HPAM溶液在石英微圆管中的流动实验

针对HPAM溶液在油藏孔隙中的复杂渗流特性问题,在内径为10～350μm微圆管中进行了部分水解聚丙烯酰胺(HPAM)溶液的流动实验.实验结果表明:在本文实验条件下,HAPM溶液在管径尺度较小的微圆管内的流动规律明显偏离常规尺度下的非牛顿流体力学流动规律,其截面平均流速高于按照常规尺度流动推算出的结果,且管径越小,偏离程度越大,微尺度效应越强.     

管道内均匀与非均匀PEO溶液湍流减阻特性研究

在自制重力式循环水槽中(最高试验Re数可达105),测试分析了管道内均匀与非均匀聚氧化乙烯(PEO)溶液的湍流减阻规律。结果表明,均匀PEO溶液减阻率(DR)随Re数和浓度增加而提高(最大减阻率70%),但浓度大于临界浓度(100p.p.m)后DR不再增加;而相同浓度均匀PEO溶液的减阻效果随时间逐渐减弱,且Re数越大,剪切破坏越严重,DR下降越明显。与均匀PEO溶液不同,非均匀PEO溶液(从管壁狭缝向内喷射PEO溶液所得)不受剪切破坏影响,减阻效果更稳定,且药品使用量小;同时通过增大喷射速率、Re数和喷射液浓度,可提高减阻率(最大减阻率50%)。对比可见,喷射减阻方式仅需消耗少量药剂,就能实现显著、稳定减阻,有望用于水下航行体湍流减阻。     

基于荧光漂白成像的润滑油膜剪切流速测量

沿润滑油膜厚度方向的剪切流速分布是影响机械部件润滑性能的重要因素.为此,搭建了基于荧光漂白成像的微间隙油膜剪切流速分布测量平台.通过对漂白区域形状演化过程进行图像分析,获得了微米量级间隙中的PB450和PAO6润滑油膜的剪切速度分布.结果表明:在设定测试条件下,厚度为8.5μm的PB450油膜沿膜厚方向的剪切流速近似为典型的线性分布,而相同厚度下的PAO6油膜流速分布表现为非线性塞流,界面附近油膜黏度较中层显著下降.研究还发现,同一滑动速度下,PAO6剪切流速偏离线性分布的程度随膜厚的降低而增加.经过比对分析,试验结果与流体动压润滑条件下的相关数据吻合.     

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.     

氮气在微米管中的微尺度流动端面效应

在压力为5～20 MPa、温度为25～60℃范围内,开展了氮气在直径31～100μm管中的流动实验。提出了用于量化微尺度流动端面效应附加压差Δ_pend的双管长压差计算法。利用该方法,基于实验结果发现：Δ_pend与管径d负相关,而与流速v正相关。在低流速条件下压力对Δ_pend的影响不明显,而在高流速条件下Δ_pend随压力的升高而增大;在本文实验条件内,温度对Δ_pend的影响不明显。据此,建立了综合表征Δ_pend与d和v关系的微尺度流动端面附加压差的经验模型。     

Herschel-Bulkley模型下磁流变减振器旁路泊肃叶流动分析

制备了羰基铁粉(CIP)的质量分数为70%的硅树脂基磁流变胶。用安东帕MCR-301型流变仪对其流变特性进行了测量,并对测试结果进行了Herschel-Bulkley本构模型参数识别。最后对MRG-70在该模型下磁流变减振器旁路中的泊肃叶(Poiseuille)流动展开了分析。试验及分析结果表明,在外加磁场下,Herschel-Bulkley本构模型中非牛顿指数n1。剪切应力随半径的增大而线性增大;当τ_rτ_y形成剪切流,反之则为柱塞流动。在剪切流动区域内,流速随半径增大而非线性降低,剪切速率随半径增大而非线性增大。磁感应强度越大,柱塞流区域越大,柱塞流区域内剪切速率恒为0。体积流量随磁感应强度增强而非线性减小。在0mT～500mT之间体积流量随磁感应强度变化剧烈,进一步增加磁感应强度则变化缓慢。     

定优胶溶液狭缝喷射减阻实验研究

定优胶具有比柔性聚合物更优越的抗剪切效果, 是一种新型高分子聚合物减阻添加剂, 目前对其研究仍相对缺乏. 这里通过开展定优胶流变和管内狭缝喷射减阻实验, 分析了流变特性与减阻行为之间的联系, 并从其喷射扩散角度解释了其减阻规律变化的原因. 实验结果表明, 定优胶溶液为剪切变稀流体, 会发生黏性到弹性转变, 且转变点与温度无关, 仅随浓度增加而前移; 定优胶减阻率随水流速度(雷诺数)呈先增后降趋势, 但随喷射速率单调递增; 相较于喷射纯水, 定优胶溶液在流场中扩散缓慢, 且喷射速率越高, 壁面附近集聚越明显. 同时, 定优胶溶液喷射减阻的变化与其扩散规律相吻合: 当流速较小时, 定优胶溶液扩散不充分, 呈非均匀聚集态, 未能充分发挥其湍流抑制效果, 减阻较弱; 随流速增加, 水流的剪切拖拽作用增强了定优胶的扩散均匀程度, 进而提升湍流抑制效果, 减阻率上升; 但当流速过大时, 定优胶的快速扩散造成其浓度被大幅稀释, 且近壁区过大剪切率可能已造成部分长链分子断裂, 致使减阻效果下降.      

颗粒材料剪切流动状态转变的环剪试验研究

颗粒材料的剪切流动行为广泛地存在于滑坡、泥石流等自然灾害以及矿物原料传输、泵送等工业过程中.颗粒材料在不同体积分数、剪切速率和应力约束下会表现出不同的流动状态并发生相互转化.对颗粒材料在剪切流动过程中力学特性的研究有助于加深理解其发生不同流动状态的内在机理,为解决相应的颗粒材料问题提供理论依据.为此,本文研制了颗粒材料剪切流动的中型环剪仪,并对颗粒材料在不同法向约束应力和剪切速率下的剪切应力和体积膨胀率进行了测试.结果表明,剪切应力和体积膨胀率均随剪切速率的增大而增大,但增长速率在临界剪切速率处发生转变,使其随剪切速率的平方呈分段式线性增长.通过对颗粒材料在不同剪切速率和惯性数下有效摩擦系数变化趋势的分析,讨论了颗粒材料由慢速流向快速流转化的基本规律,以及在临界剪切速率处发生流动状态转化的内在条件.此外,通过对不同法向应力下临界剪切速率以及快速流动下运动规律的测试,发现临界剪切速率随法向应力的增加而减小,即法向应力可促进颗粒材料由慢速流向快速流的转化,但在快速流动状态下的有效摩擦系数对法向应力不敏感.以上对颗粒材料在不同剪切速率、法向应力下流动状态的环剪试验研究有助于揭示其发生不同流动状态转化的内在机理.      

定床弯道内水沙两相运动的数值模拟

在适体同位网格中采用非正交曲线坐标系下的三维k-ε-kp固液两相双流体湍流模型研究弯道内水流和悬浮泥沙运动,主要计算了试验室S型水槽内清水流动的三维流场、120°弯道内水沙两相流动中底沙与底流的运动轨迹以及S型水槽内水沙两相流动的两相流场和泥沙浓度场. 对于S型水槽内清水流动,数值结果与试验结果吻合良好. 120°弯道内水沙两相流动中固液两相的运动轨迹在弯道直线段基本重合,在弯道内泥沙轨迹逐步偏离水体轨迹,其偏离程度随泥沙粒径增大而增大. 从S型水槽内水沙两相流动计算结果中发现泥沙纵向流速在壁面附近比水流纵向速度大,在远离壁面区域比水流纵向速度小;弯道内泥沙横向流速比水流横向流速小;垂向流速在直线段和泥沙沉速相当,在弯道内受螺旋水流影响而变化;两相流速差别随泥沙粒径增大而变大;泥沙浓度呈现下浓上稀的分布,在弯道内横向断面上呈现凸岸大凹岸小的分布,泥沙浓度随泥沙粒径增大而减小.      

MEMS稀薄气体内部流动模拟中的信息保存法

首先综述了处理低速稀薄气体流动的一些方法: 线化Boltzmann方程方法、Lattice Boltzmann方法(LBM)、加滑移边界的Navier-Stokes方程、以及DSMC方法, 并讨论它们在模拟MEMS中过渡领域低速流动特别是内部流动所遇到的困难, 其中表明了LBM现有方案不适合模拟过渡领域中的MEMS流动问题. 信息保存(IP)法通过保存一个模拟分子所代表的大量分子的平均信息,克服了流速低使得信息噪声比小而引起统计模拟的困难. 本文给出了方法的一些理论证实. MEMS中内部流动的特点, 即流速低和大的长宽比的特点, 引起椭圆性问题, 即出入口边界条件相互影响需要协调的问题. 通过对(长约几千微米的)微槽道流动应用IP方法的算例,演示了采用守恒形式的质量守恒方程和超松弛法可成功地解决这一问题. 借助同样的方法,用IP方法求解了真实长度(1\,000\,$\mu$m)硬盘驱动器读写头在过渡领域的薄膜支撑问题, 压力分布与具有严格气体动理论基础的概括化Reynolds方程完全相符, 而在此之前, DSMC方法只对短的读写头(5\,$\mu$m)与Reynolds方程做了校验. 作者建议将原来用于求解读写头润滑问题的Reynolds方程退化来求解过渡领域中的微槽道流动问题, 从而提供了一个有严格气体动理论品性的检验方法来验证求解MEMS内部流动的各种方法.      

Transition from laminar to turbulent flow in liquid filled microtubes

The transition to turbulent flow is studied for liquids of different polarities in glass microtubes having diameters between 50 and 247 µm. The onset of transition occurs at Reynolds numbers of ~1,800–2,000, as indicated by greater-than-laminar pressure drop and micro-PIV measurements of mean velocity and rms velocity fluctuations at the centerline. Transition at anomalously low values of Reynolds number was never observed. Additionally, the results of more than 1,500 measurements of pressure drop versus flow rate confirm the macroscopic Poiseuille flow result for laminar flow resistance to within –1% systematic and ±2.5% rms random error for Reynolds numbers less than 1,800.An erratum to this article can be found at     

Friction characteristics of compressible gas flows in microtubes

The characteristics of a gaseous flow of nitrogen in commercial stainless steel microtubes for gas chromatography having a nominal inner diameter of 762, 508, 254 and 127 μm are experimentally investigated. The friction factor is calculated as a function of the Reynolds number and plotted in a Moody chart. A comparison among three different methods to calculate the friction factor is made in order to evidence limitations and advantages of each method. It was observed that in the laminar regime the Poiseuille law correctly predicts the value of the pressure drop. It has been evidenced that in order to make accurate experiments on the frictional characteristics of commercial microtubes the value of the inner diameter given by the manufacturer has to be always verified. The experimental data presented in this work remark how in microtubes the compressibility effects related to the axial variation of the gas density tend to become important at large Reynolds numbers and small diameters even if the average Mach number is low. The effects due to the gas acceleration on the laminar-to-turbulent transition in microtubes are investigated by evidencing the role of the L/D (length to diameter) ratio on the transition to turbulence. No early transition to turbulence has been evidenced in the tests, instead it takes place at Reynolds numbers ranging between 1800 and 2900.     

Second law analysis of water flow through smooth microtubes under adiabatic conditions

In the study, a second law analysis for a steady-laminar flow of water in adiabatic microtubes has been conducted. Smooth microtubes with the diameters between 50 and 150 μm made of fused silica were used in the experiments. Considerable temperature rises due to viscous dissipation and relatively high pressure losses of flow were observed in experiments. To identify irreversibility of flow, rate of entropy generation from the experiments have been determined in the laminar flow range of Re = 20-2200. The second law of thermodynamics was applied to predict the entropy generation. The results of model taken from the literature, proposed to predict the temperature rise caused by viscous heating, correspond well with the experimental data. The second law analysis results showed that the flow characteristics in the smooth microtubes distinguish substantially from the conventional theory for flow in the larger tubes with respect to viscous heating/dissipation (temperature rise of flow) total entropy generation rate and lost work.     

Velocity profile characterization in sub-millimeter diameter tubes using molecular tagging velocimetry

Fluid flow through microtubes is of interest to many industries and there exists a need for detailed measurements of the velocity field. Velocity profile data are critical for momentum, mass, and heat transport analysis, and thus the design of devices utilizing microgeometries. This paper outlines a measurement technique that has led to time-resolved measurements of velocity profiles in microtubes (less than 1,000 μm). The research program was experimental in nature and consisted of an extension of molecular tagging velocimetry to the microscale. Average velocity and rms profile data in the fully developed region, in addition to mass flow rate and pressure drop data, are presented for numerous Reynolds numbers ranging from 600 to 5,000 in a tube of diameter 705 μm. Received: 20 December 1999 / Accepted: 20 March 2001     

Slug and bubble flows in a flat sheet ultrafiltration module: Experiments and numerical simulation

In this paper, the effect of gas bubbling including slug and bubble flows on enhancing shear force in an ultrafiltration (UF) process in a flat sheet module is investigated experimentally by image processing and numerically using the OpenFOAM software. In order to study characteristics of bubbles in the slug and bubble flows, the flat sheet module is analyzed by a video system facilitated with a high speed camera. The experimental results show that the average diameter of the slug flow is much larger than that of the bubble flow. The permeate flux for the slug and bubble flows is increased by 78% and 30%, respectively, compared to the case with no gas bubbling. The numerical results are shown to be in good agreement with those of the measurements both qualitatively and quantitatively. The results of simulations also demonstrate that although both flow patterns increase the shear stress by increasing the velocity gradient and/or vorticity, the shear stress induced by the slug flow is considerably larger. Therefore, the slug flow with a higher induced shear stress is more effective on the enhancement of the permeate flux in a UF process.     

Atomization of glycerin with a twin-fluid swirl nozzle

Atomization of liquids with high viscosity is always a challenge, especially when small diameter droplets and high liquid flow rates are simultaneously required. In the present research, the performance of a Venturi–vortex twin-fluid swirl nozzle is examined, attending to its capabilities to generate droplets with diameters below 20 µm when atomizing pure glycerin at room temperature. In this nozzle, air is injected tangentially in a central convergent section, and discharges suctioning the liquid fed to a coaxial chamber, here using a gear pump. The resulting spray is visualized and analyzed. Droplet size distributions are measured with a laser diffractometer. As expected, droplet diameter increases with liquid flow rate, and quickly diminishes when air flow rate is increased. Sauter mean diameters (SMD) below 15 µm can be obtained even when atomizing pure glycerin. However, these values are obtained for relatively low glycerin flow rates (∼5 l/h), and with rather wide distributions. For 10 l/h and an air-to-liquid mass flow rate ratio (ALR) of 13.7 more than 26% of the glycerin volume is atomized in droplets smaller than 20 µm. Liquid ligaments are observed near the nozzle exit, but they tend to break up while moving downstream.     

Investigations on viscous dissipation effect of liquid flow in microtubes

The experimental and numerical investigations are carried out to explore the viscous dissipation effect during de-ionized ultra pure water flowing through smooth quartz glass microtubes with inner diameters of 25 and 50 m, and the Reynolds number varies in the range from 0 to 680. The viscous dissipation characteristic in microtubes is numerically calculated by a 2-D model and the Electrical Double Layer (EDL) effect on the flow is considered. A new criterion V _c demonstrating the law of the viscous dissipation effect in microtubes is summed up with the numerical simulation results. By applying the micro-area thermal-imaging technology and a series of correction tests, the viscous heating temperature increment in the microtube can be exactly measured by an IR camera with a special magnifying lens. Furthermore, the temperature increment of the working fluid due to the heat generated by the pump is also considered in the experimental investigation. The comparisons among the experimental results, the numerical predictions and the new theoretical correlations are made in the present research, which indicates the experimental data are in rough accordance with the numerical and the theoretical results.     

Permeability Reduction by Sediment Retention in Saturated Sand Columns

Recently, researchers in coastal engineering have paid more attention to the role of sediment (particulate organic matter adsorbed on fine solid particles, diameter range 1– $100~\upmu $ m) in considering the biodiversity of estuaries. In this study, permeability reduction of saturated sand columns by sediment retention is investigated through laboratory experiments. Water-based sediment was injected through vertical sand columns under a constant water head difference, with different flow rates, porosities of the sand columns, and the chemical properties of sediment. It was found that the permeability reduction was uniquely correlated with flow rate and sediment properties; that is, increasing flow rate or using sediment containing high amounts of organic matter causes a significant reduction in permeability. Furthermore, an approximate experimental equation is proposed to predict the retention mass of sediment. This equation can predict the retention mass with a relative error of less than 5 %. In addition, a new model is proposed to determine the permeability reduction by sediment retention. This model could predict the permeability reduction with a relative error of 10 %.     

Modeling and Simulation of Local Thermal Non-equilibrium Effects in Porous Media with Small Thermal Conductivity

The two-equation model in porous media can describe the local thermal non-equilibrium (LTNE) effects between fluid and solid at REV scale, with the temperature differences in a solid particle neglected. A multi-scale model has been proposed in this study. In the model, the temperature differences in a solid particle are considered by the coupling of the fluid energy equation at REV scale with the heat conduction equation of a solid particle at pore scale. The experiments were conducted to verify the model and numerical strategy. The multi-scale model is more suitable than the two-equation model to predict the LTNE effects in porous media with small thermal conductivity. The effects of particle diameter, mass flow rate, and solid material on the LTNE effects have been investigated numerically when cryogenic nitrogen flows through the porous bed with small thermal conductivity. The results indicate that the temperature difference between solid center and fluid has the same trend at different particle diameters and mass flow rates, while the time to reach the local thermal equilibrium is affected by solid diameter dramatically. The results also show that the temperature difference between solid center and surface is much greater than that between solid surface and fluid. The values of \( \rho {\text{c}} \) for different materials have important influence on the time to reach the local thermal equilibrium between solid and fluid.