应用激光成像系统研究非牛顿流体中的气泡生成

A self-developed laser image measurement system was established to study the behavior of bubble formation at a single orifice in non-Newtonian polyacrylamide （PAAm） solutions. Images of bubbles were captured by a CCD camera and volumes of bubbles were digitally analyzed online. The effects of rheological property of PAAm solution, orifice, reservoir, and gas flowrate on bubble formation were studied experimentally. It is found that the volume of bubble increases with the concentration of PAAm solution, the diameter of the orifice, and the gas flowrate, respectively, whereas little effect of reservoir is observed in experiments.     

Analysis of deformation and internal flow patterns for rising single bubbles in different liquids

Gas–liquid multiphase flow is a significant phenomenon in chemical processes. The rising behaviors of single bubbles in the quiescent liquids have been investigated but the internal flow patterns and deformation rules of bubbles, which influence the mass transfer efficiency to a large extent, have received much less attention. In this paper, the volume of fluid method was used to calculate the bubble shapes, pressure, velocity distributions,and the flow patterns inside the bubbles. The rising behavior of the bubbles with four different initial diameters,i.e., 3 mm, 5 mm, 7 mm and 9 mm was investigated in four various liquids including water, 61.23% glycerol,86.73% glycerol and 100% glycerol. The results show that the liquid properties and bubble initial diameters have great impacts on bubble shapes. Moreover, flow patterns inside the bubbles with different initial diameters were analyzed and classified into three types under the condition of different bubble shapes. Three correlations for predicting the maximum internal circulation inside the bubbles in 86.73% glycerol were presented and the R-square values were all bigger than 0.98. Through analyzing the pressure and velocity distributions around the bubbles, four rules of bubble deformation were also obtained to explain and predict the shapes.     

非牛顿流体中的两并行运动气泡周围流场及气泡间相互作用的PIV研究

The flow fields surrounding two parallel moving bubbles rising from two identical orifices submerged in non-Newtonian fluid of carboxymethylcellulose (CMC) solution of three different mass concentration were measured experimentally by the use of particle image velocimetry (PIV). The influences of gas flowrate, solution mass concentration, orifice interval and the angle between two bubble centers line and vertical direction on the flow field surrounding bubbles were discussed respectively by analyzing the velocity vector, velocity contours as well as individual velocity components. The results show that the liquid velocity both in front of two bubbles and behind increases with gas flowrate duo to shear-thinning effect of previous bubbles, whereas decreases with the increase of CMC concentration due to the increase of drag force acting on bubbles. The effect of the orifice interval on the flow field around two moving bubbles becomes gradually obvious as the interval becomes closer. Moreover, two adjacent side-by-side bubbles repulse each other during rising, leading to the practical interval between them increased somewhat above the orifice interval. When the distance between bubbles is less than the orifice interval l0 mm, the interaction between two neighboring bubbles changed from mutual repellence to attraction with the decrease of the angle of the line of linking two bubble centers to the vertical direction.     

气液错流条件下孔口气泡直径的模型预测(英文)

The size of initial bubbles is an important factor to the developed bubble size distribution in a gas-liquid contactor. A liquid cross-flow over a sparger can produce smaller bubbles, and hereby enhance the performance of contactor. A one stage model by balancing the forces acting on a growing bubble was developed to describe the formation of the bubble from an orifice exposed to liquid cross-flow. The prediction with this model agrees with the experimental data available in the literatures, and show that orifice size strongly affects the bubble size. It is showed that the shear-lift force, inertia force, surface tension force and buoyancy force are major forces, and a simplified mathematical model was developed, and the detachment bubble diameter can be predicted with accuracy of ±21%.     

低温向上倾斜管内Taylor泡初始形成位置分布的实验研究

An experimental study was carried out to understand the phenomena of the boiling flow of liquid nitrogen in inclined tubes with closed bottom by using the high speed digital camera. The tubes in the experiment are 0.018 m and 0.014 m in inner diameter and 1.0 m in length. The range of the inclination angles is 0-45° from the vertical. The statistical method is employed to analyze the experimental data. The experiment was focused on the effect of the inclination angle on the initial position distribution of Taylor bubbles. The formation criterion of Taylor bubbles was confirmed by analyzing the images of Taylor bubbles. The experimental results show that the initial position of Taylor bubble increased first, and then decreased with the increasing inclination angle, with the maximum at 30°. The standard deviation of the initial position of Taylor bubble in tubes was different with different inner diameters. The lognormal shape was fitted to the measured the initial position distributions of Taylor bubbles in the cryogenic tubes.     

Interaction of two in-line bubbles of equal size rising in viscous liquid

The interaction of bubbles is the key to understand gas–liquid bubbling flow. Two-dimensional axis-symmetry computational fluid dynamics simulations on the interactive bubbles were performed with VOF method,which was validated by experimental work. It is testified that several different bubble interactive behaviors could be acquired under different conditions. Firstly, for large bubbles(d: 4, 6, 8, 10 mm), the trailing bubble rising velocity and aspect ratio have negative correlations with liquid viscosity and surface tension. The influences of viscosity and surface tension on leading bubble are negligible. Secondly, for smaller bubbles(d: 1, 2 mm), the results are complicated. The two bubbles tend to move together due to the attractive force by the wake and the potential repulsive force. Especially for high viscous or high surface tension liquid, the bubble pairs undergo several times acceleration and deceleration. In addition, bubble deformation plays an important role during bubble interaction which cannot be neglected.     

一串上升气泡周围流体的湍动特性(英文)

The turbulence behavior of gas-liquid two-phase flow plays an important role in heat transfer and mass transfer in many chemical processes. In this work, a 2D particle image velocimetry (PIV) was used to investigate the turbulent characteristic of fluid induced by a chain of bubbles rising in Newtonian and non-Newtonian fluids. The instantaneous flow field, turbulent kinetic energy (TKE) and TKE dissipation rate were measured. The results demonstrated that the TKE profiles were almost symmetrical along the column center and showed higher values in the central region of the column. The TKE was enhanced with the increase of gas flow and decrease of liquid viscosity. The maximum TKE dissipation rate appeared on both sides of the bubble chain, and increased with the increase of gas flow rate or liquid viscosity. These results provide an understanding for gas-liquid mass transfer in non-Newtonian fluids.     

中等Reynolds数线性剪切泡状流的液相雷诺应力

Analytical investigation of liquid Reynolds stress in shear bubbly flow with intermediate Reynolds numbers is absent. In this paper, the velocity field around a sphere bubble in linear shear liquid is assumed to be the linear superposition of the velocity field of uniform liquid passing a sphere bubble and the linear shear velocity field. The formula of shear liquid Reynolds stress was derived by averaging the velocity field in the cell-ensemble averaging method, and the formula was corrected under conditions of intermediate Reynolds number. The formula was compared with that of Sato, and the predicted results of local liquid velocity of the fully developed upward bubbly flow in pipes were compared with the experimental data. The results show that the formula is valid and accurate in prediction.     

垂直管内通气空泡掺气过程仿真分析简

A semi-empirical gas entrainment model was proposed for the ventilated cavity in vertical pipe, based on which, a complete numerical scheme was established by coupling with the Eulerian-Eulerian two-fluid model to predict the multiscale flow field created by ventilated cavity. Model predictions were validated against experimental measurements on void fraction and bubble size distributions. Simulations were carried out to explore the effect of ventilation rate and inlet turbulence intensity on the macroscale cavity shape and the bubbly flow downstream of the ventilated cavity. As the ventilation rate increasing, a reverse trend was observed for the void fraction and bub- ble size distributions. It is concluded that the average void fraction in the pipe flow region is determined by the volumetric ratio between liquid and gas. However, the bubble size evolution is dominated by the breakage effect induced by turbulence in the vortex region. Furthermore, simulations were conducted to analyze geometric scale effect based upon Froude similitude. The results imply that the velocity distributions were properly scaled. Slight scale effect was seen for the void fraction caused by faster dispersion of bubbles in the larger size model. The com- paratively greater bubble size was predicted in the smaller model, implying significant scale effects in terms of tur- bulence and surface tension effect. It reveals that empirical correlations valid in wide range are required for the ex- trapolation from small-size laboratory models.     

泡沫相塔壁对泡沫分离牛血清蛋白分离性能的影响

In order to enhance foam drainage, the column with an inner sleeve in the foam phase was designed for studying effect of the column wall of foam phase on foam separation performances using bovine serum albumin(BSA) as the research system. The effects of the wall on the liquid holdup out of the top column, bubble size, enrichment and recovery percentage were investigated. The results indicated that the experimental column with the inner sleeve decreased the liquid holdup, accelerated the coarsening and coalescence of bubbles and increased enrichment of BSA compared the contrasted column without the inner sleeve. Under the conditions of the initial concentration 0.2 g?L-1 of BSA, air flow rate 400 ml?min-1, the experimental column achieved up to a 2.06 fold increase in enrichment compared to the contrasted column. The enrichment of BSA increased with the increase of inner sleeve length. Channel theoretical analysis showed that the ratio of exterior channels to interior channels increased with the increase of bubble diameter. So the experiment column obtained the better performances at the lower concentration and the lower air flow rate. The better performances obtained by experimental column showed that the drainage rate of plateau borders on wall was greater than that of plateau borders between bubbles. So the inner sleeve provided more plateau borders on wall and improved foam drainage.     

Experimental investigation on dynamic behaviour of bubbles emerging from micro-capillary orifice in a flow channel

The flow phenomenon of liquid with bubbles is widespread in various industrial fields, which determines the mass transfer characteristics of the equipment. In this work, the dynamic behaviour of bubbles emerging from micro-capillary orifice in a flow channel was studied by a visualization experiment, while the effects of gas flow rate and liquid flow rate on these processes of bubble growth, departure, and inrush were explored. The experimental results showed that one bubble formation cycle can be divided into three stages: Waiting, departure, and inrush, as well as the dynamic behaviour of bubble emerging from micro-capillary orifice in a flow channel, were significantly affected by gas flow rate and liquid flow rate. At a higher gas flow rate, the growth time and the departure time were shorter, as well as the departure volume of the leading bubble and the inrush volume of the trailing bubble were smaller, while the transverse longitudinal ratio fluctuated more violently, and the swing amplitude of the bubble centroid was greater. With an increasing liquid flow rate, the growth time, the departure time, and the inrush time shortened, while the departure volume of the leading bubble decreased and the fluctuation of the bubble centroid weakened. These findings are conducive to improving the performance of the equipment by optimizing the design of the aerator to regulate the dynamic behaviour of bubbles.     

电场作用下冷态单气泡形成过程

对氮气泡在电场作用下的形成过程进行了可视化研究。实验观察到电场作用下气泡生长的动态图像；研究表明，电场作用下气泡沿场强方向伸长，随着场强的升高，气泡的脱离长径比增大，脱离体积减小；另外利用Matlab编程从实验中取得实际气泡生长的轮廓线，对气泡界面的电应力计算显示，气泡顶部受到电拉力作用，两侧受电压力作用，从而使气泡产生了沿场强方向的伸长变形。这表明气泡界面受到的电应力是气泡产生变形的主要原因，该结果有助于电场对沸腾传热强化机理的研究。     

非均匀电场作用下气泡生长及运动特性

气液两相流广泛应用于化工、石油等工业生产中,电场可以有效强化相间作用。为探究电场作用下气泡的生长演化特性,本文采用显微高速数码摄像技术对气泡生长、脱离过程及运动进行可视化研究,精确捕捉了非均匀电场作用下气泡产生、脱离和运动过程的显微形貌特性,结合图像处理技术定量分析了电场强度对气泡生长时间、脱离频率、体积及运动速度的影响规律。实验结果表明电场作用改变了气泡的生长方式,显著地促进了气泡的脱离及运动。电场作用下气泡的脱离频率明显增大,相较于无电场情况下增加了几十倍,最短脱离周期可达到10ms左右。气泡脱离体积显著减小,相应的最小气泡直径为毛细管直径一半。气泡初始速度大约增加4倍,横向速度达到80mm/s左右,强化了气泡在液体中的分散性。这为荷电气液两相流工业应用提供了良好的理论基础。     

非均匀电场作用下汽泡的行为

为进一步探索电场强化沸腾换热的机理,利用高速摄像仪对沸腾汽泡在非均匀电场作用下的生长过程进行了可视化实验研究,实验观察到非均匀电场作用下汽泡脱离壁面时的形态。当没有施加高压电场时,汽泡脱离壁面时基本上呈球形形状;而外加电场后,汽泡脱离壁面时呈椭球状,而且随着场强的升高,汽泡脱离壁面时汽泡沿场强方向的伸长更加明显。这是由于未加电场时,汽泡的长大主要受惯性力和表面张力的支配。外加电场后,汽泡的受力增加了电场力的作用,电应力在赤道方向压缩汽泡,在极轴方向拉伸汽泡。在电场作用下,汽泡由近似球形形状变成了椭球形形状。随着场强的增大,汽泡所受的电应力加大,使得汽泡脱离壁面时沿场强方向的伸长更加显著。施加电压后,汽泡脱离频率随着场强的增大而增大,当电场强度为1.4 kV/mm时,汽泡的脱离频率为不加电压时的1.85倍。研究结果表明,电场对汽泡的行为有显著的影响作用。     

Experimental investigation on bubbling behavior in anode channel of passive DMFC

将被动式DMFC阳极通道内气泡形成过程简化为CO₂气体注入充满静态甲醇溶液中形成气泡的过程。利用可视化实验研究了气体垂直注入静态甲醇溶液中形成气泡以及气泡脱离的过程,考察了气体流量、液层高度、孔径大小、甲醇浓度对气泡形成及长大脱离过程的影响。结果表明：在气泡的生长过程中,接触角先是快速下降后又迅速上升,然后趋于稳定值直到脱离。随着气体流量的增加,脱离时气泡的直径变化甚微,脱离时间先下降较快后趋于稳定。随着液层高度的增加,气泡的脱离直径变小,产生的频率加快,脱离时间先是减小后趋于稳定;气体流量在此条件下对脱离时间的影响减弱。随着孔径的增加,气泡的脱离直径增加,产生气泡的频率减慢,气泡的脱离时间减小。随着甲醇浓度的增加,气泡的脱离直径变小且波动明显,气泡产生的频率变快,脱离时间先是减小后趋于稳定,对低浓度的溶液影响较为明显。     

单孔及微孔曝气低气速鼓泡床内气泡行为比较

引言鼓泡床反应器被广泛应用于吸收、液相氧化、好氧生化等气液反应过程,气体在液相中的分散情况对鼓泡床的反应和传质特性都有很大影响.为了提高气液传质效率,增加生产强度,工业反应器很多都是在高气速下操作(Ug>0·05m·s-1),很多研究都集中在高气速湍动鼓泡区[1~3].但对有机     

Discrete particle simulations of an electric-field enhanced fluidized bed

Reducing the size of gas bubbles can significantly improve the performance of gas-solid fluidized reactors. However, such a control of bubbles is difficult to realize without measures that either use a lot of energy or deteriorate the fluidization behavior. In this paper, we present the results of discrete particle simulations of an electric-field enhanced fluidized bed, and compare these results to experimental data.The simulations show a significant effect on the size of bubbles, both with horizontal and vertical electric-fields applied. When the field strength is increased to values higher than those used in the experiments, the particles are found to form strings in the direction of the electric field. At very high field strengths, defluidization is observed, consistent with the experiments.Through the analysis of the bubble behavior, it is concluded that moderate strength electric fields distribute gas more evenly at the bottom of the bed. As the bubbles rise through the bed, the coalescence rate is lower because of the guiding paths, or resistance, the particles form due to the field. This results in a smaller average bubble size in the higher region of the bed. The simulations presented here show how and why the electric fields reduce bubble size in electric-field enhanced fluidized beds.     

大孔径高气速单孔气泡形成

在内径为190mm的鼓泡塔内,研究了空气-去离子水系统在大孔径高气速条件下的单孔气泡形成。考察了五个不同的孔径,分别为4、8、10、15及21mm,孔口气速范围为0.8~154.8m·s^-1。以CCD摄像记录气泡的形状及尺寸,根据气泡长径比的变化,得到气泡初始形态转变时的临界孔口气速：当孔口气速低于20m·s^-1时,孔口气泡近似于球形,长径比小于1.1;当孔口气速大于50m·s^-1时,气泡呈现椭球形,长径比大于1.5。并对气泡尺寸与孔径及孔口气速进行关联,所得关联式对孔径大于3mm、孔口气速在10~80m·s^-1范围内所形成的气泡尺寸预测效果较好。     

Numerical study of condensing bubble in subcooled boiling flow using volume of fluid model

In this numerical study, the behavior of condensing bubble was investigated using the volume of fluid (VOF) model in the FLUENT code. In order to simulate the condensing bubble with the FLUENT code, the bubble condensation was modeled using the user-defined function (UDF). For the validation of the UDF of bubble condensation, the results of CFD simulation were compared with the results of a bubble condensation experiment performed in Seoul National University (SNU). Simulation results showed good agreements with the experimental data. Moreover, the fundamental behavior of the condensing bubble was investigated in various conditions. The effects of condensation on bubble behavior were analyzed by comparing the behavior of condensing bubbles with that of adiabatic bubbles. It was found that the behavior of the condensing bubble was different from that of the adiabatic bubble in many respects including the bubble shape, velocity, rise distance and moving trajectory.