Bubble formation in cocurrently upward flowing liquid

The effects of liquid velocity, nozzle diameter, gas chamber volume and gas flow rate on volumes, shapes and growth curves of bubbles formed at a nozzle submerged in a cocurrently upward flowing liquid in a bubble column were experimentally investigated. The bubble volume decreases with increasing liquid flow velocity. The effect of liquid flow velocity on the volume of bubble increases with an increase in the gas flow rate. To simulate bubble formation at a nozzle submerged in cocurrently upward flowing liquid, a revised non-spherical bubble formation model was proposed. Bubble volumes, bubble growth curves and shapes experimentally obtained in this study, as well as in previous experimental studies, are well predicted by the present model.     

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。     

气泡在颗粒床层表面的生成脱离行为

试验研究了气泡在颗粒床层表面的生成脱离过程及其行为特性,利用高速摄像技术揭示了进气管管径、颗粒床层高度、颗粒粒径等因素对气泡脱离直径及其生成周期的影响规律,对比分析了颗粒床层表面和进气管管口的气泡生成脱离行为差异。研究结果表明：在1500～3000μm粒径范围的床层表面所生成气泡的初始形态相对更扁小,气泡也更快向扁平状演变;颗粒粒径的增大使得进气流量对气泡形态的影响减弱;管径和颗粒床层高度的增大可以有效促进气泡脱离直径的增长,但延缓了气泡的生成脱离,增加了气泡的生成周期;颗粒粒径对气泡生成周期的影响随着进气流量的增大而逐渐减弱;气泡在颗粒床层表面和管口的生成脱离行为存在显著差异,相比之下,150～300μm粒径范围的颗粒床层对气泡的生成脱离具有更明显的阻碍作用,其表面所生成气泡的脱离直径和生成周期相对较大。     

DISPERSION OF GAS AND LIQUID IN BUBBLE COLUMNS OF HOMOGENEOUS BUBBLE FLOW REGIME

Mean gas holdup, lateral distribution of gas holdup and axial mixing of gas and liquid were measured in bubble columns of 12 and 19cm i.d. The lateral distribution of gas holdup was strongly dependent on the flow regimes in the column. The axial mixing of liquid in the homogeneous bubble flow regime was much smaller than that in the heterogeneous bubble flow regime, and was not expressed by existing correlations. The axial mixing of liquid in the homogeneous bubble flow and the intermediate flow regime was simulated with a flow model based on the lateral distribution of buoyancy force and the effective viscosity. The axial mixing of gas was larger than that of liquid.     

气-液-固三相下喷环流反应器局部气相流动特性

The local gas-phase flow characteristics such as local gas holdup (εg), local bubble velocity (Vb) and local bubble mean diameter (db) at a specified point in a gas-liquid-solid three-phase reversed flow jet loop reactor was experimentally investigated by a five-point conductivity probe. The effects of gas jet flow rate, liquid jet flow rate, solid loading, nozzle diameter and axial position on the local εg, Vb and db profiles were discussed. The presence of solids at low solid concentrations not only increased the local εg and Vb, but also decreased the local db. The optimum solid loading for the maximum local εg and Vb together with the minimum local db was 0.16 × 10-3 m3, corresponding to a solid volume fraction, εS = 2.5%.     

液体中气—液射流特性的研究

在气-液两相喷射式装置中气体或/和液体是经由单个(或多个)喷咀射入塔内的。本研究用频闪摄影技术对气、液向上共同喷射时液体中的气-液射流特性进行了实验研究,并建立了气泡直径分布函教及射流特征尺寸的关联式。同时,对射流两相区内两相的流动用一两相模型进行模拟,考察了喷咀直径、初始条件、液体压头和气泡直径对器内两相流动的影响。     

鼓泡塔中气泡尺寸分布和局部气含率研究

在内径为0.38 m的鼓泡塔中采用双电导探针法对不同通气速率下的气泡尺寸分布和局部气含率进行了实验研究,分析了气泡尺寸的概率密度分布。结果表明:气泡尺寸随轴向高度的增加而增大,随径向距离增加而减小;鼓泡塔中气液流动可分为过渡流域和充分发展流域,在过渡流域气含率随轴向高度增加而增大,在充分发展流域气含率趋于均值,径向局部气含率分布呈抛物线型下降。高气速下气泡尺寸概率密度分布比低气速下宽,且随轴向高度的增加分布变宽。     

湍流中气泡破碎建模与实验研究进展

综述了充分发展湍流中气泡破碎的机理和模型,将其机理归纳为湍流涡碰撞、黏性剪切、尾涡剪切脱落过程和界面不稳定性四类。对文献中气泡破碎速率和子气泡大小分布的预测模型进行了系统总结。分析讨论了现有气泡破碎模型的发展和局限性,并提出了未来的发展方向。同时,也综述了湍流中单气泡破碎的实验研究,依据产生湍流的方法归纳为四种情况：增大液体流速产生湍流,采用内构件产生湍流,搅拌产生湍流,以及圆锥反应器结合搅拌产生湍流。总结了现有气泡破碎实验的进展和局限,并进行了分析和展望。最后,通过将文献中气泡破碎速率模型预测值和实验数据进行对比,表明文献中多个破碎模型已经有了较好的预测能力。     

并行微通道内气液相分配规律

微反应器的集成放大对于微化工技术的工业应用具有重要意义。利用高速摄像仪对4个并行微通道内气液两相流动状况及相分配规律进行了研究,考察了气液两相流量及液相黏度对两相分布均匀性的影响。实验所用液相为含0.3%表面活性剂十二烷基硫酸钠（SDS）的蒸馏水-甘油溶液,气相为氮气（N₂）。实验观察到了6种典型的两相流型。对各支通道均为弹状流情况下气泡长度和气泡速度的分布规律进行了研究。在一定气相流率下,各支通道气泡长度的相对标准偏差随液相流率的增大而增大,气泡速度的相对标准偏差值随液相流率的增大先升高到一定值然后逐渐减小。气相分配不均匀性随液相流率和黏度的增大而增大,液相分配不均匀性随液相黏度的增大而减小,气相流率的变化对于两相分布影响不明显。研究结果有助于并行微通道的结构设计与优化,以实现更为均匀的气液两相流动分配。     

采用压力传感技术测量鼓泡床中流体力学参数

引　言鼓泡床以其良好的传热、传质、相间充分接触和高效的可连续操作特性在许多领域得到了广泛应用 .在过去的 4 0多年里 ,人们采用许多测量方法(光导纤维、多普勒测速仪、电导法、压力传感技术 )对鼓泡床中的各种流动行为进行了大量研究 ,由于床层内流动的复杂性以及各测量手段间的差别 ,得出的结论不尽相同[1] ,而且工业反应多数在高温、高压、非透明体系下进行 ,这限制了许多测量技术的应用 .压力传感技术以其适用范围广、所需仪器便宜、耐用、测量结果准确的特点在鼓泡床流体力学参数测量中得到了广泛应用床层塌落法是压力传感技术在鼓泡床流体力学参数测量中的一个重要应用 .Sriram和Mann[2 ] 较早地将其应用于测量鼓泡床中的气含率 ;Fan等[3] 也曾利用此方法测量鼓泡床内的固含率 .前人大都采用压力传感技术测量床层内的平均相含率 ,而采用此方法测量大小气泡分数和气泡上升速度的报道很少 .本文根据前人在此方面的研究成果并结合本实验的特点进行了这方面的研究1　实验装置本实验结合工业对二甲苯氧化反应器的特点设计并建立了其流体力学冷模实验装置 ,如图 1所示 .鼓泡床高 6 6m ,内径 0 3m .在鼓泡床一侧自...     

High-frequency formation of bubble with short length in a capillary embedded step T-junction microdevice

The performances of slug flow gas–liquid reactors are mostly determined by slug length, especially for the high gas–liquid flow rate ratio condition. This work is the first time to report the short bubble generated with high frequency in a capillary embedded step T-junction microdevice. The aspect ratio of bubble could be around 0.5 with a frequency higher than 750 s⁻¹ when the gas–liquid flow rate ratio is even higher than 5. The specific surface area of the generated gas–liquid microdispersion system is larger than 10,400 m²/m³. The short bubble formation process includes two periods, and its formation mechanism is mainly because of the relatively higher pressure drop in the step T-junction, which provides a much higher breakup force for the squeezing flow. Finally, two models are developed to predict the bubble frequency and volume. This work provides a highly promising dispersion technology for the gas–liquid process intensification in microreactors.     

Continuous formation of microbubbles during partial coalescence of bubbles from a submerged capillary nozzle

Bubble formation from a downward-pointing capillary nozzle was investigated in this study. The experiments were conducted at gas flow rate of 40–5,400 ml/h and inner nozzle radius of 0.030–0.255 mm. Experimental results show that microbubbles are formed continuously at moderate Weber number, which was not reported in pervious investigations with injecting gas through an upward-pointing capillary nozzle. High-speed visualization indicates that the formation of microbubbles arises from the convergence of the capillary waves induced by the partial coalescence of larger bubbles. A bubbling regime map is given to identify the critical conditions for the formation of microbubbles. In the present air-water experiments, the generated microbubbles are 20–170 μm in diameter. From experimental data, a scaling law for microbubble size is proposed as a function of Weber and Bond numbers.     

超声速气体浸没射流的数值计算和实验

从实验和数值计算两方面研究了超声速气体射流在水中的喷射过程.用高速摄影机拍摄了三维水下超声速气体射流的流场.针对实验工况,基于VOF方法,建立水下超声速气体射流的二维轴对称数值计算模型,并开展了相关数值模拟.成功模拟了射流初期气泡运动演化的复杂过程;分析了水下超声速气体欠膨胀射流的流场结构,包括流场的压力和速度等参数分布以及变化规律.数值结果与实验结果对比得知数值计算结果不仅与实验数据吻合较好,而且给出了实验中没有发现的激波、膨胀波等流场结构.     

分布器对鼓泡塔中气泡直径分布的影响

采用电导探针测定了冷态鼓泡塔中不同气速下的气泡直径及气含率的轴向分布,考察了分布板对鼓泡塔操作性能的影响.结果表明:随着开孔率的减小,从均匀鼓泡区到过渡区的转变提前;在均匀鼓泡区,开孔率对气泡直径影响较小;在过渡区,开孔率大的分布器形成的稳定气泡直径较小、气含率较大;分布板开孔直径越大,形成的初始气泡直径越大,但对轴向气泡直径分布的影响仅限于分布器区.包含分布器影响的气泡直径经验关联式为d/D=140.2Bo-0.5Ga-0.12Fr0.099(h/D)-0.15T-0.34(0.5 cm/s＜ug＜7 cm/s).     

溶气式泡载分离器内减压成泡过程

通过对压力饱和溶气水减压成泡过程的热力学分析表明,溶气压力越高,形成气泡越细密,释气较完全。采用Kolmogoroff各向同性湍流理论得出湍流场中气泡稳定存在的条件和气泡大小随操作条件的变化规律。实验考察了操作条件对气泡分布的影响,采用Gamma分布能较好地描述实验。     

下喷自吸环流反应器内局部气泡平均直径及其径向分布

引　言喷射环流反应器因具有规整的流型、良好的分散效果、相对低的功耗及较好的相间传递特性等优点 ,而广泛应用于化学工业、生物及环境化工领域 ,如发酵、废水和废气处理等 ,以及化学工业中众多的多相反应[1,2 ].鉴于下喷环流反应器和自吸各自的特点 ,文献 [3]开发出了新型高效反应器———下喷自吸环流反应器 ,并对该反应器性能进行了系统的研究 .开展下喷自吸环流反应器内局部气泡大小及其分布的研究 ,不仅对了解该反应器内混合、相间作用和相间耦合的微观流动与传质机理及其传递特性具有极其重要的意义 ,而且也为此类反应器的工业放大…     

顶部浸没竖直向下平口管口气泡膨胀脱离特性

利用高速摄像机拍摄顶部浸没竖直向下管口气泡膨胀及脱离演变过程,对比分析管径、平均气速对气泡尺寸、形成时间的影响规律。引入泡龄k表征管口气泡位置特性,用椭球形气泡长短轴比L表征气泡形状特征。研究发现,气泡生成机理有间歇生成气泡、含抽吸间歇生成气泡和连续生成气泡3种;气泡脱离直径随平均气速变化过程明显存在气泡脱离形态转变点;气泡膨胀脱离模式随平均气速变化分为单个气泡形成、气泡聚并形成两种。平均气速为3.1 m·s^-1时管口处存在抽吸现象,此时最大泡龄k_max最小;脱离阶段气泡长短轴比L随脱离时间变化过程明显存在时间分界点。在分界点前,L值在2.0附近波动,气泡主要以椭球形存在。分界点后,L值明显增加,气泡形状向扁椭球体发展。     

On the transition stage of bubble formation on the orifice of a submerged vertical nozzle

The transition stage (self‐accelerating necking process) generally takes place before a bubble collapses. However, the mechanism is still not well understood. In this article, seven existing experiments dealing with bubble formation on the orifice of a submerged vertical nozzle are examined by solving the Young–Laplace equation. Multiple solution modes are found. Bubble in solution mode 2 has a neck and thus taller than mode 1 at the same volume. The present numerical result along with an experiment from Longuet‐Higgins et al. [J. Fluid Mech. 230, 365–390 (1991)] evidences an isometric transition from solution modes 1 to 2. This might account for the self‐accelerating necking process before a bubble collapses. Surprisingly, all of the seven existing experiments agree excellently with the bubble shapes from the Young–Laplace equation without the dynamic effect even when the bubble growth rate in the experiment is 2.25 times as large as the critical value. The gas flow rate (the dynamic effect) seems to play a role only after the transition stage. © 2011 Canadian Society for Chemical Engineering     

A unified theoretical model for breakup of bubbles and droplets in turbulent flows

Pressure has a significant effect on bubble breakup, and bubbles and droplets have very different breakup behaviors. This work aimed to propose a unified breakup model for both bubbles and droplets including the effect of pressure. A mechanism analysis was made on the internal flow through the bubble/droplet neck in the breakup process, and a mathematical model was obtained based on the Young–Laplace and Bernoulli equations. The internal flow behavior strongly depended on the pressure or gas density, and based on this mechanism, a unified breakup model was proposed for both bubbles and droplets. For the first time, this unified breakup model gave good predictions of both the effect of pressure or gas density on the bubble breakup rate and the different daughter size distributions of bubbles and droplets. The effect of the mother bubble/droplet diameter, turbulent energy dissipation rate and surface tension on the breakup rate, and daughter bubble/droplet size distribution was discussed. This bubble breakup model can be further used in a population balance model (PBM) to study the effect of pressure on the bubble size distribution and in a computational fluid dynamics‐population balance model (CFD‐PBM) coupled model to study the hydrodynamic behaviors of a bubble column at elevated pressures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1391–1403, 2015     

鼓泡塔中水合物法分离混合气体的数值模拟

To develop a new technique for separating gas mixtures via hydrate formation,a set of medium-sized experimental bubble column reactor equipment was constructed.On the basis of the structure parameters of the ex- perimental bubble column reactor,assuming that the liquid phase was in the axial dispersion regime and the gas phase was in the plug flow regime,in the presence of hydrate promoter tetrahydrofuran（THF）,the rate of hydrogen enrichment for CH4＋H2 gas mixtures at different operational conditions（such as temperature,pressure,concentra- tion of gas components,gas flow rate,liquid flow rate）were simulated.The heat product of the hydrate reaction and its axial distribution under different operational conditions were also calculated.The results would be helpful not only to setting and optimizing operation conditions and design of multi-refrigeration equipment,but also to hydrate separation technique industrialization.