共查询到20条相似文献,搜索用时 0 毫秒
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Akhil Rao Jennifer S. Curtis Bruno C. Hancock Carl Wassgren 《American Institute of Chemical Engineers》2012,58(5):1381-1396
A numerical study of a dilute turbulent gas‐particle flow with inelastic collisions and turbulence modulation in an Eulerian framework is described. A new interpretation is provided for the interaction/coupling terms, based on a fluctuating energy transfer mechanism. This interpretation provides for a new robust closure model for the interaction terms with the ability to predict the turbulence dampening as well as the turbulence enhancement phenomenon. Further, the model developed herein is investigated along with a variety of other published closure models used for the interaction/coupling terms, particle drag, and solid stress. The models are evaluated against several sets of benchmark experiments for fully‐developed, turbulent gas‐solid flow in a vertical pipe. © 2011 American Institute of Chemical Engineers AIChE J, 2012 相似文献
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John D. Schwarzkopf Clayton T. Crowe Prashanta Dutta 《American Institute of Chemical Engineers》2009,55(6):1416-1425
A dissipation transport equation for the carrier phase turbulence in particle‐laden flow is derived from fundamental principles. The equation is obtained by volume averaging, which inherently includes the effects of the particle surfaces. Three additional terms appear that reveal the effect of the particles; these terms are evaluated using Stokes drag law. Two of the terms reduce to zero and only one term remains which is identified as the production of dissipation due to the particles. The dissipation equation is then applied to cases where particles generate homogeneous turbulence, and experimental data are used to evaluate the empirical coefficients. The ratio of the coefficient of the production of dissipation (due to the presence of particles) to the coefficient of the dissipation of dissipation is found to correlate well with the relative Reynolds number. © 2009 American Institute of Chemical Engineers AIChE J, 2009 相似文献
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The deposition of fine silica and polystyrene spheres was measured for conditions of laminar and turbulent flow (960 ≤ Re ≤ 16040) in a rectangular channel using image analysis. The plate glass deposition surfaces were rendered positively charged by coating them with a cationic copolymer while, under the water chemistry conditions employed, both types of particles were negatively charged. It was found that, contrary to the results for laminar flow, the initial depositon rates in turbulent flow decreased with increasing Re, indicating that deposition was no longer mass-transfer controlled and that particle attachment played an increasingly important role as Re was raised. Attachment was modelled as a rate process in series with mass transfer in which the attachment rate varies inversely as the square of the friction velocity. Under the conditions of the present experiments, no particle re-entrainment was observed, so that the declining rate of particle accumulation on the wall recorded in each run could only be attributed to a declining deposition rate. Even where asymptotic accumulations were reached, particle coverages never exceeded 3.5%. 相似文献
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The subgrid-scale effects on particle motion were investigated in forced isotropic turbulence by DNS and priorLES methods. In the DNS field, the importance of Kolmogorov scaling to preferential accumulation was validated by comparing the radial distribution functions under various particle Stokes numbers. The prior-LES fields were generated by filtering the DNS data. The subgrid-scale Stokes number (StSGS) is a useful tool for determining the effects of subgrid-scale eddies on particle motion. The subgrid-scale eddies tend to accumulate particles with StSGS <1 and disperse particles with 1 < StSGS < 10. For particles with StSGS ≫ 1, the effects of subgrid-scale eddies on particle motion can be neglected. In order to restore the subgrid-scale effects, the Langevin-type stochastic model with optimized parameters was adopted in this study. This model is effective for the particles with StSGS >1 while has an adverse impact on the particles with StSGS <1. The results show that the Langevin-type stochastic model tends to smooth the particle distribution in the isotropic turbulence. 相似文献
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The problem of turbulence modulation, the process whereby the gas‐phase turbulence is modified by the presence of particles, is investigated. Experimental trends are examined and parameters affecting turbulence modulation and the mechanisms by which turbulence modulation occurs are identified. A new model that accounts for the crossing trajectory effect through fluid velocity correlations is presented. This model and the turbulence modulation models of Chen and Wood (1985), Tu and Fletcher (1994), and Mostafa and Mongia (1988) are compared directly to determine inherent differences or similarities of the models. The models are also invoked to simulate a particle‐laden pipe flow and predictions are compared to the experimental results of Tsuji et al. (1984). 相似文献
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研究聚并器内布朗聚团和湍流聚团引起超细颗粒聚团,特别考虑颗粒之间近程力(范德华引力、静电斥力)和颗粒与气体之间的流体力学作用力对颗粒聚团的影响。基于FLUENT软件UDF功能自定义聚团核,考虑颗粒之间近程力和流体力学作用力对聚并率的影响,引入碰撞效率α对聚团核进行修正,得到修正湍流聚并模型并将该模型与理想湍流聚并模型进行比较。应用群体平衡模型(population balance model,PBM)耦合CFD对颗粒聚团过程进行数值模拟,并采用微分代数积分矩量法(DAE-QMOM)求解群体平衡方程。结果表明:理想湍流聚并模型与实验结果误差为8.92%,而修正改进的湍流聚团模型与实验结果误差仅为3.35%,更加符合实际情况;微分代数积分矩量法具有较高的效率,而且误差较小,相比PD积分矩量法有明显的优势,稳定性也比较突出。 相似文献
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This work describes a theoretical and numerical study of turbulent gas-particle flows in the Eulerian framework. The equations describing the flow are derived employing Favre averaging. The closures required for the equations describing the particulate phase are derived from the kinetic theory of granular flow. The kinetic theory proposed originally is extended to incorporate the effects of the continuous fluid on the particulate phase behavior. Models describing the coupling between the continuous phase kinetic energy and particulate phase granular temperature are derived, discussed, and their effect on the flow predictions is shown.The derived models are validated with benchmark experimental results of a fully developed turbulent gas-solid flow in a vertical pipe. The effect of the models describing the influence of turbulence on the particle motion as well as the turbulence modulation due to the presence of particles is analyzed and discussed. 相似文献
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A dual-scale turbulence model is applied to simulate cocurrent upward gas–liquid bubbly flows and validated with available experimental data. In the model, liquid phase turbulence is split into shear-induced and bubble-induced turbulence. Single-phase standard k-εmodel is used to compute shear-induced turbulence and another transport equation is added to model bubble-induced turbulence. In the latter transport equation, energy loss due to interface drag is the production term, and the characteristic length of bubble-induced turbulence, simply the bubble diameter in this work, is introduced to model the dissipation term. The simulated results agree well with experimental data of the test cases and it is demonstrated that the proposed dual-scale turbulence model outperforms other models. Analysis of the predicted turbulence shows that the main part of turbulent kinetic en-ergy is the bubble-induced one while the shear-induced turbulent viscosity predominates within turbulent vis-cosity, especially at the pipe center. The underlying reason is the apparently different scales for the two kinds of turbulence production mechanisms:the shear-induced turbulence is on the scale of the whole pipe while the bubble-induced turbulence is on the scale of bubble diameter. Therefore, the model reflects the multi-scale phe-nomenon involved in gas–liquid bubbly flows. 相似文献
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This paper addresses the theoretical development and experimental validation of optimized recirculating reverse-flow gas cyclones. The simulation of these systems is based on the predictive properties of a finite diffusivity model, modified to include partial recirculation of the cyclones' emissions.Experimental validation was obtained at laboratory and pilot scales at low temperatures (up to 350 K) and for cork waste biomass boilers at higher temperatures (up to 600 K). Under certain circumstances, with recirculation, the proposed system showed a better performance than an online pulse jet bag filter, and substantially better than with multicyclone systems. The generally observed unexpected high collection of submicron particles, which occurs with inlet concentrations as low as 100 μg/m3, is attributed to turbulent dispersion, either by promoting fine particle capture by larger ones, much like what occurs in recirculating fluidized beds, or by bringing fine particles near the cyclone wall.The extremely fine particle size distributions exiting from the recirculation system, as measured off-line at laboratory, pilot and industrial scales, were confirmed at pilot scale using online measurements through a laser monitor. 相似文献
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Abhinandan Pattanayak 《Powder Technology》2009,192(3):359-6108
Macro- and giga-porous zirconia supports were prepared from a 20% colloidal sol of zirconia (ZrO2) by a combination of a polymer-induced colloid aggregation (PICA) process and the oil emulsion (OE) method. The effect of the pH of the initial sol on the size of the PICA particles, and subsequently on the final product, made by oil-emulsion assisted aggregation of the PICA particles was thoroughly investigated. Both the PICA and the OE methods were further optimized for performance. Particle morphology and porosity of the resultant particles were characterized by scanning electron microscopy, mercury intrusion-extrusion porosimetry, and nitrogen adsorption-desorption sorptometry. The supports were comprised of stable aggregates of 50-250 μm in size. The pore and throat size distributions showed narrow bi-modal distributions over two distinct size scales: 10-100 nm and 600-3000 nm. In addition, different combinations of aggregation techniques and porous supports prepared in previous steps for use in a subsequent aggregation were evaluated. Optimal amounts of zirconia sol and 10-100 micron porous spherical particles produced by the OE method in an earlier step were combined in an additional OE process to yield stable giga-porous supports. Porous zirconia particles obtained after calcination and sintering had particle sizes of 0.15-3.5 mm and multimodal pore and throat distributions over a range of 50 nm-8 μm. 相似文献
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When a dispersion of fine particles is concentrated, the product can contain clumps which arise from the aggregation of the particles. There are several drivers: sedimentating, drying, filtration, forcing the particles together to produce agglomerated structures which are much larger than the primary particles. The problem of understanding this phenomenon is twofold: on the one hand, it is difficult to measure aggregates in a concentrated slurry; on the other, there is no theory to predict when aggregates should form in an apparently-stable dispersion. This paper describes a new experimental method for measuring aggregates in concentrated suspensions, showing that the aggregation phenomenon can be followed over a wide range of experimental conditions. In particular, the results show that the aggregates exist at small concentrations in ostensibly stable dispersions even before concentrating takes place. Colloids based on polymers, ceramics, biological cells and emulsions all showed this aggregation effect. We have called these aggregated structures “multiplets” to distinguish them from the more normal flocs produced by destabilising the colloid. A theory of aggregation is proposed to fit the experimental results. This theory is based on the idea that multiplets form as a consequence of small adhesion forces between particles immersed in liquid; a molecular dynamics simulation using this concept of adhesion forces is used to demonstrate the formation of multiplet material at low concentrations. The theory seeks to show how the size of multiplets should vary with adhesion and with particle concentration. 相似文献
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J. J. Derksen 《American Institute of Chemical Engineers》2012,58(8):2589-2600
Direct numerical simulations of turbulent solid–liquid suspensions have been performed. The liquid is Newtonian, and the particles are identical spheres. The spheres have a tendency to aggregate since they are attracted to one another as a result of a square‐well potential. The size of the particles is typically larger than the Kolmogorov scale, albeit of the same order of magnitude. In such situations, the particle dynamics (including the aggregation process), and turbulence strongly interact which explains the need for direct simulations. The lattice‐Boltzmann method combined with an immersed boundary method for representing the no‐slip conditions at the spherical solid–liquid interfaces was used. The results show that the aggregate size distributions depend on both the strength of particle–particle interactions and the intensity of the turbulence. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2589–2600, 2012 相似文献
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Kenneth S. Asiagbe Michael Fairweather Derrick O. Njobuenwu Marco Colombo 《American Institute of Chemical Engineers》2019,65(4):1325-1339
Turbulent liquid–gas vertical channel flows laden with microbubbles are investigated using large eddy simulation (LES) two-way coupled to a Lagrangian bubble tracking technique. Upward and downward flows at shear Reynolds numbers of Re τ = 150 and 590 are analyzed for three different microbubble diameters of 110, 220, and 330 μm. Predicted results are compared with published direct numerical simulation results although, with respect to comparable studies available in the literature, the range of bubble diameters and shear Reynolds numbers considered herein is extended to larger values. Microbubble concentration profiles are analyzed, with the microbubbles segregating at the wall in upflow conditions and moving toward the channel centre in downflow. The various forces acting on the bubbles, and the effect of the flow turbulence on the bubble concentration, are considered and quantified. Overall, the results suggest that the level of detail achievable with LES is sufficient to predict the fluid structures impacting bubble behavior. Therefore, LES coupled with Lagrangian bubble tracking shows promise for enabling the reliable prediction of bubble-laden flows that are of industrial relevance. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1325–1339, 2019 相似文献
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从苯乙烯、丙烯酸丁酯和丙烯酸制备了表面带COOH的不同粒径的乳胶颗粒 (~ 10 0~40 0nm)。研究了因水分挥发由毛细管力引起的乳胶颗粒的聚集 ,以及这些乳胶颗粒和含N2 + 的高分子 (重氮树脂 ,Mn~ 2 5 0 0g/mol)的自组装 ,结合紫外光照 ,制备了高分子与乳胶粒交替的三明治 (sandwich)结构的超薄膜 ,结果表明 ,层与层间的光反应是高分子与乳胶粒成功组装的关键。由于光照中 ,层与层间的离子键已转变为共价键 ,这种超薄膜对极性溶剂和盐水溶液非常稳定 ,在DMF、c(NaCl) =0 1mol/L的水溶液中 ,室温下 (~ 2 0℃ )分别浸泡 2 4h ,组装膜仍完好无损。 相似文献
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Zong-Min Ma Ji-Liang Mu Jun Tang Hui Xue Huan Zhang Chen-Yang Xue Jun Liu Yan-Jun Li 《Nanoscale research letters》2013,8(1):532
In this paper, the potential sensitivity in Kelvin probe force microscopy (KPFM) was investigated in frequency modulation (FM) and heterodyne amplitude modulation (AM) modes. We showed theoretically that the minimum detectable contact potential difference (CPD) in FM-KPFM is higher than in heterodyne AM-KPFM. We experimentally confirmed that the signal-to-noise ratio in FM-KPFM is lower than that in heterodyne AM-KPFM, which is due to the higher minimum detectable CPD dependence in FM-KPFM. We also compared the corrugations in the local contact potential difference on the surface of Ge (001), which shows atomic resolution in heterodyne AM-KPFM. In contrast, atomic resolution cannot be obtained in FM-KPFM under the same experimental conditions. The higher potential resolution in heterodyne AM-KPFM was attributed to the lower crosstalk and higher potential sensitivity between topographic and potential measurements. 相似文献
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Stergios G. Yiantsios 《Desalination》2003,151(3):229-238
RO membrane colloidal fouling experiments were performed in the laboratory under well controlled and realistic conditions. Iron oxide was selected as a typical inorganic colloidal foulant, due to its importance, as evidenced from well known manufacturer recommendations on iron concentrations in feed waters and from frequently encountered problems in membrane installations. A range of iron concentrations was identified where a linear relationship existed between flux reduction rate and concentration. The performance of the Silt Density Index (SDI) was tested on the basis of the RO fouling data obtained. The range of iron concentrations where measurable and meaningful SDI values could be obtained was remarkably close to membrane manufacturer recommendations. A notable sensitivity of the SDI was also observed with particles for which retention is negligible. However, on the basis of the RO fouling data obtained, it appears that the SDI is not conservative enough. Furthermore, since the SDI cannot predict fouling rates, it cannot discriminate between different types of membranes. 相似文献