双孔连续出流气直接接触泡聚并行为数值模拟

气泡作为气液流体行为和传递特性的最基本元素,其聚并的运动特性对于化工、石油等工业具有重要作用。本文基于VOF对双孔连续出流气泡直接接触式聚并行为进行数值研究,揭示了聚并演变机理,考察了孔间距、进气速度、液面高度对聚并行为的影响。结果表明:气泡初始纵横比随进气时间增加而减小,聚并时突然增大,气泡开始聚并时刻与进气速度负相关;气泡脱离时间与孔间距、进气速度均负相关,液面高度对入口气泡聚并行为影响不大。     

气泡聚并的实验研究

气泡聚并对气泡运动速度和形状有重要影响,本文通过实验探究气泡聚并过程以及不同因素对于聚并的影响。经过多次实验得出,在相同的实验条件下,氮气的聚并时间小于空气气泡的聚并时间。液体浓度增加,气泡聚并时间增大。气泡直径的增加,加大了液膜的体积,增加了液膜脱落时间。     

表面改性剂对溶气气浮微气泡特性影响研究

研究了十六烷基三甲基溴化铵(CTAB)、壳聚糖、聚二甲基二烯丙基氯化铵(PDADMAC)3种表面改性剂对溶气气浮微气泡特性的影响。结果表明:微气泡平均直径减小主要得益于小尺寸微气泡数量的大幅增加。CTAB投加浓度能显著影响微气泡的直径及并聚概率,对微气泡上升速度有略微影响。投加阳离子聚合物可使微气泡间的并聚现象增多,对微气泡直径及上升速度影响微弱。3种改性剂在微气泡表面的附着效率以CTAB最大,PDADMAC最小。不同改性剂分子产生的电荷量及其在微气泡表面的吸附效率有较大差异。     

不同管口浸没方式下气泡生成行为特性

对三种管口浸没方式下气泡生成行为过程进行可视化实验和三维数值模拟。对比分析了管口浸没方式、管口直径、气体流量等因素对气泡生成形态、气泡脱离直径、气泡膨胀脱离时间以及气液流场速度的影响。实验与数值模拟取得较为一致的结果。研究发现,气泡生成过程可分为单气泡生成和双气泡生成聚并两种模式,两者之间存在明显的气泡脱离形态转折点;三种管口浸没方式下,气泡脱离直径均随着管径和气体流量的增大而增大;气泡膨胀脱离时间随管径的增大而增加,而随气体流量的增加先急剧下降然后趋于平缓;在底吹和侧吹方式下,气泡长短轴比C值分别在0.75和1.1附近波动,其最终脱离形式均接近于球形;而顶吹方式下,C值在1.5附近波动,气泡脱离形态为椭球形。     

幂律流体中平行上升双气泡间相互作用研究

采用流体体积(VOF)法对非牛顿幂律流体中平行上升双气泡间相互作用进行了模拟研究.结果表明:剪切变稀效应及气泡周围流场结构对气泡间相互作用的贡献分别随着气泡间距增大而减小,气泡间相互作用主要受气泡间流体涡旋结构控制.当初始间距很小时,气泡平行聚并发生;随着初始间距增大,双气泡行为转变为相互影响的平行上升;当初始间距增大...     

相间传质对气泡聚并过程影响的实验研究

设计了一套双气泡聚并实验系统,考察了相间传质对气泡聚并过程的影响,气相采用非相变的N2,液相为挥发性的丙酮和乙醇水溶液. 利用该系统测定了25~50℃温度下,不同组分浓度、鼓泡频率和气泡直径时气泡的聚并特性. 结果表明,相间传质诱导的Marangoni效应使聚并时间随温度升高呈先减小后增大的趋势,聚并时间的分散性在较高温度下显著增大,采用聚并效率描述气泡聚并特性更合适;聚并效率在温度升高到一定程度后开始降低并趋于最小值,该值随溶液浓度增加而减小,稳定因子可定量描述Marangoni效应;鼓泡速率和气泡直径增加1倍,最小聚并效率分别减小50%和67%.     

气浮接触区气泡聚并行为的数值模拟

在气浮接触区内,聚并会导致气泡直径增大,对分离效果产生影响。采用相群平衡模型对接触区气泡聚并行为进行数值模拟,研究了气泡聚并发生的原因及来液流量、回流流量对气泡聚并的影响。首先分别应用Schiller-Naumann、Grace和Tomiyama3种曳力系数模型进行模拟,所得气泡直径均与实验值吻合,无明显差异,选定Schiller-Naumann曳力系数模型对气浮中两相流动进行模拟。通过对模拟结果进行分析,表明回流入口周围上下行流过渡区域存在较大速度梯度,是导致气泡聚并的关键因素。最后研究了来液流量和回流流量对接触区气泡尺寸的影响,接触区上部气泡直径随回流流量增大而明显增大,原因在于增大回流流量使得过渡区域速度梯度升高,气泡聚并频率提高;而来液流量对气泡尺寸基本无影响。     

气浮接触区气泡聚并行为的数值模拟

在气浮接触区内,聚并会导致气泡直径增大,对分离效果产生影响。采用相群平衡模型对接触区气泡聚并行为进行数值模拟,研究了气泡聚并发生的原因及来液流量、回流流量对气泡聚并的影响。首先分别应用Schiller-Naumann、Grace和Tomiyama 3种曳力系数模型进行模拟,所得气泡直径均与实验值吻合,无明显差异,选定Schiller-Naumann曳力系数模型对气浮中两相流动进行模拟。通过对模拟结果进行分析,表明回流入口周围上下行流过渡区域存在较大速度梯度,是导致气泡聚并的关键因素。最后研究了来液流量和回流流量对接触区气泡尺寸的影响,接触区上部气泡直径随回流流量增大而明显增大,原因在于增大回流流量使得过渡区域速度梯度升高,气泡聚并频率提高;而来液流量对气泡尺寸基本无影响。     

基于FTM方法的双气泡融合特性模拟

采用界面追踪法(FTM)对气泡融合现象进行数值模拟,将模拟结果与文献结果进行对比,验证了计算模型的准确性。结果表明,同轴双气泡上升速度均高于单独气泡的上升速度,且融合后气泡与等直径单气泡上升速度相同。气泡间距较小时,跟随气泡的上升速度更高。引导气泡的厄特沃什数Eo=0.36~9,Eo较大时两气泡上升阶段时间较短,但接触阶段时间较长,接触阶段气泡间的液膜在压力作用下逐渐变薄,最终破裂,气泡融合。Eo?4.16时,气泡融合所需时间随Eo增加而增加;Eo?4.16时,气泡融合所需时间不再变化。莫顿数Mo=0.57,Eo=5.04~18.72时,存在特定的双气泡初始角度?c,当0?≤?≤?c时,双气泡相互排斥;当?c≤?≤90?时,双气泡融合,且?c随Eo增加而降低.     

不同管口浸没方式下气泡生成行为特性

对三种管口浸没方式下气泡生成行为过程进行可视化实验和三维数值模拟。对比分析了管口浸没方式、管口直径、气体流量等因素对气泡生成形态、气泡脱离直径、气泡膨胀脱离时间以及气液流场速度的影响。实验与数值模拟取得较为一致的结果。研究发现,气泡生成过程可分为单气泡生成和双气泡生成聚并两种模式,两者之间存在明显的气泡脱离形态转折点;三种管口浸没方式下,气泡脱离直径均随着管径和气体流量的增大而增大;气泡膨胀脱离时间随管径的增大而增加,而随气体流量的增加先急剧下降然后趋于平缓;在底吹和侧吹方式下,气泡长短轴比C值分别在0.75和1.1附近波动,其最终脱离形式均接近于球形;而顶吹方式下,C值在1.5附近波动,气泡脱离形态为椭球形。关键词:气液两相流;气泡;数值模拟;浸没方式;脱离直径     

The influence of approach velocity on bubble coalescence

Experiments have been carried out in which a cloud of air bubbles has been prevented from rising by downflowing water in a tube. High speed photography revealed an almost complete absence of bubble coalescence. This has been attributed to the large approach velocities of bubbles in the cloud.Further experiments in which a single bubble has been allowed to coalesce with a plane air—water interface have demonstrated the effect more clearly. Two basic types of bubble coalescence have been recognised depending on the approach velocity of the bubbles. At a low approach velocity, bubble coalescence is rapid, but coalescence times are considerably increased at large approach velocities. For pure liquids, a theory is put forward which shows that at low approach velocities film rupture can occur before the approaching bubbles are brought to rest. At large approach velocities the bubbles are brought to rest before rupture occurs. In the latter case bubble bounce can occur and the total coalescence time is thereby considerably increased.Based on observed approach velocities in a stationary bubble cloud, it is suggested that large approach velocities in a bubble column may be an important factor in limiting bubble coalescence.     

A note deriving the analytical solution for the wake theory of coalescence

The recent unsolved wake theory to describe bubble coalescence by Colella et al. [1999. A study on coalescence and breakage mechanisms in three different bubble columns. Chem. Eng. Sci. 54, 4767-4777] is solved analytically to give exact values of coalescence time in terms of bubble size and relative rise velocity. The solution is expressed using the tabulated exponential integral function.     

Theoretical prediction of flow regime transition in bubble columns by the population balance model

Theoretical prediction of flow regime transition in bubble columns was studied based on the bubble size distribution by the population balance model (PBM). Models for bubble coalescence and breakup due to different mechanisms, including coalescence due to turbulent eddies, coalescence due to different bubble rise velocities, coalescence due to bubble wake entrainment, breakup due to eddy collision and breakup due to large bubble instability, were proposed. Simulation results showed that at relatively low superficial gas velocities, bubble coalescence and breakup were relatively weak and the bubble size was small and had a narrow distribution; with an increase in the superficial gas velocity, large bubbles began to form due to bubble coalescence, resulting in a much wider bubble size distribution. The regime transition was predicted to occur when the volume fraction of small bubbles sharply decreased. The predicted transition superficial gas velocity was about 4 cm/s for the air-water system, in accordance with the values obtained from experimental approaches.     

STUDIES OF COALESCENCE OF BUBBLE PAIRS

The coalescence behaviour of bubble pairs formed on two adjacent capillary tubes was studied in aqueous solutions of n-alcohols and fatty acids at various concentrations. Coalescence frequency and coalescence time were measured. The results show that coalescence time seems to be the best physical magnitude which can describe the coalescence process, because bubble coalesce only if the contact time of the bubbles is greater than the coalescence time. The coalescence time in solutions is proportional to the surface excess concentration of the solute. Coalescence time increases with the polarity of the hydrophylic group and its chain length and decreases with decreasing bubble size and increasing temperature.     

STUDIES OF COALESCENCE OF BUBBLE PAIRS

The coalescence behaviour of bubble pairs formed on two adjacent capillary tubes was studied in aqueous solutions of n-alcohols and fatty acids at various concentrations. Coalescence frequency and coalescence time were measured. The results show that coalescence time seems to be the best physical magnitude which can describe the coalescence process, because bubble coalesce only if the contact time of the bubbles is greater than the coalescence time. The coalescence time in solutions is proportional to the surface excess concentration of the solute. Coalescence time increases with the polarity of the hydrophylic group and its chain length and decreases with decreasing bubble size and increasing temperature.     

The influence of gas velocity, salt type and concentration on transition concentration for bubble coalescence inhibition and gas holdup

The influence of gas velocity (3.5, 10, and 18 mm/s), salt type (NaCl, NaF, NaBr, NaI and CsCl) and salt concentration (0.001-3 M) on bubble coalescence in a small bubble column were studied. The bubble coalescence was determined by the relative change in the measured light intensities passing through the salt solutions and clean deionised water. It was shown that the transition salt concentration for bubble coalescence inhibition (determined at 50% of bubble coalescence) of all investigated salts decreases with increasing superficial gas velocity. The difference in the transition concentration between NaCl, NaF, NaBr and CsCl does not significantly change with the gas velocity. However that difference between NaI and the other salts significantly decreases with increasing the gas velocity. The gas holdup significantly increases with NaCl, NaF, NaBr and CsCl concentrations but does not significantly change with NaI concentration. These new results imply that the transition salt concentration for bubble coalescence and gas holdup depend not only on the salt properties (i.e. the ion type and their combination) as previously reported, but also on the hydrodynamic conditions.     

Computational study of bubble coalescence/break-up behaviors and bubble size distribution in a 3-D pressurized bubbling gas-solid fluidized bed of Geldart A particles

A computational study was carried out on bubble dynamic behaviors and bubble size distributions in a pressurized lab-scale gas-solid fluidized bed of Geldart A particles. High-resolution 3-D numerical simulations were performed using the two-fluid model based on the kinetic theory of granular flow. A fine-grid, which is in the range of 3–4 particle diameters, was utilized in order to capture bubble structures explicitly without breaking down the continuum assumption for the solid phase. A novel bubble tracking scheme was developed in combination with a 3-D detection and tracking algorithm (MS3DATA) and applied to detect the bubble statistics, such as bubble size, location in each time frame and relative position between two adjacent time frames, from numerical simulations. The spatial coordinates and corresponding void fraction data were sampled at 100 Hz for data analyzing. The bubble coalescence/break-up frequencies and the daughter bubble size distribution were evaluated by using the new bubble tracking algorithm. The results showed that the bubble size distributed non-uniformly over cross-sections in the bed. The equilibrium bubble diameter due to bubble break-up and coalescence dynamics can be obtained, and the bubble rise velocity follows Davidson’s correlation closely. Good agreements were obtained between the computed results and that predicted by using the bubble break-up model proposed in our previous work. The computational bubble tracking method showed the potential of analyzing bubble motions and the coalescence and break-up characteristics based on time series data sets of void fraction maps obtained numerically and experimentally.     

Eulerian–Lagrangian simulation of bubble coalescence in bubbly flow using the spring-dashpot model

The Eulerian–Lagrangian simulation of bubbly flow has the advantage of tracking the motion of bubbles in continuous fluid, and hence the position and velocity of each bubble could be accurately acquired. Previous simulation usually used the hard-sphere model for bubble–bubble interactions, assuming that bubbles are rigid spheres and the collisions between bubbles are instantaneous. The bubble contact time during collision processes is not directly taken into account in the collision model. However, the contact time is physically a prerequisite for bubbles to coalesce, and should be long enough for liquid film drainage. In this work we applied the spring-dashpot model to model the bubble collisions and the bubble contact time, and then integrated the spring-dashpot model with the film drainage model for coalescence and a bubble breakage model. The bubble contact time is therefore accurately recorded during the collisions. We investigated the performance of the spring-dashpot model and the effect of the normal stiffness coefficient on bubble coalescence in the simulation.The results indicate that the spring-dashpot model together with the bubble coalescence and breakage model could reasonably reproduce the two-phase flow field, bubble coalescence and bubble size distribution. The influence of normal stiffness coefficient on simulation is also discussed.     

Experimental and modelling study of gas dispersion in a double turbine stirred tank

Gas dispersion in a double turbine stirred tank is experimentally characterised by measuring local gas holdups and local bubble size distributions throughout the tank, for three liquid media: tap water, aqueous sulphate solution and aqueous sulphate solution with PEG. For all these media, bubble coalescence generally prevails over breakage. Where average bubble size decreases, this can be attributed to the difference in slip velocity between different sized bubbles. Most of the coalescence takes place in the turbine discharge stream.A compartment model that takes into account the combined effect of bubble coalescence and breakage is used to simulate gas dispersion. The model predicts spatial distribution of gas holdup and of average bubble size, with average bubble size at the turbines as an input. Reasonable agreement between experiment and simulation is achieved with optimisation of two parameters, one affecting mainly the slip velocity, the other related mainly to the bubble coalescence/breakage balance. Different sets of parameters are required for each of the three liquid systems under study, but are independent of stirring/aeration conditions. The model only fails to simulate the smaller average bubble diameters at the bottom of the tank.