Analytical and numerical evaluation of two-fluid model solutions for laminar fully developed bubbly two-phase flows

An analysis of the two-fluid model in the case of vertical fully developed laminar bubbly flows is conducted. Firstly the phase distribution in the central region of the pipe (where wall effects vanish) is considered. From the model equations an intrinsic length scale L is deduced such that the scaled system reduces to a single equation without parameters. With the aid of this equation some generic properties of the solutions of the model for pipes with diameter greater than about 20L (the usual case, since L is of the order of the bubble radius) are found. We prove that in all physically meaningful solutions an (almost) exact compensation of the applied pressure gradient with the hydrostatic force occurs (with ρ_eff the effective density and the gravity). This compensation implies flat void fraction and velocity profiles in the central region not affected by the wall, even when no turbulence effects are accounted for.We then turn to consider the complete problem with a numerical approach, with the effect of the wall dealt via wall forces. The previous mathematical results are confirmed and the near-wall phase distributions and velocity profiles are found. With the numerical code it is also possible to investigate the regime in which the pressure gradient is greater than the weight of the pure liquid, in which case a region of strictly zero void fraction develops surrounding the axis of the pipe (in upward flow of bubbles).Finally, the same code is used to study the effect of reducing the gravity. As decreases, so does the relative velocity between the phases, making the lift force increasingly dominant. This produces, in upward bubbly flows, narrower and sharper void fraction peaks that also appear closer to the wall.     

Non-contact electrostatic flow probes for measuring the flow rate and charge in the two-phase gas-solids flows

The paper presents mathematical models of a measuring system for real-time monitoring the net electric charge, the mass flow rate or concentration of charged particles, and the mean flow velocity of the particles in a pipe. The main problem lies in proper defining and establishing the unambiguous relationships between the electric parameters of a measuring system, which uses a probe, sensor or transducer based on electrostatic induction. Such devices are non-contact, non-obstructive ones and are intended for the two-phase flow metering of charged solids in pipelines. The probe's output potential (voltage) is derived as a function of an equivalent point charge representing the net charge of a charged particle column within the probe's sensing zone and flowing in a real pipe as well as a function of the net charge of this column and its dynamic space density. The relationships are established for the above three forms of charge travelling rectilinearly along or parallel to the probe's geometrical axis in a laminar or fully developed turbulent flow. The models presented here show a useful, helpful approach to the modelling of non-contact, non-intrusive measuring systems based on electrostatic induction. The substantial role the capacitance of the electrostatic flow probe itself and the whole measuring system plays in the physical process of electrostatic induction of charge and in the mathematical modelling of the probe and system is interpreted. The capacitance influence on the system bandwidth is also discussed.     

Flow structure of gas-liquid two-phase flow in an annulus

The flow structure of gas-liquid two-phase flow in vertical annulus channel has been investigated. The inner and outer diameters of the annular channel were 19.1 and 38.1 mm, respectively. The total height of the test section was 4.37 m. Nineteen inlet flow conditions were selected, which cover bubbly, cap-slug, and churn-turbulent flows. The local flow parameters, such as void fraction, interfacial area concentration (IAC), and bubble interface velocity, were measured at nine radial positions within the gap of the annulus at z/D_h=230 of the test section. The flow regimes of the flow conditions, which were based on visual observations, were compared with several flow regime maps. In addition, the local measurements were used to calculate distribution parameter, C₀ in drift-flux model, and area-averaged IAC. A new correlation of C₀ was proposed based on the experimentally obtained C₀ values. This correlation was tested in the drift-flux model successfully along with Ishii's drift velocity correlations. The area-averaged IAC values were compared with the most widely used models. The advantages and drawbacks of these models were highlighted.     

Stereoscopic PIV studies on the swirling flow structure in a gas cyclone

Understanding the swirling flow in a gas cyclone is of great importance in improving the cyclone design. Once the three-dimensional strong swirling flow is fully understood, cyclone performance such as pressure drop and separation efficiency can be improved by optimizing the cyclone design. The swirling flow was investigated by the stereoscopic particle image velocimetry (Stereo-PIV) in this work. The instantaneous whole-field tangential, axial, and radial velocities were measured simultaneously in the cylindrical and conical separation zone, and in the dust hopper area of the cyclone with gas inlet velocity of .The time-averaged flow pattern in the cylindrical and conical sections of the cyclone showed: a typical Rankine vortex with inner quasi-forced vortex and outer quasi-free vortex which is generated by tangential gas velocity; inner upward flow and outer downward flow of axial gas velocity; and centripetal flow in the region close to the wall due to the presence of radial gas velocity. In the dust hopper, a secondary longitudinal circular flow is formed in the annulus area between the conical body and the cylindrical wall. Experimental results indicate that the separated particles may be re-entrained into the cyclone from the bin to degrade the separation efficiency of the cyclone.     

倾斜油水两相流复杂网络社团结构探寻

提出一种基于延迟嵌入和模块度的复杂网络构建方法,并利用倾斜油水两相流电导波动信号构建了流型复杂网络。基于数据场理论的社团探寻算法对该网络的社团结构进行了分析,发现该网络存在分别对应于D O/W PS流型, D O/W CT流型和过渡流型的3个社团。基于复杂网络理论从全新的角度探讨了两相流流型辨识问题,并指出复杂网络是非线性时间序列分析的一个有效工具。     

气/液-固两相流中的粒子速度

通过实验研究和理论分析,考察了提升管中颗粒相的质量守恒;发现在传统的两相流研究中,对颗粒速度的时间平均和算术平均存在着误用。在传统理论中,由于引入了颗粒相的体积分率项,实际上是对离散的颗粒进行了连续化的假设。由此不可避免地造成颗粒速度时间平均和算术平均的混淆。在微观尺度上对气/液-固两相流中颗粒速度时间平均的物理意义进行了分析,给出了计算非连续流场中颗粒时均速度的表达式,并提出了分析颗粒相质量守恒的方法。     

Simulation of polydisperse multiphase systems using population balances and example application to bubbly flows

In this work the relationship between multiphase computational fluid dynamics models and population balance models is illustrated by deriving the main governing equations from the generalized population balance equation. The resulting set of equations, consisting of the well known two-fluid model coupled with a bivariate population balance model, is then implemented in the CFD code OpenFOAM. The implementation is used to simulate a particular multiphase problem: bubbly flow in a rectangular column. Results show that, although the different mesoscale models for drag force, coalescence, breakup and mass transfer, can be improved, the agreement with experiments is nevertheless good. Moreover, although the problem investigated is quite complex, as the evolution of bubbles is solved in real-space, time and phase-space (i.e. bubble size and composition) the resulting computational costs are reasonable. This is due to the fact that the bivariate population balance model is solved here with the so-called conditional quadrature method of moments, that very efficiently deals with these problems. The overall approach is demonstrated to be efficient and robust and is therefore suitable for the simulation of many polydisperse multiphase flows.     

Numerical and experimental studies on the flow multiplicity phenomenon for gas-solids two-phase flows in CFB risers

The flow multiplicity phenomenon in circulating fluidized bed (CFB) risers, i.e. under the same superficial gas velocity and solids circulation rate, the CFB risers may sometimes exhibit multiple flow structures, was numerically and experimentally investigated in this study. To investigate the flow multiplicity phenomenon, the experiments of gas-solids two-phase flows in a 2-D CFB riser with different flow profiles at the inlet of the CFB riser were conducted. Specially designed gas inlet distributors with add-ons are used to generate different flow profiles at the inlet of the CFB rise. The CFD model using Eulerian-Eulerian approach with k-ε turbulence model for each phase was employed to numerically analyze the flow multiplicity phenomenon. It is experimentally and numerically proved that for gas-solids two-phase flows, the flow profiles in the fully-developed region are dominated by the flow profiles at the inlet. The solids concentration profile is closely coupled with the velocity profile, and the inlet solids concentration and velocity profiles can largely influence the fully-developed solids concentration and velocity profiles.     

水溶解CO2过程界面对流现象的PIV/LIF测量及传质系数预测

建立了用于观测水溶解CO₂过程界面Rayleigh对流现象的粒子成像测速（PIV）和激光诱导荧光（LIF）技术联用方法,同时获得了该对流传质过程的液相速度场及浓度分布,定量观测了Rayleigh对流的发展过程。经界面对流发生条件下传质过程的分析,指出Rayleigh对流过程的涡量是影响传质过程的主要因素,给出了传质系数随涡量的变化关系和相应的关联式,进而为通过液相速度场的测量预测Rayleigh对流条件下的液相传质系数提供了一种有效的方法。     

子波分析识别环流反应器泡状结构实验

用热膜测速技术以高于对应最小湍流时间尺度的分辨率精细测量了环流反应器内气液二相流速度的时间序列信号,对气液二相流速度信号中自动识别泡状流结构进行了研究。用子波分析方法,将热膜测速仪输出的气液二相流混合速度信号在时域空间和频域空间同时进行时频双局部化多尺度分解,研究了气液二相流中液相流和泡状流结构的分尺度时域特征和分尺度能谱等统计特征,利用子波能谱分析的能量最大准则,提出了一种用子波变换自动识别气液二相流中泡状流结构的新方法。     

Circumferential mixing in one-phase and two-phase Taylor vortex flows

This work deals with the experimental study of the circumferential mixing in the Taylor vortex flow between coaxial cylinders. Each pair of vortices is considered as a closed chemical reactor and the residence time distribution in this reactor is determined by an electrochemical method. The tanks-in-series model with recirculation is applied to describe the flow in the vortex reactor; the circumferential mixing is characterized by the number of tanks in series and by the mixing time. Experiments were carried out with one-phase flow and with two-phase liquid—liquid flow. It is shown that stirring due to the liquid droplets increases the circumferential mixing.     

管道泡状流相分布模式和分布机理研究进展

对管道泡状流的相分布模式和分布机理进行了详细的回顾和介绍。从前人研究的结果发现,相分布与流动条件有关,对于小管径泡状上升流,相分布主要表现为壁面峰值分布、中间峰值分布、中心峰值分布、过渡分布和扁平分布模式;而对于小管径泡状下降流,相分布主要表现为偏离中心峰值分布、钟形分布和中心峰值分布模式。然而对于大管径泡状流,相分布与流向无关,主要表现为壁面峰值分布和中心峰值分布两种模式。除此之外,还存在双峰分布模式和双鞍分布模式。影响相分布的主要因素有气泡尺寸、管道尺寸、气液相速度、气泡的注入位置和注入方法、重力水平,而气泡尺寸为关键因素。调查发现,到目前为止仍未形成一个可以解释所有相分布模式的通用机理。部分物理现象仅通过分析升力、湍流扩散力、壁面斥力以及其他力的平衡给予定性分析。未来应进一步研究相间作用力模型、湍流相干结构对气泡输运机理等问题。     

液固及气液固三相并流系统的动力学行为和混沌产生的研究

运用时域、频域、吸引子及混沌特征参数相关维和Kolmogrov熵研究了液固并流系统动力学行为,揭示了液固并流系统是由拟周期过渡到混沌的。在拟周期行为的液固并流系统中引入气体,结果表明:低气速下,床层中只有冠状气泡,它表现为拟周期行为。随着气速的增大,床层中小气泡的出现及浆料湍动的作用使拟周期冠状气泡过渡为混沌,进而整个系统通向了混沌。     

降膜熔融结晶器内气液二相流场的数值模拟

在典型的熔融结晶过程中,脉冲式通入气泡是关键的操作条件之一。文中通过分析对甲酚熔体中形成的气液二相流动特点,建立了降膜熔融体系的气体通入过程中二相流流动数学模型,采用CFD技术对连续和脉冲过程中的气液二相流流场进行了数值模拟。利用气泡形态和上升速度的比较验证了模拟结果的可靠性。研究结果表明:脉冲通气形成的气液二相流的整体速度与连续通气条件下的相比较要小,但是在结晶器近壁面区域的速度却明显比连续通气条件下的大,且弹状气泡形成的速度场促进了流体的径向混合。在结晶过程中,脉冲通气形成的弹状气泡流动有利于熔体与晶体壁面之间的层流层与液相主体之间的传质和传热,从而有利于保持边界层稳定的浓度和温度梯度。     

油水混合物在水平管中的二相流动特性研究

对内径40mm的钢管和有机玻璃管内油水二相水平流动时的流型、摩擦压降特性进行了详细的实验研究,结果表明:油包水向水包油的转变发生在含水体积分数约0. 4时。随含水体积分数的增大,油水二相流的摩擦压降先是急剧减小,其后在含水体积分数大于0. 4时压降变化趋于平缓。油水二相流的摩擦压降受含水体积分数、管壁润湿特性、管壁粗糙度以及混合物流速的影响,当二相流体处于水包油状态时,钢管内的摩擦压降比有机玻璃管内的大;而当处于油包水时,有机玻璃管内的摩擦压降则比钢管内的摩擦压降大。     

Prediction of pressure drop and void-fraction in annular two-phase flows

The polynomial vvelocity profile due to Pai (1953), applicable to transition and turbulent conditions in single phase flow, has been used to solve the equations of two-phase annular flow with appropriate matching conditions at the interface and at the boundaries. The results obtained enable one to predict the pressure drop and the void-fraction, when the interfacial friction factor is specified. Entrainment of the liquid in the core of the flowing gas has been taken into account by using the empirical relation of Hutchinson and Whalley (1973). It has been shown that the theory provides results in agreement with horizontal tube and vertical tube adiabatic flow data, in addition to predicting the liquid film thickness accurately.     

泡状流中水合物生成预测模型及实验

针对可燃冰钻采井筒内易发生水合物生成和堵塞的工程问题,本文开展了泡状流条件下甲烷水合物生成实验,发现流速增加会提高水合物生成速率,黄原胶质量分数的增加会降低水合物生成速率。基于传质理论,构建了适用于可燃冰钻采井筒内泡状流条件下水合物生成预测模型,模型考虑了连续相流体流变性、气泡破裂、聚并和形变等因素对泡状流中气液界面分布和气液间传质规律的影响,并耦合实验数据,提出了气泡群间的综合传质系数经验公式,用于描述气泡间相互作用对气液间传质速率的影响。对比实验结果,所建立模型对水合物生成量和水合物生成速率的预测误差分别在±5%和±15%以内,满足工程计算需求。该模型的构建有助于精准预测油气和可燃冰钻采井筒内水合物风险,为建立经济、高效的井筒水合物防治方案奠定理论基础。     

两相均流板对弯管中气固两相运动分布特征的影响

针对燃煤电厂烟气污染物脱除设备入口弯管内气固两相分布不均的问题提出一种新型两相均流板,采用CFD数值模拟首先对比分析了安装两相均流板与常规导流装置的均流效果,之后详细研究了新型两相均流板对弯管水平出口管道中气流速度、颗粒质量分布规律的影响以及板型及板间夹角对两相均流板均流效果的影响,并结合Design-Expert响应面法获得两相均流板的最优结构,将最优结构应用于工业性实验中,最后将计算结果与工业性实验数据进行了比较。结果表明:弯管内安装两相均流板较安装导流板/三角翼挡板气固两相均流效果更优,既可以使气固两相均匀分布又可以有效降低系统的压阻;随板间夹角的增大,管内的气流速度分布以及颗粒的质量浓度分布呈阶段性变化,而管内的压降则随板间夹角增大而增大;最佳两相均流板结构为夹角为75.49°的直板型两相均流板。模拟与实验结果吻合较好,利用提出的数值模拟方法可以详细真实地模拟计算大型及具有复杂内部结构的除尘器入口弯管中气固两相流动。