Particle Deposition Velocity Research Articles

Bio-inspired patterned surface for submicron particle deposition in a fully developed turbulent duct

Arrays of surface ribs have been reported to significantly enhance particle collection efficiency in particle removal devices. However, the surface ribs also cause a higher pressure drop. Therefore, the overall performance needs to take into consideration the above factors. In this study, different forms of surface ribs inspired by nature were designed and parametric studies were performed to enhance deposition efficiency. Our parametric studies comprised three different aspects: geometry of the patterned surface, pitch-to-height ratio, and particle size. The flow field around patterned surfaces was simulated in a two-dimensional channel flow by using the Reynolds stress model, corrected by turbulence velocity fluctuation in the wall-normal direction. The particle trajectory was solved by using Lagrangian particle tracking. When the overall efficiency ratio was considered, a semi-circular pattern had the best overall efficiency with 1137 times increase when compared to the case without patterns. Although the open-circular pattern has the minimum particle deposition enhancement, the overall efficiency of the open-circular pattern has 862 times increase compared to the case without patterns. Surface ribs (semi-circular, triangular and rectangular) can achieve a higher particle deposition velocity, but a higher flow resistance is generated compared with applying the open-circular surface ribs. The deposition location was then investigated for different surface ribs at different pitch-to-height ratios (p/e). This study shows that the semi-circular surface pattern should be recommended to enhance the overall performance of particle removal devices, especially for submicron particles.

Improved Discrete Random Walk Stochastic Model for Simulating Particle Dispersion and Deposition in Inhomogeneous Turbulent Flows

Abstract The performance of different versions of the discrete random walk models in turbulent flows with nonuniform normal root-mean-square (RMS) velocity fluctuations and turbulence time scales were carefully investigated. The OpenFOAM v2−f low Reynolds number turbulence model was used for evaluating the fully developed streamwise velocity and the wall-normal RMS velocity fluctuations profiles in a turbulent channel flow. The results were then used in an in-house matlab particle tracking code, including the drag and Brownian forces, and the trajectories of randomly injected point-particles with diameters ranging from 10 nm to 30 μm were evaluated under the one-way coupling assumption. The distributions and deposition velocities of fluid-tracer and finite-size particles were evaluated using the conventional-discrete random walk (DRW) model, the modified-DRW model including the velocity gradient drift correction, and the new improved-DRW model including the velocity and time gradient drift terms. It was shown that the conventional-DRW model leads to superfluous migration of fluid-point particles toward the wall and erroneous particle deposition rate. The concentration profiles of tracer particles obtained by using the modified-DRW model still are not uniform. However, it was shown that the new improved-DRW model with the velocity and time scale drift corrections leads to uniform distributions for fluid-point particles and reasonable concentration profiles for finite-size heavy particles. In addition, good agreement was found between the estimated deposition velocities of different size particles by the new improved-DRW model with the available data.

Numerical simulation of coarse particle pipeline transportation based on Eulerian Lagrangian model

In this paper, based on the Eulerian Lagrangian model, the non-phase change flow process of the coarse particle slurry in the horizontal pipeline is studied. The particle motion state is tracked, the dynamic characteristics of the coarse particle slurry in the pipeline transportation are explored, and the resistance characteristics of the pipeline and the operation stability under different conditions are analyzed. The comparison of the results of velocity distribution, concentration distribution and pressure curve with the empirical formula and experimental data shows that the simulation results are in good agreement with the calculation results of wasp formula and the experimental values of Roco&Balakrishnam, and the simulation can better describe the flow characteristics in the basin. In addition, the flow characteristics of particles are further analyzed according to the key factors such as the flow stability, the transition process and the critical deposition velocity of particles in different positions of the pipeline.

Influence of surface roughness on particle deposition distribution around multi-slot cabin supply air nozzles of commercial airplanes

Enhanced soiling due to particle deposition is often observed around multi-slot cabin supply air nozzles in commercial airplanes. This study aimed to investigate the influence of surface roughness on the particle deposition distribution around multi-slot cabin supply air nozzles of commercial airplanes. This investigation constructed a half-row cabin mockup installed with three 3D-printed supply air nozzles of a twin-aisle commercial airplane. A cutting method was proposed to measure the detailed particle deposition velocity distribution on the target surface of the nozzles covered by different grades of sandpaper with different roughness heights. This study also conducted numerical calculations for the particle deposition velocity distribution using an Eulerian particle deposition model. Both the experimental and modeling results show that strong particle deposition occurred on the cut samples near the slot dividers. When the surface roughness height was greater than or equal to 6 μm, the particle deposition velocity increased significantly with the surface roughness height. However, when the surface roughness height was less than or equal to 3 μm, the particle deposition velocity was relatively insensitive to the surface roughness. The analysis indicated that polishing the surfaces of cabin supply air nozzles may not effectively solve the problem of enhanced soiling in the aircraft cabin.

Triboemission of FINE and Ultrafine Aerosol Particles: A New Approach for Measurement and Accurate Quantification

A dynamic model based on mass balance of fine aerosol particles was developed in order to tackle the problem of accurate quantification of mechanically stimulated particle emission (MSPE) from nanofunctionalized and solid lubricating materials. In contrast to the conventional approach, the model accounts for the effect of air turbulization caused by moving parts of the experimental tribological setup on the enhancement of particle deposition velocity. The increase of the velocity of the moving parts results in an increase of the deposition velocity that leads to a significant underestimation of experimentally measured particle emission rates. The developed model was experimentally verified using natural and artificial nanoparticle aerosols. Finally, the new methodology of particle emission rate quantification was employed for the analysis of fine particle emission produced when the solid lubricating materials were tested against a sliding steel surface. The developed method paves the way for defining a standard method of experimental assessment of nanoparticle triboemission enabling the experimental results obtained in various laboratories to be compared. It also bridges the gap between the phenomenological models and experimental measurements.

A Local Model and Experimental Verification of the Crossflow Filtration of a Polydispersed Slurry

In this paper, we propose a calculation model for a crossflow filtration process that is applicable to polydispersed slurry microfiltration. The deposition velocity of particles in slurry, particle distribution, and resistance of the filter cake on the surface of the filtration media can be predicted by this model, and can be used to predict the variations of filtration velocity. The theoretical prediction matched well with the experimental data, having a difference within 20%, except for the initial few seconds. However, the porosity of the filter cake used in the theoretical prediction was assigned based on the literature. It is revealed by the model that the variations in the crossflow filtration velocity are induced by the gradual domination of particles with small diameters in the filter cake. Meanwhile, the possible direction for the optimization of this model is pointed out.

Operator splitting method for numerical solving the atmospheric pollutant dispersion problem

The paper discusses the numerical modeling of transport and diffusion of air pollutants in the atmospheric boundary layer. There was developed a mathematical model of industrial emissions spread in the atmosphere taking into account the deposition velocity of fine particles. The model is described by multidimensional partial differential equations with appropriate initial and boundary conditions. The basic laws of hydrothermodynamics were used in deriving the model. In order to obtain the numerical solution of the problem, we used one of the splitting methods according to physical processes involved (transport, diffusion and absorption), as well as a second-order implicit finite-difference scheme in time. Analysis of numerical results showed that the developed computational algorithm provides sufficient accuracy of the problem solution compared with field measurement data and it has a certain advantage over other numerical methods. In the course of computational experiments, there was determined the degree of influence of such parameters as wind speed and direction, absorption coefficient and physicomechanical properties of particles on the process of atmospheric air pollutants dispersion.

Numerical investigation on thermophoretic deposition of particles in turbulent duct flow with conjugate heat transfer: Analysis of influencing factors

Thermophoretic deposition of particles in turbulent duct flow is of significant relevance in energy and thermal engineering applications. However, conjugate heat transfer (CHT) was commonly not considered in the previous studies, but may have crucial influences on particle deposition behaviors. Therefore, thermophoretic particle deposition in turbulent duct flow with and without CHT was numerically investigated by using $$\overline {v{\prime ^{_2}}} - f$$ turbulence model and discrete particle model (DPM) with a modified discrete random walk method. After grid independence study and numerical verification, several important influencing factors on particle deposition velocity were studied, such as flow Reynolds number, temperature difference between inlet hot air and cool wall, thermal conductivity ratio and width ratio of solid and fluid domain. The thermophoresis greatly increases deposition velocity of small particles but has no influence on large particles. The critical particle relaxation time $$\tau _{\rm{p}}^ + $$ for thermophoresis effect is 20, which is the same for all the cases in this study. The corresponding particle diameter is 28 µm. The thermophoretic deposition is enhanced when the flow Reynolds number and temperature difference between air and wall increase. This is because the wall-normal temperature variety is higher for large Reynolds number and temperature difference, which can enhance thermophoretic deposition. However, CHT reduces the thermophoretic deposition by decreasing temperature difference in fluid region. Besides, higher thermal conductivity ratio and width ratio of solid and fluid domain will decrease the thermophoretic deposition, as thermal conduction in solid domain becomes more intense.

On random walk models for simulation of particle-laden turbulent flows

In this investigation, the accuracy of the discrete and continuous random walk (DRW, CRW) stochastic models for simulation of fluid (material) point particle, as well as inertial and Brownian particles, was studied. The corresponding dispersion, concentration, and deposition of suspended micro- and nano-particles in turbulent flows were analyzed. First, the DRW model used in the ANSYS-Fluent commercial CFD code for generating instantaneous flow fluctuations in inhomogeneous turbulent flows was evaluated. For this purpose, turbulent flows in a channel were simulated using a Reynolds-averaged Navier–Stokes (RANS) approach in conjunction with the Reynolds Stress Transport turbulence model (RSTM). Then spherical particles with diameters in the range of 30 µm to 10 nm were introduced uniformly in the channel. Under the assumption of one-way coupling, ensembles of particle trajectories for different sizes were generated by solving the particle equation of motion, including the drag and Brownian forces. The DRW stochastic turbulence model of the software was used to include the effects of instantaneous velocity fluctuations on particle motion, and the steady state concentration distribution and deposition velocity of particles of various sizes were evaluated. In addition, the improved CRW model based on the normalized Langevin equation was used in an in-house Matlab code. Comparisons of the predicted results of the DRW model of ANSYS-Fluent with the available experimental data and the DNS simulation results and empirical predictions showed that this model is not able to accurately predict the flow fluctuations seen by the particles in that it leads to unreasonable concentration profiles and time-varying deposition velocities. However, the predictions of the improved CRW model were in good agreement with the experimental data and the DNS results. Possible reasons causing the discrepancies between the DRW predictions and the experimental data were discussed. The improved CRW model was also implemented through user-defined functions into the ANSYS-Fluent code, which resulted in accurate concentration distribution and deposition velocity for different size particles.

Hydraulic performance and deposition enhancement of ultrafine particles for in-duct twisted tapes under stationary and rotating conditions

Sustainable and energy-efficient air-conditioning systems require high-performance filtration systems with low air resistance. Experimentally, we investigated different diameters of both stationary and rotating twisted tapes (TTs) duct-inserts to characterize their hydraulic performance and level of ultrafine particles (UFP) deposition. It is an innovative study using an active approach (rotating TTs) for particle removal in the ventilation duct. Friction factors and particle deposition velocities were calculated both for empty ducts and for ducts with TT inserts in stationary and rotating conditions under constant pumping power. In addition, the overall particle deposition-hydraulic performance index was established to quantify the effectiveness of TTs under both laminar and turbulent airflows. The results showed a marginal difference in friction factor between stationary and rotating TTs under turbulent flow. The rotating TTs yielded a higher UFP deposition velocity than stationary TTs, peaking at 7.3 times compared to the deposition velocity for empty duct for particles smaller than 20 nm. Moreover, the highest performance index of a rotating TT could reach 6.5 for particles smaller than 20 nm at a Reynolds number of 10,000. Overall, a rotation or an increase in diameter of TTs both could improve the particle deposition performance. Notably, this study is the first to develop a filterless system for the removal of particles in the ventilation duct. Using in-duct rotating TTs would provide a solution for developing high filtration devices at less pressure penalty or smaller fanning energy consumption.

Particles dispersion and deposition in inhomogeneous turbulent flows using continuous random walk models

The suitability of the normalized Langevin stochastic equation with appropriate drift correction for simulation of instantaneous fluctuation velocities in inhomogeneous turbulent flows was studied. The Reynolds Stress Transport turbulence model of the ANSYS-Fluent code was used to evaluate the inhomogeneous turbulent flow properties in a two-dimensional duct flow. The simulation results were then used in an in-house Matlab particle tracking code and the trajectories of about 2 × 105 randomly distributed particles in the channel were evaluated by solving the particle equation of motion including the drag and Brownian forces under the one-way coupling assumption. The performance of the Continuous Random Walk (CRW) stochastic model using the conventional nonnormalized, as well as the normalized Langevin equations without and with the drift term for predicting a uniform distribution for the fluid-tracer particles in an inhomogeneous turbulent flow was examined. The accuracy of these models in predicting the deposition velocities and distribution of solid particles with diameters ranging from 10 nm to 30 μm was also carefully examined. In addition, the effects of including the finite-inertia coefficient in the drift term and using the corrected root mean square normal velocity in the near-wall region on the accuracy of the results were emphasized. By exploring the concentration profiles and deposition velocities of particles resulting from different CRW models, it was concluded that the Normalized-CRW model including the appropriate drift term leads to the most accurate results.

Cracks in the 3D-printed conductive traces of silver nanoparticle ink

Printing conductive traces of sinterable metallic nanoparticle (NP) based inks is one of the key components of 3D additively manufactured (printed) electronic circuits. In this study, we employ 3D aerosol jet printing to print conductive traces of silver NP based ink and provide insights into the risk of cohesive longitudinal cracking inside the printed and sintered traces. This was investigated for a wide range of carrier and sheath gas flow rates, which implies a wide range of particle deposition rates and velocities. These cracks appear for both single and double pass traces that are several microns in height and hundreds of microns in width. We examine different sources of stress that could be the potential driver for the cracking and infer that the most likely source is the classical capillary-pressure-driven crack formation mechanism for the evaporating NP-based thin films. Most importantly, this hypothesis raises the possibility of crack formation at several locations of the NP-based conductive traces printed using an extremely popular recent technique for printed electronics, AJP.

Velocity and spatial distribution of inertial particles in a turbulent channel flow

We present experimental observations of the velocity and spatial distribution of inertial particles dispersed in turbulent downward flow through a vertical channel at friction Reynolds numbers $\mathit{Re}_{\unicode[STIX]{x1D70F}}=235$ and 335. The working fluid is air laden with size-selected glass microspheres, having Stokes numbers $St=\mathit{O}(10)$ and $\mathit{O}(100)$ when based on the Kolmogorov and viscous time scales, respectively. Cases at solid volume fractions $\unicode[STIX]{x1D719}_{v}=3\times 10^{-6}$ and $5\times 10^{-5}$ are considered. In the more dilute regime, the particle concentration profile shows near-wall and centreline maxima compatible with a turbophoretic drift down the gradient of turbulence intensity; the particles travel at speed similar to that of the unladen flow except in the near-wall region; and their velocity fluctuations generally follow the unladen flow level over the channel core, exceeding it in the near-wall region. The denser regime presents substantial differences in all measured statistics: the near-wall concentration peak is much more pronounced, while the centreline maximum is absent; the mean particle velocity decreases over the logarithmic and buffer layers; and particle velocity fluctuations and deposition velocities are enhanced. An analysis of the spatial distributions of particle positions and velocities reveals different behaviours in the core and near-wall regions. In the channel core, dense clusters form which are somewhat elongated, tend to be preferentially aligned with the vertical/streamwise direction and travel faster than the less concentrated particles. In the near-wall region, the particles arrange in highly elongated streaks associated with negative streamwise velocity fluctuations, several channel heights in length and spaced by $\mathit{O}(100)$ wall units, supporting the view that these are coupled to fluid low-speed streaks typical of wall turbulence. The particle velocity fields contain a significant component of random uncorrelated motion, more prominent for higher $St$ and in the near-wall region.

Effects of blockage ratio and Reynolds number on particle deposition in duct airflow over a forward-facing step

Deposition processes of particles in duct airflow over a forward-facing step (FFS) were numerically studied by computational fluid dynamics method. Reynolds stress model with velocity fluctuation modification and discrete particle model with turbulent particle dispersion were developed to predict airflow fields and particle motions respectively. After grid independence study and numerical validation, the effects of different Reynolds numbers and blockage ratios on particle deposition characteristics were investigated. Particle deposition velocities, deposition efficiency and deposition mechanisms in duct with a forward-facing step were analyzed and discussed. The results showed that particle deposition velocity in FFS duct keeps monotonically increasing as particle diameter increases. Moreover, particle deposition profile in FFS duct increases as blockage ratio and Reynolds number increases. Deposition efficiency is higher for small particles (dp 10 µm) than uniform duct case. When air velocity is fixed as 5.5 m/s, the maximum deposition efficiency of FFS duct can reach 84 when λ = 0.25 and particle size is 1 µm. Moreover, when air velocity increases, the deposition efficiency will increase. Reduction of deposition distance, production of high TKE region and interception of windward surface by FFS are dominant mechanisms to increase particle deposition rates. Moreover, the “particle free zone” appears when particle diameter is more than 10 um and blockage ratio is 0.5.

Numerical investigation on monodispersed particle deposition in turbulent duct flow with thermophoresis

The study presents thermophoretic deposition characteristics of particles in duct air flow. The v2-f turbulence model and discrete particle model were used to simulate particle-air flow. After numerical validation, particle thermophoretic deposition with different temperature gradient and particle diameters were investigated and analyzed. It was found that thermophoretic force has obvious effect on deposition velocity for small particles ( dp < 10μm ), while almost no effect for large particles ( dp > 10μm ). Thermophoresis effect is obviously enhanced when temperature gradient increases. Besides, thermophoretic deposition is mainly caused by the dramatic temperature difference in temperature boundary layer.

Particle Deposition Characteristics and Efficiency in Duct Air Flow over a Backward-Facing Step: Analysis of Influencing Factors

Dry deposition of airborne particles in duct air flow over a backward-facing step (BFS) is commonly encountered in built environments and energy engineering. However, the understanding of particle deposition characteristics in BFS flow remains insufficient. Thus, this study investigated particle deposition behaviors and efficiency in BFS flow by using the Reynolds stress model and the discrete particle model. The influences of flow velocities, particle diameters, and duct expansion ratios on particle deposition characteristics were examined and analyzed. After numerical validation, particle deposition velocities, deposition efficiency, and deposition mechanisms in BFS duct flow were investigated in detail. The results showed that deposition velocity in BFS duct flow monotonically increases when particle diameter increases. Moreover, deposition velocity falls with increasing expansion ratio but rises with increasing air velocity. Deposition efficiency, the ratio of deposition velocity, and flow drag in a BFS duct is higher for small particles but lower for large particles as compared with a uniform duct. A higher particle deposition efficiency can be achieved by BFS with a smaller expansion ratio. The peak deposition efficiency can reach 33.6 times higher for 1-μm particles when the BFS expansion ratio is 4:3. Moreover, the “particle free zone” occurs for 50-μm particles in the BFS duct and is enlarged when the duct expansion ratio increases.

A numerical investigation on the mixing factor and particle deposition velocity for enclosed spaces under natural ventilation

The multi-zone model is widely used to predict airflow and contaminant transport in large buildings under natural or mechanical ventilation. Selecting appropriate mixing factors and particle deposition velocities for the multi-zone model can compensate for the errors resulting from the model’s well-mixing assumption. However, different room types, air change rates and ventilation modes can result in different mixing factors and particle deposition velocities. This study selected three typical room types: Z-type, L-type, and rectangle type (R-type). For each room type, the mixing factors and particle deposition velocities were investigated by the CFD model under different natural ventilation rates (0.5 h−1, 1 h−1, 3 h−1, 6 h−1, 12 h−1 and 20 h−1) and modes (door-inlet, window-inlet). The results showed that the mixing factor of the Z-type room was the highest, and the mixing factors of these rooms were 1.32, 1.28 and 1.13, respectively. In addition, the mixing factors presented a V-shaped distribution as a function of the air exchange rate under the window-inlet mode. The particle deposition velocity increased as the air change rate increased, and also demonstrated that the V-shaped curves as a function of particle size (0.05 μm, 0.1 μm, 0.5 μm, 1 μm, 2.5 μm, 5 μm) varied under different air change rates and room types. The results of mixing factors and particle deposition velocities for different room types, air change rates and ventilation modes can be used to improve the accuracy of the multi-zone model.

Experimental measurements and large eddy simulation of particle deposition distribution around a multi-slot diffuser

Enhanced soiling around multi-slot air diffusers due to particle deposition is frequently observed in commercial airplanes. The dirty black soiling is very unsightly and influences the passengers’ perception of cabin air quality. This study conducted experimental measurements and large eddy simulations with Lagrangian tracking for the distribution of particle deposition around a multi-slot diffuser. This investigation first used a relatively simple case of indoor particle deposition to compare the LES-Lagrangian model with the RANS-Lagrangian model with near-wall turbulence kinetic energy correction. The comparison shows that the LES-Lagrangian model was more robust than the RANS-Lagrangian model in predicting particle deposition indoors. The superior LES-Lagrangian model was then applied in predicting the particle deposition distribution around a multi-slot diffuser. This investigation also conducted detailed measurements of the distribution of particle deposition around the multi-slot diffuser in a laboratory chamber using a wiping method on a resolution of 3 × 20 mm2. The measurement accuracy of the wiping method was within 20%. The particle deposition distribution predicted by the LES-Lagrangian model was compared with the experimental data to validate the model. The results indicated that the LES-Lagrangian model correctly predicted the order of magnitude of the particle deposition velocity distribution around the multi-slot diffuser with an average relative error of 63.2%.

The particle dry deposition component of total deposition from air quality models: right, wrong or uncertain?

Dry deposition is an important loss process for atmospheric particles and can be a significant part of total deposition estimates calculated for critical loads analyses. However, algorithms used in large-scale air quality and atmospheric chemistry models to predict particle deposition velocity as a function of particle size are highly uncertain. Many of these algorithms, although derived from a common heritage, predict vastly different particle deposition velocities for a given particle diameter even under identical environmental conditions for major land use classes. Even more problematic, for vegetated landscapes (forests, in particular) the algorithms do not agree very well with available measurements. In this work, we perform a sensitivity study to estimate how significant the uncertainties in particle deposition algorithms may be in an air quality model’s predictions of ground-level fine particle concentrations, particle deposition and overall total deposition of nitrogen and sulfur. Our results suggest that fine particle concentration predictions at the surface may vary by 5–15% depending on the choice of particle deposition velocity algorithm, while particle dry deposition is affected to a much greater extent with differences among algorithms >200%. Moreover, if accumulation mode particle dry deposition measurements over forests are correct, then dry particle deposition and total elemental deposition to these landscapes may be much larger than is typically simulated by current air quality and atmospheric chemistry models, calling into question commonly available estimates of total deposition and their use in critical loads analyses. Since accurate predictions of atmospheric particle concentrations and deposition are critically important for future air quality, weather and climate models and management of pollutant deposition to sensitive ecosystems, an investment in new dry deposition measurements in conjunction with integrated modelling efforts seems not only justified but vitally necessary to advance and improve the treatment of particle dry deposition processes in atmospheric models.

Discrete phase-CFD simulations of asphaltenes particles deposition from crude oil in shell and tube heat exchangers

Fouling in crude preheat trains is a major cause of thermal inefficiency in petroleum refineries. The limited fundamental understanding of its causes and mechanisms led to ineffective fouling mitigation techniques. It is believed that asphaltenes precipitation and deposition is the major cause of fouling. The present work attempts to simulate the deposition of asphaltenes from crude oil in a multi-pass shell and tube heat exchanger through discrete-phase CFD simulations. The effects of bulk velocity, temperature gradients in the radial direction and particle sizes on asphaltenes deposition are investigated. In an effort to understand the effect of various forces on the rate of deposition of asphaltenes, forces such as gravity, drag, Saffman lift and thermophoretic are applied on the asphaltenes particles. The deposition velocities of the asphaltenes particles are determined based on solving the balance of these forces. The asphaltenes particles mass deposition rate is high in locations of higher particle deposition velocities. The CFD simulations indicate that the dominant forces to encourage particles deposition are gravity and drag. The deposition velocities and mass deposition rates are high for larger particle sizes. Low fluid velocities, higher temperature gradients and larger particle sizes favor higher particle deposition rates.