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.     

气液鼓泡床泡罩式气体分布器流场的CFD模拟

采用ANSYS FLUENT商用CFD软件对气液鼓泡床内泡罩式气体分布器单体进行了数值模拟。以Ug=12.34m/s的入口速度对计算模型进行单相三维稳态模拟。研究了泡罩内径、泡罩齿片和入口管开孔分别对塔内不均度和泡罩式气体分布器的压降影响规律。研究表明,随着泡罩内径的增加,塔内不均度和分布器压降减小;泡罩齿片加宽,齿片对气体的阻力增加;齿片间距增加,齿片对气体的阻力减小;入口管开孔数对塔内不均度没有任何影响。开孔数很少时,开孔数量的增加使分布器的压降减小,然而开孔数增加到一定数量时,开孔数对分布器的压降影响甚微。对泡罩式气体分布器进行了结构优化。     

Water electrolysis and pressure drop behaviour in a three-dimensional electrode

Water electrolysis from acidified solution was used as a model system to investigate the net contribution of hydrogen bubbles to the pressure drop increase in a three-dimensional electrode. A bed of silvered glass beads in both fixed and fluidized state was used, assuming an unchanging particle surface during the experiments. Pressure drop behaviour with time was measured for different experimental conditions and presented relative to the pressure drop determined for a bubble free bed. Parameters, such as current density, electrolyte velocity and particle size, greatly influence the relative pressure drop behaviour in the three-dimensional electrode. A sudden increase in the pressure drop occurs with the appearance of a gas phase in the bed, reaching a constant value (plateau) after a certain time; this plateau corresponds to steady state conditions. The pressure drop increases with increasing current density. This increase is in the range 40-150% relative to the bubble free electrolyte flow through the bed. Electrolyte flow-rate also strongly influences the pressure drop in the hydrogen evolving fixed bed electrode. It was observed that the relative pressure drop decreases with increasing electrolyte velocity. At higher flow rates, peaks occur on the pressure drop-time curves, indicating the existence of channeling inside the bed in which spouting occurs. The time to reach the pressure drop plateau decreases with increasing electrolyte velocity as do the time intervals corresponding to maximum pressure drop values. At the minimum fluidization velocity the peaks disappear and the relative pressure drop decreases with time, tending to approach a constant value. For hydrogen evolution in the fluidized bed, the pressure drop is lower than that measured in the absence of gas, and reason for this decrease being the gas hold-up in the bed.     

多微通道内两相流动阻力特性及气泡行为

采用去离子水作为实验工质,对14条水力直径为187.5 μm的平行矩形微通道内两相流动阻力特性及气泡行为进行可视化实验。研究发现,当质量流速一定时,随着热通量增大,汽相逐渐增多,平行多微通道内压降相应增加;当热通量一定时,随着质量流速减小,压降出现先减小后增大的趋势,该曲线中的最低点被认为是静态流动不稳定性起始点（OFI点）。结合可视化图像并利用气泡动力学理论对4种不同的汽相行为特征进行分析,发现汽核受限、倒流等堵塞流道现象的出现导致系统阻力增大,压降增加。通过压降实验值与Kim & Mudawar关系式在不同质量流速下进行比较发现,Kim & Mudawar关系式仅能对本实验压降变化趋势进行预估,而无法对压降数值进行有效预测。     

Water electrolysis under microgravity: Part 1. Experimental technique

Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H₂SO₄) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from –0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current–potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte.     

Water electrolysis under microgravity: Part II. Description of gas bubble evolution phenomena

Water electrolysis is carried out in both alkaline (25 and 2 wt.% KOH) and acidic (0.1N H₂SO₄) solutions for 8 s under a microgravity (μ-G) environment realized in a drop shaft. The effects of gravitational strength on gas bubble evolution behavior are analyzed in consideration of various factors (bubble movement, bubble assembly and single bubble). Under a μ-G environment, a collection of fine gas bubbles forms a froth layer in alkaline solutions, whereas bubbles frequently coalesce in acidic solution. Moreover, H₂ gas bubbles in alkaline jump from a cathode surface and O₂ bubbles often coalesce on an anode. In acidic solution both H₂ and O₂ bubbles frequently coalesce on electrode surfaces. Such gas bubble movements are reflected in the coalescence number and bubble residence time. A single bubble is characterized by the bubble size and the dynamic contact angle between a gas bubble and a Pt electrode, however, these factors are not essentially influenced by the gravitational strength.     

微通道壁面浸润性对气-液两相流的影响规律研究

通道壁面浸润性对微通道内的气-液两相流具有重要影响。利用等离子体辅助接枝改性,将甲基丙烯酰乙基磺基甜菜碱（SBMA）及1H, 1H, 2H, 2H-全氟癸基三乙氧基硅烷接枝在聚甲基丙烯酸甲酯（PMMA）材料表面,得到了10°、40°、70°和110°四种接触角的微通道,并考察了浸润性对流型、气泡长度和压降的影响。结果表明,随接触角增大,气泡截断位置下移,膨胀阶段缩短,挤压阶段变长;低流量时,气泡长度随接触角增加而增大,高流量时则减小;建立了与材料表面水接触角相关的气泡尺寸预测关联式,与Garstecki经典预测关联式相比,预测精度更高;θ<90°时,接触角增加,压降减小;θ>90°时,三相接触线使流动阻力和压降增加。     

Prediction of pressure drop in a modified gas-liquid downflow bubble column

A model for pressure drop in the ejector induced downflow bubble column based on mechanical energy balance within the framework of dynamic interaction of the phases has been formulated. The model includes the effect of bubble formation and form drag at interface on the pressure drop. It provides a functional form of equation for correlating pressure drop. The theoretical model proposed in the present study appears to predict the pressure drop satisfactorily for gas-liquid dispersed flow in the concurrent gas-liquid downflow bubble column.     

Pressure drop in monolith reactors

A theoretical model for the computation of pressure drop in bubble-train flow inside capillaries of square cross-section was developed. The model is based on three contributions: hydrostatics, viscous pressure drop, and capillary pressure drop. Capillary pressure drop is related to the shape of the fronts and ends of the bubbles. The model does not include entrance or exit effects, has no adjustable parameters, and agrees very well with available experimental data.

For a given set of flow parameters, bubble velocity and liquid slug average velocity are computed as a function of gas and liquid superficial velocities. The length of the unit cell determines the number of bubbles inside the capillary for a given flow situation. The model requires experimental information of average bubble lengths to compute the length of a unit cell consisting of a bubble and a liquid slug.

The three pressure contributions for a unit capillary length are linear functions of the number of bubbles inside the capillary. The length of the bubbles in bubble-train flows is a critical parameter in the computation of pressure drop.     

The Effect of Drop (Bubble) Size on Contact Angle at Solid Surfaces

Examples of experimental contact angle data for varying drop and bubble volumes on different solids whose surfaces are smooth and homogeneous, rough and homogeneous, smooth and heterogeneous, and covered with unstable organic films are presented. The ideas and theoretical models as proposed in the literature for the interpretation of contact angle/drop (bubble) size relationships are critically reviewed. It is shown that major factors affecting the contact angle variation with drop (bubble) size such as surface heterogeneity, roughness, and stability, have been identified in the literature. However, there is still a need for experimental work with well-defined and well-characterized solid surfaces. Theoretical models that have been proposed in the literature are still inadequate. Advanced modeling of liquid behavior at heterogeneous and rough surfaces is required to understand further, and to predict, the contact angle/drop (bubble) size relationships at imperfect surfaces.     

Water electrolysis under microgravity: Part 1. Experimental technique

Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H₂SO₄) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from -0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current-potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte.     

The Effect of Drop (Bubble) Size on Contact Angle at Solid Surfaces

Examples of experimental contact angle data for varying drop and bubble volumes on different solids whose surfaces are smooth and homogeneous, rough and homogeneous, smooth and heterogeneous, and covered with unstable organic films are presented. The ideas and theoretical models as proposed in the literature for the interpretation of contact angle/drop (bubble) size relationships are critically reviewed. It is shown that major factors affecting the contact angle variation with drop (bubble) size such as surface heterogeneity, roughness, and stability, have been identified in the literature. However, there is still a need for experimental work with well-defined and well-characterized solid surfaces. Theoretical models that have been proposed in the literature are still inadequate. Advanced modeling of liquid behavior at heterogeneous and rough surfaces is required to understand further, and to predict, the contact angle/drop (bubble) size relationships at imperfect surfaces.     

Liquid dispersion,gas holdup and frictional pressure drop in a packed bubble column at elevated pressures

The gas holdup, frictional pressure drop and liquid dispersion have been investigated in a packed bubble column at elevated pressures for the air–water system. The bubble column, which had an internal diameter of 0.15 m and which was packed with 15 mm plastic Pall rings was operated in the semibatch mode. The operating pressures ranged from 0.1 to 0.66 MPa. It was found that increasing the pressure increases both the gas holdup and the dispersion coefficient. In contradiction to the results obtained from packed bubble columns fed with a continuous net flow of liquid, a maximum point of the frictional pressure drop was observed at the transition point between bubble and pulse flow region.     

Flow regimes in a gas–liquid–solid three-phase moving bed

The gas–liquid–solid three-phase moving beds could supply a potential solution for multiphase reactions with catalyst easily deactivated, and the flow regimes in it were studied by optical method and pressure drop measurement. Results showed that taking the trickle flow as the initial flow regime, the flow channels were more obvious as the particle velocity increased. When the initial flow regimes were pulse flow and bubble flow respectively, the pulse-to-trickle and bubble-to-pulse flow transitions mainly occurred at moderate-to-high particle velocities (0.01–0.04 m s⁻¹ under conditions used in this work). Moreover, the flow regime map in the three-phase moving bed was constructed and shown that the region of trickle flow increased and the region of bubble flow decreased. Finally, the application of three-phase moving beds was discussed, and it could be suitable for those reactions, which had to operate in the pulse flow, bubble flow, and transition zone.     

Dispersed oil–water–gas flow through a horizontal pipe

An experimental study of three‐phase dispersed flow in a horizontal pipe has been carried out. The pressure drop over the pipe strongly increases with increasing bubble and drop volume fraction. Because of the presence of drops the transition from dispersed bubble flow to elongated bubble flow occurs at a lower gas volume fraction. The gas bubbles have no significant influence on the phase inversion process. However, phase inversion has a strong effect on the gas bubbles. Just before inversion large bubbles are present and the flow pattern is elongated bubble flow. During the inversion process the bubbles break‐up quickly and as the dispersed drop volume fraction after inversion is much lower than before inversion, a dispersed bubble flow is present after inversion. (When inversion is postponed to high dispersed phase fractions, the volume fraction of the dispersed phase can be as high as 0.9 before inversion and as low as 0.1 after inversion.) © 2009 American Institute of Chemical Engineers AIChE J, 2009     

微通道分流弹状流的界面过程及压力演变规律

弹状流分流不仅能调控两相流流型从而强化传热,同时也是生物化工、制药行业的传统过程。针对壁面微通道分流液相、调控两相流型的过程进行数值模拟,获得局部参数变化规律,是获得两相流流型演变机理的基础。采用VOF模型耦合动态自适应网格精准追踪气液界面,模拟气液界面在分液口的界面运动;获得轴向及壁面静压、动压的演变规律。通过模拟可知微通道分流弹状流的关键是气弹在分液口的类活塞运动;同时由于界面拉普拉斯压力差的存在,弹状流压降具有不连续性;且此不连续压力随气弹在分液口的类活塞运动具有周期波动性。而弹状流液桥部分的局部压降是影响总压降的关键;近气弹头部的液相区压降显著,近气弹尾部的液相区域由于液速降低其压降明显衰弱;此为弹状流有别于其他两相流流型的压降特点。     

Gas hold-up in non-newtonian solutions in a bubble column with internals

Bubble characteristics and gas hold-up were studied in a two phase (air-aqueous CMC solution) bubble column provided with helical coils and straight tubes as internals. The effects of superficial gas velocity, rheological properties, and volume fraction covered by the internals, on gas hold-up were studied. Hold-up values determined directly and by simultaneous pressure drop measurements matched well. Enhancement of gas hold-up values up to 55 per cent was achieved in systems using internals. The gas hold-up results were also compared with the values obtained from correlations reported in the literature.     

静态混合器气液两相流压降的数值模拟及实验研究

气泡流经过静态混合器的压降问题涉及到舰船动力系统性能的好坏。在分析了气泡平均驰豫时间的基础上,结合k ε湍流模型,利用平衡流模型模拟静态混合器内形成的稀疏湍流气泡流,得到了混合器内两相流流场。空气—水系统的压力降实验表明,计算流体力学模拟能够很好地预测SMV型静态混合器内气泡流的压降,是静态混合器设计的有效手段。     

对称分支并行微通道中气液两相流的均匀性规律

采用高速摄像系统研究了对称分支形并行微通道内气液两相流及弹状气泡均匀性规律。实验中分别采用含0.3% SDS的甘油-水溶液与氮气作为液相和气相。观察到弹状流和泡状流两种流型,作出了由两相操作条件构成的流型图及流型转变线。结果表明,气泡非均匀性主要由两微通道内流体之间的相互作用、下游通道中流体动力学的反馈作用以及通道制造误差造成。随液相黏度增大,气泡均匀性变好;在高液相流量以及低气相压力下操作,气泡尺寸分布更易达到均匀。基于压力降守恒原理和微通道内气液两相流阻力模型,构建了两通道中气泡尺寸的预测模型。