Effects of exponentially decaying and growing concentrations on particle size distribution from a scanning mobility particle sizer

Abstract

A scanning mobility particle sizer (SMPS) is one of the most widely used instruments to obtain size distribution for atmospheric particles. In an SMPS measurement, a voltage scanning process on a differential mobility analyzer is required, and it typically takes 30?s to 120?s to obtain one entire size distribution. A size distribution obtained by an SMPS measurement might have significant deviations from actual values due to the scanning process when the measured particle concentrations change over time. In this study, we introduce an analytical approach for estimating particle size distribution under exponentially decaying and growing particle concentrations. The analytical SMPS results are validated by performing experiments using exponentially decaying particle concentrations under the same conditions. Furthermore, the effects of a decay parameter, initial size distribution, and scan time are evaluated, and the deviations from actual (real or true) size distributions obtained by an exact solution are analyzed. Geometric mean diameters and standard deviations of the size distributions from SMPS results increase or decrease with exponentially decaying or growing concentrations, respectively, and total concentrations estimated by the analytical SMPS approach are significantly underestimated or overestimated compared to real total concentrations. While SMPS measurements have been widely employed in various applications such as atmospheric particle characterization in highly variable particle concentrations versus time, very few studies on the influence of changing concentrations on SMPS measurements have been conducted. Therefore, the introduced analytical approach and findings provide valuable insight into the importance of accurate SMPS measurements with changing particle concentrations.

Copyright © 2020 American Association for Aerosol Research     

Experimental study of oil particle emission rate and size distribution during milling

Metalworking fluids (MWFs) used in milling generate oil particles through impaction, action of centrifugal forces and evaporation/condensation mechanisms. The oil particles suspended in the factory atmosphere can affect the health of the labor force. In order to study the emission properties of these oil particles, this work investigates the oil particle emission rate and size distribution during milling using an environmental chamber method. Two commonly used operating modes for MWFs were selected, the minimum quantity lubrication (MQL) mode (40?ml/h) and the cooling mode (1 m³/h). The cooling mode without cutting was studied separately for comparison with the cooling mode with cutting. The results show that the oil particle emission rate in milling ranges from 7.2 to 641?mg/h, and the size distribution ranges from 0.265 to 12.5?µm. Evaporation/condensation is the main mechanism in the MQL mode. The majority of oil particles formed by evaporation/condensation are in the range of 0.265 to 1.8?µm. As the tool rotation speed increases, the particle emission rate increases, while the mass mean diameter (MMD) and the sauter mean diameter (SMD) decrease. Oil particles are mainly generated by the action of centrifugal force in the cooling mode, and mainly distributed in the range of 1.8 to 12.5?µm. The particle emission rate increases with the tool rotation speed, and the particle MMD and SMD increase with the tool rotation speed only in the cooling mode without cutting. The particle emission rate ranging from 1.8 to 12.5?µm, as well as PM5 and PM10, are a polynomial function of the square of tool rotation speed during the cooling mode. The coefficient of determination (R²) is above 0.99.

© 2018 American Association for Aerosol Research     

Effect of axial eccentricity on the performance of a cylindrical differential mobility classifier

A differential mobility classifier (DMC) is one of the core components in electrical mobility particle sizers for sizing sub-micrometer particles. A DMC in the cylindrical configuration (i.e., constructed by axial aligning of the inner and outer cylinders) is typically included in the sizers. The knowledge of construction tolerance is required in the design of a cylindrical DMC. The numerical approach was applied in this study. Our study shows that the DMC transfer function deteriorated as the axial eccentricity was increased (i.e., the peak is reduced and the width at the half peak height is broaden). At high axial eccentricity, the transfer function peak would split into two. In addition to the flow parameters such as the sheath-to-aerosol flow rate ratio and total flow rate, the effect of geometrical parameters (i.e., the length and aspect ratio of the particle classification channel, and the ratio of outer-to-inner cylinder radii) on the transfer function of an eccentric DMC were also investigated. It is found that the classification length and the sheath-to-aerosol flow rate ratio have obvious impact on the transfer function of an eccentric DMC. Furthermore, the particle diffusivity reduced the effect of axial eccentricity on DMC transfer function, especially for particles with the sizes less than 10?nm.

Copyright © 2019 American Association for Aerosol Research     

Effect of secondary gas injection on the particle entrainment rate in a gas fluidized bed

The effect of secondary gas injection on the particle entrainment rate was measured in a cold model fluidized bed (i.d. 0.1 m, height 2.25 m) and discussed. Sand particles below 0.425 mm in screen size were used as bed materials. The particle size (0.128–0.363 mm), the overall superficial gas velocity (0.78–2.76 m/s), the secondary gas fraction (0–0.5), and the static bed height (0.1–0.3 m) were considered as experimental variables. The particle entrainment rate decreased with an increase of the secondary gas fraction. The injection level of the secondary gas was shown to have an important influence on its effect. The effect of the secondary gas was appreciable for over-bed injection; the effect was reduced, however, when the injection level was placed in the splash zone or in the dense phase by means of increasing the bed height.     

Influence of inherent particle characteristics on hopper flow rate

The inherent factors influencing the shear strength of particulate materials are also believed to influence the flow rate through a hopper. These inherent particle characteristics include particle size, particle-size distribution, particle shape, angularity, hardness, and surface roughness. To determine the effect of inherent particle characteristics on flow rate and pluviated void ratio, a wide range of materials were tested. These materials ranged from manufactured sands and glass beads to natural sands. Determination of particle size, particle-size distribution, and hardness was achieved by conducting conventional tests such as sieve analyses and specific gravity tests. Quantification of the shape and angularity parameter is more difficult due to the nonavailability of any standard techniques. To achieve the objective of quantifying shape and angularity, a new technique was developed utilizing the image analyzer. Two shape parameters, namely, Shape Factor and Angularity Factor, were determined for various materials. Shape and Angularity Factors were correlated with flow rate as well as the pluviated void ratio. Overall, the results indicate that as shape and angularity of particles increase, the flow rate decreases, and pluviated void ratio increase. A good correlation between drained shear strength properties and the flow rates measured in the cone was also found to exist. Therefore, index tests such as flow rate through a flow cone (hopper) can be used to estimate the drained monotonic strength of particulate materials.     

镧对碳化硅超细粉合成温度与粒度的影响

众多研究表明,提高反应物起始原料的均匀混合程度、添加适量的添加剂是改善碳热还原法制备碳化硅超细粉体的有效途径。由于稀土元素对众多的化学反应都能起到有效的促进作用,于是,以纳米二氧化硅和活性炭为起始原料,以稀土镧为添加剂,采用碳热还原法制备碳化硅超细粉体,并借助X射线衍射仪和激光粒度仪分别对合成的粉体进行物相和粒度分析,实验结果表明:添加一定量的稀土镧有助于降低碳化硅超细粉的合成温度及粉体粒度。     

Effect of coal particle size distribution on packed bed pressure drop and gas flow distribution

This paper focuses on the role of coal particle size distribution on pressure drop and gas flow distribution through packed coal beds. This fundamental knowledge is helpful in better understanding the operational behaviour of fixed bed dry bottom gasifiers. The Sasol synfuels plants in South Africa use 80 such gasifiers to convert more than 26 million tons of coal per annum to synthesis gas, and ensuring stable operation is of primary importance to ensure high synthesis gas production rates and gasifier availability. Pressure drop measurements on laboratory scale equipment were conducted to investigate the effect of particle size distribution on packed bed pressure drop. The well-known Ergun equation for pressure drop does not accommodate the effect of size distribution on pressure drop. A novel approach was followed to model pressure drop through simulated coal bed structures using Computational Fluid Dynamics (CFD). The coal bed structures were simulated by assuming that the coal particles are represented by randomised convex polyhedra in three-dimensional space. The computational space was divided into polyhedra using statistical Voronoi tessellation technique, which have been shown to be versatile in modelling problems in many fields, e.g. filtration, molecular physics, metallurgy, geology, forestry and astrophysics. This approach of flow modelling through packed coal beds is able to accommodate size distribution effects on pressure drop and gas flow distribution. The modelling work shows large deviations from plug flow with broad size distributions. The lowest bed pressure drop with the closest approximation to plug flow is obtained with the narrowest particle size distribution. Low gas flow rates are also beneficial for reducing excessive channel flow. Combustion profiles for different particle size distributions were studied using a pilot scale combustor. The combustion profiles provide confirmation of the CFD modelling results, namely that narrow particle size distributions and low gas flow rates reduce channel burning. Excessive channel burning was observed for broad particle size distributions, and is enhanced by high gas flow rates. The experimental and modelling work which was conducted, clearly indicate that narrow coal particle size distributions are desirable for optimum gas flow distribution and lowest packed bed pressure drop.     

Effect of temperature on particle size for vapor-phase synthesis of ultrafine iron particles

For the vapor-phase synthesis of iron particles from FeCl₂ at temperatures ranging from 800 to 950‡C the reason is sought why the model based on the classical nucleation theory brought an increase of particle size with temperature increase, in reverse to experimental observation. The nucleation rate according to the classical theory should decrease with a temperature increase, due to the decrease of super-saturation ratio resulting from the increase of vapor pressure. The decrease of nucleation rate ultimately leads to an increase of particle size. Yang and Qiu’s nucleation theory was applied in place of the classical theory. However, the same result as with the classical theory was obtained : the nucleation rate decreased with the temperature increase. Finally, an Arrhenius-type nucleation rate equation was introduced. The preexponential factor and the activation energy for nucleation were determined to be 1348.2 sec_-1 and 159.1 KJ/mol, respectively. With these values put into Park et al.’s model, good agreement was obtained in temperature dependence of particle size between model prediction and experimental data.     

载气流量及升温速率对污染土壤中多氯联苯热脱附的影响

利用小型管式炉进行了载气流量和升温速率对多氯联苯污染土壤的热脱附过程影响的实验研究。结果表明流量的增大对多氯联苯含量和毒性当量的去除效率影响不大,载气流量小于400 ml·min^-1时,气相中的脱附量明显增大,载气流量大于400 ml·min^-1则脱附量变化较小,而多氯联苯的毒性当量则随着载气流量增大呈线性增加趋势。实验结果还表明污染土壤中多氯联苯变化速率与升温速率呈明显的正线性相关,随着升温速率增加,污染土壤中多氯联苯去除效率总体呈上升趋势,毒性当量去除效率降低。总体结果分析可以看到升温速率越大,总体效果也越好。污染土壤经热脱附处理后气固相的二 英毒性当量则有不同程度提升,尤其气相内检测到了大量多氯二苯并呋喃生成。     

Effect of ultrafine Al-C particle additives on the PETN sensitivity to radiation exposure

The thresholds of explosive decomposition of PETN (pentaerythrite tetranitrate) with the addition of ultrafine Al-C mechanocomposite particles were measured as a function of the concentration of the latter in the experimental samples exposed to laser pulses (1.064 nm, 12 ns). The sample density was 1.73 g/cm³, and the Al-C particle size at the distribution peak was 220 nm. The minimum threshold of explosive transformation corresponding to a 50% probability of explosion with an energy density of 4 J/cm² was reached at an optimum concentration of the mechanocomposite of 0.1–0.3%. Comparison with experimental data obtained for samples with aluminum nanoparticle additives was performed.     

Influence of the mass flow rate of secondary air on the gas/particle flow characteristics in the near-burner region of a double swirl flow burner

The influence of the mass flow rate of secondary air on the gas/particle flow characteristics of a double swirl flow burner, in the near-burner region, was measured by a three-component particle-dynamics anemometer, in conjunction with a gas/particle two-phase test facility. Velocities, particle volume flux profiles, and normalized particle number concentrations were obtained. The relationship between the gas/particle flows and the combustion characteristics of the burners was discussed. For different mass flow rates of secondary air, annular recirculation zones formed only in the region of r/d=0.3–0.6 at x/d=0.1–0.3. With an increasing mass flow rate of secondary air, the peaks of the root mean square (RMS) axial fluctuating velocities, radial mean velocities, RMS radial fluctuating velocities, and tangential velocities all increased, while the recirculation increased slightly. There was a low particle volume flux in the central zone of the burner. At x/d=0.1–0.7, the profiles of particle volume flux had two peaks in the secondary air flow zone near the wall. With an increasing mass flow rate of secondary air, the peak of particle volume flux in the secondary air flow zone decreased, but the peak of particle volume flux near the wall increased. In section x/d=0.1–0.5, the particle diameter in the central zone of the burner was always less than the particle diameter at other locations.     

Nanocrystalline ZnO films deposited by spray pyrolysis: Effect of gas flow rate

ZnO is one of the multifunctional metal oxide semiconductors suitable for use in optoelectronic devices, as an alternative to tin oxide and indium oxide. Undoped ZnO films have been prepared using simple and cost-effective spray pyrolysis technique. In this work, we present detailed analysis of the structural, morphological and optical properties of ZnO films. X-ray diffraction spectra of the films have shown that the films are polycrystalline and hexagonal wurtzite in structure. The average grain size, estimated from the (002) peak using the Scherrer formula is around 40 ± 5 nm. The average optical transmittance of undoped ZnO thin films was about 80% in the visible. The AFM analysis reveals the surface topology having highly rough nature. The enhanced extinction coefficient varies with the flow rate. The EDX spectrum registers non-stoichiometric ZnO growth with O and Zn in 28.46 and 71.56 at. %. The features of the ZnO lead to the visible light transmission and efficient charge transport.     

Effect of modulations of opposed gas flow velocity on flame spread rate over a liquid surface

The effect of modulations of the velocity of the gas flow incident on the flame on the average flame velocity over a shallow liquid is studied. It is shown that the average flame velocity depends on the modulation frequency. If the modulation frequency is higher than the flame oscillation eigenfrequency, then, upon the imposition of the modulation, the flame velocity first increases and then gradually returns to the initial value. At frequencies close to the flame oscillation eigenfrequency, the average flame velocity is constant but is higher than the initial value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 11–17, January–February, 2009.     

Short and long term particle release from surfaces under the influence of gas flow

Preliminary measurement of particle reentrainment from 5 mm i.d. and 3 m long stainless steel tubes show two distinct regimes of concentration decay with time.

In the regime (t < few minutes) the concentration n(t) is proportional to t^−a with a > 1 being a function of flow rate and particle size. The ST regime appears to correspond to particle release by shear flow in the region where F = (adhesion force)/(removal force) 1.

The concentration decay is described by the desorption model of Wen and Kasper, where n(t) is proportional to (1/t) exp (−t/T). The LT regime appears to include the release of relatively strongly adhering particles where F > 1 up to about F = 5 or 6 (Wen and Kasper, 1989) which is classically forbidden and explained only in the model Reeks et al. (1988).     

Effect of chemical reaction on particle to gas heat transfer

Particle to gas heat transfer studies were carried out by employing the endothermic cyclohexane dehydrogenation reaction. The calculated heat transfer coefficients were on the lower side of those previously reported in the literature for unreactive systems. Previous studies of other investigators with reactive systems suggest that particle to gas heat transfer coefficients may increase with exothermic reaction due to the product molecules leaving the catalyst surface with excess vibrational energy. For an endothermic reaction this effect would result in the calculation of a lower heat transfer coefficient.     

Porosity and particle size effects on the gas flow characteristics of porous metals

Several models have been proposed in the past which relate the gas flow characteristics of a porous material to the pore microstructure. Such approaches require metallographic measurements to predict the permeability or inertial flow coefficients. The present study approaches the problem through the dominant adjustable extrinsic processing variables. Specifically, porosity and mean particle size are used to predict the gas flow coefficients for porous 316L stainless steel. Although such an approach proves successful, it is established that traditional processing variables (compaction pressure, sintering temperature and sintering time) have a significant influence on pore shape and therefore on gas flow. Such factors must be included in any comprehensive future predictive models of gas flow through porous metals.     

Effect of multi-inlet flow on particle classification performance of hydro-cyclones

The effect of multi-inlet flow on particle classification performance of hydro-cyclones was examined experimentally and via a simulation study. Cut size of the type A cyclone with two inlets flow indicated smaller cut size and sharp separation performance compared to the standard cyclone. Numerical simulation indicated a nearly uniform rotational fluid velocity profile for the type A cyclone. On the other hand, the standard cyclone showed a non-uniform rotational velocity profile near the inlet part of the main flow. The type B cyclone with a small additional flow injection area, showed smaller cut sizes as the flow rate of the additional flow increased. The type B cyclone showed smaller cut size compared to the standard cyclone without the additional flow. The use of a multi-inlet cyclone is quite effective at improving particle separation performance compared to the standard one.     

Effect of particle acceleration/deceleration on particle clustering behavior in dilute gas-solid flow

Effect of particle acceleration/deceleration on particle clustering behavior in dilute gas-solid flow was studied by experimental measurements and mathematical modeling. Calculation results of the model are in good agreement with experimental data from Phase Doppler Particle Analyzer (PDPA) measurements. The variation of voidage inside particle clusters is strongly dependent upon the change of the number of particles within the clusters. During acceleration, particle clusters are gradually disaggregated into smaller clusters with increasing voidage, while during deceleration, particle clusters are aggregated into larger clusters with decreasing voidage.