A Transition from Heterogeneous to Homogeneous Nucleation in the Turbulent Mixing CNC

A new method for changing the supersaturation in the Turbulent Mixing CNC has been developed and used to examine the transition from heterogeneous nucleation of test particles to homogenous nucleation of working fluid: dibutylphthlate (DBP). Supersaturation was controlled by changing the DBP vapor pressure in the nozzle flow by saturating only a predetermined part of the flow, while the total flow and temperature remain constant. This approach allows for the changing of the initial DBP vapor pressure, while keeping the flow structure and temperature field unchanged. The DBP concentration in the outlet of the vapor generator was measured experimentally for different ratios of saturated and bypass flows and found to be close to estimated values. Experimental results for transitions from heterogeneous nucleation to homogeneous nucleation are presented for NaCl and WO_x particles at various DBP vapor pressures. With an increasing of the DBP vapor pressure, the concentration of enlarged particles increases until it reaches a plateau. At higher initial values of DBP pressure, homogeneous nucleation prevails, and the number concentration of particles follows a curve typical for homogeneous nucleation recorded in the absence of nuclei. Nuclei with different mobility diameters were activated at different values of vapor pressure. There are significant differences in the slopes of particle activation curves for NaCl and WO_x particles. The reasons for such differences are a subject for continuing research.     

Large Eddy Simulation of Turbulent Reacting Mixing of Liquids in a Coaxial Jet Mixer

Large eddy simulation is used to investigate passive and reactive scalar mixing at high Reynolds Re and Schmidt Sc numbers in order to prove capability of the LES‐SGS micromixing approaches based on the eddy dissipation and DQMOM‐IEM models properly simulating liquid reacting flows. Simulations were performed for a fast neutralization reaction in a confined jet reactor. The mean profiles for passive scalar agree well with measurements. It was shown that the most contribution to the scalar variance is made by large scale motions whereas the contribution of fine scales smaller than typical inertial range scales is negligible. Thus, the existing LES models are capable of predicting the scalar variance at large Sc numbers. The results obtained for reactive transport revealed discrepancies in the determination of micromixing rate and product concentration. A special study was performed to investigate the dynamics of fine structures using locally refined box embedded into global grid. Typical statistical properties of fine structures were reproduced numerically.     

射流泵湍流场的数值模拟与实验研究

采用k-ε湍流模型和非等间距加密网格,对射流泵流场进行了数值模拟和分析,并对相应的流场进行了实验研究.结果表明,流场轴向速度剖面在扩散管段具有较好的自相似性,而在喉管段则不然;这种速度剖面变化的转折点与喉管的长度有关;流场的湍动能分别在喷嘴出口与扩散管入口处产生峰值,并且前者远大于后者,可见射流泵流场中,湍流主要发生在喉管入口处,湍动能的不平衡将导致额外的能量损失.本研究结果对工程应用有指导意义.     

Monte Carlo Simulation of Droplet Nucleation in RAFT Free Radical Miniemulsion Polymerization

Abstract

The early stages of the reversible addition/fragmentation transfer (RAFT) miniemulsion polymerization were simulated, focusing on the effect of the RAFT agent on droplet nucleation. For highly reactive RAFT agents, a large number of free radicals (N_c ) needed to be captured by a droplet in order to initiate polymerization in the droplet, which was totally different from the behavior of regular miniemulsion polymerization. More interestingly, it was found that droplet size had a significant influence on N_c value. It was shown that the RAFT agent has a significant influence on miniemulsion polymerization, leading to long induction periods and retardation of polymerization. In addition, miniemulsion droplets with different sizes are nucleated at different times, which could lead to very low nucleation efficiency. The results would be very helpful in understanding and designing a RAFT miniemulsion polymerization system.     

Diffusion of a Submicronic Spray in an Homogeneous Turbulent Flow

The prediction of the diffusion of a spray in a gaseous environment is still a difficult problem to solve, even when the droplets are small enough to be considered a passive contaminant. The present work proposes to use a simulation of the random trajectories of the individual particles based on the method proposed by Ormancey (1983), and Ormancey and Martinon (1983). It is applied to the case of a nozzle of finite dimensions discharging passive particles in homogeneous turbulent airstream at a low enough concentration to avoid neighboring effects. The results of this simulation compare favorably with those of an experiment designed to test the method. They are also in agreement with the Lagrangian diffusion theory of Taylor (1936).     

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

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

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

Combined Homogeneous and Heterogeneous Crystal Nucleation in Glasses

The temperature dependence of combined homogeneous and heterogeneous crystal nucleation is examined within the framework of the classical theory. It is demonstrated that nucleation can have a local maximum rate at one or two temperatures. The physical parameters which cause large changes in the structure of the nucleation curve are elucidated. Also, the influence of saturation effects on the structure of nucleation curves is illustrated. The utilization of these results for the interpretation of experiments and for the production of controlled crystal nucleation in glass is considered.     

Ammonium Chloride Aerosol Nucleation and Growth in a Cross-Flow Impinging Jet Reactor

The nucleation and growth of an ammonium chloride aerosol starting from gaseous ammonia and hydrogen chloride was investigated experimentally and with the use of a mathematical model. The reactor was composed of 2 opposed jets perpendicular to a main stream and was operated under laminar/transition flow conditions. The reactants were segregated when they entered the reactor. The parameter observed was the particle size distribution of the aerosol described by its moments. Considerable scatter in the experimental results complicated their analysis, but some important trends could be identified. The model results were strongly affected by the fluid mechanics model, which influenced the predicted mixing inside the reactor. This work shows the importance of fluid mixing in controlling the aerosol size distribution.     

均相成核-水热法制备纳米氧化锌

制备纳米氧化锌的关键是成核与生长控制。采用尿素为均相沉淀剂 ,结合水热处理 ,温度 130℃ ,反应时间为 3～ 5h制备了粒径小 ,分布窄的纳米氧化锌。Zn2 +的均相成核与其浓度、OH- 的浓度有关 ,体系中Zn(OH) 2 的过饱和度越大 ,则成核数量越多 ,所制备的氧化锌颗粒越小 ;与以氢氧化锌胶体为前驱体的共沉淀技术相比 ,均相沉淀技术有利于氧化锌纳米晶粒均匀生长 ;此外 ,添加有机物作分散剂可以减弱微小晶粒间的叠合生长 ,有利于制备均一性较好的纳米氧化锌。     

乳液聚合粒子成核和粒子大小的影响因素

介绍了乳液聚合中胶束,均相和单体液滴三种不同的粒子成核机理。讨论了引发速率和乳化剂浓度以及不同聚合方法和各种条件对乳胶粒子大小的影响。     

The Effects of Unresolved Scalar Fluctuations During Homogeneous Nucleation

The effects of the subgrid-scale (SGS) scalar interactions on nanoparticle nucleation are investigated via a priori analysis of direct numerical simulation data. The formation of dibutyl-phthalate (DBP) particles via homogeneous nucleation is simulated in a planar wake. Classical nucleation theory is used to model particle nucleation and the Navier-Stokes equations are coupled with the scalar transport equations to provide the fluid, thermal, and chemical fields. The data shows that particle nucleation is initially confined to the thin interfacial region or shear layers, where molecular diffusion is dominant. As the flow becomes turbulent nucleation increases significantly and the rate of particle formation increases by several orders of magnitude. To assess the effect of SGS scalar interactions on DBP particle nucleation, the temperature and mass-fractions are filtered and the resulting quantities are used to compute the nucleating particle field. Two filter widths are used to obtain varying levels of SGS interactions. Particle size distributions are computed to examine the particle fields produced. This work shows that the SGS interactions’ effect on nucleation has two distinct trends. In the proximal region of the wake, the unresolved interactions act to decrease particle formation. However, as the flow transitions or becomes turbulent the effect of the SGS interactions act to increase particle formation.

Copyright 2013 American Association for Aerosol Research     

Analytical Approximation Schemes for Mean Nucleation Rate in Turbulent Flows

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

Copyright 2014 American Association for Aerosol Research     

新型穿流式搅拌桨研究 I.全混状态下的水射流实验

穿流式搅拌桨桨叶上的开孔所引起的射流对于提高搅拌效率、降低搅拌能耗起着重要的作用。采用模拟实验方法,研究了全混状态下喷孔出口流速、孔径和孔长对水射流半扩展角β和源点xp/D的影响。研究结果表明:喷孔孔径、孔长以及喷孔前流体的湍动程度对射流发展有很大影响,存在一临界喷孔长径比(H/D)0,1.31 (H/D)0 1.67。当H/D≤(H/D)0时,β随速度(或Re)增加而增大,xp/D随Re增加变化很小;当(H/D)>(H/D)0时,β随速度(或Re)增加而减小,xp/D随Re增加而减小。为了获得较好的混合效果,搅拌桨桨叶上的开孔所引起的射流应有尽量大的扩展效果,因此桨叶上开孔的长径比应小于(H/D)0。     

湍动流化床中的颗粒减速现象

采用PV6D型光纤探针测量了直径为200 mm与95 mm的2套湍动流化床装置中的颗粒速度分布,据此将湍动床在轴向上分成6个区域,分析了上部出口减速区与下部过渡段减速区的产生机理,考察了流化段高度、静床高、表观气速对颗粒减速区的影响。结果表明:上部减速区是扩大段所产生的,下部减速区则是由于固含率急剧变化所引起的;随流化段高度减小,上部减速区下降,相邻的充分发展区与加速区缩短乃至消失;下部减速区的位置随静床高的增加而升高,随表观气速的增大而下移,与过渡段的变化趋势一致。     

Particle Motion in Two-Dimensional Confined Turbulent Flows

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

Particle Mixing and Diffusion in the Turbulent Wake of a Sphere

The mixing and diffusion of monodisperse aerosols from a turbulent free stream into the wake created by a solid sphere were studied experimentally and analytically. Experimental measurements with regard to wake velocity profiles and momentum transport were taken in a tubular flow system by hot-wire anemometry. Particle concentration profiles, measured in the wake by means of a screen collection technique, were used to calculate mixing and diffusion parameters for the aerosols used. Experimental results are presented for the mean velocity and particle concentration profiles, wake centerline velocity and particle concentration, wake velocity and concentration half widths, and momentum and particle diffusivities. It is shown that the experimental results may be reasonably described in terms of two phenomenological theories, i.e., Prandtl's mixing length and constant diffusivity in the radial cross section of the wake. Certain departures from predicted concentration behavior are empirically analyzed resulting in the ability to predict overall particle mixing and diffusion characteristics in a turbulent wake.     

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

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

Homogeneous Nucleation of 2, 4-Dichlorophenol in Acid Aqueous Solutions

The interfacial tension of 2, 4-dichlorophenol/acidic solution, 23 ergs/cm², was calculated from Nielsen's equations which combine homogeneous nucleation theory and diffusion controlled growth. The parameters measured were induction periods in the millisecond range and the technique employed was photographing the appearance of turbidity on a film moving at 50cm/sec velocity. The calculated value was compared with direct measurements of 2, 4-dichlorophenol's surface tension in air and in solution.     

Particle Nucleation Loci in Styrene-Butyl Acrylate Miniemulsion Copolymerization

Summary Miniemulsion polymerizations of styrene and butyl acrylate in the presence of sodium dodecyl sulfate (SDS) as the surfactant and stearyl methacrylate (SMA) as reactive cosurfactant were carried out by using water-soluble (potassium persulfate) and oil-soluble (2,2-azobisisobutyronitrile) (AIBN) initiators respectively. Effects of the two initiators on the particle nucleation mechanisms are investigated. By comparison with the mean diameter of monomer droplets and polymer particles, it is shown that both homogeneous nucleation and monomer droplets nucleation coexist in the presence of a water-soluble initiator but homogeneous nucleation becomes less significant with SDS concentration increasing. Meanwhile, using oil-soluble initiator, the possibility of homogeneous nucleation is depressed effectively. As a result, monomer droplets are definitely the main loci of particle nucleation.     

Effect of NO 2 on Particle Formation in SO 2 /H 2 O/Air Mixtures by Ion-Induced and Homogeneous Nucleation

Effects of NO 2 on particle formation in SO 2 /H 2 O/Air mixtures by f -ray irradiation as a source of hydroxyl radicals are investigated to evaluate the contribution of homogeneous and ion-induced nucleation in the nanometer-sized aerosol particle generation. The total particle number concentration, the charged-particle fraction, and the electrical mobility distribution of particles and hydrated and solvated ions generated in NO 2 /SO 2 /H 2 O/Air mixtures were measured at NO 2 levels of 0-1.98 ppm, SO 2 levels of 0-3.87 ppm, and H 2 O levels of 6480-9889 ppm. Total particle number concen2 tration first tends to increase with increasing NO 2 concentration up to a certain NO 2 level and then decreases for NO 2 concentrations beyond this point. The charged-particle fraction, indicative of particle formation by ion-induced nucleation, tends to decrease with increasing NO 2 concentration, reaching a minimum value. The peak intensity of the electrical mobility distribution for particles significantly exceeded that in the absence of NO 2 at a relatively low concentration of NO 2 . The intensity decreased with increasing NO 2 concentration and fell below that in the absence of NO 2 . The results indicate that 2 opposing mechanisms that enhance and suppress the particle formation work simultaneously in NO 2 /SO 2 /H 2 O/Air mixtures, and the dominant mechanism depends on the NO 2 concentration.