多层流体的Marangoni对流

介绍微重力环境下矩形液池中多层不相混液体的Marangoni对流及其不稳定性.Marangoni对流流动的形成是由于在该系统施加一个与液体交界面相垂直的外加温度梯度.Marangoni对流的线性不稳定性分析是基于无限延长矩形液池内的两层流体系统.应用数值模拟研究了Marangoni对流的流体动力学和热传输特性,将多层流体的Marangoni对流同两层流体中外加温度梯度与流体交界面平行时引起的热毛细对流的主要特征进行了比较.     

静水中气泡上升运动及阻力系数研究

采用模型计算法与实验法结合的方式对静水中气泡上升运动行为进行研究。通过牛顿运动定律,基于不同物理模型,建立气泡在水中运动的微分方程;假设气泡在运动过程中的关键参数取值,推导小气泡在水中浮升过程中的气泡行为预测公式;针对不同流态下的气泡上升关键参数进行适应性分析和算例计算。通过设计气泡上升运动实验,对气泡上升运动公式进行适应性分析,修正关键参数的取值。据此提出一种小气泡上升运动规律的计算方法以及关键参数取值方式及参考区间。     

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.      

惰性颗粒抑爆过程的数值模拟

对高温火球诱导的爆炸及惰性颗粒的抑爆过程进行了数值研究。由于扰动特征时间 ,两相弛豫特征时间和化学平衡特征时间的差异 ,这种带激波的两相化学反应流守恒方程具有刚性。利用处理包括组元守恒方程的全耦合TVD格式和可处理方程源项的Lax Wendroff Rubin格式分别求解气相和颗粒相 ,并根据各特征时间的比值用分步法处理方程的刚性。计算结果反映了均温燃烧火球在可燃环境中形成加速火焰和激波 ,进而成长为爆轰波的过程 ,以及惰性颗粒对爆轰波的抑制过程 ,反映了激波、化学反应和惰性颗粒之间的相互作用和耦合。计算结果表明 ,仅当颗粒浓度大于某值时才能有效抑制爆轰 ,否则无论多大的颗粒散布区域 ,皆不能使爆轰波完全抑制 ,爆轰波或是在其中发展为两相爆轰波 ,或是穿越该区域后重新成长为气相爆轰。     

平面激波冲击并排液滴的三维数值模拟研究

采用三维守恒清晰界面数值方法, 研究平面激波冲击并排液滴的动力学过程. 研究的焦点在于激波接触液滴后的复杂波系结构生成, 以及并排液滴相互耦合作用诱导的单个液滴非对称界面演化. 首先, 分析并排液滴之间界面通道内的波系结构发展, 发现在冲击初期由于反射激波相交而形成新的反射激波以及马赫杆; 这些流动现象与液滴另外一侧 (非通道侧) 由激波反射所形成的弯曲波阵面截然不同, 而且所导致的液滴横向两侧流场差异是中后期冲击过程液滴两侧界面非对称演化的主要原因. 其次, 研究冲击中期时, 特别是入射激波已运动至液滴下游并远离并排液滴, 界面形态的演化过程和规律, 揭示通道下游出口处由于气流膨胀导致的界面闭合、以及随后气流阻塞导致的界面破碎等新的流动现象. 最后, 研究液滴间距对并排液滴相互作用的影响规律, 发现液滴间距大小与通道内压力峰值具有明显的关联关系. 研究表明, 更小的液滴间距不仅带来更大的压力峰值, 而且使得峰值出现的时间更早.      

稠密颗粒床中点火传火过程的两维两相流数值模拟

本文结合火炮工程背景,建立伴随化学反应的稠密颗粒群气固两相非定常两维轴对称流和点火器内的非定常一维两相流数学模型,并考虑点火器对主装药床的耦合作用。采用MacCormack两步差分格式,模拟点火传火过程中火焰传播及其物理量沿轴向和径向变化规律。部分计算结果与实验结果相吻合。发现点火开始阶段存在明显的径向效应,当火焰波传到固壁后,轴向流占主要优势。     

气液非混相驱替过程中的卡断机理及模拟研究

研究气液非混相驱替过程中的相界面卡断机理及其影响因素在气驱, 气水交替及泡沫驱等提高油气采收率领域具有重要意义. 本文在原始伪势格子玻尔兹曼模型的基础上, 改进流体-流体作用力格式, 添加流-固作用力, 耦合RK状态方程, 并采用精确差分方法将外力添加到LBM框架中. 通过校准模型的热力学一致性以及模拟测试界面张力, 静态平衡接触角及液相在角隅的滞留等一系列两相体系验证模型的准确性. 基于改进的伪势格子玻尔兹曼模型, 在孔-喉-孔系统中开展气液非混相驱替模拟, 结果表明: 卡断现象与驱替压差, 孔喉长度比及孔喉宽度比有关, 只有当驱替压差处于一定范围内时, 气液两相驱替过程中才会发生卡断现象; 当驱替压差大于临界驱替压差上限时, 即使达到了经典静态准则所预测的卡断条件, 卡断也会被抑制; 当驱替压差小于临界驱替压差下限时, 无法克服毛管“钉扎”作用, 形成无效驱替. 对于固定孔喉宽度比的孔-喉-孔结构, 随着孔喉长度比的增大, 发生卡断现象的驱替压差范围增大; 对于固定孔喉长度比的孔-喉-孔结构, 随着孔喉宽度比的减小, 发生卡断现象的驱替压差范围增大.      

高含能颗粒床中瞬变点传火过程的三维两相流数值模拟

结合火炮工程背景，建立高含能稠密颗粒床中非定常三维两相流与点火器中非定常一维两相流的数理模型，并考虑主装药床与点火器的相互耦合作用。采用分数步法（ＦＳＭ）数值求解。发现在点传火阶段，膛内存在明显的三维效应，并伴随波动现象，火药颗粒向壁面聚集。部分计算结果与试验结果相一致。     

垂直上升圆管内等温泡状流的数值模拟

介绍了描述等温状态下垂直气泡流流动的总体平衡方法(population balanceapproach). 将平均气泡数密度控制方程引入到双流体模型中实现总体平衡. 介绍了描述气泡聚并与破裂机制的Yao和Morel理论模型以及Hibiki等人的实验方法. 利用商业计算流体力学软件ANSYSCFX10, 对圆管特定位置上的5个基本变量:气含率、气泡平均直径、相间表面积浓度以及气体和液体速度沿半径分布进行了模拟计算. 预测结果与实验数据的比较表明,预测结果和实测数据之间有很好的一致性.      

气泡-水流两相流的激光多普勒法测量

为区分激光多普勒稀态两相流动测量中出现的流体相信号和颗粒(气泡、液滴)相信号,本文提出可见度和基座的复合判别法,并设计了专用的信号预处理器,此预处理器能将原始多普勒信号分为两路,分别代表两相,并可用计数型信号处理器测量每路的速度及其它统计量。使用本方案对矩形管道內低速气泡-水两相流的实验和测量表明,对两相的区分和测量是成功的。文中给出了两相速度、湍流度和气泡滑移速度的测量结果。     

Numerical modelling and validation of Marangoni and surface tension phenomena using the finite volume method

Surface tension induced flow is implemented into a numerical modelling framework and validated for a number of test cases. Finite volume unstructured mesh techniques are used to discretize the mass, momentum and energy conservation equations in three dimensions. An explicit approach is used to include the effect of surface tension forces on the flow profile and final shape of a liquid domain. Validation of this approach is made against both analytical and experimental data. Finally, the method is used to model the wetting balance test for solder alloy material, where model predictions are used to gain a greater insight into this process. Copyright © 2000 John Wiley & Sons, Ltd.     

Unsteady model of drop Marangoni migration in microgravity

The experiments of drop Marangoni migration have been performed by the drop shift facility of short period of 4.5 s, and the drop accelerates gradually to an asymptotic velocity during the free fall. The unsteady and axisymmetric model is developed to study the drop migration for the case of moderate Reynolds numberRe=O(1), and the results are compared with the experimental ones in the present paper. Both numerical and experimental results show that the migration velocity for moderate Reynolds number is several times smaller than that given by the linear YGB theory. The project supported by the National Natural Science Foundation (19789201) and the Ministry of Science and Technology of China (95-Yu-34)     

Pumping of a fluid through a microchannel by means of a bubble driven by a light beam

A new method of pumping a fluid through a microchannel device using a gas bubble-piston, set in motion by the thermocapillary force induced by a light beam, is proposed. To demonstrate the method, a model micropump has been assembled. The model consists of two reservoirs connected by two channels with a bubble-piston driven by a light beam. The pumping rate and the volume per piston stroke are evaluated experimentally. The method proposed is compared with known microfluid pumping methods. Some advantages of the new method are indicated.     

Marangoni convection and weld shape variation in A-TIG welding process

The flux effect on TIG weld shape variations is investigated by application of the heat transfer and fluid flow model. The simulation makes use of Nimonic 263 alloy, TiO, TiO₂ and Ti₂O₃ as the flux. The arc constriction and the reversed Marangoni convection are considered to be the two main factors for increasing penetration of A-TIG weld pool. And the simulated results show that the latter is the main factor for changing weld shapes. The surface tension temperature coefficient is sensitive to the active elements and affects the pattern of the fluid flow. By controlling the category and quantity of the active elements, different kinds of the weld shapes are obtained. The experimental result shows that increase of active flux on the weld bead tends to increase the penetration of the weld pool at first and then decreases steeply. This does not coincide with the simulated results. It is probably because part of the oxide in the flux is not totally decomposed when the flux reaches a critical value. The solid oxide particles in the weld pool act as the obstacles of the fluid flow and reduce the velocity of the flow.     

Experimental and numerical study of Marangoni convection in shallow liquid layers

An experimental and numerical study of thermal Marangoni convection in shallow liquid layers was carried out for a range of temperature differences and layer depths. This was done to permit earth based experiments to be undertaken in situations where Marangoni convection dominated the flow. Particle image velocimetry (PIV) was used to obtain the flow patterns and velocity vectors. The experimental results were compared to numerical models created using FLUENT V6. Both results are in good agreement. The liquid free surface profile due to the presence of the menisci is shown to be critical for good quantitative validation. The layer depths are also proven to be shallow enough for Marangoni convection to dominate over buoyancy under normal gravity conditions.     

Influences of Marangoni effect on solidification of curved interfaces with constant curvature

The asymptotic method and phase field simulation are applied to study the influences of variation of the surface tension with temperature on the movements of solid-liquid interfaces in the solidification process of spherical and cylindrical nuclei, respectively. Results indicate that Marangoni effect will increase the critical nucleation radius, slow up the movement of interface. The tip speed of dendrite decreases linearly with Marangoni number for melt without convection. The results of phase field simulation are qualitatively in accord with that of asymptotic method.     

Numerical simulation of a single rising bubble by VOF with surface compression

The capability of the direct volume of fluid method for describing the surface dynamics of a free two‐dimensional rising bubble is evaluated using quantities of a recently published benchmark. The model equations are implemented in the open source computational fluid dynamics library OpenFOAM®. Here, a main ingredient of the numerical method is the so‐called surface compression that corrects the fluxes near the interface between two phases. The application of this method with respect to two test cases of a benchmark is considered in the main part. The test cases differ in physical properties, thus in different surface tension effects. The quantities centre of mass position, circularity and rise velocity are tracked over time and compared with the ones given in the benchmark. For test case one, where surface tension effects are more pre‐eminent, deviations from the benchmark results become more obvious. However, the flow features are still within reasonable range. Nevertheless, for test case two, which has higher density and viscosity ratios and above all a lower influence of the surface tension force, good agreement compared with the benchmark reference results is achieved. This paper demonstrates the good capabilities of the direct volume of fluid method with surface compression with regard to the preservation of sharp interfaces, boundedness, mass conservation and low computational time. Some limitation regarding the occurrence of parasitic currents, bad pressure jump prediction and bad grid convergence have been observed. With these restrictions in mind, the method is suitable for the simulation of similar two‐phase flow configurations. Copyright © 2012 John Wiley & Sons, Ltd.     

Three‐dimensional numerical simulation of Marangoni instabilities in liquid bridges: influence of geometrical aspect ratio

Oscillatory Marangoni convection in silicone oil–liquid bridges with different geometrical aspect ratios is investigated by three‐dimensional and time‐dependent numerical simulations, based on control volume methods in staggered cylindrical non‐uniform grids. The three‐dimensional oscillatory flow regimes are studied and compared with previous experimental and theoretical results. The results show that the critical wavenumber (m), related to the azimuthal spatio‐temporal flow structure, is a monotonically decreasing function of the geometrical aspect ratio of the liquid bridge (defined as the ratio of length to diameter). For this function, a general correlation formula is found, which is in agreement with the previous experimental findings. The critical Marangoni number and the oscillation frequency are decreasing functions of the aspect ratio; however, the critical Marangoni number, based on the axial length of the bridge, does not change much with the aspect ratio. For each aspect ratio investigated, the onset of the instability from the axisymmetric steady state to the three‐dimensional oscillatory one is characterized by the appearance of a standing wave regime that exhibits, after a certain time, a second transition to a travelling wave regime. The standing wave regime is more stable for lower aspect ratios since it lasts for a long time. This behaviour is explained on the basis of the propagation velocity of the disturbances in the liquid phase. For this velocity, a general correlation law is found as a function of the aspect ratio and of the Marangoni number. Copyright © 2001 John Wiley & Sons, Ltd.     

A Mechanistic Model of Gas-Condensate Flow in Pores

Recent experimental results reported in the literature indicate that the relative permeability of gas-condensate systems increases with rate (velocity) at some conditions. To gain a better understanding of the nature of the flow and the prevailing mechanisms resulting in such behaviour flow visualisation experiments have been performed, using high pressure micromodels. The observed flow behaviour at the pore level has been employed to develop a mechanistic model describing the coupled flow of gas and condensate phases. The results of the model simulating the observed simultaneous flow of gas and condensate phases have been compared with reported core experimental results. Most features of the reported rate effect are predictable by the developed single pore model, nevertheless, its extension to include multiple pore interaction is recommended.