Bubble formation in cocurrently upward flowing liquid

The effects of liquid velocity, nozzle diameter, gas chamber volume and gas flow rate on volumes, shapes and growth curves of bubbles formed at a nozzle submerged in a cocurrently upward flowing liquid in a bubble column were experimentally investigated. The bubble volume decreases with increasing liquid flow velocity. The effect of liquid flow velocity on the volume of bubble increases with an increase in the gas flow rate. To simulate bubble formation at a nozzle submerged in cocurrently upward flowing liquid, a revised non-spherical bubble formation model was proposed. Bubble volumes, bubble growth curves and shapes experimentally obtained in this study, as well as in previous experimental studies, are well predicted by the present model.     

Bubble formation from a flexible hole submerged in an inviscid liquid

In the waste water treatment industry, a novel gas sparger based on flexible membranes has been used for the last 10 years. The objective of the present work is to study the bubble formation generated from a flexible orifice (membrane). Firstly, the membranes are characterised with regard to their properties: wetting critical surface tension, expanding hole diameter, orifice coefficients, flexibility, critical and elastic pressures. The bubble formation phenomenon in an inviscid liquid at rest is studied experimentally for different membranes and gas flow rates. The variation in the bubble diameter, the bubble centre of gravity and the bubble spread on the membrane are determined as a function of time. An analytic model is proposed to describe the bubble growth and its detachment at a flexible orifice. This theoretical approach, developed by Teresaka and Tsuge (J. Chem. Eng. Jpn. 23 (1990) 160) for rigid orifices, is adapted to take into account the membrane features (elastic behaviour and wettability). The predicted bubble diameters at detachment agree with the experimental measurements; however, the model underestimates slightly the bubble formation times. The calculation of the various forces acting on the bubble in the vertical direction indicates that the real forces governing the bubble growth are the buoyancy force, the surface tension force, and near detachment the inertial force.     

Bubble formation and dynamics in a quiescent high‐density liquid

Gas bubble formation from a submerged orifice under constant‐flow conditions in a quiescent high‐density liquid metal, lead–bismuth eutectic (LBE), at high Reynolds numbers was investigated numerically. The numerical simulation was carried out using a coupled level‐set and volume‐of‐fluid method governed by axisymmetric Navier–Stokes equations. The ratio of liquid density to gas density for the system of interest was about 15,261. The bubble formation regimes varied from quasi‐static to inertia‐dominated and the different bubbling regimes such as period‐1 and period‐2 with pairing and coalescence were described. The volume of the detached bubble was evaluated for various Weber numbers, We, at a given Bond number, Bo, with Reynolds number . It was found that at high values of the Weber number, the computed detached bubble volumes approached a 3/5 power law. The different bubbling regimes were identified quantitatively from the time evolution of the growing bubble volume at the orifice. It was shown that the growing volume of two consecutive bubbles in the period‐2 bubbling regime was not the same whereas it was the same for the period‐1 bubbling regime. The influence of grid resolution on the transition from period‐1 to period‐2 with pairing and coalescence bubbling regimes was investigated. It was observed that the transition is extremely sensitive to the grid size. The transition of period‐1 and period‐2 with pairing and coalescence is shown on a Weber–Bond numbers map. The critical value of Weber number signalling the transition from period‐1 to period‐2 with pairing and coalescence decreases with Bond number as , which is shown to be consistent with the scaling arguments. Furthermore, comparisons of the dynamics of bubble formation and bubble coalescence in LBE and water systems are discussed. It was found that in a high Reynolds number bubble formation regime, a difference exists in the transition from period‐1 to period‐2 with pairing and coalescence between the bubbles formed in water and the bubbles formed in LBE. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3996–4012, 2015     

Bubble formation from a free-standing tube in microgravity

The bubble characteristics and the bubble detachment mechanisms during injection of air from a free-standing capillary tube submerged in water were studied in microgravity. The experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A tube, 0.51 mm in diameter and 150 mm long, in a rectangular test section ( long) served as the injector. Images of the bubbles during the drops were acquired using a high-speed camera for various gas flow rates. Bubble detachment was observed for all the Weber numbers tested (0.28-31.12). This observation was different from previous studies using plate orifices in microgravity when bubble detachment was observed only for Weber numbers larger than 8. In order to resolve these differences, experiments were carried out using plate orifices. It was found that the bubbles detached from the orifice for all Weber numbers and that the bubbles formed were larger than those formed with the tube injector, particularly at low gas flow rates. The availability of a large area for the bubble to anchor itself and the presence of the chamber underneath the orifice could cause these differences. The effects of the chamber volume on the unsteadiness of bubble formation in plate-orifices at low gas flow rates are discussed.     

Bubble characteristics and aerosol formation in electrowinning cells

The nature of the aerosol emitted from a scale model zinc electrowinning system, operated under industrial conditions, has been established as a function of bubble formation rate, electrode surface characteristics, coalescence of bubbles, and control strategies. The emitted aerosol was collected and characterized using an Andersen Ambient Impactor. The effect of the relative humidity of the ambient air on the composition and density of the aerosol droplets was determined, permitting comparison of emissions on a standard basis. The size distribution of the aerosol was found to be of a bimodal nature indicating the presence of both film and jet droplets of geometric mean size 2 and 30m respectively. The amount and distribution of the aerosol were found to depend on the bubble size distribution in the cell which in turn depends on the anode surface, the amount and nature of the MnO₂ scale and surface active substances in the electrolyte. The gross emission rate was about 2–3 mgm^–2 s^–1 without any control. A new method of control of the acidic emissions is proposed based on observations made and results obtained from the study of the aerosol characteristics. Providing means of enhancing bubble coalescence below the electrolyte surface results in fewer and larger bubbles which yield less aerosol. The method has been evaluated in both zinc and copper electrowinning and leads to reduction of aerosol emission by over 90%.Paper presented at the International Meeting on Electrolytic Bubbles organized by the Electrochemical Technology Group of the Society of Chemical Industry, and held at Imperial College, London, 13–14 September 1984.     

Bubble formation and gas leakage in fluidized beds

The Harrison and Leung model for spherical bubble formation in fluidized beds has been extended to account for the dynamics of the emulsion phase. The simplified Davidson-Harrison model is used to obtain the gas and particle velocities in the emulsion phase and includes the effect of the bubble pressure, the change in hydrostatic pressure, grid orifice discharge rate, and gas leakage into the emulsion phase. Computations for the constant flow rate case agree well with measured gas leakage for beds of large particles. For large grid orifices, a simplified model explains the low frequencies observed experimentally for small flow rates.     

Bubble formation in non-Newtonian fluids in a microfluidic T-junction

The aim of this work is to investigate the bubble formation in non-Newtonian fluids in a microfluidic T-junction by crossflowing rupture technique, using a high-speed digital camera. Experiments were conducted in a glass microchannel with 120 μm wide and 40 μm deep. N₂ bubbles were generated in different concentrations of polyacrylamide (PAAm) solutions. Various flow patterns were observed at the T-junction by varying gas and liquid flow rates. The breakup mechanism for bubbles was investigated to gain insight into the effects of flow rates and concentrations of PAAm solutions on bubble size. The gaseous thread collapses at a constant speed in the collapse stage; while during the final pinch-off stage, the variation of the minimum width W_m of the gaseous thread with the remaining time (T − t) could be scaled as W_m ∼ (T − t)^0.21. The bubble size increases non-linearly with the gas/liquid flow rates ratio, and decreases with the concentration of PAAm solutions.     

Bubble formation in quiescent liquids under constant flow conditions

In this investigation, a simple equation has been theoretically developed to predict the bubble detachment diameter in quiescent liquids under constant volumetric gas flow conditions. The equation is valid in the bubbling regime up to transition to the jetting regime and for liquids with very low up to very high viscosities. Known experimental measurements were used to examine the validity of the developed equation. The present measurements cover gas Weber numbers from about 0 up to 4, liquid dynamic viscosities from 10⁻³ up to 1 Pa s, and nozzle diameters from 0.2 up to 6 mm. The comparison between the theoretical predictions and the experimental measurements is satisfactory; the deviations lie in most cases within ± 10%.     

Bubble formation in viscous liquids under constant flow conditions

Bubble formation from single nozzles has been studied in liquids of different viscosities. The viscosity is varied in the range 50 to 500 c.p., whereas flow rates up to 100 cm³/second have been investigated. A model based on a two step (viz. expansion and detachment) mechanism has been proposed for bubble formation. The model explains the results of the present investigation as well as those reported in the literature for similar systems. As an extreme case, the model is tested on liquids of very low viscosity after dropping the viscosity terms from the equations and is found to explain even these data quite well.     

微通道内浆料体系中的气泡生成特性及尺寸预测

利用高速摄像仪对T型微通道内浆料体系中的气泡生成频率和气泡尺寸进行了研究。以氮气作为分散相,含0.35%(质量分数)表面活性剂(SDS)不同浓度玻璃珠的甘油-水溶液为连续相。实验考察了弹状流下气液两相流量、颗粒浓度以及浆料表观黏度对气泡生成频率及气泡尺寸的影响。结果表明:在弹状流下,当分散相流量一定时,随着连续相流量的增大,气泡的生成频率增大而气泡尺寸减小。当连续相流量一定时,随着分散相流量的增大,气泡生成频率和气泡尺寸均增大。随着颗粒浓度的增大,浆料的表面张力减小,表观黏度增大,气泡生成频率增大而气泡尺寸减小。提出了T型微通道内浆料体系中生成气泡尺寸的预测模型,模型具有良好的预测精度。     

Bubble formation and breakup mechanism in a microfluidic flow-focusing device

The aim of this study is to investigate the bubble formation mechanism in a microfluidic flow-focusing device using a high-speed digital camera and a micro-particle image velocimetry (μ-PIV) system. Experiments were conducted in a PMMA square microchannel with 600 μm wide and 600 μm deep. Gas bubbles were generated in glycerol-water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Various flow patterns were obtained at the cross-junction by changing gas and liquid flow rates. The formation mechanism of slug bubble at the cross-junction was investigated to gain insight into the effects of liquid and gas flow rates, and viscosity of the liquid phase on the breakup rate of the gas thread, and on the collapse time. The velocity fields in the liquid phase around the thread were determined by μ-PIV measurements. The experimental data of the breakup rate and the collapse time of the gas thread were described as a function of the liquid superficial velocity u_l, the ratio of the gas and liquid flow rates Q_g/Q_l and Reynolds number Re=ρul/μ.     

Bubble size effect on low liquid input drift-flux parameters

We investigated the effect of bubble size on the drift-flux parameters at low liquid flow conditions by measuring the radial profiles of void fraction and phase velocities in a vertical bubbly pipe flow of diameter and height . To study the effect of the bubble size we used two different types of bubble inlets. We measured the local bubble fraction and velocity U_g by using single and four-point-optical fibre probes, and we used Laser Doppler Anemometry to determine the liquid velocity U_l. The distribution parameter C₀ and the weighted mean drift velocity |U_drift| were directly computed from the local measurements at a height on our experimental set-up. Both parameters were influenced by the bubble size. Provided no liquid flow reversal occurred at the near wall region, the distribution parameter reached a below unity minimum plateau value of C₀=0.95 for wall peaking void fraction profiles. At low liquid input conditions both the liquid input and bubble size had an influence on the distribution parameter. Extreme values such as C₀>2 were measured. From these measurements we developed models for the drift-flux parameters to take into account the effect of bubble size and input-flow conditions for our intermediate pipe diameter value. These models were tested and validated with separately collected experimental data.     

Experimental study of a liquid film flowing over a perforation

Perforations are one of the recognized geometrical features that contribute to liquid redistribution in corrugated sheet packings. Our experimental study focuses on a simplified but relevant configuration: a thin liquid film flowing on either side of a vertical plate with a circular perforation. We focus on the curtain mode when the liquid fills the perforation. Confocal chromatic imaging reveals a capillary ridge upstream of the perforation, an inertial ridge downstream, and a varicose capillary wave standing on the liquid curtain. We show that the wavelength is selected such that the velocity of the wave both satisfies Taylor's dispersion relation and matches the curtain local speed. We examine the effect of perforation size, supply conditions, and liquid properties on the curtain transition. Lastly, we propose a simple model based on a momentum balance that describes the effect of these parameters on the Reynolds number at which curtain forms.     

流动液膜的数值模拟研究综述

液膜流动现象广泛存在于自然界中,作为一种高效传热传质技术,其在化工等领域有着广泛的应用。近几年来,国内外学者越来越热衷于运用数值模拟技术来研究液膜的流动特性及传热传质特性。本文归纳分析了数值模拟研究中液膜自由液面的追踪方法。总结了不同壁面结构、不同壁面倾角、液体物性、液相流量与气相流速4个方面对液膜的流动特性的影响规律,以及改变壁面倾角、入口雷诺数、入口添加扰动时表面波呈现的波动特性。此外,还论述了流动液膜的传热传质特性的研究现状。所得结论对流动液膜的数值模拟研究具有一定的参考价值,最后提出了用数值模拟方法研究液膜流动的缺陷与不足,展望了更加科学合理地研究流动液膜的方法。     

Single-stage flowing liquid film impactor for continuous on-line particle analysis

A single-stage “wet impactor” is presented, where the impaction occurs on a regenerated water surface. The developed impactor is equipped with an impaction liquid support plate of etched glass and a drain spout providing a continuous liquid flow covering the impaction area. Subsequent transport of the impaction liquid makes an on-line determination possible. With multiple nozzles (74 holes, 0.3 mm i.d.) and an air flow of 101/min the cut-off was determined to 0.41 ± 0.02 μm. The impactor was also investigated for its particle loss. The cut-off function, regarding the consequences of letting impaction occur in a liquid film is discussed and compared to conventional impactors. The analysis technique was tested in an ambient air measurement study with an ion chromatograph attached to the sampling system.     

Electroviscous effects in a Carreau liquid flowing through a cylindrical microfluidic contraction

Electroviscous effects in steady, pressure-driven flow of a Carreau shear-thinning liquid in a cylindrical microfluidic 4:1:4 contraction–expansion at low Reynolds number are investigated numerically by solving the equations governing the flow, the electric field, and ion transport, using a finite volume method. The channel wall is considered to have a uniform surface charge density and the liquid is assumed to be a symmetric 1:1 electrolyte solution. Predictions are presented for a range of values of the shear-thinning parameters in the Carreau model for various surface charge densities and Debye lengths. The apparent/physical viscosity ratio is shown to increase as the degree of shear-thinning increases. Thus the electroviscous effect is stronger in shear-thinning liquids than it is when the liquid is Newtonian, a result previously obtained for uniform pipe flow of power-law liquids. The trend holds true regardless of the choice of surface charge density or Debye length, although the magnitude of the trend decreases as the surface charge density and/or the Debye length is reduced. Comparison between uniform pipe flow of a Carreau liquid and the corresponding power-law liquid that approximates it at large shear rates shows that the apparent/physical viscosity ratios for the two models are almost identical. A previous prediction that a near-wall region of reduced velocity can occur for pipe flow of a shear-thinning power-law liquid when EDLs are overlapping and surface charge density is elevated is confirmed for a Carreau liquid.