气泡夹带对壁面油膜流动特性的影响

油-气润滑系统工作过程中,润滑油膜受微油滴冲击和压缩空气扰动影响易形成气泡夹带现象,气泡夹带行为将对壁面润滑油膜层的形成及流动过程产生重要影响。基于VOF数值模拟方法,对含气泡油膜沿倾斜壁面的流动行为进行研究,考察了气泡的存在对油膜形态和流动速度的影响规律,以及气泡破裂阶段空腔邻域内流体压力变化特性。研究表明,油膜夹带气泡的形变和迁移诱发气泡周围微流场的速度扰动现象,导致气液界面处产生非均匀速度梯度分布,进而引发油膜表面的形态波动。气泡发生破裂时,油膜空穴部位发生明显的正负压力波动现象,气泡附近壁面将承受一定的交变载荷作用。     

Robust numerical analysis of the dynamic bubble formation process in a viscous liquid

The dynamics of bubble formation from a submerged nozzle in a highly viscous liquid with relatively fast inflow gas velocity is studied numerically. The numerical simulations are carried out using a sharp interface coupled level set/volume-of-fluid (CLSVOF) method and the governing equations are solved through a hydrodynamic scheme with formal second-order accuracy. Numerical results agree well with experimental results and it is shown that the sharp interface CLSVOF method enables one to reproduce the bubble formation process for a wide range of inflow gas velocities. From numerical results, one can improve their understanding of the mechanisms regarding the dynamics of bubble formation. For example, it is found that for some sets of parameters that the bubble formation process reaches steady state after several bubbles are released from the nozzle. At steady state, bubbles uniformly rise freely in the viscous liquid. It is observed that the fluid flow around a formed bubble has a significant role in determining the overall dynamic process of bubble formation; e.g. the effect of the fluid flow from the preceding bubble can be seen on newly formed bubbles.     

Forced oscillations of two gas bubbles in a fluid in the vicinity of bubble contact

For a theoretical derivation of bubble coalescence conditions, nonlinear forced oscillations of two closely spaced spherical bubbles subjected to the action of a periodic external pressure field are considered. The equations, asymptotic with respect to a small distance between the bubble surfaces, are derived to describe the approach of the bubbles under the action of (i) the Bjerknes attraction force averaged over the oscillation period and (ii) the viscous drag. It is shown that due to nonlinear interaction of the viscous drag with the radial and translational oscillations of the bubbles a unidirectional repulsive force is generated, which prevents the approach of the bubbles. The coalescence of the bubbles is possible when the nondimensional parameter combined from the amplitude and frequency of the external pressure field, the bubble radius, and the fluid viscosity is greater than a certain critical value. The obtained coalescence condition is qualitatively confirmed by experiments.     

Multivelocity effects in high-pressure-gradient flows of dilute bubbly media

The multivelocity effects associated with the behavior of gas or vapor bubbles in a region with high pressure gradients typical of the flows around a cavity in which the pressure is higher than that in the surrounding space are considered. For a low volume bubble concentration, the problem of fluid flow perturbation by the bubbles is examined. For gas bubbles, it is shown that taking multivelocity effects into account considerably reduces the additional jet momentum. It is found that, with time, the temperature distribution in the wake behind a vapor bubble becomes nonmonotonic and the maximum temperature may even exceed the initial bubble temperature. It is demonstrated that the bubbles may accumulate and a flow regime with a sharply pronounced two-phase jet extending to the outer edge of the main liquid jet may develop. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 87–100, January–February, 1998. The work received financial support from the Russian Foundation for Fundamental Research (project No.96-01-01442).     

Quasi-static bubble in a yield stress fluid: elasto-plastic model

The influence of the yield stress of Carbopol® gel dispersions on the behaviour of quasi-static bubbles was investigated. Many fluids, from many different industrial fields, have yield stress behaviour. Most of them contain gas bubbles. To study bubble behaviour in such suspensions, a transparent model fluid (dispersion of Carbopol® in water) was used. The experimental device allowed to quasi-statically increase bubble internal pressure with small pressure step to reach a maximum target internal pressure and the pressure setpoint was inverted to return to the initial pressure. Hysterical behaviour of the bubbles was highlighted as they did not regain their initial shape because of yield stress. We show that the rheological behaviour is related to the internal pressure, bubble geometry and yield stress in quasi-static conditions. A modification of the Laplace law depending on the yield stress of the fluid and bubble sphericity was proposed and validated.     

含气泡油滴撞击油膜壁面时气泡的变形与破裂

油--气润滑过程中润滑油液滴受高速气流扰动易形成含气泡油滴,微气泡将对油滴撞击壁面时的运动过程以及壁面油膜 层的形成质量产生重要影响. 基于耦合的水平集--体积分数 方法,对含气泡油滴撞击油膜壁面行为进行数值模拟研究, 考察含气泡油滴撞击油膜壁面时气泡的变形运动过程,探讨气泡破裂的动力学机制,分析气泡大小、碰撞速度和液体黏度等因素对含气 泡油滴撞壁过程中气泡变形特征参数的影响规律. 研究表明:含气泡油滴撞击油膜壁面后气泡会发生变形,并破裂形成膜液滴;气泡随同 液滴运动过程中,气泡内外压力和速度梯度变化是使气泡发生破裂的主要诱因. 气泡大小对气泡破裂方式影响较大,气泡较小时发生单 点破裂,而气泡较大时更容易发生多处破裂. 不同大小气泡受力差异较大,气泡大小与破裂发生时刻没有明显相关性. 碰撞速度和液体 黏度对气泡的变形、破裂和破裂发生时刻都具有一定的影响. 碰撞速度越大,油滴动能越大,更容易产生气泡变形和破裂现象. 液体黏 度增大,在油滴撞壁运动前期促进气泡变形,而在运动后期可以阻延气泡破裂行为发生.      

Shock structure in bubbly liquids: comparison of direct numerical simulations and model equations

Shock wave structure in a bubbly mixture composed of a cluster of gas bubbles in a quiescent liquid with initial void fractions around 10% inside a 3D rectangular domain excited by a sudden increase in the pressure at one boundary is investigated using the front tracking/finite volume method. The effects of bubble/bubble interactions and bubble deformations are, therefore, investigated for further modeling. The liquid is taken to be incompressible while the bubbles are assumed to be compressible. The gas pressure inside the bubbles is taken uniform and is assumed to vary isothermally. Results obtained for the pressure distribution at different locations along the direction of propagation show the characteristics of one-dimensional unsteady shock propagation evolving towards steady-state. The steady-state shock structures obtained by the present direct numerical simulations, which show a transition from A-type to C-type steady-state shock structures, are compared with those obtained by the classical Rayleigh–Plesset equation and by a modified Rayleigh–Plesset equation accounting for bubble/bubble interactions in the mean-field theory.      

DYNAMICAL BEHAVIORS OF DOUBLE CAVITATION BUBBLES UNDER ULTRASONIC HONING

为了探讨超声珩磨作用下磨削区的空化机理,基于速度势叠加原理,考虑超声珩磨速度和珩磨压力,建立了磨削区两空化泡的动力学模型. 数值模拟了磨削区空化泡初始半径、两空化泡间距、超声声压幅值、珩磨压力、珩磨头转速对磨削区两空化泡动力学特性的影响. 研究表明,考虑两空化泡之间的相互作用时,要想获得良好的空化效果,可将两空化泡初始半径之比控制在3 倍以内;选择较高的超声波声压幅值与较低的珩磨压力,并且使超声波声压幅值与珩磨压力和液体静压力之差介于0.66~1.89MPa 之间;增大珩磨头转速空化泡溃灭也略有加速;通过试验测量材料表面粗糙度的方法间接验证了理论分析的合理性.     

超声珩磨作用下两空化泡动力学特性

为了探讨超声珩磨作用下磨削区的空化机理,基于速度势叠加原理,考虑超声珩磨速度和珩磨压力,建立了磨削区两空化泡的动力学模型. 数值模拟了磨削区空化泡初始半径、两空化泡间距、超声声压幅值、珩磨压力、珩磨头转速对磨削区两空化泡动力学特性的影响. 研究表明,考虑两空化泡之间的相互作用时,要想获得良好的空化效果,可将两空化泡初始半径之比控制在3 倍以内;选择较高的超声波声压幅值与较低的珩磨压力,并且使超声波声压幅值与珩磨压力和液体静压力之差介于0.66~1.89MPa 之间;增大珩磨头转速空化泡溃灭也略有加速;通过试验测量材料表面粗糙度的方法间接验证了理论分析的合理性.      

Characterization of the interaction of two oscillating bubbles near a thin elastic membrane

Oscillating bubbles appear in the bodily fluid during many medical treatments, for example in Extracorporeal Shockwave Lithotripsy. We report a systematic study on the complex interaction between two such bubbles and an elastic membrane, which could be a biological membrane in the human body. We have grouped our analysis into similarly sized bubbles, and differently sized bubbles. All bubbles are created at the same time. For the similarly sized bubbles, it can be broadly characterized as the splitting up of two bubbles in vertical direction perpendicular to (vertical split) and at an angle to (oblique split) the membrane surface, jetting towards each other and bubble coalescence. For the two differently sized bubbles, there is the jetting towards or away from the large bubble for the small bubble and the ??catapult?? effect observed. The two bubbles dynamics depend on the relative bubble sizes, the distance from the membrane, and the inter-bubble distance.     

自由场中液氮单空泡动力学特性的实验研究

本文专门设计搭建了低温介质空泡演化实验测试平台,对液氮单空泡非定常演化过程和动力学特性开展了实验研究.实验中利用电火花瞬态放电激发液氮汽化形成单空泡,通过高速摄影系统对单空泡的瞬态特征进行了精细化捕捉.为了进一步揭示低温介质独特的物理性质以及强热力学效应对单空泡演化过程的影响机制,对比分析了在相同环境压力下, 77.41 K液氮和298.36 K水单空泡的演化过程和动力学特性.基于实验得到空泡半径与界面速度等定量数据,阐明了液氮单空泡球形与非球形演化阶段的非定常特性.研究结果表明:(1)在相同输入电压下,液氮单空泡的整体尺寸比常温水更小,当输入电压为400 V时,液氮空泡的最大半径约为常温水空泡的0.69倍;同时,液氮单空泡经历了膨胀阶段-收缩阶段-振荡阶段以及上升阶段的演化过程.(2)液氮空泡的收缩过程主要由相界面的热传导主导,没有明显的塌陷现象,收缩阶段液氮空泡的最小收缩半径约为常温水的5.5倍.(3)在液氮空泡振荡初期,空泡相界面传热增强, Rayleigh-Taylor不稳定与热力学效应共同引起了空泡界面的表面粗化效应;在整个振荡阶段,空泡界面附近存在破碎的小泡.当输入电压较高...     

波浪作用下直立结构物附近强湍动掺气流体运动的数值模拟

波浪破碎卷入气体易对建筑物受力产生压力振荡, 了解波浪作用下建筑物附近掺气水流的运动特性是精确计算建筑物受力的前提. 基于OpenFOAM开源程序包和修正速度入口造波方法建立三维数值波浪水槽, 模型采用S-A IDDES湍流模型进行湍流封闭, 并采用修正的VOF 方法捕捉自由液面, 数值模拟了规则波在1:10的光滑斜坡上与直立结构物的相互作用过程, 重点分析了结构物附近的水动力和掺气水流运动特性. 结果表明, 建立的数值模型能精确地捕捉波浪作用下直立结构物附近的自由液面的变化以及气泡输运过程, 较好地描述气体卷入所形成的气腔形态以及多气腔之间的融合、分裂等过程; 波浪与直立结构物相互作用产生强湍动掺气水流, 其运动过程十分复杂; 掺气流体输运过程中水气界面周围一直伴随着涡的存在, 其中, 气泡的分裂与周围正负涡量剪切作用密切相关, 且其输运轨迹主要受周围流场的影响; 研究揭示了结构物附近湍动能与掺气特性的关系, 发现波浪作用下直立结构物附近湍动能的分布与掺气水流特征参数(气泡数量、空隙率)整体呈现一定的线性关系.      

A numerical strategy for the direct 3D simulation of the expansion of bubbles into a molten polymer during a foaming process

In the framework of the foam process modelling, this paper presents a numerical strategy for the direct 3D simulation of the expansion of gas bubbles into a molten polymer. This expansion is due to a gas overpressure. The polymer is assumed to be incompressible and to behave as a pseudo‐plastic fluid. Each bubble is governed by a simple ideal gas law. The velocity and the pressure fields, defined in the liquid by a Stokes system, are subsequently extended to each bubble in a way of not perturbing the interface velocity. Hence, a global velocity–pressure‐mixed system is solved over the whole computational domain, thanks to a discretization based on an unstructured first‐order finite element. Since dealing with an Eulerian approach, an interface capturing method is used to follow the bubble evolution. For each bubble, a pure advection equation is solved by using a space–time discontinuous‐Galerkin method, coupled with an r‐adaptation technique. Finally, the numerical strategy is achieved by considering a global mesh expansion motion, which conserves the amount of liquid into the computational domain during the expansion. The expansion of one bubble is firstly considered, and the simulations are compared with an analytical model. The formation of a cellular structure is then investigated by considering the expansion of 64 bubbles in 2D and the expansion of 400 bubbles in 3D. Copyright © 2007 John Wiley & Sons, Ltd.     

Bubble characteristics in a bubbling fluidized bed with a rotating distributor

In this paper, the effect of a novel rotating distributor for fluidized beds on the bubble size is studied. The distributor is a perforated plate that rotates around the vertical axis of the column.The formation of the bubbles on the rotating distributor is theoretically analyzed. The pierced length of the bubbles ascending in the bed were measured using optical probes. The probability distribution of bubble diameter was inferred from these experimental measurements using the maximum entropy method. The radial profile of the bubble diameter is presented for the static and rotating configurations at different gas velocities. The frequency of bubble passage and the distribution of bubbles in the cross section of the bed are also reported. Results were finally shown for different heights above the distributor.A radial decrease in the bubble size when the distributor rotates is found. The bubble growth with the bed height is also lower in the rotating case.     

均匀电场中气泡上升特性的实验研究

电场中的气泡对于强化传热有显著的作用, 对于电场中气泡动力学特性的研究对增强换热器效率, 提高能源利用率有重要意义. 为了获得外加电场作用下气泡的动力学特性, 设计与搭建了可视化实验平台. 采用50 kV高压直流电源构建均匀电场, 高清摄像机拍摄实验图像. 引入电场强度、气泡体积与溶液介电常数作为变量, 探究其对于气泡动力学特性的影响. 观测了竖直与水平均匀电场中气泡的上升过程, 分析了不同变量下气泡变形状况与上升速度的变化. 引入气泡长宽比L/D用于表示气泡拉伸变形程度, 截取单个气泡上升过程分时段图像展示形态变化过程. 研究结果表明, 气泡沿电场方向伸长, 且电场强度越大, 变形越明显; 竖直电场中气泡伸长导致上升速度增大, 而水平电场中气泡上升速度减小. 气泡尺寸增大, 浮升力作用增强, 气泡上升速度增大. 溶液介电常数增加, 电场力作用明显增加, 气泡变形更加明显.      

INVESTIGATION OF BUBBLE-BUBBLE INTERACTION EFFECT DURING THE COLLAPSE OF MULTI-BUBBLE SYSTEM

通过直接数值模拟方法对多泡在压力驱动下的溃灭过程进行了研究。气相满足理想气体正压模型,液相为不可压 流体,采用基于压力的方法求解多泡的两相流场。数值研究表明,在多泡流场中,中心气泡的溃灭过程明显不同于单泡,存在总体溃灭延迟现象和后期加速现象。随着周围气泡数的增多或气泡间距的减小,中心气泡的溃灭时间增长,溃灭时的压力峰值增大。结合理论定性分析发现,气泡运动不仅受远场压力的驱动,还受周围气泡诱导压力场的影响。周围气泡的诱导压力经历先减小后增大的过程,从而使受其影响的中心气泡产生先延迟后加速的特征。     

Bubble behaviors of geldart B particle in a pseudo two-dimensional pressurized fluidized bed

Pressurized fluidized beds have been developed in quite a few industrial applications because of intensified heat and mass transfer and chemical reaction. The bubble behaviors under elevated pressure, strongly influencing the fluidization and reaction conversion of the whole system, are of great research significance. In this work, the bubble behaviors of Geldart B particle in a pseudo two-dimensional (2D) pressurized fluidized bed were experimentally studied based on digital image analysis technique. The effects of pressure and fluidization gas velocity on the general bubble behaviors (i.e., size, shape and spatial distribution) and the dynamic characteristics, such as the time-evolution of voidage distribution and local flow regimes, were comprehensively investigated. Results show that increasing pressure reduces the stability of bubbles and facilitates gas passing through the emulsion phase, resulting in the “smoother” fluidization state with smaller bubbles and declined bubble fraction and standard deviation. The equivalent bubble diameter and bubble aspect ratio increase with the increasing gas velocity while decrease as pressure rises. The elevated pressure reduces bubbles extension in the vertical direction, prohibits the “short pass” of fluidization gas in large oblong bubbles/slugs and benefits the gas–solid interaction. The flow regimes variation with gas velocity is affected by the elevated pressure, and demonstrates different features in different local positions of the bed.