纳米阵列中气体驱替液体的流动特征

纳米尺度下气体驱动液体流动特征在纳流控芯片及页岩气开发中具有广泛的应用前景.利用管径规格为292.8 nm,206.2 nm,89.2 nm,67.0 nm,26.1 nm的氧化铝膜为纳米阵列,进行气驱水实验和单相气体流动实验,分析纳米尺度下气驱水流动特征.实验表明,纳米阵列中气驱水时气体流量随驱动压力变化经历三个阶段:第一阶段流量缓慢增大,且比单相气体流量降低约一个数量级;第二阶段纳米阵列中的水被大量驱替出,流量迅速增大;第三阶段纳米阵列中的水全部被驱替出,流动特征与单相气体流动保持一致.分析表明,气驱水第一阶段存在气液界面毛细管力的"钉扎"作用及固液界面相互作用力的影响,是产生非线性流动的主要原因;而一旦"钉扎"作用破坏,气体进入管道推动界面运动,气柱与液柱之间的毛细曲面曲率变化,毛细管力减小,气体流量急剧增大,其中毛细管力随驱替压力增大急剧变化,是造成第二阶段气体流量突变的主要原因.     

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.      

微管中非混溶两种流体运动界面的特征

以平流泵为压力源,在不同管径的石英微管中进行流动试验,显微镜观察和拍摄水-气界面和油-水界面,在微米尺度下进行了不同流速的运动界面实验,研究了微管中非混溶两种流体运动界面的特征,以及润湿性对流体在微管中流动界面的影响.实验中观察到了润湿界面的滞后现象,即界面随流速的不同而改变的现象.实验结果表明:水在微管中流动的气液界面随着流速的不同形状发生改变,流速较小时,界面基本保持为凹液面;随着流速的增加,液面由凹液面向平液面发展,进而发展为凸液面.在表面张力的作用下,微管的尺寸越小,两种流体的性质差别越大,界面的润湿滞后现象越不明显,讨论了界面和润湿滞后存在的问题和可能的应用.     

气体速度对液膜在预膜板表面流动形态影响的LBM模拟

为了探究气流剪切作用对航空燃油在气动雾化喷嘴预膜板上流动形态的影响,首先对基于相场理论的两相流格子Boltzmann模型进行修正,并通过经典算例验证了修正后模型的准确性和可靠性.随后利用该模型模拟了同向气流驱动下液膜在水平预膜板表面上的流动,分析了气流速度对液膜流动形态的影响规律.研究表明,该模型可准确追踪具有大密度比的气液相界面的形态变化;气液剪切速度差会诱发两相界面出现Kelvin-Helm-holtz不稳定性现象,因而当气体速度升高时,气液剪切速度差增大,不仅液膜流动速度随之增高,且在铺展阶段液膜会产生较高振幅的波浪面并加快铺展过程,自由表面波动增强.     

片状颗粒间液桥力变化规律的计算研究

研究颗粒间液桥力有助于揭示非饱和土持水特性的内在机理. 为探究片状颗粒间液桥力演化规律, 从细观尺度研究非饱和土的水力特性机理, 使用Surface Evolver软件在两平行的片状颗粒间构建出三维液桥模型, 分析了液桥拉伸过程中接触角、液桥体积、分离距离以及固液接触线钉扎效应等对液桥力变化规律的影响. 基于圆弧假定, 计算相应条件下液桥力以及接触半径的大小, 并与上述模拟结果进行对比分析. 结果表明: 片状颗粒间液桥力随液桥体积增大而递增, 随分离距离的增大而递减, 随固液接触角的增大先增后减或一直递减; 液桥体积一定时, 在钉扎状态下, 其液桥力随着分离距离的增大迅速递增达到峰值, 而后逐渐降低; Surface Evolver模拟与液桥界面环形近似的计算结果相对比, 当固液接触角较大时(θ = 60°和θ = 80°), 二者相对误差在6%以内, 而当固液接触角减小到30°及以下时, 相对误差随之增大, 且颗粒间分离距离越大, 相对误差越大.      

基于核磁共振技术的非均质岩心中泡沫动态稳定性评价方法及应用

基于有效孔隙体积守恒和核磁共振技术建立了泡沫在岩心中的动态稳定性的评价方法. 利用油、水标定方法测量了岩心中油相和泡沫液的体积, 计算了泡沫在岩心驱替过程中的动态稳定因子. 测试了双层非均质岩心的横向弛豫谱线及核磁共振图像, 比较了纳米颗粒强化泡沫和表面活性剂泡沫的驱油效果和动态稳定因子. 结果表明, 岩心中的含水体积在注入2.0 PV泡沫前快速上升随后基本稳定; 而含气体积逐渐上升, 注入5.0 PV泡沫后上升速率变小. 泡沫的动态稳定因子经历了骤减、递增和平稳3个阶段. 泡沫前期的驱油效果主要依赖于水相, 随着含水体积基本稳定, 岩心的产油速率和泡沫动态稳定因子的增长速率具有明显正相关关系, 即中后期取决于泡沫气体对剩余油的驱替能力. 与表面活性剂泡沫相比, 纳米颗粒强化泡沫提高了低渗层的波及能力和驱油效率, 抑制了泡沫发展的不稳定阶段并且提高了动态稳定因子最终的平衡值. 该稳定性评价方法可用于反映泡沫渗流特点并筛选适合储层特征的稳定泡沫体系.      

湿润性对孔隙介质两相渗流驱替效率的影响

孔隙介质中多相渗流的驱替效率对二氧化碳封存效率和石油采收率具有决定性影响, 是实际工程调控中的一个关键指标. 湿润性是影响多相渗流驱替模式及其效率的一个重要因素. 本文通过微流体模型-显微镜-高速相机可视化实验平台, 对基于真实砂岩孔隙结构的微流体模型进行湿润性修饰, 开展了5种流量和2种湿润性的两相驱替可视化实验, 研究了湿润性对砂岩孔隙结构中两相渗流驱替模式及其效率的重要影响. 实验结果表明: 随着流速的增大, 两相渗流驱替模式由毛细指流向稳定流发生转变; 在低流速条件下, 由于毛细力的主导效应, 亲水性介质中指进的宽度和被驱替流体团簇的数目均小于疏水性介质, 而被驱替流体团簇的最大半径、平均半径和方差均大于疏水性介质. 实验结果还证实了亲水性介质中由于单支优势通道和"绕流"现象的发生, 驱替效率显著小于疏水性介质. 最后, 通过考虑接触角效应对毛细管数进行修正, 建立了考虑湿润性影响的驱替效率和毛细管数之间的统一关系式, 为不同湿润性条件下驱替效率的预测提供了一种潜在方法.      

储层含水条件下致密砂岩/页岩无机质纳米孔隙气相渗透率模型

页岩及致密砂岩储层富含纳米级孔隙,且储层条件下页岩孔隙(尤其无机质孔隙)及致密砂岩孔隙普遍含水,因此含水条件下纳米孔隙气体的流动能力的评价对这两类气藏的产能分析及生产预测具有重要意义.本文首先基于纳米孔隙内液态水及汽态水热力学平衡理论,量化了储层孔隙含水饱和度分布特征;进一步在纳米孔隙单相气体传质理论的基础上,考虑了孔隙含水饱和度对气体流动的影响;最终建立了含水饱和度与气相渗透率的关系曲线. 基于本文岩心孔隙分布特征,计算结果表明:储层含水饱和度对气体流动能力的影响不容忽视,在储层含水饱和度20%的情况下,气相流动能力与干燥情况相比将降低约10%;在含水饱和度40% 的情况下,气相流动能力将降低约20%.      

多孔介质中稳定泡沫的流动计算及实验研究

将多孔介质简化为一簇变截面毛管束,根据多孔介质的颗粒直径、颗粒排列方式、孔喉尺度比及束缚水饱和度,计算出变截面毛细管的喉道半径和孔隙半径. 在考虑多孔介质喉道和孔隙中单个气泡的受力和变形基础上,利用动量守恒定理,推导出单个孔隙单元内液相的压力分布和孔隙单元两端的压差计算公式,最终得到多孔介质的压力分布计算公式. 利用长U型填砂管对稳定泡沫的流动特性进行了实验研究. 研究结果表明:稳定泡沫流动时多孔介质中的压力分布呈线性下降,影响泡沫在多孔介质中流动特性的因素包括:多孔介质的孔喉结构、泡沫流体的流量和干度、气液界面张力、气泡尺寸,其中孔喉结构和泡沫干度是影响泡沫封堵能力的主要因素.关键词: 稳定泡沫;多孔介质;变截面毛管;流动;表观粘度;压力分布;实验研究      

确定微纳米尺度金属薄膜拉伸分叉点的实验研究

为了确定微纳米尺度金属薄膜的拉伸分叉点,本文使用磁控溅射镀膜技术,在PI(聚酰亚胺)基底上沉积500nm厚的铜薄膜,制作薄膜/基底结构拉伸试件。在单轴拉伸作用下,通过测量拉伸加载过程中铜薄膜的电阻变化情况,得到薄膜电阻随应变变化的关系,并与理论推导的结果进行对比分析,从而确定了塑性阶段理论曲线与实验曲线分离的点,即铜薄膜的分叉点。以此为基础,研究了铜薄膜在单轴拉伸作用下的分叉行为。研究结果表明,沉积于PI基底上的微纳米尺度铜薄膜在单轴拉伸下,经过弹性变形阶段后,很快就发生分叉,然后产生破坏,而塑性变形阶段和局部化阶段较短;弹性阶段薄膜的电阻变化速率很小,塑性阶段薄膜的电阻变化速率稍有增大,而当薄膜表面开始出现微裂纹后,电阻变化速率急剧增大。     

Two-phase flow distributors for fuel cell flow channels

All existing proton exchange membrane （PEM） fuel cell gas flow fields have been designed on the basis of single-phase gas flow distribution. The presence of liquid water in the flow causes non-uniform gas distribution, leading to poor cell performance. This paper demonstrates that a gas flow restrictor/distributor, as is commonly used in two-phase flow to stabilize multiphase transport lines and multiphase reactors, can improve the gas flow distribution by significantly reducing gas real-distribution caused by either non-uniform water formation in parallel flow channels or flow instability associated with negative-slope pressure drop characteristic of two-phase horizontal flow systems.     

Experimental study on a two-phase critical flow with a non-condensable gas at high pressure conditions

An experimental study was performed on a two-phase critical flow with a non-condensable gas at high pressure conditions. Experimental data for the critical flow rates were generated by using sharp-edged stainless steel pipes with an inner diameter of 10.9 mm, a thickness of 3.2 mm, and a length of 1000 mm. The test conditions were varied by using the stagnation pressures of 4.0, 7.0, and 10.0 MPa, water subcoolings of 0.0, 20.0, and 50.0 °C, and nitrogen gas flow rates of 0.0–0.22 kg/s. The experimental results show that the critical mass flux decreases rapidly with an increase of the volumetric non-condensable gas fraction. Also the critical mass flux increases with an increase of the stagnation pressure and a decrease of the stagnation temperature. An empirical correlation of the non-dimensional critical mass flux, which is expressed as an exponential function of the non-condensable gas fraction of the volumetric flow, is obtained from the experimental data.     

A Fickian diffusion model for the spreading of liquid plumes infiltrating in heterogeneous media

Infiltration of water and non-aqueous phase liquids (NAPLs) in the vadose zone gives rise to complex two- and three-phase immiscible displacement processes. Physical and numerical experiments have shown that ever-present small-scale heterogeneities will cause a lateral broadening of the descending liquid plumes. This behavior of liquid plumes infiltrating in the vadose zone may be similar to the familiar transversal dispersion of solute plumes in single-phase flow. Noting this analogy we introduce a mathematical model for ‘phase dispersion’ in multiphase flow as a Fickian diffusion process. It is shown that the driving force for phase dispersion is the gradient of relative permeability, and that addition of a phase-dispersive term to the governing equations for multiphase flow is equivalent to an effective capillary pressure which is proportional to the logarithm of the relative permeability of the infiltrating liquid phase. The relationship between heterogeneity-induced phase dispersion and capillary and numerical dispersion effects is established. High-resolution numerical simulation experiments in heterogeneous media show that plume spreading tends to be diffusive, supporting the proposed convection-dispersion model. Finite difference discretization of the phase-dispersive flux is discussed, and an illustrative application to NAPL infiltration from a localized source is presented. It is found that a small amount of phase dispersion can completely alter the behavior of an infiltrating NAPL plume, and that neglect of phase-dispersive processes may lead to unrealistic predictions of NAPL behavior in the vadose zone.     

Pressure drop in two phase slug/bubble flows in mini scale capillaries

A segmented two phase slug/bubble flow occurs where a liquid and a gas are pumped into the same tube over a range of Reynolds numbers. This segmented two phase flow regime is accompanied by an increase in pressure drop relative to the single phase flow where only one fluid is flowing in a capillary. This work experimentally and theoretically examines the pressure drop encountered by the slug/bubble flow with varying slug lengths in mini channels. In the experimental work the dimensionless parameters of Reynolds number and Capillary number span over three orders of magnitude, and dimensionless slug length ranges over two orders of magnitude to represent flows typical of mini- and micro-scale systems. It is found, in agreement with previous work, that these dimensionless groups provide the correct scaling to represent the pressure drop in two phase slug/bubble flow, although the additional pressure drop caused by the interface regions was found to be ∼40% less than previously reported.     

Primary and Secondary Infiltration of Wetting Liquid Sessile Droplet into Porous Medium

The infiltration of a wetting droplet into the porous medium is a two-step process referred to as primary and secondary infiltration. In the primary infiltration there is a free liquid present at the porous medium surface, and when no fluid is left on the surface, the secondary infiltration is initiated. In both situations the driving force is the capillary pressure that is influenced by the local medium heterogeneities. A capillary network model based on the micro-force balance is developed with the same formulation applied to both infiltrations. The only difference between the two is that the net liquid flow into the porous medium in the secondary infiltration is equal to zero. The primary infiltration starts as a single-phase (fully saturated) flow and may proceed as a multiphase flow. The multiphase flow emerges as the interface (flow front) becomes irregular in shape. The immobile clusters of the originally present phase can be locally formed due to entrapment. Throughout the infiltration, it was found that portions of the liquid phase can be detached from the main body of the liquid phase forming some isolated liquid ganglia that increase in number and decrease in size. The termination of the secondary infiltration occurs once the ganglia become immobile due to their reduction in size. From the transient solution, the changes in the liquid saturation and capillary pressure during the droplet infiltration are determined. The solution developed in this study is used to investigate the droplet infiltration dynamics. However, the solution can be used to study the flow in fuel cell, nano-arrays, composites, and printing.     

不同发射深度下导弹水下点火气水流体动力计算

从流体动力角度研究了不同发射深度下，导弹水下点火这一非定常非线性过程。整个系统分为外部水流场、喷管流场和燃气泡流场三个区域加以考虑。水流场采用不可压势流模型，用边界元方法求解；喷管内流场采用非定常一元流动模型，用特征线差分法求解，并设置了激波检测功能；燃气泡采用基于质量和能量守恒的零维计算模型。在时间域中用步进方法实现了三个流场的耦合求解。给出了四种发射深度下的数值计算结果，展示了导弹水下点火的一     

Horizontal capillary flow of a Newtonian liquid in a narrow gap between a plane wall and a sinusoidal wall

An experimental and theoretical study of the capillary flow of a Newtonian liquid (mineral oil) in a Hele-Shaw cell in which the gap varies sinusoidally in one coordinate direction, and flow takes place in the direction of constant channel cross-sectional area is reported. The geometric non-uniformity of the gap is observed to produce interface fingering. Finger length is observed to increase with decreasing spacing between plates of fixed shape, and with increasing gross penetration distance. In the regime of interest, finger length is observed to increase slowly with increasing interface advancement, motivating a quasi-steady model in which gross interface advancement is predicted by a Lucas–Washburn model and interface fingering is predicted by a Hele-Shaw model of steady flow. The steady interface velocity in the Hele-Shaw model is set equal to the instantaneous interface velocity predicted by the Lucas–Washburn model. Fingering predicted by the quasi-steady model matches the experimentally observed trends with regards to plate spacing and gross penetration distance.     

Flux calculation at the interface between two rock types for two-phase flow in porous media

When simulating two-phase flow in porous media, one has to consider the case where there is a discontinuity in the medium. There relative permeabilities and capillary pressure functions may change and we address the problem of calculating the convective part of the numerical flux at the interface between the two rock types. Several solutions are compared.     

Enhancing Immiscible Fluid Displacement in Porous Media by Capillary Pressure Discontinuities

Fluid displacement in porous media plays an important role in many industrial applications, including biological filtration, carbon capture and storage, enhanced oil recovery, and fluid transport in fuel cells. The displacement front is unstable, which evolves from smooth into ramified patterns, when the mobility (ratio of permeability to viscosity) of the displacing fluid is larger than that of the displaced one; this phenomenon is called viscous fingering. Viscous fingering increases the residual saturation of the displaced fluid, considerably impairing the efficacy of fluid displacement. It is of practical importance to develop suitable methods to improve fluid displacement. This paper presents an experimental study on applying the discontinuity of capillary pressure to improve immiscible fluid displacement in drainage for which the displacing fluid (air) wets the porous media less preferentially than does the displaced fluid (silicone oil). The concept involves using a heterogeneous packing system, where the upstream region features large pores and small capillary pressure, and the downstream region features small pores and large capillary pressure. The increase in capillary pressure prevents fingering from directly crossing the media interface, thus enhancing the displacement. The experimental apparatus was a linear cell comprising porous media between two parallel plates, and glass beads of 0.6 and 0.125 mm diameter were packed to compose the heterogeneous porous media. The time history of the finger flow was recorded using a video camera. Pressure drops over the model from the inlet to the outlet were measured to compare viscous pressure drops with capillary pressures. The results show that the fluid displacement was increased by the capillary discontinuities. The optimal displacement was determined through linear regression by adjusting the relative length of the large- and small-pore region. The results may assist in the understanding of fingering flow across the boundaries of different grain-sized bands for the gas and oil reservoir management, such as setting the relative location of the injection and production wells. The findings may also serve as a reference for industrial applications such as placing the grain bands in an adequate series to improve the displacement efficacy in biological filtration.