A novel theoretical breakup kernel function for bubbles/droplets in a turbulent flow

Several models for the daughter bubble/droplet size distribution are reviewed and a detailed discussion is given to get a better understanding of the daughter size distribution. A novel theoretical breakup kernel function for bubbles/droplets in a turbulent flow, based on an eddy-bubble/droplet collision method, is developed. It takes into account the energy distribution of turbulent eddies, effect of capillary pressure and surface energy increase during bubble/droplet breakup. An increase in the mother bubble/droplet size and energy dissipation rate increases the probability of unequal breakup. The model prediction is in good agreement with experimental results and the underlying physical situation.     

湍流中气泡破碎建模与实验研究进展

综述了充分发展湍流中气泡破碎的机理和模型,将其机理归纳为湍流涡碰撞、黏性剪切、尾涡剪切脱落过程和界面不稳定性四类。对文献中气泡破碎速率和子气泡大小分布的预测模型进行了系统总结。分析讨论了现有气泡破碎模型的发展和局限性,并提出了未来的发展方向。同时,也综述了湍流中单气泡破碎的实验研究,依据产生湍流的方法归纳为四种情况：增大液体流速产生湍流,采用内构件产生湍流,搅拌产生湍流,以及圆锥反应器结合搅拌产生湍流。总结了现有气泡破碎实验的进展和局限,并进行了分析和展望。最后,通过将文献中气泡破碎速率模型预测值和实验数据进行对比,表明文献中多个破碎模型已经有了较好的预测能力。     

Experiments on breakup of bubbles in a turbulent flow

The breakup of air bubbles in a turbulent water flow is studied experimentally. Water flows from a nozzle array, generating intense turbulence, and then flows downward through a cell. The velocity field is measured by PIV, and the local dissipation rate is estimated using a large‐eddy PIV technique. Bubbles (1.8 to 5 mm) are injected in the bottom of the cell and rise toward the region of intense turbulence, where they break. The time spent by bubbles in various zones without breaking and the number of breakups are evaluated, providing information about the breakup frequency. The number of daughter bubbles and their size distribution are determined. The number of daughters depends on a Weber number , where ? is the turbulent energy dissipation rate, D′ is the mother particle size, ρ and σ are the liquid density and surface tension. The daughter size distribution is a function of their number. © 2017 American Institute of Chemical Engineers AIChE J, 64: 740–757, 2018     

Experimental measurement of bubble breakup in a jet bubbling reactor

The bubble breakups in a jet bubbling reactor are captured using a high-speed camera and the velocity field is measured by particle image velocimetry. Two typical breakup patterns, jet breakup and jet-vortex breakup are observed. The breakup time interval of the jet-vortex breakup is two orders of magnitude higher than the jet breakup. The probability of the jet-vortex breakup and the jet breakup accounting in the total breakup events increases and decreases with the jet velocity and the mother bubble size, respectively. The bubble breakup region increases with the jet velocity. The bubble breakup frequency increases with the turbulent dissipation rate and the mother bubble size. The average number of daughter bubbles increases with the Weber number. An L-shaped daughter bubble size distribution is observed. Empirical correlations are established for the bubble breakup frequency, the average number of daughter bubbles and daughter bubble size distribution, and fitted well with the experimental results.     

微通道内液滴/气泡破裂动力学分析

微化学工程与技术是现代化学工程学科的前沿领域。微通道内液滴及气泡破裂动力学是决定多相过程并行微通道数目放大的基础与难点。破裂流型转换条件、界面动力学和尺寸调控等三方面是微通道内液滴与气泡破裂动力学的主要研究对象。讨论了对称微通道、非对称微通道、多级微通道、旁路微通道、含有障碍物的微通道内气泡和液滴破裂行为及影响因素,指出了目前微尺度下气泡与液滴破裂行为相关研究工作存在的不足,并对该领域未来的发展进行了展望。     

Instability and breakup of cavitation bubbles within diesel drops

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 ...     

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.     

Theoretical model for multiple breakup of fluid particles in turbulent flow field

Generalized phenomenological model, based on the theories of probability and isotropic turbulence, is developed for multiple breakup of fluid particles in turbulent flow field. The approach uses a series of successive binary breakup events occur at a time scale comparable to the colliding eddy turnover time. It was found that the use of energy density, instead of energy, will increase the predicted binary breakup rate which is usually underestimated by the existing models in the literature. Generalization of the binary breakup model for multiple fragmentations is performed by defining a “remaining energy function” for the colliding eddy which means the contribution of original eddy to the later breakup events. For ternary breakage, the model shows a reasonably good agreement with the experimental data. The quaternary fragmentation frequency, however, is of negligible importance at lower energy dissipation rates but its contribution to breakage fraction at higher energy dissipation rates becomes considerable. The results also show that ternary and quaternary breakups have a considerable 90% contribution to the overall fragmentation, while pentenary and further fragmentations are of lower importance at low energy dissipation rates. At higher levels of energy dissipation rate, fragmentations up to six daughter particles contribute to more than 95% of the overall fragmentations. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4508–4525, 2016     

A theoretical model for droplet breakup in turbulent dispersions

A theoretical model for the prediction of droplet breakup rate and daughter size distribution in turbulent dispersions has been developed. It considers the breakup contributed by a novel breakup criterion based on surface energy density increase, the droplet surface oscillation from previous collision and the eddies larger than original droplets. The breakup rate and daughter size distribution predicted by this model show a good agreement with the experimental data reported recently.     

气流式雾化过程的有限随机分裂模型

滴径分布是描述喷嘴雾化性能的一个重要指标.从液滴分裂的自相似性出发,提出了描述气流式雾化过程的液滴有限随机分裂模型.实验和模拟计算结果表明,该模型能较好地模拟三通道喷嘴气流式雾化过程的平均滴径和滴径分布的非线性关系.     

Interface‐shrinkage‐driven breakup of droplets in microdevices with different dispersed fluid channel shape

A new droplet breakup mechanism is previously proposed—interface‐shrinkage‐driven breakup. In coaxial microdevices, when the contact angle between the continuous phase and dispersed fluid channel (DFC) is sufficiently low, the new mechanism instead of the classic shear‐driven mechanism dominates the breakup. The present study further investigated the new mechanism in microdevices with DFCs of different shape. Critical contact angles in different devices were determined by theoretical analysis and verified by experiments. It was found that the critical contact angle for the new mechanism depends on the shape of the DFC. The droplet size was measured for different devices when the new mechanism dominated the breakup. In contrast to the case for the shear‐driven mechanism, the droplet size is little affected by the capillary number. Mathematical models were established to predict the droplet size in different devices and results were found to agree well with experimental results. © 2017 American Institute of Chemical Engineers AIChE J, 63: 367–375, 2018     

Transformation of single drop breakup from binary to ternary and multiple in turbulent jet flows

By releasing liquid drops in turbulent jet flows,we investigated the transformation of single drop breakup from binary to ternary and multiple.Silicone oil and deionized water were the dispersed phase and con-tinuous phase,respectively.The probability of binary,ternary,and multiple breakup of oil drops in jet flows is a function of the jet Reynolds number.To address the underlying mechanisms of this transfor-mation of drop breakup,we performed two-dimensional particle image velocimetry(PIV)experiments of single-phase jet flows.With the combination of drop breakup phenomenon and two-dimensional PIV results in a single-phase flow field,these transformation conditions can be estimated:the capillary number ranges from 0.17 to 0.27,and the Weber number ranges from 55 to 111.     

A model for drop and bubble breakup frequency based on turbulence spectra

In this article, a new Eulerian model for breakup frequency of drops induced by inertial stress in homogeneous isotropic turbulence is developed for moderately viscous fluids, accounting for the finite response time of drops to deform. The dynamics of drop shape in a turbulent flow is described by a linear damped oscillator forced by the instantaneous turbulent fluctuations at the drop scale. The criterion for breakup is based on a maximum value of drop deformation, in contrast with the usual critical Weber criterion. The breakup frequency is then modeled as a function of the power spectrum of Weber number (or velocity square), based on the theory of oscillators forced by a random signal, which can be related to classical statistical quantities, such as dissipation rate and velocity variance. Moreover, the effect of viscosities of both phases is included in the breakup frequency model without resorting to any additional parameter. © 2018 American Institute of Chemical Engineers AIChE J, 65: 347–359, 2019     

A literature review of theoretical models for drop and bubble breakup in turbulent dispersions

This paper presents a literature review on mechanisms and models for the breakage of bubbles and drops (fluid particles) in turbulent dispersions. For the mechanisms, four categories are summarized, namely, turbulence fluctuation, viscous shear stress, shearing-off process and interfacial instability. The models for breakup frequency and daughter size distribution available in literature are reviewed thoroughly. The development and limitation of the existing models are studied and possible improvements are proposed.     

宽Re范围内气泡和液滴曳力系数的关联模型

流粒(气泡或液滴)的曳力系数CD和上升/终端速度因有助于准确预测反应器内相含率分布、液相速度分布、流粒停留时间和传质速率而具有重要意义.但现有用于估算流粒CD的关联式大多分段且只在低雷诺数Re区间内有效,并难以同时准确预测不同实验体系和操作条件下的实验结果.针对这些不足,基于实验测量和理论分析,本工作提出了一个能够在整...     

A CFD-PBM coupled model under entire turbulent spectrum for simulating a bubble column with highly viscous media

To account for the effect of liquid viscosity, the bubble breakup model considering turbulent eddy collision based on the inertial subrange turbulent spectrum was extended to the entire turbulent spectrum that included the energy-containing, inertial, and energy-dissipation subranges. The computational fluid dynamics-population balance model coupled model was modified to include this extended bubble breakup model for simulations of a bubble column. The effect of turbulent energy spectrum on the bubble breakup and hydrodynamic behaviors was studied in a bubble column under different liquid viscosities. The results showed that when the liquid viscosity was <80 mPa s, the bubble breakup and hydrodynamics were almost independent on the turbulent spectrum. At liquid viscosity >80 mPa s, the bubble breakup rate and gas holdup were significantly under-predicted when the inertial turbulent spectrum was used, and when using the entire turbulent spectrum the predictions were more consistent with experimental data.     

A model for turbulent binary breakup of dispersed fluid particles

Accurate predictions of particle size distributions, and therefore of the underlying processes of fluid particle breakup and coalescence are of vital importance in process design, but reliable procedures are still lacking. The present paper aims at developing a modular formulation for the turbulent particle breakup process. The model is to be included in a population balance model which is formulated such as to facilitate the direct future implementation into a full multifluid CFD model.The breakup process is described without introducing adjustable parameters. The current model is a further development of an existing model by Luo and Svendsen (AIChE J. 42 (5) (1996) 1225), which has been expanded and refined, and where an inherent weakness regarding the breakup rate for small particles and small daughter particle fragments are removed. A new criterion regarding the kinetic energy density of the colliding turbulent eddy causing breakup has been introduced. This new criterion is a novel concept describing the breakup process. The details are thoroughly discussed together with possible further modifications. The results from the new model are encouraging because the breakup rate is greatly reduced when the dispersed fluid particles are reduced in size. Further, the response to changes in system variables is reasonable and the distribution of daughter sizes vary in a reasonable way for the different collision possibilities.     

Consideration of low viscous droplet breakage in the framework of the wide energy spectrum and the multiple fragments

An improved model for low viscous droplet breakage has been developed. Unlike the previous work that considered the inertia subrange and adopted the assumption of binary breakage, this work considered the breakage of droplets in the framework of the multiple fragments and the wide energy spectrum (i.e., including the dissipation range, the inertia subrange, and the energy containing range simultaneously). The previous interactions between the droplet and the surrounding fluids have been considered through introducing the interaction forces. The effect of the surface deformation and oscillation resulting from these interactions on the constraints of multiple breakages has been accounted for. These factors have been neglected in the existing models. The wide energy spectrum distribution was found to have an important effect on the nonmonotone evolution of breakage frequency with increasing parent droplet size. The cumulative volume fractions predicted by this work showed a better agreement with the experimental data. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2147–2168, 2015     

微通道内气泡和液滴自组织行为的研究进展

微流体技术良好的可控性为制备高通量的单分散性气泡或液滴提供了新的途径,气泡和液滴的流动行为因在材料领域具有较大的应用前景而受到关注。综述了近年来微通道内气泡和液滴自组织行为的研究进展。气泡或液滴自组织晶格具有周期性的流动特征,自组织行为受分散相体积分数、液滴或气泡尺寸、聚并效应和通道构型的影响。展望了气泡和液滴自组织行为研究过程中待解决的关键科学问题,为进一步的模拟和实验研究提供了参考。     

Direct measurement of droplet breakage in a pulsed disc and doughnut column

The lack of experimental data for the droplet breakup has been one of the limitations for the application of population balance model (PBM). In this work, a high‐speed camera was used to directly measure the droplet breakup frequency and daughter size distribution in a pulsed disc and doughnut column. It was found from the captured video that multiple breakup events were more frequently observed than binary breakup. The multiple breakup was treated as an original breakup and several intermediate breakups to characterize the process quantitatively. The effects of pulsation intensity, dispersed phase flow rates, and the spatial locations were investigated in detail. Empirical correlations were finally established for both the breakup frequency function and the daughter droplet size distribution function and fitted well with the experimental data. The correlation equations were then used in a simplified PBM to calculate the droplet number density, which further proved the feasibility of the correlations. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4188–4200, 2017