A New Turbulent-Burst-Based Model for Particle Resuspension from Rough Surfaces in Turbulent Flow

Particle resuspension could affect human exposure to particulate matter (PM) and serves as a potential route for infection transmission in indoor environments. A new resuspension model incorporating the effects of turbulent bursts, depletion of resuspendable particles, adhesion force distribution, and environmental relative humidity (RH) is proposed. In the proposed model, Monte Carlo simulation is used to model the occurrence of turbulent bursts and the depletion of resuspendable particles on surfaces. The adhesion force distribution and RH effects were included by employing the recently proposed adhesion force distribution model. Model validation is conducted by comparing model predictions against reported experimental data found in the literature. The effects of RH and particle size on resuspension are investigated using the proposed model. The threshold free stream velocity increases by two and three times when the RH increases from 36% to 61% and 67%, respectively. The threshold friction velocity decreases by five times when the particle size increases from 30.1 μm to 111 μm. The proposed model provides a physically reasonable framework for describing particle resuspension under turbulent flow. The capability of predicting the effect of RH greatly enhances the practical application of current resuspension models.

Copyright 2014 American Association for Aerosol Research     

Studies on detachment behavior of micron sized droplets: A comparison between pure fluid and nanofluid

Resuspension is considered as a source of indoor air pollutants. These airborne pollutants can be in the form of liquid or solid. It has been previously found that the detachment mechanism of liquid droplets is different from the solid particles on the poly(methyl methacrylate) (PMMA) surface. Liquid droplets detach by portion when they are under an increasing normal force field while droplets detach completely when under a tangential force field. In this research, droplet detachment experiments are extended to different substrate materials, which are PMMA, glass, and stainless steel by the means of centrifuge. Also, the differences in detachment between pure glycerol-water (pure fluid) and a glycerol solution with the addition of nanoparticles (nanofluid) are investigated under different substrate materials. It is found that liquid droplets, again, detach by portion under normal force for all the substrate materials. For tangential force, the droplets detach completely if the exerted force was sufficiently large and the threshold values are material dependent, which is further elaborated by retention theory. After the addition of nanoparticles, a higher removal force was required compared to the droplets of pure fluid within the same size range. Also, solid residues with a negligible amount of fluid were found on the substrate after each removal of droplets under both normal and tangential force. The involvement of nanoparticles could be the pioneer work for future studies on commonly found liquid pollutants, which are prone to be contaminated by solid particles, such as in salivary excretion.

Copyright © 2018 American Association for Aerosol Research     

Detachment of droplets by air jet impingement

Surface cleaning using air jets is an appropriate method to remove particles from surfaces especially when cleaning by mechanical methods is not suitable. The detachment behavior of droplets using an air jet is not necessarily the same as solid particles and there is a lack of studies regarding this behavior. In this article, the detachment of droplets on a plastic substrate by air jet impingement was investigated experimentally. Droplets of two different size ranges were impinged by an air jet with different impinging angles. For micrometer-sized droplets, a smaller horizontal velocity was required to detach large droplets. Moreover, the horizontal velocity required to detach 50% number fraction of droplets decreased when the air jet impinging angle increased. Millimeter-sized droplets split into many portions. Most portions remained on the substrate and only a few were resuspended. The remaining portions were distributed in a fan shape, with larger droplets traveling further on the substrate. A linear lower bound of traveled distance was observed. Due to the splitting and the small fraction of resuspension, it should not be expected that air jet cleaning of droplets is the same as that for solid particles.

Copyright © 2017 American Association for Aerosol Research     

Transient aerodynamic atomization model to predict aerosol droplet size of pressurized metered dose inhalers (pMDI)

Pressurized metered dose inhalers (pMDI) produce large numbers of droplets with smaller sizes than 5 μm to treat asthma and other pulmonary diseases. The mechanism responsible for droplet generation from bulk propellant liquid is poorly understood, mainly because the small length scales and short time scales make it difficult to characterize transient spray formation events. This article describes the development and findings of a numerical atomization model to predict droplet size of pharmaceutical propellants from first principles. In this model, the velocity difference between propellant vapor and liquid phase inside spray orifice leads to formation of wave-like instabilities on the liquid surface. Two variants of the aerodynamic atomization model are presented based on assumed liquid precursor geometry: (1) cylindrical jet-shaped liquid ligaments surrounded by vapor annulus; (2) annular liquid film with vapor flow in the core. The growth of instabilities on the liquid precursor surfaces and the size of the subsequently formed droplets are predicted by numerical solutions of dispersion equations. The droplet size predictions were compared with phase doppler anemometry (PDA) data and the predictions were in good agreement with the number mean diameter D₁₀, which is representative of the respirable droplets. The temporal behavior of droplet size production was captured consistently well during the period of the first 95% of the propellant mass emission. The outcome of our modeling activities also suggests that, in addition to saturated vapor pressure of the propellant, its viscosity and surface tension are also key properties that govern pMDI droplet size.

© 2017 American Association for Aerosol Research     

Characteristics of particles deposited on a single free-fall charged droplet

Particles deposited on a free-fall charged droplet were experimentally studied. A droplet, charged under 40% Rayleigh limit, fell through the particle chamber to capture particles by electrostatic attractions. The velocity of the droplet was smaller than 2.1 m/s. The particle-laden droplet eventually spread on a glass slide, which was further analyzed using optical microscope. It was found that the equivalent number of particles captured by the charged droplet were larger than that of uncharged ones by one order of magnitude at least. Remarkably, particles on the charged droplet agglomerated into a large cluster, which indicates that the agglomerated cluster can be actively precipitated due to the gravity force if the droplet completely evaporates. The front side of the charged droplet was the predominant region to capture the particles. However, the actual area of capture was smaller than hemispheric surface.

Copyright © 2017 American Association for Aerosol Research     

GAS HOLDUP IN UNIFORMLY AERATED BUBBLE COLUMN REACTORS

Conditions of the stable performance of gas distributing plates were studied and the effect of plate geometry on gas holdup of uniformly aerated gas-liquid beds was investigated. The ratio of plate holes opened for gas passage was determined as a function of gas hole velocity and critical values of gas hole velocity corresponding to the onset of stable performance of distributing plates were obtained.

Two regimes of bubbling were observed under conditions of stable uniform gas distribution; the regions of their existence being determined by the values of gas flow velocity and distributing plate parameters. Considerable increase of gas holdup was observed in the region of “foam” bubbling compared to the “turbulent” bubbling regime commonly encountered in bubble column reactors. The character of the bed and hence its gas holdup value were affected by the geometry of distributing plates in the “foam” bubbling region while no such effect was observed under “turbulent” bubbling conditions.     

Thermophoresis of a particle in a concentric cavity with thermal stress slip

The thermophoretic motion of a spherical particle situated at the center of a spherical cavity filled with a gaseous medium under a prescribed temperature gradient is studied analytically. The Knudsen number is small for the gas motion in the slip-flow regime, and the temperature jump, thermal creep, frictional slip, and particularly, thermal stress slip are allowed on the solid surfaces. After solving the equations of heat conduction and fluid motion, an explicit formula for the migration velocity of the confined particle is obtained for different temperature conditions of the cavity with arbitrary values of the particle-to-cavity radius ratio and other parameters. Contributions from the thermoosmotic flow along the cavity wall and from the wall-corrected thermophoretic force to the particle velocity are equivalently important and can be linearly superimposed. With either or both of these contributions, the particle velocity in general is a decreasing function of the particle-to-cavity radius ratio and vanishes in the limit. The effects of the thermal stress slip at the solid surfaces to the migration velocity of the confined particle can be significant and interesting, dependent on the thermal and interfacial properties of the particle and surrounding gas. The wall effect on the thermophoretic migration of the particle in a cavity is qualitatively different from that on the motion of the particle in a circular tube.

Copyright © 2018 American Association for Aerosol Research     

Aerosol Generation by a Spray-Drying Technique Under Coulomb Explosion and Rapid Evaporation for the Preparation of Aerosol Particles for Inhalation Tests

In this study, nanosized (<100 nm) aerosol particles with high mass concentrations for inhalation tests were generated by a spray-drying technique with combining Coulomb explosion and rapid evaporation of the droplets. Under typical spray-drying conditions, aerosol particles with average diameter of 50–150 nm were prepared from a suspension of NiO nanoparticles with a primary diameter of 15–30 nm. Under the Coulomb explosion method, the sprayed droplets were charged by being mixed with unipolar ions to break up the droplets, which resulted in the generation of smaller aerosol particles with diameters of 15–30 nm and high number concentrations. Under the rapid evaporation method, the droplets were heated immediately after being sprayed to avoid inertial impaction on the flow path due to shrinkage of the droplet, which increased the mass concentration of the aerosol particles. The combination of the Coulomb explosion and rapid evaporation of droplets resulted in the generation of aerosol particles with sizes less than 100 nm and mass concentrations greater than 1 mg/m³; these values are often necessary for inhalation tests. The aerosols generated under the combined method exhibited good long-term stability for inhalation tests. The techniques developed in this study were also applied to other metal oxide nanoparticle materials and to fibrous multiwalled carbon nanotubes.

Copyright 2014 American Association for Aerosol Research     

Transient flashing propellant flow models to predict internal flow characteristics,spray velocity,and aerosol droplet size of a pMDI

Despite the popularity of the pMDI as an asthma remedy, the mechanism leading to spray generation is elusive, mainly due to small length scales and short time scale, causing experimental difficulties to obtain flow information. This mechanism involves transient development of two-phase flashing propellant flow inside pMDI actuator as well as transfer of heat, mass, and momentum between the liquid and vapor phase. Variations in the rate of such interphase phenomena dictate the two-phase mass flow rate emission, which itself determines spray velocity and droplet size. In this work, we compare the performance of existing two-phase flow models to predict the flow conditions and the rate of propellant flow through a pMDI actuator: the homogenous equilibrium model (HEM), the slip equilibrium model (SEM), and the homogenous frozen model (HFM). The velocity prediction of the HFM was found to be in good agreement with phase Doppler anemometry (PDA) data indicating the metastable nature of the emitted propellant spray. This work also considers Clark's correlation for the aerosol droplet size based on the results of the flow model. The results of the correlation were compared with PDA droplet size measurements. Clark's correlation was found to be effective in predictions of the temporal droplet size variations. However, the value of an empirical constant had to be tuned to fix the droplet size for a given combination of formulation, device, and to a lesser extent also the distance from the spray orifice where predictions are compared with PDA data. This highlights the need to develop first principle atomization models without the need for case-by-case adjustment.

© 2017 American Association for Aerosol Research     

Growth rate and oscillation frequency of electrified jet and droplet: Effects of charge and electric field

Electrified jets are applied industrially in agriculture, automobiles, targeted drug delivery systems, spacecraft propulsion units, liquid metal sprayers, ion sources, emulsifiers, dust scavenging systems, and ink-jet printers. Electrified columnar jets experience instability caused by electrohydrodynamic interactions of the charged liquid surfaces with electric fields. Electrostatic and surface tension forces competing along the liquid surface create surface pressure differences. The temporal rise and fall of the surface pressure induce oscillations of jets and droplet. A linear theory was derived to yield a dispersion equation determining the most dominant wavelength of oscillation for a given charge level and electric field; this enabled the estimation of the diameter of an atomized droplet. In addition, the frequency of oscillation was derived for a cylindrical jet and spherical droplet. Parametric studies were performed for various charging levels and electric field strengths.

© 2018 American Association for Aerosol Research     

Controlling Blown Polyethylene Film Optical Properties

The conventional blown process imparts an inherent haze to the product. The percentage of haze varies with certain process variables:

1. Surface irregularities caused by melt flow phenomena

2. Crystallization behavior

3. Melt drawing phenomena in certain types of polyethylene     

HEAT TRANSFER IN PACKED BED REACTORS WITH ONE PHASE FLOW

An extensive array of literature data on the heat transfer from a reactor wall to a fluid flowing through a packed bed and those obtained from some experimental runs were interpreted with a model containing two parameters: k_e, (effective radial thermal conductivity within the bed) and h_w (heat transfer coefficient at the wall).

Both parameters were considered in terms of a stagnant contribution (due to the heat conduction through the solid particles and the fluid in the void space) and a radial mixing contribution (due to the heat convection by turbulent mechanism.

The stagnant contribution was interpreted with a model similar to that proposed by Kunii and Smith (1966) for heat transfer in a packed bed with motionless fluid.

General correlating equations for calculating the stagnant and the turbulent contributions of both k_e, and h_w are proposed.     

An integrative model for the filtration efficiencies in realistic tests with consideration of the filtration velocity profile and challenging particle size distribution

Many well-established models can be applied to calculate the filtration efficiencies. In these models the filtration velocity and challenging particle size are assumed to be known accurately. However, in realistic filtration tests, the filtration velocity has profiles dependent on the filter holder geometry and experimental conditions; the challenging particles have size distributions dependent on the instruments and operation conditions. These factors can potentially affect the measured filtration efficiency and lead to discrepancies with the models.

This study aims to develop an integrative model to predict the filtration efficiencies in realistic tests by incorporating the effects of the filtration velocity profile and challenging particle size distribution classified by a differential mobility analyzer (DMA) into the existing filtration models. Face velocity profile is modeled with fluid mechanics simulations; the initial generated particle size distribution, the particle charging status and the DMA transfer function are modeled to obtain the challenging particle size distribution. These results are then fed into the filtration models. Simulated results are compared with experimental ones to verify the model accuracy. This model can be used to reduce filtration test artifacts and to improve the experimental procedure.

The results reveal that the face velocity upstream the filter exhibits high degree of homogeneity not affecting the filtration efficiency if the filter pressure drop is not very low. The generated particle size distribution and the DMA selection size window could influence the challenging particle size distribution and therefore the measured filtration efficiency.

Copyright © 2017 American Association for Aerosol Research     

MECHANISM OF CLASSIFICATION IN A STURTEVANT-TYPE AIR CLASSIFIER

The estimation of air velocity distributions and particle trajectories is inevitable to analyse the mechanism of classification, but the direct measurement of il is extremely difficult.

The authors, here report three dimensional air velocity distributions within the inside drum of model Sturtevant-type air classifier measured by a spherical five-holed Pitot-lube, and also two dimensional particle ejecting velocities on a model distributor determined by photography.

Using those results, the cut size calculated from particle trajectories in the classifier is compared with the experimental results and theoretical values.     

Influence of surfactants on growth of individual aqueous coarse mode aerosol particles

Understanding the links between aerosol and cloud and radiative properties remains a large uncertainty in predicting Earth's changing energy budget. Surfactants are observed in ambient atmospheric aerosol particles, and their effect on cloud droplet growth is a mechanism that was, until recently, neglected in model calculations of particle activation and droplet growth. In this study, coarse mode aqueous aerosol particles were created containing the surfactant Igepal CA-630 and NaCl. The evaporation and condensation of these individual aqueous particles were investigated using an aerosol optical trap combined with Raman spectroscopy. For a relative humidity (RH) change from 70% to 80%, droplets containing both Igepal and NaCl at atmospheric concentrations exhibited on average more than 4% larger changes in droplet radii, compared to droplets containing NaCl only. This indicates enhanced water uptake in the presence of surfactants, but this result is unexpected based on the standard calculation of the effect of surfactants, using surface tension reduction and/or hygroscopicity changes, for particles of this size. One implication of these results is that in periods with increasing RH, surfactant-containing aqueous particles may grow larger than similarly sized aqueous NaCl particles without surfactants, thus shifting atmospheric particle size distributions, influencing particle growth, and affecting aerosol loading, visibility, and radiative forcing.

Copyright © 2018 American Association for Aerosol Research     

Ozone Treatment in Cooling Water Systems

Ozone treatment for preventing the biofouling in cooling water systems is investigated.

In the fresh water system, the separating effect of the ozonated water on the microorganisms such as the sphaerotilus and the zoogloea which adhere to the piping and form the slime is recognized. When the ozonated water is supplied intermittently to the piping without stopping the flow of the cooling water, a constant volume of cooling water can be maintained. At the velocity of 1 m/sec, the amount of metal corrosion produced by the ozonated water is reduced on the mild steel, increased on the copper and does not change on the cast iron, when compared with that produced by the water containing no ozone.

In the seawater system, since many substances are oxidized by the ozone, the same treatment as that in the fresh water system cannot be applied. However, if the seawater in the cooling system can be replaced with ozone-containing air intermittently once a week, the adhesion of organisms such as barnacles and mussels to the piping can be prevented without having a bad influence on the other living oceanic organisms.     

HEAT TRANSFER IN TURBULENT RECIRCULATORY FLOWS AFFECTED BY BUOYANCY FORCES IN RECTANGULAR CAVITIES

An analysis is carried out to investigate the heat transfer in a turbulent recirculatory flow affected by buoyancy forces.

Transport equations for turbulence kinetic energy, energy dissipation Tate and mean square temperature fluctuations are derived and solved numerically together with the mass, momentum and energy conservation equations. The flow pattern and temperature distribution are shown for different Reynolds numbers. The buoyancy effect on flow pattern and temperature distribution is analyzed. Mean Nusselt numbers, with and without buoyancy effect are presented. Turbulence characteristics such as turbulence kinetic energy and effective viscosity arc discussed.     

Opto-aerodynamic focusing of aerosol particles

We describe a new method for focusing and concentrating a stream of moving micron-sized aerosol particles in air. The focusing and concentrating process is carried out by the combined drag force and optical force that is generated by a double-layer co-axial nozzle and a focused doughnut-shaped hollow laser beam, respectively. This method should supply a new tool for aerosol science and related research.

Copyright © 2018 American Association for Aerosol Research     

Analytical expression for the friction coefficient of DLCA aggregates based on extended Kirkwood–Riseman theory

We use a self-consistent field method, which we have previously validated, to calculate the translational friction coefficient of fractal aerosol particles formed by diffusion-limited cluster aggregation (DLCA). Our method involves solving the Bhatnagar–Gross–Krook model for the velocity around a sphere in the transition flow regime. The velocity and drag results are then used in an extension of Kirkwood–Riseman theory to obtain the drag on the aggregate. Our results span a range of primary sphere Knudsen numbers from 0.01 to 100 for clusters with up to N = 2000 primary spheres. Calculated friction coefficients are in good agreement with experimental data and approach the correct continuum and free molecule limits for small and large Knudsen numbers, respectively. Results show that particles exhibit more continuum-like behavior as the number of primary spheres increase, even when the primary particle is in the free molecule regime; as an illustrative example, the friction coefficient for aggregates with primary sphere Kn = 1 is approximately equal to the continuum friction coefficient for N > 500. We estimate that our calculations are within 10% of the true values of the friction coefficients for the range of Kn and N presented here. Finally, we use our results to develop an analytical expression (Equation (38)) for the friction coefficient over a wide range of aggregate and primary particle sizes.

Copyright © 2017 American Association for Aerosol Research