Basset力对液体中易溶性气泡运动的影响

为从理论上分析Basset力对液体中易溶性气泡运动过程的影响, 综合考虑气泡上浮速度和传质速率间的耦合关系, 构建了适于描述易溶性气泡上浮过程的动态耦合模型. 利用该模型对氨气鼓泡吸收过程的计算分析表明, Basset力对易溶性气泡运动行为影响显著. 为此, 又提出了衡量Basset力对气泡运动全过程影响大小的评价指标(无量纲$\eta $数)及其算法. 基于上述模型和指标对水中气泡上浮过程的分析表明: 气体溶解度系数$H$对Basset力影响气泡运动的强度具有决定性作用, 当气泡内气体的$H<10^{-4}$时, 可以忽略Basset力的影响, 而当$H>10^{-4}$时, Basset力的影响迅速增强, 不应忽略; 且气泡半径越小的, Basset力的影响越显著; 而气泡所处深度与Basset力的影响强度关系不大.      

离散型湍流多相流动的研究进展和需求

离散型多相流动,指气体-颗粒(气-固)、液体-颗粒(液-固)、液体-气泡、气体-液雾以及气泡-液体-颗粒等两相或三相流动.这种类型的多相流动广泛存在于能源, 航天和航空, 化工和冶金,交通运输, 水利, 核能等领域.本文阐述了离散型多相流动的国内外基础研究,包括颗粒/液滴/气泡在流场中受流体动力作用力的研究, 颗粒-颗粒,液滴-液滴,气泡-气泡之间以及颗粒/液滴和壁面之间碰撞和聚集规律的研究,颗粒-气体和气泡-液体湍流相互作用的研究, 和数值模拟的研究,包括多相流动的雷诺平均模拟、大涡模拟和直接数值模拟的研究进展.最后, 归纳了目前尚待研究的需求.      

非定常流场中颗粒运动所受Basset力的性质解析

鉴于目前工程固液两相流研究中颗粒运动所受Ｂａｓｓｅｔ力性质的不明确性，应用典型基函数试验法从理论上对其进行了解析．结果表明，在颗粒相对流体的变速运动过程中，Ｂａｓｓｅｔ力是一项比较重要的作用力，一般不宜忽略．但对于湍流场中的高频分量而言，颗粒在其中受到的Ｂａｓｓｅｔ力则很小，可忽略不计．     

格子Boltzmann方法分析气泡的运动及其相互作用

利用格子Boltzmann方法对气泡在液体中的运动规律及相互作用的气液两相流问题进行了研究;采用了基于自由能模型且适用于大密度比多相流问题的Z-S-C模型,计算模拟了处于不可压缩流体中的气泡在浮力作用下的运动特性;通过算例验证了Z-S-C模型的可行性;研究了不同放置位置两气泡的运动形态及其相互作用情况,分别分析了沿y方向同轴放置的两气泡间距变化对合并过程和沿x方向并列放置的两气泡的间距变化对气泡相互作用的影响。结果表明:两气泡的初始间距在一定范围内时,初始间距对上升过程中气泡间的相互作用具有重要影响;当初始间距超过一定量值时,上升过程中两气泡相互独立,不存在相互作用。     

激光多普勒测速技术在气液两相流中的应用

本文采用激光颗粒动态分析仪(PDA)测量了钢包底部喷吹气液两相流中气泡的直径和气泡上升速度的分布;采用激光多普勒测速仪(LDA)测量了气液两相区和液体单相循环区液体速度场的分布。测量结果表明:气泡在脱离喷嘴上浮一定距离后,其大小基本保持不变;在气液两相区中,气泡速度和液体速度的分布均服从高斯分布;液体在单相区作循环流动,在侧壁与底部交接处,存在液体流动的“死区”。     

鼓泡流化床中流动特性的多尺度数值模拟

鼓泡流化床因其较高的传热特性以及较好的相间接触已经被广泛应用于工业生产中,而对鼓泡流态化气固流动特性的充分认知是鼓泡流化床设计的关键.在鼓泡流化床中,气泡相和乳化相的同时存在使得床中呈现非均匀流动结构,而这种非均匀结构给鼓泡流化床的数值模拟造成了很大的误差.基于此,以气泡作为介尺度结构,建立了多尺度曳力消耗能量最小的稳定性条件,构建了适用于鼓泡流化床的多尺度气固相间曳力模型.结合双流体模型,对A类和B类颗粒的鼓泡流化床中气固流动特性进行了模拟研究,分析了气泡速度、气泡直径等参数的变化规律.研究表明,与传统的曳力模型相比,考虑气泡影响的多尺度气固相间曳力模型给出的曳力系数与颗粒浓度的关系是一条分布带,建立了控制体内曳力系数与局部结构参数之间的关系.通过模拟得到的颗粒浓度和速度与实验的比较可以发现,考虑气泡影响的多尺度曳力模型可以更好地再现实验结果.通过A类和B类颗粒的鼓泡床模拟研究发现,A类颗粒的鼓泡床模拟受多尺度曳力模型的影响更为显著.      

固液两相流体中的空泡溃灭计算

本文导出了固液两相流体中球空泡溃灭的运动方程,计算并讨论了空泡溃灭过程中的颗粒运动和颗粒对空泡溃灭的影响,得到了固相浓度、颗粒尺寸等因素与空泡溃灭之间的定性关系。在分析过程中,考虑了液体与固体颗粒之间的阻力耦合作用。     

气液二相流研究概述

气液二相流研究液体和气体(或蒸气)两相介质共存条件下的流动特性。二相体系可以是液体中含有气体或(蒸气)泡或者气体,(或蒸气)中含有液体微滴,也可能因其中气泡或液滴的聚并,两相间形成更复杂的分布状态。气液二相流是自然界、日常生活中常见的现象,在许      

变开度岩体裂隙多相渗流实验与有效渗透率模型

岩体裂隙的有效渗透率是描述岩体非饱和或多相渗流的关键参数,而裂隙开度是影响有效渗透率的重要因素.通过自主研发的粗糙裂隙多相渗流可视化实验平台,针对天然岩体裂隙复制而成的裂隙模型开展变开度条件下的多相渗流可视化实验,研究开度变化对多相渗流流动结构以及有效渗透率的影响.研究表明:非湿润相流体运动通道,在低流量比条件下呈现出气泡流流动结构,而在高流量比条件下呈现较为稳定的通道流流动结构.随着开度的增加,非湿润相流动通道的分支变少、等效宽度增加,两相流体的有效渗透率均增大,流动结构趋于稳定.可视化结果还阐明了柱塞流流动结构下,两相流体交替占据裂隙空间的竞争机制:当非湿润相流体通道由连续转变为不连续时,裂隙进出口压差显著增加;反之,当该通道由不连续转变为连续时,压差显著减小.最后,基于分形理论以及渗透率统计建模方法,建立了考虑开度效应的岩体裂隙多相渗流有效渗透率理论模型,并通过实验测定的有效渗透率数据验证了该模型的正确性与有效性.     

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。     

Efficient calculation of the history force at finite Reynolds numbers

Well-acknowledged problems associated with modeling the history force in large, many-particle simulations are related to the need to store and integrate over the entire lifetime of the particle. To address this concern, a computationally efficient method for calculating the history force (the “window model”) was developed based on the assumption of weak changes in acceleration in the recent relevant history of the particle. This assumption leads to the design of a model with a truncated integration interval which requires storage of and integration over a much shorter period of the particle’s history compared to other history force models. The truncation of the integration window can yield more than an order of magnitude savings in CPU time. In a related study, the two empirical coefficients of the Mei & Adrian history force kernel have been optimized (based on comparison with experimental data for falling particles) to give improved predictions of the data. Both the new history force kernel and the window model have been investigated for a large range of experimental data yielding, to the authors’ knowledge, the most extensive comparison yet conducted. For falling particles, the new history force kernel shows good predictions for particle Reynolds numbers ranging from 9 to 853 and density ratios from 1.17 to 9.32. Good predictions were also obtained using the window model when changes in particle relative acceleration over the window period were modest. For particles under forced oscillating in a quiescent fluid, the history kernel was generally reasonable but did not predict the peak forces well in all cases. This may be explained by noting that the assumption of a t⁻² long-time dependence for the finite Reynolds number history force kernel may become invalid during rapid deceleration and wake ingestion (which can lead to exponential or t⁻¹ behavior). However, the finite Reynolds number kernel gives better predictions in all cases than those made using the Basset history force. The window model was only reasonable for the oscillating particle cases when the changes in the relative particle acceleration over the integration window were small.     

Direct measurement of fluid force on a particle in liquid by telemetry system

For real-time measurement of the fluid force acting on a particle which moves freely in liquid, we have made a “sensor particle” with a built-in wireless sensor. The sensor particle contains a 3-axis acceleration sensor, 3-axis magnetometer, microcomputer, wireless module and cells. The MEMS acceleration sensor detects the gravitational acceleration in addition to the dynamic acceleration of particle. In order to remove the gravity from acceleration signals, we applied the external magnetic field which is oriented in the same direction as the gravity field. The 3-axis magnetometer detects the direction of external magnetic field and consequently we can remove gravitational acceleration from the sensor signals with arbitrary attitude of particle. A preliminary experiment was performed for checking the accuracy of the measurement system. Firstly we measured the force acting on a settling particle toward a solid wall in liquid. The obtained signals of acceleration (force divided by mass) agrees well with the theoretical prediction of particle motion in liquid for each instant of time. The sensor particle also detects the lubrication force which is exerted on particle for a very short time when it comes close to the wall. Moreover, we applied our system to the gravitational settling of the particle assemblage and detected the fluid force acting on a particle in multi-particle system.     

Simultaneous velocimetry/accelerometry measurements in a turbulent two-phase pipe flow

A two-color digital particle image velocimetry and accelerometry (DPIV and DPIA) measurement technique is described that records the velocity and acceleration fields of both the solid and liquid phases simultaneously. Measurements were taken at turbulent conditions of a vertical pipe flow using glass spheres as the solid phase and fluorescent particles to indicate fluid phase motion. Nd-YAG pulse lasers acted as illumination sources and images were recorded by two monochrome CCD cameras. The two-color aspect of the technique was realized by placing optical filters in front of the cameras to discriminate between the phases. Cross-correlations and auto-correlations were applied to determine velocity and acceleration fields of the two phases. Results showing some of the capabilities of the technique as applied to a two-phase pipe flow experiment are provided. For the condition studied, it was found that there was turbulence suppression due to the solid phase and that the statistics associated with the acceleration probability distribution were different for the solid and fluid phases.     

垂直湍流液-固流中大颗粒的相对速度

通过量纲分析和实验测量,对于垂直、局部均匀的湍流稀态液一固流中,大颗粒的相对速度,建立了无量纲参数表达式．用分析和实验相结合的方法,确定了表达式中无量纲参数的幂次及有关系数．实验中用激光多普勒分相测量技术,分别测出流体和颗粒的时均速度结果表明,大颗粒相对速度强烈依赖于流体雷诺数,当流体雷诺数较高时,其幂次渐近于1.5。     

On the use of Csanady’s formulae in a turbulent gas–solid channel flow

The paper examines the use of expressions proposed by Csanady to predict the influence of the crossing trajectory and continuity effects on the decorrelation time scales of the fluid along solid particle trajectories in horizontal and downward vertical channel flows. The model is evaluated using data provided by a direct numerical simulation (DNS) of the carrier phase combined with a Lagrangian simulation of discrete particle (LS). Two particle relaxation times and two values of the gravity acceleration are considered. The results show the possibility of using Csanady’s expressions in a turbulent channel flow provided that the spatial and temporal correlations anisotropy is included in the model. As in isotropic homogeneous turbulence, a decrease of the decorrelation time scales is found to be more important in the directions perpendicular to the mean relative velocity.     

Stokes flow between two confocal rotating spheroids with slip

The steady axisymmetric flow problem of a viscous fluid confined between two confocal spheroids that are rotating about their axis of revolution with different angular velocities is considered. A linear slip, of Basset type, boundary condition on both surfaces of the spheroidal particle and the container is used. Under the Stokesian assumption, a general solution is constructed from the superposition of basic solutions in prolate and oblate spheroidal coordinates. The boundary conditions on the particle’s surface and spheroidal container are satisfied by a collocation technique. The torque exerted on the spheroidal particle by the fluid is evaluated with good convergence for various values of the slip parameters, the relative angular velocity and aspect ratios of the spheroids. The limiting case of no-slip is in good agreement with the available values in the literature.     

A numerical study of the influence of the Basset force on the statistics of LDV velocity data sampled in a flow region with a large spatial velocity gradient

 The motion of small particles, such as those typically used as seeding particles for tracer particle flow velocity measurement techniques, is studied numerically for a flow region with a large spatial velocity gradient. The influence of the Basset history integral on the statistics of results of particle motion calculations which are based on multi-disperse particle size distributions is investigated. The biasing of the measured velocity data, with regard to the actual flow velocity, which results as a consequence of such particle size distributions is discussed. It is found that the net effect of the Basset integral on the calculations is indeed to reduce the maximum RMS deviation associated with the multi-disperse distribution and that the relative reduction increases with a decreasing particle density. The main result of this study is, thus, that it is desirable to use light tracer particles not only because they more readily adjust to a changed flow velocity but in particular also because they tend to contribute less to the overall RMS deviation of velocity data sampled in a region with a large spatial gradient of the flow velocity. Received: 9 August 1996/Accepted: 25 January 1997     

变速运动颗粒所受非恒定作用力分析

本文利用典型函数试验法对变速运动颗粒所受非恒定作用力（Ｂａｓｓｅｔ力和附加质量力）进行了深入分析和比较，结果表明：Ｂａｓｓｅｔ力对重质颗粒运动的影响很大，不能忽略；对于轻质颗粒，则以附加质量力的作用为主，Ｂａｓｓｅｔ力可忽略不计．     

Force acting on a particle in unsteady flow of a pseudoplastic fluid

The accelerated flow of a pseudoplastic fluid around a quiescent sphere at Reynolds numbers Re = 0–200 and dimensionless acceleration Ga = 10–10⁴ is studied by numerical simulation. It is shown that the analytical expression of the added mass force for an ideal fluid is appropriate for a pseudoplastic fluid. An expression for calculating the hereditary Basset force for a pseudoplastic fluid is proposed.