Wind Tunnel Studies of Particle Transport and Deposition in Turbulent Boundary Flows

In this study, particle transport and deposition were studied in a wind tunnel by measuring both the airflow turbulence characteristics and deposition of monodisperse uranine particles of 2.0-4.5 w m diameter on smooth, horizontal surfaces. Turbulence characteristics behind a 2.54 cm high rectangular bar were investigated for free stream velocities ranging from 3.3 m/s to 15.3 m/s. The well-developed boundary layer thickness was approximately four times the height of the rectangular bar at a distance of about 55 cm from the bar. Results of the wind tunnel experiments show the complex nature of deposition in turbulent flows due to the interactions between particles and turbulence. In general, the particle deposition flux is larger in the near wake region than in the far wake region. The particle deposition flux is roughly independent of the dimensionless particle relaxation time when the relaxation time is less than one, but increases rapidly as the relaxation time increases above one.     

Eulerian Model for Prediction of Particle Transport and Deposition in Turbulent Duct Flows with Thermophoresis

The Reynolds-averaged equations for turbulent particle population/transport in an Eulerian framework must be closed by specifying models for several terms: a turbophoretic force; a turbulent thermophoretic force; and a turbulent particle-diffusion term. In this article, new models are proposed for the turbophoretic term, as a particle-size dependent extrapolation of the corresponding turbulent fluid-velocity correlation, and for the turbulent thermophoretic term as an eddy-viscosity-scaled multiple of the corresponding mean thermophoretic term, appropriate for small low-inertia particles with τ⁺_p < 10. When the turbophoresis model is incorporated in a system of equations that describes particle motion within the surrounding fluid, it predicts particle deposition velocities that are in good agreement with experimental data over a range of particle sizes. When this equation system is included in a computational model to predict particle transport in turbulent pipe flows, the efficiency of particle deposition in pipes with upstream heating and downstream cooling is found to be in fair agreement with experimental measurements at two different Reynolds numbers, and over a range of particle sizes and temperature differences.

Copyright 2015 American Association for Aerosol Research     

Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows

Thermophoretic deposition of aerosol particles (particle diameter ranges from 0.038 to 0.498 μm) was measured in a tube (1.18 m long, 0.43 cm inner diameter, stainless steel tube) using monodisperse NaCl test particles under laminar and turbulent flow conditions. In the previous study by Romay et al., theoretical thermophoretic deposition efficiencies in turbulent flow regime do not agree well with the experimental data. In this study, particle deposition efficiencies due to other deposition mechanisms such as electrostatic deposition for particles in Boltzmann charge equilibrium and laminar and turbulent diffusions were carefully assessed so that the deposition due to thermophoresis alone could be measured accurately. As a result, the semiempirical equation developed by Lin and Tsai in laminar flow regime and the theoretical equation of Romay et al. in turbulent flow regime are found to fit the experimental data of thermophoretic deposition efficiency very well with the differences of less than 1.0% in both flow regimes. It is also found that Talbot's formula for the thermophoretic coefficient is accurate while Waldmann's free molecular formula is only applicable when Kn is greater than about 3.0.     

An Analytical Study of Thermophoretic Particulate Deposition in Turbulent Pipe Flows

The presence of a cold surface in non-isothermal pipe flows conveying submicron particles causes thermophoretic particulate deposition. In this study, an analytical method is developed to estimate thermophoretic particulate deposition efficiency and its effect on overall heat transfer coefficient of pipe flows in transition and turbulent flow regimes. The proposed analytical solution has been validated against experiments conducted at Oak Ridge National Laboratory. Exhaust gas carrying submicron soot particles was passed through pipes with a constant wall temperature and various designed boundary conditions to correlate transition and turbulent flow regimes. Prediction of the reduction in heat transfer coefficient and particulate mass deposited has been compared with experiments. The results of the analytical method are in a reasonably good agreement with experiments.     

Direct Numerical Simulation of Curly Fibers in Turbulent Channel Flow

Wall deposition of rigid-link fibrous aerosols in a turbulent channel flow is studied. The instantaneous turbulent velocity vector field is generated by the direct numerical simulation of the Navier-Stokes equation with the aid of a pseudospectral code. It is assumed that the fiber is composed of five rigidly attached ellipsoidal links. The dynamic behavior of these elongated and irregular shaped particles is markedly different from the spherical ones. The hydrodynamic forces and torques acting on the fiber are evaluated and the equations governing the translational and rotational motions of the fiber are analyzed. Euler's four parameters are used, and motions of fibrous particles in the turbulent channel flow field are studied. Ensembles of 8000 fiber trajectories are generated and are used for evaluating various statistics. Root mean-square fiber velocities and fiber concentrations at different distances from the wall are evaluated and discussed. Empirical models for the deposition rate of curly fibers are also developed. The model predictions are compared with the simulation data and good agreement is observed.     

High-Resolution Analysis of Particle Deposition and Resuspension in Turbulent Channel Flow

Particle deposition and resuspension during turbulent flow were investigated using a rectangular channel with glass side walls. Micrometer-sized alumina particles were used in the experiments. Particle behavior in the rectangular channel was observed through a high-speed microscope camera with a resolution of 0.3 μm and a speed of 87,600 fps, and particle deposition and resuspension fluxes were quantified using digital image analysis. The experimental results showed that particle resuspension was caused by the collision of airborne particles with those deposited on the surface. The resuspension flux was found to be correlated with the deposition flux. Furthermore, the average residence time between particle deposition and resuspension was several tens of milliseconds, which was very short but much longer than the contact time at the collision. Additionally, the residence time decreased as the particle diameter increased.

Copyright 2015 American Association for Aerosol Research     

Charged Particle Trajectory Statistics and Deposition in a Turbulent Channel Flow

The statistical properties of charged particles and their wall deposition in a turbulent channel flow in the presence of an electrostatic field is studied in this paper. For a dilute concentration, the influence of small particles on the fluid motion is neglected. The instantaneous velocity field is generated by a direct numerical simulation of the Navier-Stokes equation via a pseudospectral method. The case in which each particle carries a single unit of charge and the case in which the particles have a saturation charge distribution are analyzed. Ensembles of 8192 particle trajectories are used for evaluating various statistics. Effects of size and electric field intensity on particle trajectory statistics and wall deposition rate are studied. RMS particle velocities and particle concentrations at different distances from the wall are evaluated and discussed. The results for deposition rates are compared with those obtained from empirical equations.     

Direct Numerical Simulation of Kinematics and Thermophoretic Deposition of Inhalable Particles in Turbulent Duct Flows

Three-dimensional, incompressible turbulent air-particle flows in a channel with a temperature gradient are simulated by direct numerical simulations (DNS). The calculations used the fractional projection method to directly solve the Navier-Stokes equations. For obtaining more accurate results, the Oberbeck-Boussinesq model was used for considering the convective heat transfer and applied two-way coupling between the particles and the air phase to accurately simulate flow field state. The particles motions including mutual collisions were calculated with the direct simulation Monte-Carlo method (DSMC). The particles agglomeration and deposition in the turbulent channel flow with a temperature gradient were simulated by the Dahneke model. The research focused on the effects of the Reynolds number, the temperature gradient and particle concentration which simultaneity affect particle kinematics, impacts, agglomerations, and deposition characteristics. The numerical results show that the thermophoresis dominates the particle deposition, which agrees well with the experimental data, the particle concentration determines the particle collision and agglomeration rate, the Reynolds number determines the particle distribution in the duct and the 2.5 μm particles do not obviously affect the air phase motion under comparatively low concentration referred in this research.     

Dispersion and Deposition of Spherical Particles from Point Sources in a Turbulent Channel Flow

The dispersion and deposition of particles from a point source in a turbulent channel flow are studied. An empirical mean velocity profile and the experimental data for turbulent intensities are used in the analysis. The instantaneous turbulence fluctuation is simulated as a continuous Gaussian random field, and an ensemble of particle trajectories is generated and statistically analyzed. A series of digital simulations for dispersion and deposition of aerosol particles of various sizes from point sources at different positions from the wall is performed. Effects of Brownian diffusion on particle dispersion are studied. The effects of variation in particle density and particle-surface interaction are also discussed.     

Aerosol Particle Removal and Re-entrainment in Turbulent Channel Flows - A Direct Numerical Simulation Approach

Aerosol particle removal and re-entrainment in turbulent channel flows are studied. The instantaneous fluid velocity field is generated by the direct numerical simulation (DNS) of the Navier - Stokes equation via a pseudospectral method. Particle removal mechanisms in turbulent channel flows are examined and the effects of hydrodynamic forces, torques and the near-wall coherent vorticity are discussed. The particle resuspension rates are evaluated, and the results are compared with the model of Reeks. The particle equation of motion used includes the hydrodynamic, the Brownian, the shearinduced lift and the gravitational forces. An ensemble of 8192 particles is used for particle resuspension and the subsequent trajectory analyses. It is found that large-size particles move away roughly perpendicular to the wall due to the action of the lift force. Small particles, however, follow the upward flows formed by the near-wall eddies in the low-speed streak regions. Thus, turbulent near-wall vortical structures play an important role in small particle resuspension, while the lift is an important factor for reentrainment of large particles. The simulation results suggests that small particles (with τ_p⁺ ≤ 0.023) primarily move away from the wall in the low-speed streaks, while larger particles (with τ_p⁺ ≥ 780) are mostly removed in the high-speed streaks.     

Effect of Sub-Grid Scales on Large Eddy Simulation of Particle Deposition in a Turbulent Channel Flow

Large-eddy simulations (LES) of particle transport and deposition in turbulent channel flow were presented. Particular attention was given to the effect of subgrid scales on particle dispersion and deposition processes. A computational scheme for simulating the effect of subgrid scales (SGS) turbulence fluctuation on particle motion was developed and tested. Large-eddy simulation of Navier-Stokes equations using a finite volume method was used for finding instantaneous filtered fluid velocity fields of the continuous phase in the channel. Selective structure function model was used to account for the subgrid-scale Reynolds stresses. It was shown that the LES was capable of capturing the turbulence near wall coherent eddy structures.

The Lagrangian particle tracking approach was used and the transport and deposition of particles in the channel were analyzed. The drag, lift, Brownian, and gravity forces were included in the particle equation of motion. The Brownian force was simulated using a white noise stochastic process model. Effects of SGS of turbulence fluctuations on deposition rate of different size particles were studied. It was shown that the inclusion of the SGS turbulence fluctuations improves the model predictions for particle deposition rate especially for small particles. Effect of gravity on particle deposition was also investigated and it was shown that the gravity force in the stream wise direction increases the deposition rate of large particles.     

Particle Motion in Two-Dimensional Confined Turbulent Flows

This article introduces (1) the basic idea of a computer code on the basis of a Lagrangian model and (2) a method of computing particle concentration by the model. This computer program permits us to calculate the spatial distributions of particle velocity and concentration as well as the capture efficiency of the ventilation system in the two-dimensional confined turbulent flows. As a validation of the computer code, the experiment of Ruck and Makiola (1988), where the turbulent air flow passes a single-sided backward-facing step, is simulated. Then, we simulate a bounded turbulent field with a strong recirculation region to demonstrate the capability of the present program. The simulation is conducted for particles with diameters of 1, 60, and 100 μm and the predicted particle velocity and concentration distributions are given. The concentration of the 1-μm particles is also compared with that computed by the prediction program EOL (Fontaine et al., 1991).     

离散型多相流动的研究进展(英文)

Dispersed multiphase flows, including gas-particle (gas-solid), gas-spray, liquid-particle (liquid-solid), liquid-bubble, and bubble-liquid-particle flows, are widely encountered in power, chemical and metallurgical, aeronautical and astronautical, transporta-tion, hydraulic and nuclear engineering. In this paper, advances and research needs in fundamental studies of dispersed multiphase flows, including the particle/droplet/bubble dynamics, particle-particle, droplet-droplet and bubble-bubble interactions, gas-particle and bubble-liquid turbulence interactions, particle-wall interaction, numerical simulation of dispersed multiphase flows, including Reynolds-averaged modeling (RANS modeling), large-eddy simulation (LES) and direct numerical simulation (DNS) are reviewed. The research results obtained by the present author are also included in this review.     

Particle Detachment from Rough Surfaces in Turbulent Flows

Two flow structure-based models for particle resuspension from rough surfaces in turbulent streams are developed. It is assumed that the real area of contact is determined by elastic deformation of asperities and the effect of topographic properties of surfaces are included. The JKR adhesion model is used to analyze the behaviour of individual asperities. The theories of rolling and sliding detachment are used and the flow-induced resuspension is studied. The effects of the near-wall coherent eddies, and turbulence urst/inrush motion are included in the model development. The critical shear velocities needed to detach different sized particles from rough surfaces under various conditions are evaluated and discussed. The model predictions are compared with the available experimental data and good agreement is obtained.     

Effects of Turbulent Fluctuations on Nanoparticle Coagulation in Shear Flows

The effects of fluid turbulence on the coagulation of aerosols are studied quantitatively and qualitatively. Direct numerical simulation data is used to isolate the effect of the small or subgrid-scale (SGS) particle–particle interactions on nanoparticle coagulation in three-dimensional flows. The rate of particle growth is decomposed into the contribution of the large-scales and small-scales interactions. The contribution of the small-scale interactions is presented as a function of time, space, flow dynamics, and coagulation Damköhler number. Results show that small-scale interactions act to both increase and decrease particle growth. The probability density functions (PDFs) of the SGS growth rate exhibit a negative bias, which increases with time and coagulation Damköhler number. Additionally, PDFs conditioned on the Q-criterion suggest that the contribution of the small-scale interactions primarily act to reduce particle growth in regions characterized by fluid rotation.     

碳纤维表面化学涂覆NiO

采用均相沉淀法在碳纤维表面进行了NiO涂覆,研究了沉淀剂种类、脱水方式、沉淀剂浓度、沉淀剂添加速度和沉积反应时间对涂覆效果的影响. 采用SEM和XRD对涂层进行了表征,得到了制备NiO涂层较为合适的工艺条件为：以尿素为沉淀剂,采取缓慢升温的方式脱水,沉淀剂浓度0.20 mol/L,沉淀剂滴加速度2 mL/min,反应时间120 min. 在此条件下制备的NiO涂层厚度均匀,无脱落现象. 抗氧化性测试结果表明,涂覆NiO涂层后,碳纤维的抗氧化性有明显的提高.     

Analytical Approximation Schemes for Mean Nucleation Rate in Turbulent Flows

Nucleation rate is a very sensitive function of the temperature and vapor mole fraction. Analytical approximation schemes for the mean nucleation rate in turbulent flows are derived using Laplace’s approximation method. The schemes only require the derivative of the nucleation rate function and the probability density function (pdf) of the vapor mole fraction and/or temperature at the point of maximum nucleation rate. Based on the relation between the mole fraction and temperature, i.e., linearly correlated or not, different approximation schemes are developed. Numerical examples are constructed to investigate the accuracy of these approximation. In the examples, the pdfs of mole fraction and/or temperature in various turbulent flows are assumed to come from the beta distribution with five distinct forms. The mean nucleation rate of dibutyl phthalate (DBP) aerosol in these turbulent flows are calculated from the approximation schemes, and compared with exact numerical integration. The relative errors are less than 1% for cases when nucleation rate diminishes at the bounds of temperature fluctuations, and no more than 50% for all studied examples. Furthermore, the approximation schemes are not sensitive to the precise form of the pdfs. Hence, these developed approximation schemes can be used to estimate the mean nucleation rate in a broad range of turbulent flows conveniently.

Copyright 2014 American Association for Aerosol Research     

The Macroscopic Kinetics of Rapid Processes of Polymerization in Turbulent Flows

The peculiarities of the macrokinetics of rapid polymerization processes in turbulent flows (when the times of the chemical reaction are less than or compatible with those of the reagent mixing) have been discussed. The characteristic specific phenomena have been elucidated, namely: dependence of the molecular characteristics output (MW and MWD) of the polymer formed on the geometry of the reaction volume and on the parameters of the processes of mixing and heat transfer (flow rates, turbulization, boiling, etc.). Such reactions require special methods of investigation and control including those in industries with principally new material-, resource-, and power-saving technology, in particular, for the production of the isobutylene polymers. It is expedient to distinguish the rapid processes of polymerization as a separate class.     

Motion and Sedimentation of Particles in Turbulent Atmospheric Flows above Sources of Heating

In this paper, theoretical investigation of the problem of particle evolution and sedimentation in turbulent gas flows above the zones of large-scale instabilities caused by heating from below is undertaken. The mathematical model takes into account the two-way coupling effects in gas-particle interactions and combines both deterministic and stochastic approaches. To simulate the gas-phase flow the k -epsilon model is used with accounts of the mass, momentum, and energy fluxes from the particulate phase. The equations of motion for particles take into consideration random turbulent pulsations in the gas flow. The mean characteristics of those pulsations are determined with the help of solutions obtained within the frames of the k -epsilon model. Contrary to the existing theories, the present approach enables us to take into account the polydispersed character of the mixtures. The models for phase transitions and chemical reactions take into account thermal destruction of dust particles, vent of volatiles, chemical reactions in the gas-phase, and heterogeneous oxidation of particles influenced by both diffusive and kinetic characteristics. The obtained results make it possible to analyze the influence of inert and chemically reacting particles on the flow field induced by heating from below and by sedimentation and to determine the influence of sources of heat release on dispersion of particles and dynamics of the reaction zone.     

Deposition of Man-Made Fibers in a Human Nasal Airway

Many occupational lung diseases are associated with exposure to aerosolized fibers in the workplace. The nasal airway is a critical route for fiber aerosol to enter the human respiratory tract. The fiber deposition efficiency in the nasal airway could be used as an index to indicate the fraction of the inhaled fibers potentially transported to the lower airways. In this research, experiments of fiber deposition in the human nasal airway were conducted. Man-made carbon, glass, and titanium dioxide fibers in the inertia regime were used as the test fiber materials. The deposition studies were carried out by delivering aerosolized fibers into a human nasal airway replica at constant human inspiratory flow rates ranging from 15 l/min to 43.5 l/min. The deposition results were compared in detail between these fiber materials to study how the fiber characteristics affected the nasal airway deposition. The results showed that the deposition efficiency of the carbon fiber increases as the fiber impaction parameter increases. Many carbon fibers deposited in the anterior region of the nasal airway. In contrast, very few glass or titanium dioxide fibers deposited in the nasal airway, but relatively more of these two fibers deposited in the turbinate region. This result implies that, if a fiber in the inertia regime is inhaled during normal human breathing, the smaller the fiber, the more easily it could enter the human lower respiratory tract, possibly causing harm to the human respiratory tract.