Experiments Measuring Particle Deposition from Fully Developed Turbulent Flow in Ventilation Ducts

Particle deposition in ventilation ducts influences particle exposures of building occupants and may lead to a variety of indoor air quality concerns. Experiments have been performed in a laboratory to study the effects of particle size and air speed on deposition rates of particles from turbulent air flows in galvanized steel and internally insulated ducts with hydraulic diameters of 15.2 cm. The duct systems were constructed of materials typically found in commercial heating, ventilating, and air conditioning (HVAC) systems. In the steel duct system, experiments with nominal particle sizes of 1, 3, 5, 9, and 16 μm were conducted at each of three nominal air speeds: 2.2, 5.3, and 9.0 m/s. In the insulated duct system, deposition rates of particles with nominal sizes of 1, 3, 5, 8, and 13 μm were measured at nominal air speeds of 2.2, 5.3, and 8.8 m/s. Fluorescent techniques were used to directly measure the deposition velocities of monodisperse fluorescent particles to duct surfaces (floor, wall, and ceiling) at two straight duct sections where the turbulent flow profile was fully developed.

In steel ducts, deposition rates were higher to the duct floor than to the wall, which in turn were greater than to the ceiling. In insulated ducts, deposition was nearly the same to the duct floor, wall, and ceiling for a given particle size and air speed. Deposition to duct walls and ceilings was greatly enhanced in insulated ducts compared to steel ducts. Deposition velocities to each of the three duct surface orientations in both systems were found to increase with increasing particle size or air velocity over the ranges studies. Deposition rates measured in the current experiments were in general agreement with the limited observations of similar systems by previous researchers.     

Modeling Large-Particle Deposition in Bends of Exhuast Ventilation Systems

A new model is presented to describe particle deposition in 90° bends of exhaust ventilation systems. This model accounts for non-Stokes particle motion and for variable deposition patterns as a function of particle Stokes number. Estimates made with the new model and with two models previously published were compared to measurements of deposition in bends with geometries, particle characteristics, and airflow conditions similar to those found in industry—large duct diameters (15.4 and 20.3 cm), large particle sizes (19–140 μm), and turbulent airflow (Re = 203,000 and Re = 368,000). Whereas the two models published previously explain 30% or less of the variability in the data, the new model explains 85%. The mean residual with the new model, 0.6%, is nearer to zero than that of the two other models, 3.6% and 9.8%. The new model is applicable to mists and to solid particles that stick to bend walls.     

PARTICLE DEPOSITION IN A NEARLY DEVELOPED TURBULENT DUCT FLOW WITH ELECTROPHORESIS

This study is concerned with deposition of neutral and charged particles in nearly developed turbulent duct flows. The cases that the duct is vertical or horizontal and when the particles carry Boltzmann, static electrification, as well as saturation charge distributions are analyzed. The mean turbulent flow field is evaluated with the aid of the FLUENT code, using the Reynolds stress transport model. Deposition rate of particles in the size range of 0.01–100 μm are studied and the effects of electric field intensity on particle deposition velocity are evaluated. The simulation results are compared with the available experimental data, the earlier numerical results and those obtained from empirical equations for fully developed duct flows. It is shown that the electrostatic effect significantly increases the particle deposition rate.     

Effects of air temperature and humidity on particle deposition

Particle deposition in a fully developed turbulent duct flow was studied. The random walk model of Lagrangian approach was used to predict the trajectories of 3000 particles with a density of 900 kg/m³. The effects of thermophoretic force and air humidity were also considered. The results were compared with the previous studies with a particle size range of 0.01–50 μm and air flow velocity of 5 m/s. The profile of dimensionless deposition velocity with relaxation time presents a V-shaped curve and the results are in good agreement with the previous studies.The effects of air temperature and humidity on particle deposition with a particle size of 1 μm were also investigated. The results show that thermophoretic force accelerates particle deposition onto the duct walls with increasing temperature difference between air flow and the duct wall surface. Meanwhile, it was found that particle deposition velocity increases with air humidity.     

How Particle Resuspension from Inner Surfaces of Ventilation Ducts Affects Indoor Air Quality—A Modeling Analysis

Dust particles deposited on the inner surfaces of the ventilation ducts can be resuspended by passing airflow. A physical-science-based model is developed to understand how particle resuspension affects the indoor air quality. This integrated model takes into consideration particle mass balance models for straight ventilation duct, duct bend, ventilation room, and air filter. The straight duct and room models have been validated using experimental data. With the integrated model, we find that in-duct resuspension of particles could lead to significant increase in exposure to airborne particles for indoor occupants. It is also found that indoor particle exposure is a linear function of dust mass loading. Greater ventilation rate, which means higher air speed above the dust particles, would lead to greater exposure ratio, while fresh air ratio has little influence. Possible control methods are discussed as well.     

Particle deposition in turbulent duct flows—comparisons of different model predictions

Numerical studies of transport and deposition of nano- and micro-particles in turbulence flow field have been studied in the past few decades. In most current industrial applications, Reynolds averaged turbulence models were used due to its relative simplicity and computational efficiency. In this work, a series of numerical simulations were conducted to study the transport and deposition of nano- and micro-particles in a turbulent duct flow using different turbulence models. Commercial software (FLUENT^TM 6.1.22) was used for turbulence mean flow simulation. Simulations of the instantaneous turbulence fluctuation with and without turbulence near wall correction, and particle trajectory analysis were performed with the in-house PARTICLE (object-oriented C++) code, as well as with FLUENT^TM code with and the use of user's defined subroutines. The simulation results for different cases were compared with the available experimental data, and the accuracy of various approaches was evaluated. In addition, the importance of turbulence model, boundary conditions, and turbulence fluctuation particularly near wall on particle transport and deposition were carefully evaluated. It was shown that when sufficient care was given to the modeling effort, the particle deposition rates could be predicted with reasonable accuracy. The presented results could provide guidelines for selecting appropriate procedure for simulating nano- and micro-particle transport and deposition in various applications.     

Deposition and coagulation of polydisperse nanoparticles by Brownian motion and turbulence

Turbulent and Brownian coagulation rates as well as deposition coefficients of polydisperse nanoparticles were measured experimentally. The coagulation rates were obtained from the change in the total number concentration of polydisperse sodium chloride aerosols, with geometric mean diameters ranging from 30 to 120 nm, in a closed chamber at atmospheric pressure. The geometric standard deviations of the experiments were in the range of 1.55–1.65. The experimental coagulation rates took deposition rates into account, because coagulation and deposition occur simultaneously in a closed chamber. As a result of deposition, it was shown that the experimental deposition coefficients of polydisperse aerosols were agreed well with the theoretical data of Park et al. [(2001). Wall loss rate of polydispersed aerosols. Aerosol Science and Technology, 35, 710–717]. It was shown that the effect of the coagulation was much greater than that of the deposition in the high particle number concentration. In addition, the results represented that bigger turbulent coefficients, caused by higher fan rotation speeds, make the turbulent coagulation process become stronger. In the small particle size range, however, the coagulation rates tend to converge though turbulent coefficients are different. In conclusion, it was shown that experimental coagulation rates followed the values of Lee and Chen [(1984). Coagulation rate of polydisperse particles. Aerosol Science and Technology, 3, 327–334], which were calculated for polydispere aerosols in the gas-slip regime and free-molecule regime.     

通风管道结构形式对颗粒物沉积的影响

在矩形断面通风管道无因次颗粒物沉积速率计算结果与相关实验数据验证的基础上,对弯头、变径、三通等通风管道结构内的颗粒物沉积进行了数值模拟. 管道流动采用RSM湍流模型,并应用拉格朗日随机轨道模型描述气固两相流动中颗粒运动. 结果表明,直管段内无因次颗粒物沉积速率与相关研究结果变化趋势相近,直管段侧壁、顶面无因次颗粒物沉积速率在无因次松弛时间大于1时(粒径约10 mm)呈下降趋势. 弯头、变径及三通管段内颗粒物沉积率随斯托克斯数(St)增加而升高,当St<0.1时,3种管段结构内颗粒物沉积率均较小且相差较小;当St>0.1时,相同St下弯头内颗粒物沉积率最高,其次为三通和变径. 直管段内小粒径颗粒物(<10 mm)主要受湍流扩散作用而沉积,对于大颗粒的沉积则主要受重力影响;弯头、变径及三通管段内颗粒St>0.1时,颗粒物的沉积主要受惯性碰撞影响.     

Turbulent deflagrations, autoignitions, and detonations

Measurements of turbulent burning velocities in fan-stirred explosion bombs show an initial linear increase with the fan speed and RMS turbulent velocity. The line then bends over to form a plateau of high values around the maximum attainable burning velocity. A further increase in fan speed leads to the eventual complete quenching of the flame due to increasing localised extinctions because of the flame stretch rate. The greater the Markstein number, the more readily does flame quenching occur. Flame propagation along a duct closed at one end, with and without baffles to increase the turbulence, is subjected to a one-dimensional analysis. The flame, initiated at the closed end of the long duct, accelerates by the turbulent feedback mechanism, creating a shock wave ahead of it, until the maximum turbulent burning velocity for the mixture is attained. With the confining walls, the mixture is compressed between the flame and the shock plane up to the point where it might autoignite. This can be followed by a deflagration to detonation transition. The maximum shock intensity occurs with the maximum attainable turbulent burning velocity, and this defines the limit for autoignition of the mixture. For more reactive mixtures, autoignition can occur at turbulent burning velocities that are less than the maximum attainable one. Autoignition can be followed by quasi-detonation or fully developed detonation. The stability of ensuing detonations is discussed, along with the conditions that may lead to their extinction.     

Investigation of particle deposition on a micropatterned surface as an energy-efficient air cleaning technique in ventilation ducting systems

Abstract

Building ventilation ducting systems play a core role in controlling indoor air quality by recirculating the indoor air and mixing with ambient air. The ventilation system can serve as an air cleaning system itself either through the filtration system or integrating other means, while at the same time, attention to energy consumption is needed. The high-efficiency fibrous filters in a conventional filtration system not only cause high-pressure drops that consume fan energy but also add to the high operation cost. This article proposes an air cleaning technique, aimed at submicron particles, by means of installing patterned surfaces on the walls of ventilation ducts, which can be easily cleaned by water and reused. The effect of patterned surfaces on particle deposition was studied numerically. In the numerical simulation, the Reynolds stress turbulent model was correlated at the near-wall regions by turbulent velocity fluctuation at the normal direction. Particle trajectory was solved by using Lagrangian particle tracking. The numerical model was then validated with a particle deposition experiment. A wind tunnel experiment was carried out to quantify the particle deposition on the semicircular micropatterns for a wide range of heights. Based on our numerical results, the semicircular pattern height of 500?µm with a pitch-to-height ratio (p/e) of 10 has 8.58 times enhancement of the energy efficiency compared with a high-efficiency particulate air filter. Our results indicated that adding surface micropatterns to ventilation ducting for submicron particle deposition is a possible energy-efficient air cleaning technique for practical usage.

Copyright © 2020 American Association for Aerosol Research     

Aerosol Deposition Measurements as a Function of Reynolds Number for Turbulent Flow in a Ninety-Degree Pipe Bend

An experimental and numerical investigation of the effect of the Reynolds number (Re) on the deposition of aerosol particles in a 90° pipe bend for turbulent flow was performed. Deposition fraction data were measured for a range of Stokes numbers (Stk) at different flow Re (10,250, 20,500, and 30,750) higher than those of most previous studies where Re was ?10,000. The data show good agreement with previous studies for Stk > 0.4, demonstrating that increased Re does not significantly alter the trend of deposition fraction with Stokes number (Stk) in this range. However, a noticeable increase in deposition was detected for 0.1 ? Stk ? 0.4. At Stk = 0.15, an increase in Re from 10,250 to 30,750 caused a factor of 2.6 increase in deposition fraction from 0.14 to 0.36. Numerical simulations were completed, using the Reynolds Averaged Navier-Stokes (RANS) equations with the Shear Stress Transport turbulence model. Modeling with inertial impaction only (i.e., neglecting turbulent dispersion), the results accurately reproduced the general trends seen in the experimental data; however, they failed to detect the Re effect at low Stk seen experimentally. The inclusion of turbulent particle tracking in the RANS simulation via an eddy interaction model did not improve the results. However, an analytical analysis of the particle tracking equation drawing data from the numerical results, showed that the experimentally observed effect of Re at low Stk can be attributed to damped particle response to velocity fluctuations at the eddy frequency scale.     

Particle Deposition in Museums: Comparison of Modeling and Measurement Results

Deposition of airborne particles may lead to soiling and /or chemical damage of objects kept indoors, including works of art in museums. Measurements recently were made of the deposition velocity of fine particles (diameter range: 0.05–2.1 μm) onto surfaces in five Southern California museums. In this paper, theoretical predictions of particle deposition velocities onto vertical surfaces are developed for comparison against the experimental results. Deposition velocities are calculated from data on surface-air temperature difference and near-wall air velocity using idealized representations of the air flow field near the wall. For the five sites studied, the wall-air temperature differences were generally in the range of a few tenths to a few degrees Kelvin. Average air velocities measured at 1 cm from the wall were in the range 0.08–0.19 m s^?1. Based on a combination of modeling predictions and measurement results, the best estimate values of deposition velocity for the wall studied at each site are obtained. These values are in the range (1.3–20) × 10^?6 m s^?1 for particles with 0.05–μm diameter and (0.1–3.3) × 10^?6 for particles with 1-μm diameter. The range of 15–30 in deposition velocity for a given particle size is due primarily to differences among sites in the near-wall air flow regime, with the low and high values associated with forced laminar flow and homogeneous turbulence in the core of the room, respectively.     

A Numerical and Experimental Study of Spray Dynamics in a Simple Throat Model

An inhalation airflow through a simple model of the human larynx and trachea, containing dispersed drug spray droplets, is studied numerically using the Computational Fluid Dynamics (CFD) code KIVA-3V (Amsden 1997) and experimentally using phase doppler interferometry. Flow conditions within the larynx and trachea affect the delivery of inhaled medications to the lungs. Deposition in these regions is considered undesirable and has been shown to be a particular problem for pediatric patients. The larynx geometry is represented by a constricted portion inside a straight tube. This constriction simulates the vocal folds within the larynx. The experimental model was 3.2 cm in diameter (approximately twice human scale) and 90 cm long. The constriction was 0.7 cm thick and was placed 30 cm from the inlet of the tube. The area of the constricted opening is approximately 40% of the tube area. Water droplets are introduced into the low-turbulence upstream airflow using a jet nebulizer. Measurements of axial velocity and axial turbulence intensity were made through an array of points between 2 diameters upstream and 4 diameters downstream of the constriction. Steady flows were used and the flow rates scaled to match in vivo tracheal Reynolds numbers simulating two different breathing conditions. The KIVA-3V code is specifically designed to analyze transient, two- and three-dimensional, chemically reactive fluid flows with sprays. The analysis considers spray dynamic effects such as coalescence, evaporation, deposition, and turbulent dispersion. The numeric simulation is carried out in a model consisting of a 21.7 cm long pipe simulating the measurement region. All other dimensions are identical to the experimental model. Several significant spray deposition mechanisms were notable in both the experimental and the numerical results.     

Respiratory Deposition of Fibers in the Non-Inertial Regime—Development and Application of a Semi-Analytical Model

A semi-analytical model describing the motion of fibrous particles ranging from nano- to micro scale was developed, and some important differences in respiratory tract transport and deposition between fibrous particles of various sizes and shapes were elucidated. The aim of this work was to gain information regarding health risks associated with inhalation exposure to small fibers such as carbon nanotubes. The model, however, is general in the sense that it can be applied to arbitrary flows and geometries at small fiber Stokes and Reynolds numbers. Deposition due to gravitational settling, Brownian motion and interception was considered, and results were presented for steady, laminar, fully developed parabolic flow in straight airways. Regarding particle size, our model shows that decrease in particle size leads to reduced efficiency of sedimentation but increased intensity of Brownian diffusion, as expected. We studied the effects due to particle shape alone by varying the aspect ratios and diameters of the microfibers simultaneously, such that the effect of particle mass does not come into play. Our model suggests that deposition both due to gravitational settling and Brownian diffusion decreases with increased fiber aspect ratio. Regarding the combined effect of fiber size and shape, our results suggest that for particles with elongated shape the probability of reaching the vulnerable gas-exchange region in the deep lung is highest for particles with diameters in the size range 10–100 nm and lengths of several micrometers. Note that the popular multi-walled carbon nanotubes fall into this size-range.     

The effect of electrically charged dielectric walls on the diffusive deposition of submicron aerosol particles

The effect of an electrical charge on a dielectric wall on the deposition of charged aerosol particles is investigated. The dielectric walls used in the deposition experiments, poly(vinyl chloride) or polytetrafluoroethylene disks, are prepared to have known uniform surface charge distributions by using a corona discharge procedure. This avoids any effects on deposition caused by nonuniformity of the surface charge. The surface charge densities are based on measurements of the surface voltage in the center of the sample. Deposition experiments using a turbulently-mixed stirred tank demonstrate that particle deposition rates to the wall decrease as the sample's thickness increases or its relative permittivity decreases, even at a constant surface voltage. The electric field over the sample wall is calculated using the surface charge density, and the convection-diffusion equations describing particle deposition by Brownian and turbulent diffusion and electric migration are solved numerically. The calculated deposition rates agree well with those measured, indicating that quantitative prediction of particle deposition is possible if the surface charge, material, thickness and the effect of adjacent walls are taken into account.     

可吸入颗粒物在管内湍流流动时的动力学特性及热泳沉积

The kinematical characteristics and thermophoretic deposition of inhalable particles with the diameters of 0-2.5μm （hereafter referred to as PM2.5） suspended in turbulent air flow in a rectangular duct with temperature distribution were experimentally studied. Particle dynamics analyzer （PDA） was used for the on-line measurement of particle motion and particle concentration distribution in the cross-sections of the duct. The influences of the parameters such as the ratio of the bulk air temperature to the cold wall temperature and the air flow rate in the duct on the kinematical characteristics and the deposition efficiencies of PM2.5 were investigated. The experimental re- sults show that the deposition efficiencies of PM2.5 mainly depend on the temperature difference between the air and the cold wail, wffile the air flow rate and the particlecon~centration almost affect hardly tile clep0si-tion-effi ciency. The radial force thermophoresis to push PM2.5 to the cold wail is found the key factor for PM2.5 deposition.Based on the experimental results, an empirical modified Romay correlation for the calculation of thermophoretic deposition efficiency of PM2.5 is presenlext. The empirical correlation agrees reasonably well with the experimental data.     

Deposition of Ultrafine Aerosols in a Human Oral Cast

This paper reports experimental measurements of the total deposition of ultrafine aerosols in a human oral airway cast. A clear polyester resin cast of the upper airways of a normal human adult, including the nasal airways, oral cavity, tongue, nasopharynx, and larynx, was made from a postmortem solid cast. Measured pressure drop in the oral airway was slightly lower than in the nasal airway. The measured oral flow resistance was similar to the values reported for human volunteers breathing through the mouth at rest and for spontaneously opening of the mouth. Aerosol deposition data in the cast for monodisperse NaCl aerosols between 0.2 and 0.005 μm in diameter deposited in the cast were obtained for inspiratory and expiratory flow rates of 4, 20, and 40 L/min. Deposition efficiency increased with decreasing particle size and flow rate indicating that turbulent diffusion was the dominant mechanism for deposition. Higher deposition efficiency was observed for inspiratory flow in the oral airway than for expiratory flow. Oral deposition and nasal deposition for inspiratory flow were similar, but oral deposition was lower for expiratory flow. Deposition efficiency can be expressed as a function of the flow rate and diffusion coefficient of the particle.     

Particle deposition in the guinea pig respiratory tract

Equations relating particle size of aerosols to deposition by impaction, diffusion and sedimentation are applied to a previously established model of the guinea pig lung using a tidal volume of 4.44 cm³ and a respiratory rate of 60 breath min⁻¹. These calculated deposition values are combined with measured values of nasal deposition to give an estimate of the particle deposition characteristics of the guinea pig respiratory tract. The nasopharyngeal-tracheobronchial (NP-TB) region removes 99% of unit density spherical particle 10 μm or more in diameter. Deposition in this region reaches a minimum of 10% at a particle diameter of 0.8 μm. For particles less than 0.8 μm, deposition increases because of diffusion. Deposition in the pulmonary region is about 17% for particle diameters from 0.08 to 4 μm. For typical polydisperse aerosols with mass median diameters above 1 μm, a greater fraction of the mass than of the count is deposited in the NP-TB region, while a smaller fraction of the mass than of the count is deposited in the pulmonary region. Aerosol clouds with mass median diameters less than 0.1 μm deposit a greater fraction of the count than of the mass in the NP-TB region and a smaller fraction of the count than of the mass in the pulmonary region.     

A numerical study of particle motion within the human larynx and trachea

In this paper, particle trajectories are calculated using a stochastic model for turbulent fluctuations incorporated into the particle momentum equation, in combination with the time-averaged solutions of flow fields in the larynx and trachea. The manner in which turbulence may affect overall deposition is investigated through illustrative numerical experiments of the effects of flow rate, initial particle location, density, and size, from which results are given in the form of probability density histograms of final particle locations (i.e. deposition sites). The histogram bins are defined in a unique manner that highlight the deposition mechanisms associated with turbulent dispersion. It is observed that turbulence may play a key role in enhancing particle deposition in the larynx and trachea.     

Particle deposition in a hollow cast of the human tracheobronchial tree

The deposition of particles within the human airways was studied using a hollow silicone rubber cast of the larynx and tracheobronchial tree which extended to bronchi of approximately 0.2 cm dia. The cast was exposed to radioactively tagged, ferric oxide aerosols, having mass median aerodynamic diameters ranging from 2.5 to 8.1 μm. at three constant “inspiratory” flow rates. The detection system was designated for the determination of deposition within airways of all sizes and at various branch levels, and to allow selective measurements of the deposited activity within bifurcation and length regions of individual bronchi. Deposition efficiencies were determined and classified according to branch generation. Bifurcations were sites of preferential deposition over the range of particle sizes and flow rates used; bifurcation deposition generally peaked in generation 3.