剪切稠化流体内气泡上浮运动特性

剪切稠化流体是一种典型的非牛顿流体,研究气泡在其中的运动特性对优化设备结构、提高反应效率具有重要意义。文中采用流体体积(VOF)法,通过改变Gallilei数(Ga)、E?tv?s数(Eo)与流变指数(n),对牛顿流体(n=1)及剪切稠化流体(n>1)内气泡的形状、尾涡、终端速度和气泡周围液相黏度分布的变化进行了深入的数值研究。结果表明:气泡变形程度和尾涡尺寸随着Ga数或Eo数的增大而增加;剪切稠化效应会阻碍尾涡的形成,减小气泡的尾涡尺寸;气泡周围剪切速率的差异会导致气泡上方及尾部产生高黏度区域,该高黏度区域会随剪切稠化效应的增加而增大;气泡终端速度随Ga数的增大或流变指数n,Eo数的减小而增大。     

旋转圆环内分散气泡对液相表观黏度的影响

泡状液体广泛存在于化工、食品和生物医学等领域,深刻理解气泡对液相流变特性的影响对于产品质量改进和过程强化具有重要意义。文中以旋转流变仪作为物理模型,基于VOF和动网格相结合的方法,详细研究了分散气泡对液相表观黏度的影响。目前的研究发现:在相同气泡体积分数下,当毛细数较小时,由于表面张力,气泡几乎保持为球形,气泡的加入使流线扭曲,从而导致液相的相对黏度增大(即η_r1);然而,当毛细数较大时,气泡由圆形变为细长的椭圆形,并且沿最大法线方向气泡所占比例增加,气泡的加入使液相的相对黏度减小(即η_r1);而且对于相同毛细数的工况,体积分数越大,气泡对液相表观黏度的影响也越大。     

二维平行平板内分散气泡对液相表观黏度影响的数值研究

泡状液体广泛存在于各种工业领域,因此深入了解泡状液体的物理特性对产品质量改进和过程强化具有重要意义。本文为了理解气泡对液相表观黏度的影响,揭示液相表观黏度变化的原因,利用流体体积函数（VOF）与动网格结合的方法对二维平行板模型内气泡对液相表观黏度的影响进行了深入的数值研究;分别研究了相同体积分数、不同毛细数工况下,气泡加入对液相表观黏度的影响。结果表明：毛细数和气泡体积分数对液相的表观黏度具有巨大的影响作用;相同的体积分数下,当毛细数较大（Ca>1）时,气泡的加入会使液相的相对黏度减小,体积分数越大,液相的相对黏度减小得越严重;另外,相同的体积分数下,当毛细数较小（Ca<1）时,气泡的加入会使液相的相对黏度增大,体积分数越高,液相的相对黏度增大得越明显。     

壁面润湿性对微通道内Taylor流动特性的影响

壁面润湿性不仅影响着Taylor气泡的形状,同时对通道内流体流动、相变换热等有着关键的作用。采用VOF模型对T型微通道内气液两相Taylor流动进行三维数值模拟,重点研究了接触角改变对Taylor气泡流体动力学特性的影响。模拟结果与他人实验数据对比基本吻合,验证了模型的有效性。结果表明：随着接触角增大,气泡周围液含量逐渐降低,相界面也由外凸形变为内凹形。壁面越接近润湿（或疏水）状态,气液接触面的曲率就越大;当120°≤θ≤150°时Taylor气泡稳定性变差。当θ≥150°时“拖曳流态”出现,分析指出在大接触角下气体更易贴附壁面导致接触区内流场发生变化,形成的涡流减弱了水对气相的水平剪切作用,进而引起流型转变。接触角对通道内压力有着重要影响,通道中心轴向压力曲线以θ=90°为过渡,润湿状态下呈凸函数递减且p _G>p _L,疏水状态下气液进口处的压力分配改变,曲线趋势相反。     

高填充固体推进剂模拟物加工流变行为的实验研究

针对高填充固体推进剂模拟物,围绕其在稳态剪切及动态剪切模式下的流变特性,分别采用出口增压式毛细管流变仪及旋转流变仪,在100~120 ℃及50~100 ℃下考察了压力与剪切振荡幅度对物料剪切黏度（η）的影响,计算了熔体压力对η的敏感性参数并提出了针对该类物料真实η的有效修正方法及低剪切法向力下的测试方法。稳态剪切测试结果表明,熔体压力介于12~121 MPa之间时,其压力敏感性参数介于4.81~9.65 GPa^-1发生变化,相同温度及剪切速率下熔体η随压力表现出幂指数的递增变化规律;动态剪切测试表明,即使在不高于50 N的熔体剪切法向力作用下（法向力15~42 N）,高填充模拟物η在0.01~100 s^-1内也会表现出大约2倍的差异;对于表现出强类固体特征的高填充模拟物复杂体系,动态剪切测试中的剪切应变振幅对η的测试结果影响明显,尤其在低温大振幅剪切情况下,高填充物料与剪切平板之间还易发生“打滑”。     

剪切稀化流体中微粒子聚集行为研究

在化工过程、生物工程等领域中,实现颗粒分离至关重要。通过整合微流控技术、高速显微图像采集技术和数字图像处理技术,探究微粒尺寸、通道流量和液相流变特性对微粒聚集的影响规律。结果表明,在剪切稀化流体羧甲基纤维素钠(CMC)水溶液中,随着通道流量和颗粒粒径增大,微颗粒聚集位置逐渐向剪切速率较高的一侧偏移;随着CMC质量分数增加,聚集位置不断向剪切速率较低的一侧偏移。利用求解的幂律型剪切稀化流体速度和剪切速率方程,结合对微颗粒的受力分析,证明稠度系数越大,聚集位置越偏向剪切速率较低处;剪切稀化特性越强,聚集位置越偏向剪切速率较高处,说明在幂律型剪切稀化流体中的黏度变化是微颗粒发生特殊迁移聚集行为的一个重要原因。     

温度和剪切协同作用对十六烷基三甲基氯化铵溶液表观黏度的 影响

深入研究表面活性剂的流变特性，对于理解湍流减阻机理和改进制药、化工等领域的产品质量具有重要意义。本文研究了相同升温速率、不同剪切速率以及相同剪切速率、不同升温速率对阳离子表面活性剂十六烷基三甲基氯化铵（CTAC）溶液表观黏度的影响。研究表明：当温度较低时，剪切速率对表面活性剂溶液表观黏度的影响起主导作用；随着温度的升高，高温对胶束的影响增强导致溶液在高温处出现剪切稀化现象。当浓度为0.3125mmol/L时，溶液临界温度随剪切速率的升高保持恒定；但当浓度升高至0.6250～1.2500mmol/L时，在高温和高剪切的作用下，黏度曲线出现“平台”和短暂的增稠现象。对于中等浓度表面活性剂溶液，浓度升高导致蠕虫状胶束出现分支并抑制了“平台”的产生；随着升温速率的增大，胶束结构的响应时间滞后于升温速率，但当γ=150s^?1时，滞后效应减弱，高剪切对胶束结构的破坏占主导作用。     

温度和激发剂浓度对碱激发矿渣净浆流变特性的影响

为了获得温度和激发剂浓度对碱激发矿渣净浆流变特性的影响,以NaOH为激发剂,S95粒化高炉矿渣为被激发组分,探讨了不同激发剂浓度(1 mol/L、2 mol/L和3 mol/L)和温度(20 ℃、30 ℃和40 ℃)下矿渣净浆在凝结硬化过程中屈服应力和表观黏度的变化特征。结果表明:以NaOH为激发剂的矿渣净浆表现出非牛顿体特征,屈服应力的发展符合Bingham流体模型。激发剂浓度和温度越高,碱激发矿渣净浆的屈服应力发展越快,并且屈服应力平稳期和增长期时长均随激发剂浓度和的提高而缩短;表观黏度时变模型可通过η=η₀+e^-btⁿ表示,且初始表观黏度η₀、b和n值均随温度和激发剂浓度的提高而提高。     

质子交换膜燃料电池微孔层浆液微观结构与流变性

微孔层（MPL）是质子交换膜燃料电池（PEMFC）水管理的重要部件。本文系统性选择了4种典型炭黑[ACET、XC-72R、BP2000、天然气裂解炭黑（NG）]，借助原子力显微镜（AFM）、场发射透射电镜（TEM）和动态光散射粒度分析仪（DLS PSD）对浆液微观结构的研究，给出团聚性这一浆液评价标准，并结合流变性揭示了定量标准。研究结果表明，不同炭黑的团聚程度大小依次为ACET>XC-72R>BP2000>NG。剪切稀化指数n越大，MPL浆液团聚程度越低，颗粒分散越均匀；n越小，团聚程度越高。具体而言，固含量α=6%时，添加NG、BP2000、XC-72R、ACET炭黑的MPL浆液的剪切稀化指数n分别为0.554、0.320、0.118、0.039；测得的ACET、XC-72R、BP2000、NG的团聚体粒径分别为1.358μm、1.149μm、0.732μm、0.406μm，所以浆液的剪切稀化指数与其团聚程度成反比。总之，NG-MPL剪切稀化程度最低，NG更加适合作为MPL的碳材料，也为今后MPL的制备和优化提供指导。     

Lightnin静态混合器内气泡分散流体动力学特性实验研究

以Lightnin静态混合器（LSM）内水-空气气液两相体系为研究对象,在连续相水表观速度U_L=0.071~0.127 m/s和离散相空气表观速度U_G=0.007~0.042 m/s的条件下,研究内径100 mm的LSM内气液两相湍流流动阻力与气泡分散水动力学行为。使用分辨率为1920×1080的高速相机Revealer-2F04M采集混合器内不同轴向窗口的气泡群演化过程。结果表明：当U_L<0.085 m/s和U_G=0.025~0.042 m/s时,LSM内的流型为泡状流。随着气泡群流经混合元件数的增加,气泡群的Sauter平均直径d₃₂逐渐减小。当液体表观速度U_L≤0.085 m/s时,Sauter平均直径d₃₂随气体表观速度的增加先减小后增大;U_G =0.028 m/s时d₃₂达到局部最小值,53%的气泡直径d_B/D₀在0.02~0.05范围内。Sauter平均直径、内径与无量纲停留时间τ之间的关系满足d₃₂/D₀=0.031τ^-0.14We^-0.41。平均气含率α的增大显著增加了单位体积内气泡数量密度,加剧气泡与元件表面碰撞频率,增大旋涡二次流强度,导致摩擦系数显著降低;采用Lockhart-Martinelli方法对实验数据回归,得到气液两相流压降预测常数C的关联式：C=5.26×10⁵U_G^-0.91/Re^0.74。     

Numerical study on hydrodynamic characteristics of spherical bubble contaminated by surfactants under higher Reynolds numbers

It is of significance to investigate deeply the hydrodynamic features of the bubble contaminated by impurities in view of the fact that the industrial liquid is difficult to keep absolutely pure. On the basis of the finite volume method, the bubble interface contaminated by the surfactant (1-pentanol) is achieved through solving the concentration transport equations in liquid and along the bubble interface, and solving the absorption and desorption equation at the bubble interface. And the three-dimensional momentum equation is solved at the same time. It is investigated in detail on the influence of interfacial contamination degrees (described with the cap angle θ) on hydrodynamic characteristics of the spherical bubble when the bubble Reynolds number (Re) is larger than 200. The θ is realized by changing the surfactant concentration (C₀) in liquid. The present results show that the hydrodynamic characteristics, such as interfacial concentration, interfacial shear stress, interfacial velocity and wake flow, are related to both Re and C₀ for the contaminated bubble. When C₀ is relatively low in liquid (i.e., the contamination degree of the bubble interface is relatively slight), the hydrodynamic characteristics of the bubble can still keep the 2D features even if Re > 200. The decrease of θ or the increase of Re can promote the appearance of the unsteady wake flow. For the present investigation, when Re > 200 and θ ≤ 60°, the hydrodynamic characteristics of the bubble show the 3D phenomena, which indicates that axisymmetric model is no longer valid.     

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.     

Hydrodynamics of air–kerosene bubble column under elevated pressure in homogeneous flow regime

A multiphase computational fluid dynamics(CFD) model coupled with the population balance equation(PBE) was developed in a homogeneous air–kerosene bubble column under elevated pressure(P). The specific pressure drop(DP/L), gas holdup(a_G), and Sauter mean diameter(d_(32)) were experimentally measured in the bubble column with 1.8 m height and 0.1 m inner diameter, which was operated at a superficial gas velocity of 12.3 mm·s~(-1), and P = 1–35 bar(1 bar = 10~5 Pa). A modified drag coefficient model was proposed to consider the effect of bubble swarm and pressure on hydrodynamics of the bubble column.The Luo breakage model was modified to account for liquid density, viscosity, surface tension and gas density. The DP/L, a_G, and d_(32) obtained from the CFD model were compared with experimental data,and the gas density-dependent parameters of the CFD model were identified. With increasing P from 1 to 35 bar, the aGvaried from 5.4% to 7.2% and the d_(32) decreased from 2.3 to 1.5 mm. The CFD-PBE model is applicable to predict hydrodynamics of pressurized bubble columns for gas–organic liquid in the homogeneous regime.     

SEBS高黏度溶液气液鼓泡塔的流体力学研究

针对SBS加氢反应器开发与设计，以SEBS-1650己烷溶液为液相，采用差压法和床层塌落法研究了气液鼓泡塔中高黏度溶液的流体力学行为，考察了黏度对低表面张力溶液的气含率、大小气泡气含率、大小气泡上升速度和比表面积等因素的影响。结果表明，随黏度增加，大气泡增多，气含率明显降低，塔内流型处于湍流区；由床层塌落曲线确定鼓泡塔内存在三种类型的气泡：大气泡、小气泡及细小气泡，随黏度增加，小气泡与细小气泡逐渐减少；黏度对大小气泡的上升速度略有影响，比表面积随黏度增加而明显降低。根据实验结果给出了大小气泡气含率与平均气含率的计算公式。     

Y型微通道内纳米颗粒稳定气泡的完全阻塞破裂动力学

研究了Y型微通道吸附型纳米颗粒稳定气泡的完全阻塞破裂的动力学,破裂过程可划分为挤压阶段和快速夹断阶段,两阶段内无量纲气泡最小颈部宽度与时间均呈幂率关系。气泡破裂过程的颈部动力学表明颗粒的存在并不影响两阶段转变的临界颈部宽度,但吸附在气泡表面的颗粒层会减弱挤压阶段中连续相对气泡颈部的挤压作用,以及快速夹断阶段角区中连续相液体回流对气泡的挤压作用,进而阻碍气泡颈部的形变,延长了气泡的破裂过程。纳米颗粒稳定的气泡的指前因子m及幂率指数α均小于常规气泡,但其差值随着毛细管数Ca和气泡长度l₀的增大而减小,颗粒对气泡破裂过程的影响逐渐减弱。此外,纳米颗粒稳定的气泡的头部曲率略小于常规气泡,颗粒对完全阻塞破裂过程气泡头部动力学的影响可以忽略。     

柴油液滴内空化气泡的破碎研究(英文)

A modified mathematical model is used to study the effects of various forces on the stability of cavitation bubbles within a diesel droplet. The principal finding of the work is that viscous forces of fluids stabilize the cavitation bubble, while inertial force destabilizes the cavitation bubble. The droplet viscosity plays a dominant role on the stability of cavitation bubbles compared with that of air and bubble. Bubble–droplet radius ratio is a key factor to control the bubble stability, especially in the high radius ratio range. Internal hydrodynamic and surface tension forces are found to stabilize the cavitation bubble, while bubble stability has little relationship with the external hydrodynamic force. Inertia makes bubble breakup easily, however, the breakup time is only slightly changed when bubble growth speed reaches a certain value (50 m·s?1). In contrast, viscous force makes bubble hard to break. With the increasing initial bubble–droplet radius ratio, the bubble growth rate increases, the bubble breakup radius decreases, and the bubble breakup time becomes shorter.     

黏弹性表面活性剂溶液中颗粒沉降特性研究

黏弹性表面活性剂溶液悬浮颗粒流广泛存在于自然界和工业生产中,黏弹性表面活性剂溶液的非线性流变性质及应力松弛效应对其中颗粒沉降有着显著影响。采用FENE-P和Giesekus黏弹性本构模型对表面活性剂溶液中颗粒沉降特性进行研究,发现两种本构模型不仅表现出剪切稀化,而且出现拉伸硬化。颗粒在沉降初期的不稳定性主要是由溶液自身的弹性效应引起,弹性效应越强,颗粒沉降速度不稳定性越强,而剪切稀化效应会减弱颗粒沉降速度的不稳定。颗粒沉降过程中在其尾部形成一个“负尾迹”,随着剪切稀化和拉伸硬化效应增强,负尾迹区增大,弹性效应增加,负尾迹增强,负尾迹区流体内部反向速度分布导致的表面活性剂溶液中微观胶束的拉伸断裂和重构可能是引起颗粒沉降速度持续波动的原因。     

低电导率工质中气泡的极化运动实验研究

为探讨电场作用下气泡在低电导率工质中的极化运动特性,采用高速数码摄像技术对气泡在正庚烷溶液中的生长和分散过程进行了可视化研究,并结合无量纲数分析了不同气体流量和施加电压下的气泡演变特征以及极化力主导的气泡运动规律。结果表明,增大电场强度可导致气泡生长周期缩短,气泡尺寸显著减小,产生频率加快。在低电场强度下,气泡运动主要表现为流体动力学特性;而在强电场作用下,气泡首先受极化力主导而表现为电流体动力学特性,其直线轨迹高度随Bo_E增大而增大。但随着电场强度在竖直方向上的衰减以及液相阻力影响,气泡运动速度不断减小;当气泡脱离极化力主导区域后,其运动再次表现为流体动力学特性,受尾迹诱导和气泡间相互作用影响,气泡在竖直方向上沿毛细管轴向四周扩散。     

Bubble Behavior of a Large‐Scale Bubble Column with Elevated Pressure

Gas holdups and the rising velocity of large and small bubbles are measured using the dynamic gas disengagement approach in a pressured bubble column of 0.3 m in diameter and 6.6 m in height. The effects of superficial gas velocity, liquid surface tension, liquid viscosity, and system pressure on the gas holdups and the rising velocity of small and large bubbles are investigated. The holdup of large bubbles and the rising velocity of small bubbles increase with increasing liquid viscosity and liquid surface tension. Meanwhile, the holdup of small bubbles and the rising velocity of a swarm of large bubbles decrease. Moreover, the holdup of large bubbles and the rising velocity of a swarm of small bubbles decrease with increasing system pressure. A correlation for the holdup of small bubbles is obtained from experimental data.