水力旋流器湍流场数值模拟

<正> 引言 水力旋流器作为一种简便、易行和高效率的分离、分级和离心沉降设备,已被广泛应用于化工、冶金、石油等众多工业领域中.以往的水力旋流器设计主要是根据大量物理模型试验得出的经验准数方程来求出旋流器的几何结构参数和操作参数.然而,随着水力旋流器应用范围的迅速扩大和人们对其分离(级)性能指标的要求日益提高,传统的按经验或半经验公式进行旋流器设计方法的局限性越来越明显,以及物模试验的耗时费钱,已促使人们开始采用数值模拟的方法,通过对旋流器内部流体运动的深入研究,弄清旋流器的分离机理,以便为提高水力旋流器的分离效率和分级准确度予以理论指导.本文采用了适于水力旋流器液相(水)流场的K-ε湍流数学模型,对水力旋流器内的湍流运动规律进行了数值模拟并根据激光实测结果对部分模型常数进行了修正.     

HEAT TRANSFER IN A SUBCOOLED WATER FILM FALLING ACROSS A HORIZONTAL HEATED TUBE

A numerical simulation and an experimental study were carried out for sensible heat transfer for a subcooled water film falling across a horizontal heated tube. A laminar model and a turbulent model were adopted to calculate the heat transfer coefficient. The whole wetting zone on the tube surface was divided into two zones: the top stagnation zone and the lateral free film flow zone. The initial boundary conditions for the free film flow zone were determined by calculating the fluid and the temperature fields in the stagnation zone. A modified wall function method was used for the turbulent model. The comparisons between the experimental data and the numerical solutions show that the experimental data agree reasonably well with the laminar model solutions. Finally, two simple correlations were proposed for predicting the convective heat transfer of a falling film for engineering applications.     

A CFD-PBM coupled model under entire turbulent spectrum for simulating a bubble column with highly viscous media

To account for the effect of liquid viscosity, the bubble breakup model considering turbulent eddy collision based on the inertial subrange turbulent spectrum was extended to the entire turbulent spectrum that included the energy-containing, inertial, and energy-dissipation subranges. The computational fluid dynamics-population balance model coupled model was modified to include this extended bubble breakup model for simulations of a bubble column. The effect of turbulent energy spectrum on the bubble breakup and hydrodynamic behaviors was studied in a bubble column under different liquid viscosities. The results showed that when the liquid viscosity was <80 mPa s, the bubble breakup and hydrodynamics were almost independent on the turbulent spectrum. At liquid viscosity >80 mPa s, the bubble breakup rate and gas holdup were significantly under-predicted when the inertial turbulent spectrum was used, and when using the entire turbulent spectrum the predictions were more consistent with experimental data.     

PREDICTION OF A GAS-PARTICLE TURBULENT JET WITH THE FLUCTUATION-SPECTRUM-RANDOM-TRAJECTORY MODEL

A numerical treatment for determining the particle velocity and the trajectories in a two-phase flow is described herein and this new fluctuation-spectrum-random-trajectory (FSRT) model is proposed to account for the turbulent diffusion of particles. It is predicted for the flow of a turbulent axisymmetric gaseous jet laden with spherical solid particles of nonuniform size. The particle velocity and the concentration field are obtained by the revised volume average method. The predictions are compared with experiment.     

A mechanistic model of turbulent drag reduction by additives

The phenomenon of drag reduction in walled turbulent flows of polymer solutions is theoretically modeled. A new mechanistic model of a polymer molecule in a turbulent flow field is suggested. It is argued that the dominant forces on a polymer fiber in the turbulent flow field are elastic and centrifugal. According to this model, an additional route of dissipation exists, in which eddy kinetic energy is converted to polymer elastic energy by the centrifugal elongation of the rotated polymer, which in turn is viscously damped by the surroundings, when the polymer relaxes. A novel approach is then illustrated, where it is shown that this mechanistic model can be accounted for as a turbulent scale alteration, instead of addition, which enables the classical dimensional analysis of a turbulent boundary layer to apply. Using this dimensional analysis with the equivalent altered scale yields remarkable results. Correct-form velocity profiles are obtained, and Virk's asymptote and slope are predicted with no empirical constants. Drag-flow rate curves are also calculated, and compared favorably with Virk's experiments. The onset of drag reduction phenomenon is also explained by this model, and calculations of it are also compared with Virk's data. The parametric dependencies of the onset point agree well with Virk's conclusions.     

基于REDIM方法构建的亚网格燃烧模型及其应用

将近年发展的详细反应机理简化方法:反应-扩散流形(REDIM)方法与假定滤波密度函数相结合,构造了新的亚网格燃烧模型。由于REDIM方法既能描述快速反应状态又能描述扩散过程起主导作用的慢速反应状态,因此新构造的燃烧模型理论上可用于计算扩散和化学反应均起重要作用的湍流部分预混燃烧及分层燃烧模式。为了验证其性能,利用新模型对基准的Darmstadt湍流分层火焰燃烧器进行了大涡模拟(LES)计算:首先分别采用粗、细两种网格对无反应湍流状态进行了计算,以检验网格精度;随后对一个有分层效应的湍流燃烧状态进行了计算。LES计算得到的速度、温度、组分等统计信息与实验结果吻合良好,验证了新亚网格燃烧模型在预测湍流分层火焰方面的能力。     

A coupled CFD-Monte Carlo method for simulating complex aerosol dynamics in turbulent flows

A coupled computational fluid dynamics (CFD)-Monte Carlo method is presented to simulate complex aerosol dynamics in turbulent flows. A Lagrangian particle method-based probability density function (PDF) transport equation is formulated to solve the population balance equation (PBE) of aerosol particles. The formulated CFD-Monte Carlo method allows investigating the interaction between turbulence and aerosol dynamics and incorporating individual aerosol dynamic kernels as well as obtaining full particle size distribution (PSD). Several typical cases of aerosol dynamic processes including turbulent coagulation, nucleation and growth are studied and compared to the sectional method with excellent agreement. Coagulation in both laminar and turbulent flows is simulated and compared to demonstrate the effect of turbulence on aerosol dynamics. The effect of jet Reynolds (Re_j) number on aerosol dynamics in turbulent flows is fully investigated for each of the studied cases. The results demonstrate that Re_j number has significant impact on a single aerosol dynamic process (e.g., coagulation) and the simultaneous competitive aerosol dynamic processes in turbulent flows. This newly modified CFD-Monte Carlo/PDF method renders an efficient method for simulating complex aerosol dynamics in turbulent flows and provides a better insight into the interactions between turbulence and the full PSD of aerosol particles.

Copyright © 2017 American Association for Aerosol Research     

A stochastic model for turbulent diffusion

An improved stochastic model for turbulent diffusion is developed. The model is free of arbitrary adjustable constants. The central objective, as in Taylor's approach, is to predict the turbulent diffusion (mean square displacement and/or concentration distribution) given certain statistical information about the turbulent velocity field.The applicability of the model to problems of turbulent diffusion in shear flows is explored. When the cross correlation between the fluctuating velocity and the stochastic forcing function is zero and the forcing function is white noise in character, the conditions necessary to describe Brownian motion are satisfied. However, when the cross correlation is other than zero and when, at the same time, the energy of the velocity fluctuation is concentrated at the lower frequencies, the equation can be used to describe turbulent diffusion. The deterministic coefficient β in the model and the nature of the stochastic forcing function can be calculated from known Eulerian statistics of the flow field.The method is applicable to non-homogeneous flows in three dimensions where correlation exists between fluctuating velocity components. Implementation of the model is most readily accomplished on a modern hybrid computer. Good agreement is obtained in comparisons of predicted results from the model with (a) theoretical solutions for homogeneous turbulent diffusion (b) experimental measurements of diffusion in a turbulent boundary layer and (c) measurements of diffusion in the atmospheric boundary layer.     

A METHOD OF ESTIMATION OF TURBULENT DIFFUSION IN A CIRCULAR PIPE BASED ON SPATIAL-DEPENDENCE MATRIX

Considering that a scalar quantity (mass or heat) is transported on a turbulent lump where fluid particles show almost same behavior, a new method of numerical calculation for the turbulent diffusion is proposed. This method is based on the spatial-dependence matrix, which is one of the tools of pattern analysis. The usefulness of the method is examined by experiments on the turbulent diffusion of the tracer which is injected into the source point on the pipe axis and it is confirmed that the reasonable feature of turbulent diffusion is estimated by the method.     

A model for turbulent binary breakup of dispersed fluid particles

Accurate predictions of particle size distributions, and therefore of the underlying processes of fluid particle breakup and coalescence are of vital importance in process design, but reliable procedures are still lacking. The present paper aims at developing a modular formulation for the turbulent particle breakup process. The model is to be included in a population balance model which is formulated such as to facilitate the direct future implementation into a full multifluid CFD model.The breakup process is described without introducing adjustable parameters. The current model is a further development of an existing model by Luo and Svendsen (AIChE J. 42 (5) (1996) 1225), which has been expanded and refined, and where an inherent weakness regarding the breakup rate for small particles and small daughter particle fragments are removed. A new criterion regarding the kinetic energy density of the colliding turbulent eddy causing breakup has been introduced. This new criterion is a novel concept describing the breakup process. The details are thoroughly discussed together with possible further modifications. The results from the new model are encouraging because the breakup rate is greatly reduced when the dispersed fluid particles are reduced in size. Further, the response to changes in system variables is reasonable and the distribution of daughter sizes vary in a reasonable way for the different collision possibilities.     

A stochastic model for breakage frequency of viscous drops in turbulent dispersions

In industrial liquid-liquid mass-transfer equipment, many a times the dispersed phases involved are highly viscous. The viscosity of dispersed drops influences the rate processes, especially their breakage rate. A new stochastic model for predicting the breakage frequency of viscous drops in a turbulent dispersion applying the random behavior of the turbulent fluctuations, has been proposed. It has been assumed that the correlation time of turbulent fluctuations across the viscous drops is so small compared to the time scale of drop deformation, that the turbulent fluctuation can be considered as a white-noise process.     

共轴反转型生物反应器内流场数值模拟与性能分析

使用生物可降解塑料是解决白色污染的有效手段,然而在生物反应器中生产可降解塑料过程中会面临气体传质能力不足和能耗过大等问题,导致生产成本居高不下。为解决这些问题,提出了一种共轴反转型机械搅拌式生物反应器,并通过数值模拟对新型反应器内两相流场进行了仿真及定量分析。通过模拟气泡羽流、鼓泡塔及搅拌器系统内流场,并与实验结果对比,在双流体模型中引入了曳力、升力及湍流扩散力以及基于TroshkoHassan模型的两相湍流模型,验证了双流体模型在该问题中的有效性。对新设计的反应器内流场模拟结果表明,两相作用力模型对模拟准确性影响较大,而共轴反转能够在流场中形成更好的剪切效应,增强气体分散能力,从而提高整体气含率及相对功率准数。     

A multidimensional model for annular gas-liquid flow

A finite volume method-based CFD model has been developed to simulate steady, turbulent, two-dimensional annular gas-liquid flow in a duct. The gas flow is treated as being equivalent to flow through a rough-walled duct. The effect of the liquid film on the gas phase is included in the form of modified wall functions which incorporate the well-known triangular relationship (Annular Two-Phase Flow, Pergamon Press, Oxford, 1970) that exists among wall shear stress, film flow rate and film thickness in annular flow. The presence of droplets is accounted for by solving an additional scalar transport equation for the mass fraction of the droplets. Entrainment and deposition of droplets are included as source term and boundary condition, respectively, in the mass fraction equation. It is shown that the resulting model, while retaining simplicity of formulation, gives good predictions of the literature data of annular flow parameters under equilibrium and non-equilibrium conditions.     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

一个用于精馏塔模拟的新模型

A new computational mass transfer model is proposed for simulating the distillation process by solving the fluctuating mass flux for the closure of turbulent mass transfer equation in order to obtain the concentration profile and the separation efficiency of distillation column. The feather of the proposed model is to abandon the conventional way of introducing the turbulent mass transfer diffusivity (dispersion coefficient) to the turbulent mass transfer equation. To verify the validity of the proposed model, a commercial scale packed column and a sieve tray column were simulated and compared with published experimental data. The simulated results were satisfactorily confirmed in both concentration distribution and separation efficiency.     

Application of a modified Eulerian model to study the particle deposition on a vertical surface in turbulent flow

In this study the v2-f model was used with the two-phase Eulerian approach to predict the particle deposition rate on a vertical surface in a turbulent flow. The standard Eulerian particle model was adopted from the literature and modified, considering the majority of particle transport mechanisms in the particle deposition rate. The performance of the modified model was examined by comparing the rate of particle deposition on a vertical surface with the experimental and numerical data in a turbulent channel flow available in the literature. The model took into account the effects of drag force, lift force, turbophoretic force, electrostatic force, inertia force and Brownian/turbulent diffusion on the particle deposition rate. Electrostatic forces due to mirror charging and charged particles under the influence of an electric field were considered. The predictions of the modified particle model were in good agreement with the experimental data. It was observed that when both electrostatic forces are present they are the dominant factor in the deposition rate in a wider range of particle sizes.     

A collision model for fine particles in a turbulent system

A model is proposed to describe the collision rate of small particles suspended in a turbulent system. The model combines the possible collision mechanisms: 1) collisions due to the relative velocity between fluid and particles, and 2) collisions due to the turbulent diffusion of particles. This model also accounts for the effect of particle concentration on the collision rate. It was found that the turbulent diffusion of particles plays an important role in the collision of equally sized particles as well as of unequally sized particles. The model predictions also show that the collision rate of particles is strongly affected by the concentration of solid particles and by the turbulence intensity. Much more reliable predictions than previously possible have been obtained with the present model.     

A Stochastic Model for Particle Deposition and Bounceoff

A new model for particle deposition and bounceoff that combines current knowledge of turbulent bursts with the stochastic properties of turbulent fluctuations is presented. The model predictions for deposition velocities agree with experimental results in the literature for dimensionless particle relaxation time τ_p ⁺ > 2. For τ_p ⁺ > 10, most of the particles delivered to the edge of the viscous sublayer are able to deposit onto the surface due to their inertia; the deposition velocity approaches an asymptotic value because the process becomes limited by the rate of turbulent delivery to the viscous sublayer. Because of the penetration of turbulent fluctuations into the viscous sublayer, the minimum values of vertical velocities needed for particles to deposit onto the surface are smaller than those predicted by the free flight model. Most of the deposition occurs from those turbulent fluctuations at the upper tail of the distribution of the vertical component of air velocity.

In addition to the deposition velocity, the model is able to provide the distribution of particle velocities on reaching the surface which is used to compute the fraction of particle bounceoff. The model predictions for the fractions of rebound agree reasonably with the measured results from a wind tunnel experiment for τ_p ⁺ > 2. However, both the deposition velocity and the fraction of rebound are underestimated by the model for τ_p ⁺ < 2. Other mechanisms such as Brownian diffusion must be included in further revisions to this model in order to obtain satisfactory predictions for smaller values of τ_p ⁺.     

A new model of turbulent fibre suspension and its application in the pipe flow

A new model of turbulent fibre suspension in pipe flow is developed by deriving the equations of Reynolds averaged Navier‐Stokes, turbulence kinetic energy and turbulence dissipation rate with the additional term of the fibres, and the equation of probability distribution function for mean fibre orientation. The equations are solved numerically. The numerical mean velocity is in agreement with the experimental data. The effects of Reynolds number, fibre concentration, and fibre aspect‐ratio on the mean velocity, turbulent kinetic energy and turbulent dissipation rate are analysed. The results show that the effect of Reynolds number on the flow behaviour is insignificant. The turbulent kinetic energy and turbulent dissipation rate increase with an increasing fibre concentration and fibre aspect‐ratio. © 2012 Canadian Society for Chemical Engineering     

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.