)). Understanding of atmospheric fluxes, estimated from the very few available data, would be improved by more frequent measurement of the levels of aliphatic hydrocarbons in air and precipitation and a better knowledge of the particle deposition velocities and precipitation scavenging coefficients.

A size-segregated particle dry deposition scheme for an atmospheric aerosol module

A parameterization of particle dry deposition has been developed for the Canadian Aerosol Module (CAM).This parameterization calculates particle dry deposition velocities as a function of particle size and density as well as relevant meteorological variables. It includes deposition processes, such as, turbulent transfer, Brownian diffusion, impaction, interception, gravitational settling and particle rebound. Particle growth under humid conditions is also considered. Sensitivity tests show that the parameterization provides deposition velocities comparable with recent field observations, especially for sub-micron particles. The present parameterization has also been evaluated using two empirical bulk resistance models, which were originally developed from field observations. The present parameterization has been implemented in CAM, with meteorological input provided by the Canadian Regional Climate Model (RCM) to the eastern North America. A comparison of the modelled dry deposition velocities to a variety of recent measurements that have been reported in the literature demonstrated that the current parameterization produces reasonable results. The main improvement of the current parameterization compared to earlier size-dependent particle dry deposition models is that the current one produces more realistic deposition velocities for sub-micron particles and agrees better with recently published field measurements.

Effective Dry Deposition Velocities for Gases and Particles over Heterogeneous Terrain

Abstract Dry deposition velocities of gases and particles are highly dependent on surface type. In a numerical model, each grid cell may contain multiple surface types, each with a different deposition velocity. Therefore, some kind of averaging technique generally is used to compute the average of the subgrid-scale deposition velocities within a grid cell. In this paper, effective surface parameters are suggested to relate the mean properties of concentration and wind speed to the mean surface fluxes. An effective deposition velocity is computed subject to these effective surface parameters and a weighted-average technique. This effective deposition velocity is compared with an alternate weighted-average deposition velocity that has been used widely in numerical air quality models. For particles, the effective deposition velocity can be significantly different from the weighted-average deposition velocity. For some gases, for which biological factors often control the deposition process, the difference b...

Aerosol particle transport and deposition in vertical and horizontal turbulent duct flows

Aerosol particle transport and deposition in vertical and horizontal turbulent duct flows in the presence of different gravity directions are studied. The instantaneous fluid velocity field is generated by the direct numerical simulation of the Navier–Stokes equation via a pseudospectral method. A particle equation of motion including Stokes drag, Brownian diffusion, lift and gravitational forces is used for trajectory analysis. Ensembles of 8192 particle paths are evaluated, compiled, and statistically analysed. The results show that the wall coherent structure plays an important role in the particle deposition process. The simulated deposition velocities under various conditions are compared with the available experimental data and the sublayer model predictions. It is shown that the shear velocity, density ratio, the shear-induced lift force and the flow direction affect the particle deposition rate. The results for vertical ducts show that the particle deposition velocity varies with the direction of gravity, and the effect becomes more significant when the shear velocity is small. For horizontal ducts, the gravitational sedimentation increases the particle deposition rate on the lower wall.

Dry deposition of large, airborne particles onto a surrogate surface

Simultaneous measurements of particle dry deposition flux and airborne number concentration in the open atmosphere were made using three different types of artificially generated particles in the size range 10–100 μm – perlite, diatomaceous earth and glass beads. A combination of gravimetric analysis, automated microscopy and sonic anemometry provided size-resolved estimates of both the inertial and gravitational components of the quasi-laminar layer particle deposition velocity, ( V d) b, as a function of size. Eddy inertial deposition efficiency ( η dI) was determined as a function of dimensionless eddy Stokes number (Stk e). In the range 3<Stk e<100, the observed η dI agree with the model of Slinn and Slinn (1980, Atmospheric Environment 14, 1013–1016). In addition, the observed ( V d) b values agree well with the predictions of Pleim et al. (1984, ADOM/TADAP Model Development Program. Volume 4. The Dry Deposition Module. Ontario Ministry of Environment, Rexdale, Ontario). However the dry deposition model of Sehmel and Hodgson (1978, a model for predicting dry deposition of particles and gases to environmental surfaces. DOE Report PNL-SA-6721, Pacific Northwest Laboratories, Richland, WA), used in several regulatory models, significantly under-predicted (up to seven times) ( V d) b for large particles ( d a>10 μm).

The effect of humidity and state of water surfaces on deposition of aerosol particles onto a water surface

Deposition processes of particles with dry diameter larger than about 10 μm are dominated by gravitational settling, while molecular diffusion and Brownian motion predominate the deposition processes of particles smaller than 0.1 μm in dry diameter. Many air pollution derived elements exhibit characteristics common to sub-micron particles. The objective of the present study is to examine the effects of meteorological conditions within the turbulent transfer layer on the deposition velocity of particles with dry diameter between 0.1 and 1 μm. It is for these sub-micron particles that particle growth by condensation in the deposition layer, the broken water surface effect and the enhanced transfer process due to atmospheric turbulence in the turbulent transfer layer play important roles in controlling the particle deposition velocity. Results of the present study show that the `dry air’ assumption of Williams’ model is unrealistic. Effects of ambient air relative humidity and water surface temperature cannot be ignored in determining the deposition velocity over a water surface. Neglecting effects of ambient air relative humidity and water surface temperature will result in defining atmospheric stability incorrectly. It is found that the largest effect of air relative humidity on deposition velocity occurs at an air–water temperature difference corresponding to the point of `displaced neutral stability'. For a given wind speed of U=5 m s −1 the additive effects of water surface temperature, T w, changes from 5 to 25°C and ambient air relative humidity variations from 85 to 60%, respectively, lead to a maximum difference in v d of about 20%. For a higher wind speed of 10 m s −1, however, the corresponding change in v d reduces to less than 5%. This is further confirmation that wind speed is one of the strongest variables that governs the magnitude of v d. The present study also found that the broken surface transfer coefficient, k bs, given as a multiple of the smooth surface transfer coefficient, k ss, is physically more meaningful than assigning it a constant value independent of particle size. The method used in this study requires only a single level of atmospheric data coupled with the surface temperature measurement. The present method is applicable for determining deposition velocity not only at the conventional measurement height of 10 m but also at any other heights that are different from the measurement height.

Dry deposition of particles to building surfaces and soiling

A separated turbulent flow around a cube with slightly rough surfaces of 3×3 cm area was produced to simulate particle deposition on edges or other small scale structures of building walls. Polydisperse soot particles with a mass mean diameter of 0.8 μm produced by atomising a suspension of India ink and monodisperse fluorescent 0.6 μm latex spheres were used. The deposition velocity of the soot particles was determined by measuring the change in reflectance of the surrogate surfaces and the light absorption coefficient of the aerosol. The deposited fluorescent spheres were counted by means of a fluorescence microscope. Deposition velocities of the fluorescent spheres used in outdoor experiments were equal to deposition velocities of the soot particles used in a small wind tunnel. The density of the deposit was different on each side of the cube and also inhomogeneously distributed on each individual surface. The highest deposition was always found on the edges of the cube. This peculiar deposition pattern could be explained by the characteristics of the turbulent flow field around the surfaces which was measured by laser Doppler anemometry. Modelled soiling constants calculated with the help of the measured deposition velocities were up to a factor of 10 higher than values obtained for a flat plate in a simple boundary layer flow.

A Lagrangian Stochastic Model for the Dispersion and Deposition of Brownian Particles

A Lagrangian stochastic model for the dispersion and deposition of submicron-size particles is formulated and validated. The model satisfies the well-mixed condition, incorporates molecular diffusivity, and accounts for the effects of Reynolds number upon Lagrangian particle statistics. Reynolds number effects are found to be significant in the viscous sublayer and the buffer zone of a turbulent shear flow. The effects are due almost entirely to the change in the Lagrangian integral time scale. Sawford's correction to first-order Lagrangian stochastic models for the effects of Reynolds number is found to be appropriate for inhomogeneous turbulence even when the Taylor–Reynolds number Rλ ∼ O(0.1). The model predicts, in close accord with experiment and the results of direct numerical simulations, that the nondimensional particle deposition velocity K+ = 0.06Sc−2/3, where Sc is the Schmidt number. When Reynolds number effects are neglected, K+ is overpredicted by several orders of magnitude.

THE EFFECT OF THERMOPHORESIS ON PARTICLE DEPOSITION FROM A MIXED CONVECTION FLOW ONTO A VERTICAL FLAT PLATE

A theoretical study of the effect of thermophoresis on aerosol particle deposition onto a vertical flat plate is proposed. The flow was modeled as a two-dimensional, incompressible and steady-state laminar flow driven by a combination of forced convection and natural convection. The particle deposition mechanisms were coupled using convection, Brownian diffusion and thermophoresis. Similarity analysis with the box method and block-elimination was used to determine the velocity and temperature fields. Particle deposition velocities were solved using the finite difference method or the approach of numerical integration. The effects of thermophoresis and convection were predicted to be particularly important for submicron particles moving toward a cold surface or blowing away from a hot surface at a given temperature gradient. The results also showed that the particle deposition rates were strongly influenced by thermophoresis and buoyancy force, particularly for opposing flow and hot wall surfaces.