文丘里管空化器内空泡动力学特性的数值模拟

研究了文丘里管空化发生器内空泡的成长、溃灭特性,根据基本的R-P空泡运动方程,考虑了液体粘性、表面张力和可压缩性等因素的影响,运用四阶Runge-Kutta法对空泡径向非线性运动方程进行求解,得到空泡径向演变过程以及溃灭压力的变化趋势.讨论了初始汽泡半径、入口压力和文丘里管的喉径比等因素对空泡演变过程的影响.结果表明,流体的可压缩性对空泡溃灭的影响最大;空化发生器结构参数以及操作参数均对空泡运动特性产生影响,从而影响空化强度.所得结果对空化流场中空化泡演变规律的研究以及水力空化发生器的设计具有指导意义.     

Characterization of an ultrasonic system using wavelet transforms

A new method has been developed for the determination of the spatial distribution of the cavitation intensity in an ultrasound processor. The method uses wavelet transform analysis of the acoustic emission profiles. The periodic modulation of the acoustic pressure field in an ultrasound processor causes unsteady radial motion of the bubbles, resulting in non-stationary acoustic emission profiles that cannot be analyzed by Fourier transform. The cavitation intensity has been judged experimentally and numerically. The experimental method used the “cavitation noise coefficient” defined as the sum of the energy at different scales (or levels) in the wavelet transform of the measured signal containing the subharmonic and harmonics of the fundamental frequency. The numerical method involved the simulation of the radial motion of a bubble and the pressure waves radiated by it, applying experimentally measured acoustic pressure signals as the forcing function. The numerically predicted spatial variation of the cavitation intensity was in agreement with the experimental measurements. It is proposed that the conical divergence of the acoustic waves from the transducers and the differences in the electrical and acoustical characteristics of the adjacent transducers in the bath give rise to a non-uniform cavitation intensity distribution.     

Modeling of hydrodynamic cavitation reactors: a unified approach

An attempt has been made to present a unified theoretical model for the cavitating flow in a hydrodynamic cavitation reactor using the nonlinear continuum mixture model for two-phase flow as the basis. This model has been used to describe the radial motion of bubble in the cavitating flow in two geometries in hydrodynamic cavitation reactors, viz., a venturi tube and an orifice plate. Simulations of the bubble dynamics in a venturi flow demonstrate the stable oscillatory radial bubble motion due to a linear pressure gradient. Due to an additional pressure gradient due to turbulent velocity fluctuations the radial bubble motion in case of an orifice flow is a combination of both stable and oscillatory type. The results of numerical simulations have been explained on the basis of analogy between hydrodynamic cavitation and acoustic cavitation.     

Numerical investigation on relationship between injection pressure fluctuations and unsteady cavitation processes inside high-pressure diesel nozzle holes

The relationship between injection pressure fluctuations and unsteady cavitation processes inside the high-pressure diesel nozzle holes have been numerically analyzed by using a two-fluid approach. In order to improve prediction of nozzle hole cavitation content under high-pressure injection conditions, a new adjustment model of bubble number density has also been developed through the analysis of cavitation bubble dynamics and internal flow characteristics of nozzle hole. Model validation results verify that this model is applicable for a wide range of diesel injection pressures. Based on simulation results, it has been found that cavitation bubbles in recirculation zone and its wake flow show totally different responses to the variations of upstream pressure, and the evolution of cavitation content shows a close association with the time derivatives of upstream pressure.     

A physical insight into sonochemical emulsion polymerization with cavitation bubble dynamics

This paper tries to explain the physical features of the sonochemical emulsion polymerization process by coupling experiments with different conditions (such as monomer type, saturation level of the medium and the type of bubbling gas) with a mathematical model for the radial motion of cavitation bubble. Experiments have been performed without any added chemical initiator or surfactant. Time variation of the mean size and size distribution of polymer particles in the emulsion have been used as a measure for the analysis. This measure is found to be governed by various parameters such as rate of radical production from the cavitation bubbles, magnitude of the microturbulence and shock waves produced by the cavitation bubbles, glass transition temperature of polymer and the population density of polymer particles. The relative magnitudes of these parameters vary significantly with the experimental conditions. This variation has been explained on the basis of results of simulation of radial motion of cavitation bubble. It is revealed that the mean particle size and size distribution of particles are manifestation of simultaneous and resultant influence of these parameters.     

Flow regime maps and optimization thereby of hydrodynamic cavitation reactors

Hydrodynamic cavitation reactors are known to intensify diverse physical and chemical processes. In this article, flow regime maps have been proposed that give an overview of the operation of hydrodynamic cavitation reactor for different combinations of design and process parameters. These maps are based on simulations of cavitating flow using mathematical model that couples continuum mixture model with diffusion limited model. Specific flow regimes have been identified depending on the energetics of the collapse of cavitation bubble as sonophysical, sonochemical, and stable oscillatory (no physical or chemical effect). The radial motion of the bubble in the cavitating flow is governed by the mean and turbulent pressure gradients, which in turn, are decided by the design parameters. An analysis of variations in the pressure gradients in the cavitating flow with design parameters has been given. The flow regime maps form a useful tool for identification of most optimum set of design parameters for hydrodynamic cavitation reactor for a physical or chemical process. © 2012 American Institute of Chemical Engineers AIChE J, 58: 3858–3866, 2012     

Bubble flow mechanisms in trickle beds—an experimental study using image processing

The mechanisms of bubble motion in concurrent gas-liquid down flow through trickle beds are investigated. The laboratory reactor is a structured quasi-two-dimensional porous medium with an average pore diameter close to the values encountered in trickle beds. The accuracy of the reactor design is demonstrated by hydrodynamic investigations on the reactor scale where it is shown that the flow regimes encountered and the experimental pressure drop are comparable to those observed in trickle beds. The investigations on the pore scale are focused on the dispersed bubble flow regime where the liquid flow is continuous and the gas is divided into elongated bubbles. The bubble motion is recorded with the aid of a high-speed video camera and the images are processed and analysed in a quantitative manner. The investigations clearly show that in dispersed bubble flow, the bubbles are frequently pulsing on the pore scale. The mechanism of this flow pattern is discussed.     

柴油液滴内空化气泡的破碎研究(英文)

A modified mathematical model is used to study the effects of various forces on the stability of cavitation bubbles within a diesel droplet. The principal finding of the work is that viscous forces of fluids stabilize the cavitation bubble, while inertial force destabilizes the cavitation bubble. The droplet viscosity plays a dominant role on the stability of cavitation bubbles compared with that of air and bubble. Bubble–droplet radius ratio is a key factor to control the bubble stability, especially in the high radius ratio range. Internal hydrodynamic and surface tension forces are found to stabilize the cavitation bubble, while bubble stability has little relationship with the external hydrodynamic force. Inertia makes bubble breakup easily, however, the breakup time is only slightly changed when bubble growth speed reaches a certain value (50 m·s?1). In contrast, viscous force makes bubble hard to break. With the increasing initial bubble–droplet radius ratio, the bubble growth rate increases, the bubble breakup radius decreases, and the bubble breakup time becomes shorter.     

HYDRODYNAMIC CHARACTERISTICS OF THE PROCESS OF DEPRESSURIZATION OF SATURATED WATER

Experiments were carried out to find the effects of dissolved gas pressure,liquid flow rateand nozzle geometry on the bubble generation when saturated water was depressurized through anozzle.A new method,high speed camera system was developed to measure the generated microbubblesdynamically.On the basis of the laws of ideal gas and solution,theoretical generated gas flow ratewas deduced,while the Smoluchowski′s equation was applied to describe the kinetics of bubblenucleation.It was found that the size distribution of nucleated bubbles was of skewed distribution.An explanation to this phenomenon was made and the Gamma function distribution was employedfor mathematical simulation.The results show good agreement between the experimental data and thepredictions by proposed model.     

Application of Hydrodynamic Cavitation to Wastewater Treatment

Hydrodynamic cavitation is a promising application in wastewater treatment due to its simple reactor design and capacity in large‐scale operation. Theoretical studies including the basic mechanism of pollutant degradation, modeling of pressure distribution in the cavitation reactor, and bubble dynamics models coupled with chemical reactions are evaluated. Experimental setups with different cavitation reactors and operation parameters are compared for degrading specific kinds of pollutants. The effort directions for both theoretical and experimental investigations are suggested on the basis of the reviewed contents. Easy large‐scale operation, effective combination with intensified strategies, and capability to deal with biorefractory or toxic compounds contribute to the great potential of hydrodynamic cavitation.     

Modeling a fluidized bed reactor by integrating various scales: Pore,particle, and reactor

This work proposes a novel population-balance based model for a bubbling fluidized bed reactor. This model considers two continuum phases: bubble and emulsion. The evolution of the bubble size distribution was modeled using a population balance, considering both axial and radial motion. This sub-model involves a new mathematical form for the aggregation frequency, which predicts the migration of bubbles from the reactor wall toward the reactor center. Additionally, reacting particles were considered as a Lagrangian phase, which exchanges mass with emulsion phases. For each particle, the variation of the pore size distribution was also considered. The model presented here accurately predicted the experimental data for biochar gasification in a lab-scale bubbling fluidized bed reactor. Finally, the aggregation frequency is shown to serve as a scaling parameter.     

低电导率工质中气泡的极化运动实验研究

为探讨电场作用下气泡在低电导率工质中的极化运动特性,采用高速数码摄像技术对气泡在正庚烷溶液中的生长和分散过程进行了可视化研究,并结合无量纲数分析了不同气体流量和施加电压下的气泡演变特征以及极化力主导的气泡运动规律。结果表明,增大电场强度可导致气泡生长周期缩短,气泡尺寸显著减小,产生频率加快。在低电场强度下,气泡运动主要表现为流体动力学特性;而在强电场作用下,气泡首先受极化力主导而表现为电流体动力学特性,其直线轨迹高度随Bo_E增大而增大。但随着电场强度在竖直方向上的衰减以及液相阻力影响,气泡运动速度不断减小;当气泡脱离极化力主导区域后,其运动再次表现为流体动力学特性,受尾迹诱导和气泡间相互作用影响,气泡在竖直方向上沿毛细管轴向四周扩散。     

SEBS高黏度溶液气液鼓泡塔的流体力学研究

针对SBS加氢反应器开发与设计,以SEBS-1650己烷溶液为液相,采用差压法和床层塌落法研究了气液鼓泡塔中高黏度溶液的流体力学行为,考察了黏度对低表面张力溶液的气含率、大小气泡气含率、大小气泡上升速度和比表面积等因素的影响。结果表明,随黏度增加,大气泡增多,气含率明显降低,塔内流型处于湍流区;由床层塌落曲线确定鼓泡塔内存在三种类型的气泡：大气泡、小气泡及细小气泡,随黏度增加,小气泡与细小气泡逐渐减少;黏度对大小气泡的上升速度略有影响,比表面积随黏度增加而明显降低。根据实验结果给出了大小气泡气含率与平均气含率的计算公式。     

Investigations on hydrodynamics and mass transfer in gas–liquid stirred reactor using computational fluid dynamics

In this work, the hydrodynamics and mass transfer in a gas–liquid dual turbine stirred tank reactor are investigated using multiphase computational fluid dynamics coupled with population balance method (CFD–PBM). A steady state method of multiple frame of reference (MFR) approach is used to model the impeller and tank regions. The population balance for bubbles is considered using both homogeneous and inhomogeneous polydispersed flow (MUSIG) equations to account for bubble size distribution due to breakup and coalescence of bubbles. The gas–liquid mass transfer is implemented simultaneously along with the hydrodynamic simulation and the mass transfer coefficient is obtained theoretically using the equation based on the various approaches like penetration theory, slip velocity, eddy cell model and rigid based model. The CFD model predictions of local hydrodynamic parameters such as gas holdup, Sauter mean bubble diameter and interfacial area as well as averaged quantities of hydrodynamic and mass transfer parameters for different mass transfer theoretical models are compared with the reported experimental data of [Alves et al., 2002a] and [Alves et al., 2002b] . The predicted hydrodynamic and mass transfer parameters are in reasonable agreement with the experimental data.     

催化裂化进料喷嘴技术的研究进展

综述了催化裂化进料喷嘴的作用及特点,详细介绍了喉管类、靶式类、气泡雾化类、旋流式类、三次雾化类、气动雾化和气泡雾化结合类等进料喷嘴的结构特点、雾化机理及雾化效果。进料喷嘴的发展方向是深入了解气液两相流理论和气液初始作用,以及喷雾场存在雾化不均匀性,进一步研究出口形式使雾化粒度合适,压降分配合理。     

Experimental study on the influence of the orifice size on hydrodynamic characteristics and bubble size distribution of an external loop airlift reactor

The orifice size has a significant influence on hydrodynamic characteristics and bubble size distribution (BSD) in gas-liquid flow. However, the mechanism of the influence of orifice size on BSD and hydrodynamic characteristics in an external loop airlift reactor (EL-ALR) has not been fully revealed. In this work, the effects of the orifice size on hydrodynamic characteristics and BSD in a laboratory scale EL-ALR were investigated using the particle image velocimetry (PIV) technique and digital image analysis (DIA). The results show that the transition superficial gas velocity becomes greater when the orifice size is reduced. The time-averaged bubble velocity profiles along the riser radius are parabolic, and the shape of the time-averaged bubble velocity curve is strongly dependent on the orifice diameter. The larger the orifice diameter, the steeper the parabolas. For sparger with lager orifice diameters, the BSD curve is bimodal even at low superficial gas velocity, and its peaks shift to a larger equivalent bubble diameter. The bubble aspect ratio appears to be related only to the equivalent diameter of the bubbles, regardless of the diameter of the orifice. It has a defined relationship between the bubble aspect ratio and the bubble equivalent diameter, and a new correlation is obtained based on the experimental data. This study will help to gain an understanding of the influence of sparger orifice size on the hydrodynamic characteristics and BSD and provide a basis for numerical simulation.     

Experimental detection of bubble-wall interactions in a vertical gas-liquid flow

Bubble motions and bubble-wall interactions in stagnant liquid were experimentally investigated by high-speed CCD and PIV technique with the main feature parameters such as E(o)tv(o)s numbers Eo =0.98-1.10,Morton number Mo =3.21 × 10-9 and Reynolds numbers Re =180 ～ 190.The effect of bubble injecting frequency and the distance S between the gas injection nozzle and the wall on the statistical trajectory of bubbles,average velocity distribution of flow field and Reynolds shear stress were studied in detail.It was shown that the combination of bubble injecting frequency and the distance S caused different bubble motion forms and hydrodynamic characteristics.When the normalized initial distance was very little,like S* ≈ 1.2 (here S* =2S/de,and de is the bubble equivalent diameter),bubbles ascended in a zigzag trajectory with alternant structure of high and low speed flow field around the bubbles,and the distribution of positive and negative Reynolds shear stress looked like a blob.With the increase of distance S*,bubbles' trajectory would tend to be smooth and straight from the zigzag curve.Meanwhile,with the increase of bubble injecting frequency,the camber of bubble trajectory at 20 ＜ y ＜ 60 mm had a slight increase due to the inhibitory effect from the vertical wall.Under larger spacing,such as S* ≈ 3.6,the low-frequency bubbles gradually moved away from the vertical plane wall in a straight trajectory and the high-frequency bubbles gradually moved close to the vertical wall in a similar straight trajectory after an unstable camber motion.Under the circumstances,high-speed fluid was mainly distributed in the region between the wall and the bubbles,while the relative large Reynolds shear stress mainly existed in the region far away from the wall.     

垂直圆管内湍流泡状流的数值研究

在经典Euler/Euler型水动力模型基础上,引入考虑不同直径气泡的种群平衡方程来描述气液两相泡状流,对液相和气相分别建立了基本方程,通过对气泡的受力分析并考虑气泡之间聚合和破碎效应后给出了本构方程,建立了封闭的双流体模型并用于垂直管道湍流泡状流的三维数值模拟.模型预测值与实验数据的比较结果表明该模型能较好地模拟垂直管道湍流泡状流中的相含率分布、速度分布、湍动能分布、气泡直径分布以及气泡直径分布的演变过程.     

Three-dimensional vortex simulation of bubble dispersion in excited round jet

This study is concerned with the three-dimensional numerical simulation of a bubbly jet, injected vertically upward from a circular nozzle in still water, when the axial and helical disturbances are imposed. The water flow is simulated by a vortex method, and the equation of motion for a bubble is solved on the flow by the Lagrangian scheme. The disturbances markedly change the vortical structure of water in the developing region. Since the bubbles accumulate on the high vortical region, their dispersion remarkably varies owing to the disturbances. The single helical disturbance causes the larger dispersion of bubble. The combined two helical disturbances make the bubble concentrate on a line in the jet cross-section. The present simulation suggests the possibility of the active control for the bubble dispersion in bubbly jet.     

Neue Aspekte zum Einfluß des Blasenwakes auf die Hydrodynamik und den Stoffaustausch in einer Blasensäule

New aspects of the influence of the wake of bubbles on fluid dynamics and mass transfer in bubble columns. Wake-phenomena behind solid and fluid particles moving in a continuous liquid are an important fluid-dynamic effect which has a pronounced influence on heat- and mass transfer in two-phase dispersions. In this paper results reported in the literature are used to analyse the influence of wakes behind bubbles on the hydrodynamics of bubble columns. A new class of model is introduced to describe the flow and mixing of the liquid phase, taking into account major hydrodynamic effects.