气液界面对流传质的观测与定量分析

采用光学纹影系统对乙醇和水双组分解吸传质过程的对流结构的界面湍动进行了定性观察和定量分析。建立了一套水平非稳态气液传质设备,试验观测了乙醇和水体系中液相组分向气相传质过程的Marangoni界面对流结构。还通过对传统纹影方法的改进,对乙醇解吸传质过程的浓度(本文用质量分数表示)梯度场进行了定量测量。定量分析表明乙醇和水系统解吸过程中,引发界面湍动的原因是局部较大的表面张力梯度。定量分析的结果很好的解释了伴随Marangoni效应的传质过程的混乱的对流结构,为进一步对界面湍动现象的分析提供了帮助。     

Computational study of planar extrudate swell flows with a viscous liquid–gas interface

We present a computational study of planar extrudate swell flows of Newtonian liquids with a viscous liquid–gas interface. The model consists of the equations of motion coupled with the Boussinesq–Scriven constitutive equation for the interfacial stress tensor. The resulting set of equations is solved with the finite element method coupled with an elliptic mesh generation strategy to capture the free surface. The results show a detailed parametric study in terms of the capillary number and the Boussinesq number, a dimensionless parameter used to measure the ratio of viscous forces at the interface to viscous forces in the bulk liquid. The predictions reveal that the extrudate swells dramatically as the interfacial viscosity grows. The interfacial viscosity slows down the flow both in the bulk liquid and at the interface, and thus the extrudate size increases to conserve mass in the slow plug flow that develops under the free surface.     

Energy dissipation and macro instabilities in a stirred square tank investigated using an LE PIV approach and LDA measurements

In this study, the hydrodynamics and turbulence in a square tank stirred with a hydrofoil impeller, a Lightnin A310, is investigated using a large eddy particle image velocimetry (LE PIV) approach. The particle image velocimetry data are used as the large scale part of a large eddy simulation and the small scales are modelled assuming that the turbulent kinetic energy production is limited to the large scale structures, the turbulent energy dissipation is confined to the small scale structures and the transfer of energy takes place in the inertial sub-range. The small scale turbulence was modelled by direct calculation of the turbulent stress tensor using filtered particle image velocimetry data. The spatial distribution of the calculated dissipation rate tensors showed good agreement with previous work.

The macro instabilities of the flow structure were investigated by means of spectral analysis. Low frequency phenomena separated from the mean flow were detected. The cause of these could partly be explained by the circulation time for the tank, which corresponded to the low frequency phenomena found at 0.03N Hz, where N is the rotational speed.     

质量传递过程中界面湍动现象的光学观察

采用光学纹影摄影方法观察了多组分蒸发、吸收和解吸过程中相界面处的对流现象 .自然蒸发过程所研究的物系为苯 /甲苯二元混合物和乙酸 /乙酸乙酯 /乙二醇三组分混合物 ,气液传质过程包括甲醇、异丁醇等分别吸收二氧化碳的吸收过程和二氧化碳从有机溶剂中解吸的过程 .结果发现 ,在多组分有机液体混合物蒸发和气液传质过程中存在规则有序滚桶形或多边形细胞状对流 .这说明有序对流运动的产生是相际传质过程中普遍存在的现象 ,其结构特点取决于实验条件和体系的性质     

倾斜圆管内气液两相分层流界面的稳定性分析

<正>l 弓l言 气液两相流分层流界面波特性及界面稳定性分析对理论和应用具有重要意义.50年代,Banjamin”’、Miles”’和 Hanratty’‘’“开始了此方面的研究.以往研究中,均采用 Jeffreys”’的方法,即利用液相动量方程的积分形式,气相的影响只作为液相动量方程的边界条件.另外也有研究者从N-S方程出发,通过对实际问题的适当简化以研究界面稳定性”‘. 本文利用双流体模型研究了倾斜圆管内气液两相分层流的界面稳定性,系统研究了管道几何条件及流动状况对稳定性的影响.2 数学模型 对如图1所示的倾斜管内分层流,假定两相间不存在热量传递和相变过程,其一维双流体流动模型为”-“ ～r 入八人）乃t十别八U。人）/肚一0（I） ltw 8（pkA。U。）/9 t + 3（pkA。厂。ZtZ）/sx 、_/.\—一,。J。士。/;+八。A。slip J>>一二卜 卜。二.t 一洪人八）/gx+AdA/a工（2） 一人二二>二v抑Q厂二厂了 式（1）、式（2）中k—l或g,当k—g时,取 pry——“一”号,k—l时贩“十”号.素流速度分布均 匀,厂。一 l“’‘;层流根据液膜内速度分布可知, 图 1 分层流示意图——”———一’—””—””——～ 八一4/3““二 气液两相静压项用文献〔幻的方 F。Ic0d屯＊r＊donof盯r＊1m叨n0w”‘’-’“‘一‘””—’““——一‘一     

气液传质界面湍动现象投影观察

In gas-liquid mass transfer processes,interfacial turbulence may occur due to the surface tension gradient and the density gradient produced by mass transfer near the interface.The interfacial turbulence can enhance the mass transfer since it intensifies the movement of interfacial fluid.By means of the shadowgraph optical method,the interfacial turbulence patterns vertical to the interface were observed directly in the volatilization process of binary systems.The images of the amplified interfacial turbulence showed the variation of concentration and the fluid movement under the interface.Two patterns of interfacial turbulence were observed in the experiments：plume and vortex.With the plume,the interfacial fluid moved slowly and penetrated the liquid deeply.With the vortex,the interfacial turbulence occurred in the vicinity of the liquid interface and the fluid moves quite fast.A qualitative analysis was carried out based on the mechanism of Rayleigh-Bénard convection induced by density gradient and Marangoni convection induced by surface tension gradient.     

Prediction of gas–liquid interfacial area in valve trays

An accurate prediction of gas–liquid interfacial area is very important for the design and optimization of column trays. However, the difference of the gas–liquid flow regimes operating at different scale trays significantly affect the interfacial area calculation. Therefore, in this study, an interfacial area model operating at small column was established using the Kolmogorov's isotropic turbulence hypothesis. According to the analyzes of the gas–liquid flow phenomena of different scale columns, an assumption that the similarity principle of flow characteristics of gas–liquid in full contact was proposed. Moreover, a new model that can be used to predict the gas–liquid interfacial area of a large column with exiting the nonideal flow was obtained through the extension of the small tray interfacial area model based on this principle. Finally, the new model was tested by comparisons with the experimental results of references. The prediction accuracy significantly improved with the maximum deviation of approximately 40%. © 2015 American Institute of Chemical Engineers AIChE J, 62: 905–915, 2016     

LES/FDF simulation of a gas-particle backward-facing step flow

A newly developed filtered density function (FDF) transport equation was used to describe the fluid velocity seen by the particles in two-phase flows and the interaction between the two phases was considered. Numerical simulation of a gas-particle two-phase backward-facing flow was carried out with the LES/FDF model, in which the large eddy simulation (LES) was used for the gas-phase. The numerical results were compared with the experimental data. The comparison shows that the LES/FDF model can distinctly improve the precision of the simulation. Further investigation has been carried out on the turbulence modification by particles with different particle parameters.     

AN ANALYSIS OF PHASE DISTRIBUTION AND TURBULENCE IN DISPERSED PARTICLE/LIQUID FLOWS

An analysis of dilute, turbulent particle/liquid two-phase flow is presented. The three-dimensional conservation equations that govern turbulent motion in solid/fluid flows are derived using ensemble averaging, and the unknown terms in these equations are constituted to achieve closure. These closure terms include the shear stress due to interparticle collisions, the corresponding terms in the Reynolds stress equation, the force and dissipation due to particle-wall collisions, and the interfacial work due to particle/turbulent eddy interaction. The resultant two-fluid model was then evaluated using a computational fluid dynamic (CFD) solver and the predictions were compared with experimental data. Good agreement was observed for a variety of flow conditions.     

Liquid wall friction in two-phase turbulent gas laminar liquid stratified pipe flow

The main result of the present work is an analytic expression for the mean liquid wall shear stress in two-phase turbulent gas/laminar liquid stratified pipe flow. The Navier-Stokes equations are solved assuming a flat fluid-fluid interface subject to a constant interfacial shear — approximating the interfacial drag exerted by the gas. The effect of a pipe inclination is accounted for, thereby retaining the interesting two-phase phenomenon of backflow in upwardly inclined pipes. The corresponding expression for the wall shear stress distribution is obtained by formal differentiation. Its limiting behaviour in the triple points, where the fluid-fluid interface meets the pipe wall, is determined by residue calculus. The expression for the mean wall shear stress is given by integration. It is found to be a linear combination of two terms. The first term accounts for the free surface liquid flow in the absence of the gas. The corresponding approximate hydraulic diameter model is found to be in surprisingly good agreement with this term. The second term represents the shear flow associated with the interfacial drag exerted by the gas (not accounted for by the hydraulic diameter approximation). The shear flow increases the flow rate near the interface on behalf of the flow rate near the pipe wall, thus reducing the wall shear stress below the free surface flow value. Expedient evaluation of the expression for the mean wall shear stress, suitable for use in a 1-D multiphase pipe flow simulator, is facilitated by replacing certain one-parameter integrals with highly accurate rational approximations.     

CFD SIMULATION ON INFLUENCE OF SUPERFICIAL GAS VELOCITY,COLUMN SIZE,SPARGER ARRANGEMENT,AND TAPER ANGLE ON HYDRODYNAMICS OF THE COLUMN FLOTATION CELL

The use of flotation columns in the mineral processing industry has experienced a remarkable growth over the years. The detailed hydrodynamics study of a column flotation cell demands the solution of mass, momentum, phase-transfer, and turbulence quantities. Simulations have been carried out to examine the influence of superficial air velocity, column size, column taper angle, and sparger arrangement on hydrodynamics of the column flotation cell. A commercial CFD software package (ANSYS CFX 10.0) has been used to predict the complex unsteady air-water flow. The k-ε turbulence model for shear-induced turbulence, Sato's eddy viscosity model for bubble-induced turbulence, and the effect of interfacial momentum transfer terms (lift force, wall force) were considered. Present findings suggest use of low height-to-diameter ratio, low airflow rate, small column taper angle, and uniformly distributed sparger to achieve good separation in a column flotation cell.     

基于能量守恒的鼓泡塔双尺度流体力学模型

提出了同时描述鼓泡塔宏观流动及气泡尾涡小尺度湍动的双尺度流体力学模型,其中大尺度剪切流通过经典k-ε模型描述,而小尺度尾涡湍动则由“尾涡温度”传输方程确定。通过在运动方程添加“尾涡压力”源项,构建大小双尺度流体运动的相互作用,解释鼓泡塔内含率非均匀分布的机制。模型将鼓泡塔内的能量耗散分解3种作用机制：大尺度剪切流引起湍动耗散;小尺度尾涡湍动耗散;尾涡与壁面作用的能量耗散,较好地解决了现有鼓泡塔模型能量不守恒问题。模型计算稳定性高,模拟结果与实验结果吻合良好。     

液体中可压缩气体射流的瞬态特性

针对水下超声速气体射流实验装置,分别采用高速摄影对水下超声速气体射流的形态及发展过程进行了可视化观察分析,采用VOF方法建立了二维轴对称两相数值计算流模型,对实验工况进行数值模拟,得到详细的水下超声速射流流场结构。两者结合得以研究水下气体超声速射流的形态及发展过程。研究结果表明:超声速水下射流流场明显包含射流区、过渡区和羽流区3个不同特征区域,射流区内气相的胀鼓和回击现象导致了严重的振荡流模式。气液界面不稳定性引起射流局部颈缩,从而引起颈缩上游气相截面的扩张、收缩甚至断流。可观测的小幅度的颈缩导致上游的胀鼓现象;稍大幅度的颈缩导致上游的回击现象;大幅度的颈缩甚至导致射流中断,并在随后重建射流。     

Experimental and numerical study of stratified viscous oil–water flow

Stratified two-phase flows of oil and water are important to the energy industry, and models capable of predicting this type of flow are primordial. Many studies focus on fluids with low viscosity, but a high viscosity oil in the mixture significantly changes its behavior. We gathered experimental data of pressure drop, volumetric fractions, and flow-pattern data of a stratified liquid–liquid flow with high viscosity ratio. In addition, a wire-mesh sensor provided tomographic views of the flow. The data were compared with computational fluid dynamics (CFD) models using OpenFOAM and a one-dimensional model. CFD simulations used an interface capturing method, and turbulence damping was introduced to avoid high eddy viscosity at the interface region. Reynolds Average Navier–Stokes and large eddy simulations were used to account for turbulence, and they showed significant differences. The comparisons showed good overall results for pressure drop, volumetric fractions, and phase distributions between CFD and experiments.     

同心套管包围的多跨换热器传热管在静水中阻尼特性的实验研究

通过求解同心套管包围的带N-1个同心支承板传热管的运动方程,导出计算挤压膜阻尼和粘滞阻尼的公式.为了验证理论分析结果,对同心套管包围的四跨传热管在静水中的挤压膜阻尼和粘滞阻尼进行了实验研究,得出了粘滞阻尼与同心套管内径以及管子与同心套管偏心率的关系;挤压膜阻尼与支承板厚度、传热管与支承板管孔间隙、与支承板管孔偏心率之间的关系.在分析实验结果的基础上,得出了传热管总阻尼的公式,并为换热器设计提供了传热管管壁间距离、支承板厚度以及支承极管孔与管子间隙的参考尺寸.     

同心套管包围的多跨换热器传热管在静水中阻尼特性的实验研究

通过求解同心套管包围的带N-1个同心支承板传热管的运动方程,导出计算挤压膜阻尼和粘滞阻尼的公式.为了验证理论分析结果,对同心套管包围的四跨传热管在静水中的挤压膜阻尼和粘滞阻尼进行了实验研究,得出了粘滞阻尼与同心套管内径以及管子与同心套管偏心率的关系;挤压膜阻尼与支承板厚度、传热管与支承板管孔间隙、与支承板管孔偏心率之间的关系.在分析实验结果的基础上,得出了传热管总阻尼的公式,并为换热器设计提供了传热管管壁间距离、支承板厚度以及支承极管孔与管子间隙的参考尺寸.     

Stratified two-phase flog of molten polymers in circular dies

A theoretical study has been carried out on the steady, axisymmetric two-phase flow of molten polymers in circular dies. For both fluids, the shear rate dependence of the viscosity is described by a power law and the temperature dependence by an exponential function. Taking into account viscous dissipation, a numerical program has been developed to predict the radial position of the interface between the two fluids, the developing temperature and velocity fields, the shear rates, the shear stresses, the pressure drop as well as the field of shear deformation and the residence time distribution for different thermal boundary conditions. The numerical program is applied to the flow of a high-viscous polymer melt, (high density polyethylene), surrounded by a small annulus of less viscous polymer ni lt, (low density polyethylene). Computed results are shown graphically.     

水平管气液两相分层流界面剪切预测研究进展

水平管气液两相分层流虽流型简单,但由于界面存在复杂的动量和能量传递,分层流的界面剪切预测至今没有一致的结论。本文从理论模型、实验模型、数值计算3个角度出发,详细阐述水平管气液两相分层流界面剪切预测的研究现状,得出不同研究方法的优势和缺陷。针对3种研究方法,指出理论模型通过模型简化和经验关联式来建立封闭模型,实验模型则在封闭关系上修正经验关联式,但由于简化假设和实验条件的限制,使得这两种研究方法对界面剪切应力的预测具有一定的局限性;数值计算能够弥补机理模型在流场细节等方面的不足,但能够提供界面剪切预测或封闭关系的工作很少。此外,对比了5种不同形式的已有模型对气液两相分层流持液率和压降预测的结果。最后展望了水平管气液两相分层流界面剪切预测的研究趋势,提出理论和实验研究需要提出更详细的局部模型,并考虑工程实际工况进行研究,发展针对气液界面计算的新方法,并为分层流提供封闭关系则是数值计算研究面临的挑战。     

Pressure drop and bubble-liquid interfacial shear stress in a modified gas non-Newtonian liquid downflow bubble column

A functional form of equation for predicting pressure drop in a modified non-Newtonian downflow bubble column has been formulated. The equation has been developed based on the bubble formation, drag at interface and the wettability effect of the liquid. Also the bubble-liquid interfacial shear stress in two-phase flow is analyzed and correlated with the dynamic, geometric and physical variables. The functional form of equation appears to predict the pressure drop satisfactorily for two-phase dispersed flow in the co-current modified downflow bubble column with carboxy methyl cellulose (CMC) solution in water with different concentrations.     

Onset of pulsing in gas-liquid trickle bed filtration

When liquid suspensions containing low concentration of fine solids are treated in catalytic packed bed gas-liquid-solid reactors, which are operated in trickle flow or near the transition between trickle and pulse flow, plugging develops and increases the resistance to two-phase flow. Also due to obstruction, such accumulation of fines in the catalyst bed shifts progressively the flow pattern from trickling to pulsing flow. The progressive onset of pulsing flow along the packed bed was estimated using a sequential approach based on combining a “large time-scale” unsteady-state filtration solution of two-phase flow with a “short time-scale” solution of a linear stability analysis of two-phase flow. Space-time evolution and two-phase flow of the deposition of fines in trickle bed reactors under trickle flow regime was described using a one-dimensional two-fluid model based on the volume-average mass and momentum balance equations and volume-average species balance equation for the fines. The model hypothesized that plugging occurred via deep-bed filtration and incorporated physical effects of porosity and effective specific surface area changes due to the capture of fines, inertial effects of phases, and coupling effects between the fines filter rate equation and the interfacial momentum exchange force terms. The transition between trickle flow and pulse flow regimes was described from a stability analysis of the solution of the transient two-fluid model around an equilibrium state of trickle flow under pseudo steady state conditions. The impact of liquid superficial velocity, viscosity and surface tension, gas superficial velocity and density, feed fines concentration, and fines diameter on the transition between trickle and pulse flows in the presence of fines deposition was analyzed.