基于界面追踪法的Kelvin-Helmholtz不稳定性直接数值模拟

利用界面追踪法对三组分不混溶流体的开尔文-亥姆霍兹不稳定性(Kelvin-Helmholtz instability,KHI)进行直接数值模拟。并针对中间流体层密度及黏度、重力、表面张力及剪切力对KHI的影响进行了着重探讨。研究结果表明,理查德森数增大不仅能够抑制界面的演化,同时还会限制界面的退化。除此之外,卷起高度和KHI的增长速率与弗劳德数之间呈正相关关系。表面张力能够抑制KHI向内发展,随着表面张力系数的增大,KHI向内的发展会逐渐变慢。但当波数较小时,表面张力对KHI向上发展的影响很小,且当界面的波数大于4时不会触发KHI的典型形式。剪切力越大,界面卷起高度和增长速率越大。     

流体界面不稳定性耦合作用的格子Boltzmann模拟

使用格子玻尔兹曼方法对二维不混溶、不可压缩流体的Kelvin-Helmholtz(K-H)不稳定性进行数值模拟。以卷起高度H作为参考值,研究了密度比、表面张力、切应力对流体K-H不稳定性内产生Rayleigh-Taylor(R-T)不稳定性的影响。研究结果显示,密度比对两种不稳定性耦合起决定性作用。密度比接近1时,K-H不稳定性中不会产生R-T不稳定性,随着密度比增大,K-H不稳定性中开始产生R-T不稳定性。表面张力系数的增大对流体产生K-H不稳定性及两种不稳定性的耦合的卷起高度变化没有影响,但会对流体向内运动起抑制作用,且卷起流体的厚度明显增加。切应力对两种流体不稳定性的耦合起抑制作用。     

Marangoni效应对圆管内膜状凝结的影响

对圆管内膜状凝结换热过程进行了理论分析.探讨了重力、表面张力梯度引起的Marangoni力以及气液界面剪切力对凝结换热Nusselt数的影响.建立了含有凝结液膜的物理模型和基于边界层方程组的数学模型,应用相似理论进行了无量纲变换.结果表明,表面张力梯度对凝结换热过程的影响不可忽略,梯度越大,液膜越薄,Nu数越大,换热越好.     

单液滴撞击薄液膜产生二次雾化过程的数值模拟

应用数值模拟方法研究单液滴撞击薄液膜的动力学行为.在二维轴对称坐标系内,采用VOF方法与网格局部瞬时加密技术相结合,跟踪液滴和液膜与空气间的气液两相界面.结果表明,液滴撞击薄液膜的演化行为主要受液滴初始动能、表面张力以及液体黏性的影响.初始动能越大,则形成的空间液膜最大高度越大,达到稳定状态越晚,飞溅开始时刻越早,飞溅生成的二次液滴数量也越多;在扩展后期及回缩阶段,空间液膜的形成主要受液体黏度影响,增加液体黏度会阻碍空间液膜飞溅;表面张力增大,形成的空间液膜高度减小、厚度增加,同时阻碍二次液滴的生成.     

液层厚度比对环形腔内双层液体的热毛细对流稳定性的影响

为了了解微重力条件下、水平温度梯度作用时,上部为固壁的环形腔内双层流体系统中液层厚度比对流动稳定性的影响,采用隐式重启Arnoldi方法(IRAM)对环形池内5cSt硅油/HT-70双层流体的热对流过程进行了线性稳定性分析,获得了不同液层厚度比下的临界Marangoni数、临界波数、临界相速度,并通过计算特征向量,得到了临界Marangoni数附近液-液界面的热流体波形态。     

幂律流体气流雾化射流不稳定性分析

基于射流不稳定性理论系统研究了一个圆柱形可压缩气流喷入有限厚度的幂律流体的时间模式不稳定性.在对幂律流体本构方程线性近似的基础上,推导了表征轴对称模式幂律流体气流雾化射流不稳定性的色散方程.通过数值计算,分析了液相雷诺数、韦伯数、气/液密度比和速度比、气流马赫数以及幂律指数对于剪切变稀流体与剪切变稠流体两种情形气流雾化射流的不稳定性影响.结果表明:无论是剪切变稀情形还是剪切变稠情形,液体的黏性力总是抑制其不稳定性,减小幂律指数均可促进幂律流体气流雾化射流的不稳定性.随着气流速度的不断增大,由气/液相互运动导致的剪切波逐渐主导幂律流体气流雾化射流的不稳定性与破碎过程.当液相的参数保持不变,增大气流密度、气流速度以及气流可压缩性均可有效地促进幂律流体气流射流的破碎.空气动力是促进幂律流体射流破碎的有效措施.同时,对于一个小韦伯数,表面张力促进气体射流不稳定性;而随着韦伯数增大到临界值后,表面张力将会逐渐抑制其不稳定性.     

液滴撞击液膜动力学特性的FTM模拟

采用界面追踪法(front-tracking method)对液滴撞击液膜动力学特性进行数值模拟,通过撞击后的形态演变及内部物理场信息分析,研究了韦伯数和无量纲液膜厚度对界面运动过程的影响,并阐述了撞击过程中形成卷吸现象的机理。研究表明,液滴撞击之后,会在颈部区域产生小射流,该射流是后期水花形成的基础;水平方向上,在撞击影响不到的区域压力不变化,而在射流形成处的颈部附近存在局部压差;液滴撞击液膜时其间的气体层被压缩,在流体黏性和剪切力的作用下,压力高的气层中的气体逃逸速度减慢,从而形成卷吸现象。     

多孔泡沫内纳米流体非平衡传热及体积分数分布

研究了纳米流体在金属泡沫内的对流换热,建立了局部非热平衡数学模型,得到了金属泡沫内纳米流体速度、温度和纳米颗粒体积分数分布,分析了纳米流体和金属泡沫的强化换热效果。当使用纳米流体或在通道内填充金属泡沫时,截面速度和温度变得更均匀。随着纳米颗粒体积分数的增大,努塞尔数先增大然后又逐渐减小,即存在一个合适的体积分数能使换热效果达到最好;当金属泡沫孔隙率增加时努塞尔数也会减小,有利于换热的进行。纳米流体和金属泡沫对换热具有明显强化作用,但压降随纳米颗粒体积分数增大而急剧增大。此外,还考虑了布朗扩散和热泳扩散等因素的影响。     

浮升力对T型管道内冷热流体混合过程热波动的影响

利用Fluent软件对T型管道内冷热流体混合过程进行了大涡模拟,考虑了温差引起的浮升力,获得了混合过程的瞬时温度和瞬时速度,通过定义时均值和均方根值来描述温度和速度的平均大小和波动程度.数值模拟结果表明,理查德森数对冷热流体的混合有着重要的影响:当理查德森数为负值且绝对值越大,则温度和速度的波动也就越大;相反,当理查德...     

用格子Boltzmann方法模拟液滴合并过程

用格子Boltzmann方法中的伪势模型对两个液滴的合并过程进行了数值模拟。详细研究了两液滴能否合并临界尺寸和液滴合并过程中液桥的形成与演化的关系，并研究了表面张力对合并速度的影响。研究结果发现当两个液滴之间距离小于2倍界面厚度时，两个液滴在不受外力的作用下能够自动合并；液桥的宽度与演化时间有一定的指数关系；表面张力越大，合并速度越快，这个结果与前人的理论预测和实验结果一致。     

Thermal effect of surface tension on the inward solidification of spheres

Effect of surface tension (Gibbs-Thomson effect) on the inward solidification of a liquid in a spherical container is investigated analytically by solving the unsteady heat equation via a small-time series expansion technique. A nonlinear Shanks transformation is adopted to improve the convergence property of the series solution at large time. The results show that at fixed Stefan number, the effect of surface tension is to increase the growth rate of the freezing front. A local minimum in the freezing rate is found to develop for all surface tension parameter values considered in this study. Also, analytic expressions for the relations between the growth rate of the freezing front, Stefan number and surface tension parameter are derived under the asymptotic condition of small Stefan number.     

Effects of liquid viscosity on inception of disturbance waves and droplets in gas–liquid annular two‐phase flow

The structure of gas–liquid two‐phase flow is investigated in order to establish a reliable criterion for the development of disturbance waves and droplets considering the effects of liquid viscosity. The structure of the gas–liquid interface and the flow rate of droplets entrained in gas are measured simultaneously at five kinematic viscosities (1.0, 3.2, 9.9, 30, 70 mm²/s). The time‐series traces of liquid film thickness measured by five holdup probes reveal that the inception of disturbance waves occurs at a liquid Reynolds number of 200 or a non‐dimensional liquid film thickness of 6.5. It is also shown that droplets are generated before the inception of disturbance waves with increasing liquid kinematic viscosity at a liquid velocity of 0.02 to 0.03 m/s. As previously published criteria for the inception of droplets are found to be unsatisfactory, a new critical condition for droplet generation balancing the interfacial shear stress $τi$ with the wave height h and surface tension σ is proposed: $τih/σ=0.025$. This relation describes the action of shear force and surface tension on wave crests, and is notably independent of liquid viscosity. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 529–541, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20176     

New entrainment rate correlation in annular two-phase flow applicable to wide range of flow condition

Assuming the rate of droplet entrainment is characterized by the ratio of the interfacial shear force to the surface tension force acting on the phase interface, new correlation representing the rate of droplet entrainment in annular-dispersed two-phase flow was developed. Although the correlation is based on the simple assumption, the quasi-equilibrium droplet flow rates measured in many experiments were predicted reasonably well (root mean square error of entrainment fraction was roughly halved comparing with several existing correlations). Its applicability to the non-equilibrium situation was also demonstrated by the numerical calculations using a one-dimensional three-fluid model.     

基于VOF方法的弹簧喷嘴雾化数值模拟研究

弹簧喷嘴在除氧器中广泛应用,其雾化质量显著地影响着传热传质速率。采用欧拉VOF方法追踪气液两相交界面的动力学特性,研究喷嘴出口处液膜破裂和液滴的形成过程。结果表明,射流一开始呈连续的液膜,液膜开始破碎位置在垂直位移约20 cm左右,破碎后液滴的粒径和破碎时液膜厚度很接近。液膜在气体剪切力和表面张力的作用下,液膜表面首先产生褶皱,继而产生一些不规则分布的孔洞,然后孔洞的数量和尺寸不断增加,最后导致液膜撕裂,产生小液柱或液滴。     

Comparison of volume-of-fluid methods for surface tension-dominant two-phase flows

The capabilities of three volume-of-fluid methods for the calculation of surface tension-dominant two-phase flows are tested. The accurate calculation of the interface remains a problem for the volume-of-fluid method if the surface tension force plays an important role and the density ratios of the fluids in different phases are high. The result can be an artificial velocity field at the interface (“parasitic currents”), which can destabilize the interface significantly. The three different algorithms compared can be distinguished by their methods to compute the surface tension force, namely, the method using a kernel function for smoothing the discontinuity at the interface, a combined level-set and volume-of-fluid approach and a parabolic reconstruction of surface tension. The test cases consist of an equilibrium rod, a capillary wave and the Rayleigh–Taylor instability. The analytical solutions for each problem serve to examine the accuracy and the convergence behavior of each approach. Finally, the slow formation of a gas bubble at an underwater orifice was computed with the combined level-set and volume-of-fluid method and the results are compared with an analytical solution based on the Young–Laplace equation.     

Energy analysis of evaporating thin falling film instability in vertical tube

The Kelvin-Helmholtz instability of evaporating thin falling film flow in vertical tube is studied by method of energy analysis. Based on the rules that the interfacial capillary waves come from the balance of works done by inertial force, surface tension on phase-change interface, and also capillary force on tube wall, the stability behaviors of falling film with different Reynolds number and different perturbation wavelength are explored in detail. The analysis indicates that the main reason of film breakup by increasing tube wall heat flux is that, the stability effect of capillary adsorbability on tube wall is weakened as surface tension waving is enhanced by improving tube wall temperature.     

Study of Hydrate Formation Kinetics in Petroleum Pipes by the Phase Field

We report the influence of phase field parameters on the modeling of gas hydrates formation. Also, the interface of the surface tension, super-cooling, and homogeneous and heterogeneous nucleation on the interface morphology and growing kinetics is evaluated. The mathematical model consists of simultaneous energy and phase field equations and is solved using the finite volume method. Results indicated that decreasing the surface tension leads to an increase of interface roughness and higher interface instability. It is also observed that an increase in the surface tension occurs together with an increase of surface thickness and lower growing kinetics. In this case, to promote the hydrate growth, it is necessary to impose a super-cooling of 2 K. Regarding homogeneous and heterogeneous nucleation, two conditions were simulated: a random distribution of nuclei, where the evolution of hydrate formation shows the occurrence of coalescence and the growing kinetics of coalesced was lightly decreased in comparison to the isolated portions of the hydrate; and heterogeneous nucleation along all the extension of the wall, where hydrate grows inward, the liquid region by mean of a homogeneous advance of the interface. In this paper, a model is reported that can be used to predict the hydrate growth process and asses parameters that are difficult to obtain experimentally.     

Simulation of microchannel flows using a 3D height function formulation for surface tension modelling

Simulations of multiphase flow and phase change heat transfer in microchannels require accurate calculation of the surface tension force to provide accurate interface locations and to avoid spurious velocities that distort the flow at the interface. Building on previous work, where we implemented a 2D height function technique, the 3D height function method has been modified and implemented in ANSYS Fluent to enable such calculations to be performed on structured meshes. Firstly, simulations are presented for test cases that demonstrate the correct and accurate calculation of the surface tension force. Then the model is used to investigate both Taylor bubble formation in a square section channel and the development of an elongated vapour bubble from a small spherical nucleated bubble in a heated channel.     

加热条件下液体燃料射流破碎机理的研究

利用线性热不稳定性理论,对黏性液体射入高温气体介质模型所对应的色散方程进行了数值求解。利用所得到的计算结果,研究了加热条件下轴对称模式扰动液体射流破碎机理,探讨了表征各种影响射流破碎作用力的无量纲Weber数（We）、密度比（Q）、Marangoni数（Ma）和Ohnesorge数（Z）对液体射流破碎最大扰动增长率及占优波数的影响。研究结果表明,液体和气体介质之间的温度梯度对液体射流稳定性有着非常显著的影响,表明热毛细力对于液体射流的破碎有促进作用,这种作用对处于Taylor模式下的液体射流尤为显著,并且这种热力作用可使液体射流从一种模式进入另一种模式,并可以大大改变射流的破碎尺序。