EVALUATION OF LAGRANGIAN PARTICLE DISPERSION MODELS IN TURBULENT FLOWS

This work evaluates the performance of Lagrangian turbulent particle dispersion models based on the Langevin equation. A family of Langevin models, extensively reported in the open literature, decompose the fluctuating fluid velocity seen by the particle in two components, one correlated with the previous time step and a second one randomly sampled from a Wiener process, i.e., the closure is at the level of the fluid velocity seen by the particle. We will call those models generically the “standard model.” On the other hand, the model proposed by Minier and Peirano (2001 Minier , J. P. and Peirano , E. ( 2001 ). The PDF approach to turbulent polydisperesed two-phase flows , Phys. Rep. , 352 , 1 – 214 .[Crossref], [Web of Science ®] , [Google Scholar]) is considered; this approach is based on the probability density function (PDF) and performs the closure at the level of the acceleration of the fluid seen by the particle. The formulation of a Langevin equation model for the increments of fluid velocity seen by the particle allows capturing some underlying physics of particle dispersion in general turbulent flows while keeping simple the mathematical manipulation of the stochastic model, avoiding some pitfalls, and simplifying the derivation of macroscopic relations. The performance of the previous dispersion models is evaluated in the configurations of grid-generated turbulence (Snyder and Lumley, 1971 Snyder , W. H. and Lumley , J. L. ( 1971 ). Some measurements of particle-velocity autocorrelation functions in a turbulent flow , J. Fluid Mech. , 48 , 41 – 71 .[Crossref], [Web of Science ®] , [Google Scholar]; Wells and Stock, 1983 Wells , M. R. and Stock , D. E. ( 1983 ). The effect of crossing trajectories on the dispersion of particles in a turbulent flow , J. Fluid Mech. , 136 , 31 – 62 .[Crossref], [Web of Science ®] , [Google Scholar]), simple shear flow (Hyland et al., 1999), and confined axisymmetric jet flow laden with solids (Hishida and Maeda, 1987 Hishida , K. and Maeda , M. ( 1987 ). Turbulent characteristics of gas-solids two-phase confined jet: Effect of particle density , Jpn. J. Multiph. Flow , 1 , 56 – 69 . [Google Scholar]).     

EVALUATION OF LAGRANGIAN PARTICLE DISPERSION MODELS IN TURBULENT FLOWS

This work evaluates the performance of Lagrangian turbulent particle dispersion models based on the Langevin equation. A family of Langevin models, extensively reported in the open literature, decompose the fluctuating fluid velocity seen by the particle in two components, one correlated with the previous time step and a second one randomly sampled from a Wiener process, i.e., the closure is at the level of the fluid velocity seen by the particle. We will call those models generically the “standard model.” On the other hand, the model proposed by Minier and Peirano (2001) is considered; this approach is based on the probability density function (PDF) and performs the closure at the level of the acceleration of the fluid seen by the particle. The formulation of a Langevin equation model for the increments of fluid velocity seen by the particle allows capturing some underlying physics of particle dispersion in general turbulent flows while keeping simple the mathematical manipulation of the stochastic model, avoiding some pitfalls, and simplifying the derivation of macroscopic relations. The performance of the previous dispersion models is evaluated in the configurations of grid-generated turbulence (Snyder and Lumley, 1971; Wells and Stock, 1983), simple shear flow (Hyland et al., 1999), and confined axisymmetric jet flow laden with solids (Hishida and Maeda, 1987).     

A PSEUDO-THREE-DIMENSIONAL TWO-PHASE TURBULENT FLUID MECHANICAL MODEL FOR THE PREDICTION OF LIQUID FLOW PATTERNS WITH A FREE SURFACE IN A SHALLOW CHANNEL

A closed-loop analysis is presented for a turbulent multiphase fluid mechanical model of flow along a shallow channel, which is capable of accurately predicting free surface elevations at any point within the computational domain. It is shown that simulations based on a finite difference method and the mixing length hypothesis, effectively account for turbulent effects, with good agreement between theoretical predictions and recently performed experiments. The proposed model has many applications in everyday engineering.     

SIMULATION OF PARTICLE TRANSPORT AND DEPOSITION IN A COMBUSTOR

A computational model for Lagrangian particle tracking for studying dispersion and deposition of particles in a combustor with swirling flow and chemical reaction is developed. The model accounts for the effect of thermophoretic force, as well as the drag and lift forces acting on particles, in addition to the Brownian motion and gravitational sedimentation effects. The mean turbulent gas flow, temperature fields and chemical species concentration in the combustor are evaluated using the stress transport turbulent model of the FLUENT code. The instantaneous fluctuation velocity field is generated by a Gaussian filtered white noise model.

The simulated axial, radial and tangential mean gas velocities are compared with the existing experimental data. Ensembles of particle trajectories are generated and statistically analyzed. The effects of size and initial distribution on particle dispersion and deposition are studied. The particle concentration at different sections are also evaluated and discussed. The results shows that the turbulence dispersion effect is quite important, while the thermophoresis effect is small.     

IMPROVEMENT OF DEEP-BED CROSSFLOW MODEL BY A DIFFUSION COMPONENT FOR THE GRAIN PARTICLE AND LOG MODEL FOR EVALUATION OF RECIRCULATING CROSSFLOW DRYER

This paper presents the improved versions of -bed cross now and log simulation models. A diffusion model descries ibing internal diffusion of moisture within a grain kernel to compute moisture removal rate is developed and incur porsted in crossflow model to improve its flexlbillty and versetility. The lag model is suilably modified and employed to evaluate its accuracy and applicability compared to crossflow drying model for evaluating recirculating crossflow drying operation. Experiments were conducted on large scale recirculating crossflow dryer for corn drying. Simulation results are compared with experimental results to test the validity of drying models.     

PREDICTION OF DENSE TURBULENT PARTICLE LADEN RISER FLOW WITH A EULERIAN AND LAGRANGIAN COMBINED MODEL

This paper formulates a new numerical model for the dense gas-solid flows, combining Eulcrian/Eulerian approach and Eulerian/Lagrangian approach together. The model takes account of inter-particle interactions in Eulerian coordinate using the kinetic theory of granular flow based on the Chapman-Enskog theory of dense gases. A stochastic particle dispersion model is incorporated in the model to calculate the turbulence diffusion of particles in Lagrangian coordinate. A comparison with published experimental results is made, and good agreement is shown.     

IMPROVEMENT OF DEEP-BED CROSSFLOW MODEL BY A DIFFUSION COMPONENT FOR THE GRAIN PARTICLE AND LOG MODEL FOR EVALUATION OF RECIRCULATING CROSSFLOW DRYER

This paper presents the improved versions of -bed cross now and log simulation models. A diffusion model descries ibing internal diffusion of moisture within a grain kernel to compute moisture removal rate is developed and incur porsted in crossflow model to improve its flexlbillty and versetility. The lag model is suilably modified and employed to evaluate its accuracy and applicability compared to crossflow drying model for evaluating recirculating crossflow drying operation. Experiments were conducted on large scale recirculating crossflow dryer for corn drying. Simulation results are compared with experimental results to test the validity of drying models.     

釜式再沸器管束中循环流动换热的预报模型

提出了釜式再沸器管束中循环流动换热的分相预报新模型,考虑了管束下部的过冷沸腾及循环液体的侧面引入。与以往的模型相比,本文模型与现有实验数据符合更好。     

A BASIC CODE FOR THE PREDICTION OF TRANSIENT THREE-DIMENSIONAL TURBULENT FLOWFIELDS

A primitive pressure-velocity finite difference code has been developed to predict transient three-dimensional turbulent flow. The code is a simplified yet effective prediction procedure for use by persons with little experience in computational fluid dynamics, and into which user-oriented complexities can easily be added. The method is based on the transient 2-D Los Alamos SOLA prediction technique for laminar flows. Turbulence is simulated by. means of the two-equation κ - ε turbulence model; species diffusion and buoyancy are also included. Two applications of the code are presented to local destratification near the release structure of a reservoir and to the deflection of a jet entering normally into a uniform cross-flow. Predicted results exhibit good agreement with experimental data, showing that a useful characterization of fully three-dimensional flows is now available     

注塑件熔接痕强度预测及提高新技术

介绍了注塑件熔接痕强度预测及提高的几种新技术。综述了基于扩散理论和Flory—Huggins晶格理论的预测模型及基于BP神经网络的预测模型。给出了模型的适用范围。简述了热流道多浇口进料顺序控制、注射成型和模具设计等几种提高熔接痕强度、减少或避免熔接痕出现的新技术的作用机理及应用实例,指出进一步完善熔接痕强度预测模型和强度提高技术是今后熔接痕研究的一个重要方向。     

液－固流化床的流体流动和颗粒循环模型

基于对流动特性的观察，从二相流理论出发，提出了流体流动和颗粒循环模型，在φ１０４ｍｍ×１５００ｍｍ的流化床内，以水为流化介质，通过微机连续测定示踪剂浓度，得到不同颗粒体系，不同液体速度下床层内液体的平均停留时间分布和颗粒循环的平均速度，确定了模型参数，并给出了关联式，从而得到床层内径向流体速度分布和颗粒循环运动速度分布以及流体运动与颗粒运动之间的相互影响的关系式。将拟合结果与实验值比较，证实了模型的可行性及其新颖性，该模型较好地反映了液－固流化床内液体运动和颗粒运动的实际情况。     

预测水平管气-液环状流周向液膜厚度分布的理论模型

针对水平环状流液膜厚度沿周向分布的不对称性 ,用 5组双平行电导探针测量了水平管内气液环状流沿周向的瞬态液膜厚度 ,得到了平均液膜厚度沿周向的分布规律 .分析了液膜向上输送的机理 .为更为准确地预测液膜厚度 ,综合考虑了扰动波抽吸作用机理、气体二次流机理及液滴沉降和携带机理 ;通过量级比较和应力平衡的方法 ,提出了一个预测液膜厚度沿周向分布的理论模型 .数值模拟结果表明预测值与实验测量结果符合较好     

FINITE ELEMENT SOLUTION OF THE MAXWELL EQUATIONS FOR ABSORPTION AND SCATTERING OF ELECTROMAGNETIC RADIATION BY A COATED DIELECTRIC PARTICLE

The problem of absorption and scattering of electromagnetic radiation by particles can be solved analytically for only the simplest cases, but established numerical methods can allow a straightforward extension to particles with arbitrary inhomogeneities, shapes, and nonlinear response. In this paper a recently developed finite element technique for the scattering and absorption by a homogeneous sphere is extended to the problem of a coated dielectric sphere of arbitrary size parameter. Numerical results showing good agreement with analytical solutions for the size parameter of 5.93 and various core radii are presented. Results obtained suggest that finite element methods have promise for analytically intractable scattering/absorption problems and show that the Debye amplitude formulation of the problem offers some advantages in a numerical scheme.     

湖南界牌土的化学组成和颗粒组成与干燥强度间定量关系的数学模型

本工作,选用湖南界牌土作原料,对所选原料进行颗粒分级和量化处理,依据最优化的基本理论,经二次回归正交试验设计,测量其干燥强度,最后应用回归技术建立了原料的干燥强度与其化学组成和颗粒组成之间的定量关系的数学模型,并用数学方法和工艺原理对其影响因素进行了分析讨论,结果表明界牌土的化学组成和颗粒组成对其干燥强度有着重大的影响。     

单向复合材料弹性模量预测新式

根据复合材料单层的实际构造，提出了串并联组合模型，采用微观力学与宏观力学相结合的方法，推导出一组计算和复合材料弹性模量的新公式，其优点是：Ｅ１和ｖ１２，Ｅ２和Ｇ１２的计算式形式对称，结构简单，计算容易，物理意义明确，我们把由世界上许多著名学者提出的多组预测公式和本文导出式计算所得的结果与国内外已发表的硼／环氧、碳／环氧、玻璃／环氧复合材料的实验数据相比较，结果表明：导出式比其他公式精确，更有价值，     

MODEL FOR THE PREDICTION OF DEHYDRATION AND CALCINATION RATES OF BAYER PLANT GIBBSITE

A model for the prediction of rates of dehydration of Chemically Bound Moisture of Gibbsite is proposed. The theoretical dehydration rates evaluated from the model are compared with experimental values. The correlation between the experimental data and theoretically predicted values is found to be satisfactory. The curves obtained for differential thermal analysis of Gibbsite are found to resemble the DTA curves predicted theoretically using the proposed rate equations.     

MODEL FOR THE PREDICTION OF DEHYDRATION AND CALCINATION RATES OF BAYER PLANT GIBBSITE

A model for the prediction of rates of dehydration of Chemically Bound Moisture of Gibbsite is proposed. The theoretical dehydration rates evaluated from the model are compared with experimental values. The correlation between the experimental data and theoretically predicted values is found to be satisfactory. The curves obtained for differential thermal analysis of Gibbsite are found to resemble the DTA curves predicted theoretically using the proposed rate equations.     

提高文丘里水膜除尘器除尘效率可能途径探讨

介绍了文丘里水膜除尘器应用和除尘性能研究现状,结合近期对某厂除尘器实测和实验结果,讨论了影响该类除尘器除尘效率进一步提高的主要因素。提出了提高文丘里水膜除尘器除尘效率的可能途径,认为继续进行提高文丘里除尘器除尘效率的研究具有现实意义。     

LIMITATIONS AND EMPIRICAL EXTENSIONS OF THE k-ε MODEL AS APPLIED TO TURBULENT CONFINED SWIRLING FLOWS

Shortcomings and recommended corrections to the standard two-equation k-ε turbulence model suggested by previous investigators are presented. They are assessed regarding their applicability to turbulent swirling recirculating flow. Recent experimental data on swirling confined flows, obtained with a five-hole pilot probe and a six-orientation hot-wire probe, are used to obtain optimum values of the turbulence parameters C_μ, C₂, and σ_ε, for swirling flows. General predictions of moderately and strongly swirling flows with these values are more accurate than predictions with the standard or previous simple extensions of the k-ε turbulence model.     

A CONTRIBUTION TO THE PREDICTION OF PHASE SEPARATION IN BRANCHING CONDUITS

A phenomenological model for phase separation in branching conduits (Hwang el al., 1988) has been extended and improved. These modifications involve new models for the phase distribution at the inlet of a branching junction corresponding to four different flow regimes, and a correction for flooding in vertical junctions. This mechanistically-based model gives good agreement when compared with representative experimental data.