Bubble Size Measurements Research Articles

Aeration and bubble measurements of coastal breaking waves

The air entrainment process of surf zone waves is studied experimentally to understand as a first step of two-phase characteristics of surf zone breaking waves. A set of laboratory experiments of free surface elevation, water velocity, void fraction and bubble distribution is conducted simultaneously for regular gravity wave breaking on a plane slope. The in situ data are collected by using a measurement array of wave gages, a dual-tip resistivity void fraction probe and an acoustic Doppler velocimeter. Two-dimensional projected bubble size measurements are conduced by the high speed camera with imaging technique. The experimental data show a linear relationship between the void fraction and turbulent intensity. In addition, the bubble size distributions are proportional to the bubble size to the power of - 1 and - 3.4 independent of the distance from breaking point and water depth. The length scale separating two power laws is the Hinze scale which corresponds to the wave energy dissipation scale. This result will be important for modeling and implication for the study of surf zone dynamics.

Influence of surfactant concentration on the ink removal selectivity in a laboratory flotation column

A laboratory flotation column equipped with Venturi aerators and an adjustable froth removal system was used to study the effect of an alkyl phenol ethoxylate surfactant on water, particle and surfactant transfer during flotation de-inking of recycled papers. Wash water was not used in order to emphasize the influence of surfactant concentration on water/particle transport in the froth. Surface tension, bubble size and contact angle measurements showed that the increase in surfactant concentration boosted pulp frothing, lowered bubble coalescence and ink/pulp contact angle, thus increasing water hold-up in the froth, gas hold-up in the aerated pulp and depressing ink floatability, respectively. Cross-linked phenomena determining the yield of the flotation process where interpreted using model equations describing particle removal in terms of flotation, entrainment and drainage in the froth. An exponential decrease in the flotation rate constant and in the drainage coefficient of ink, fillers, fine elements and in the fibre entrainment coefficient was observed when increasing the surfactant concentration. Whereas, the surfactant removal rate constant rose, in agreement with the increase in the surface area flux due to a reduction in bubble coalescence.

Experimental analysis of bubble velocity in a rotating fluidized bed

The bubble velocity in a two-dimensional rotating fluidized bed (RFB) was experimentally analyzed. The motion of bubbles was observed by means of a high-speed video camera, and the radial and angular components of bubble velocity were experimentally measured. The radial bubble velocity ( U Br) and angular bubble velocity ( ω B) were expressed as a function of actual centrifugal acceleration acting on the bubble ( g ′ B ) , bubble diameter ( D b), and angular velocity of the rotating vessel (ω v): U Br = K r ( g ′ B D b ) 0.5 and ω B = K θ ω v, respectively. The effects of the operating parameters (gas velocity and centrifugal acceleration) on the bubble velocity coefficients ( K r and K θ ) were analyzed experimentally. The distribution of both bubble velocity coefficients could be well correlated by the log-normal distribution function. The distributions of K r and K θ showed almost unchanged with the gas velocity and centrifugal acceleration, because the buoyancy force acting on a bubble under high centrifugal force field is so high, and the interaction from other bubbles can be neglected. The bubble velocity coefficients in an RFB could be empirically obtained as K r = 0.52 and K θ = 0.96. The experimental mean bubble velocities at the various operating conditions were compared with the predicted ones by using the obtained bubble velocity coefficients and our proposed model for the bubble diameter [H. Nakamura, T. Iwasaki, S. Watano, Modeling and measurement of bubble size in a rotating fluidized bed, AIChE J. 53 (2007) 2795–2803]. The radial and angular bubble velocities could be predicted only by the operating parameters.

DNA Bending in Transcription Initiation

Electrophoretic mobility shift (bandshift) phasing analysis and rotational variant topological analysis were performed on initiation complexes formed on the bacteriophage lambda PR promoter. Both the open complex and an abortive complex containing a short RNA primer extending to +3 were characterized. The two methods were used to analyze a series of constructs containing tandemly repeated copies of the PR promoter, with the repeat length increased in single base pair increments to progressively change the rotational setting of adjacent copies. The phasing effect observed in bandshift analysis of open complexes formed on this set of constructs provided qualitative evidence for the presence of a bend. Subsequent rotational variant topological analysis confirmed this and quantified the overall bend angle in the open complex as well as in the +3 abortive complex: a bend of 49 degrees +/- 7 degrees was measured for the open complex, while a bend of 47 degrees +/- 11 degrees was measured for the +3 complex, i.e., the two bends are the same. However, the topological results are not consistent with extensive superhelical wrapping of DNA on either complex as has been proposed. The two complexes do differ in the size of the transcription bubble: the open complex contains a 10.4 +/- 0.1 bp bubble, while that of the +3 complex is 12.2 +/- 0.1 bp, a result consistent with "DNA scrunching" during the onset of transcription. A model for the overall path of the DNA in the open complex is presented that is consistent with the measured bend angle. Measurement of both bubble size and overall bend angle complements the results of crystal structures in providing an enhanced description of the solution structures of the intact initiation complexes.

Diesel oil removal from water by froth flotation under low interfacial tension and colloidal gas aphron conditions

The objective of this work was to investigate the relationship between the froth characteristics, the system interfacial tension (IFT), and the efficiency of diesel oil removal by froth flotation under colloidal gas aphron (CGA) conditions. Branched alcohol propoxylate sulfate sodium salt (C 14–15(PO) 5SO 4Na) as an extended surfactant, was used to form CGA and microemulsions with diesel oil. For the CGA studies, the effects of salinity, surfactant concentration, stirring speed, and stirring time were investigated in order to determine the optimum conditions of the CGA formation which were further used for the froth flotation experiments. The air bubble size measurement was carried out in order to correlate the air bubble size to the froth flotation performance. The results showed that the use of CGA enhanced the process performance of froth flotation in terms of both the removal and the enrichment ratio of diesel oil since CGA increased both froth formation and stability. The froth flotation column which was operated at an air flow rate of 0.30 l/min with the feed solution prepared under non-equilibrium and the CGA conditions at 0.1 wt.% C 14–15(PO) 5SO 4Na, 3 wt.% NaCl, with a stirring speed of 5000 rpm and a stirring time of 5 min, gave the highest oil removal of 97%.

Bubble velocity, size, and interfacial area measurements in a bubble column by four‐point optical probe

AbstractThe four‐point optical probe is applied in a bubble column with an air–water system to investigate the bubble properties (local gas holdup, velocity, chord length, specific interfacial area, and frequency) over a range of gas superficial velocities. Both bubbles moving upward and downward are recorded and measured as opposed to only upward bubbles measured and reported in other studies involving probes. The probe worked efficiently in both bubbly flow and highly churn‐turbulent flow at very high superficial gas velocities. Bubble properties at the conditions of churn‐turbulent flow are obtained and investigated for the first time. The changes in the bubble velocity distribution, bubble chord length distribution, and specific interfacial area with superficial gas velocity, sparger design, and with axial and radial positions in the column are discussed. © 2007 American Institute of Chemical Engineers AIChE J, 2008

In-pile Xe diffusion coefficient in UO2 determined from the modeling of intragranular bubble growth and destruction under irradiation

Intragranular bubbles grow in the nuclear fuel by diffusion and precipitation of fission gases, mainly xenon; and are ultimately destroyed, under irradiation, by fission fragments. This article will attempt to determine the in-pile bubble distributions taking into account the evolution of the concentration profile around a bubble during its growth and the destruction process by fission fragments. From these distributions a relation between the bubble mean radius and the diffusion coefficient of xenon can be established, allowing the determination, from experimental measurements of intragranular bubble sizes, of the in-pile Xe diffusion coefficient in UO2. The estimated activation energy (0.9eV) is about one order of magnitude lower than the widely used value of 3.9eV determined from out-of-pile experiments. This effect can be attributed to the presence of point defects created by the irradiation.

Lift force on bubbles in a bubble column reactor: Experimental analysis

The lift force acting on bubbles in a swarm has been estimated by analyzing the instantaneous velocity–time data obtained using LDA in a cylindrical bubble column. Phase distinction was achieved through the multiresolution analysis of the velocity–time data. Several important issues related to the transverse motion of bubbles subjected to a shear field have been discussed quantitatively. The actually measured bubble sizes, the respective slip velocity values in transverse and axial directions and the local shear rates ( γ ) enabled the verification of known formulations for the lift coefficient ( C L ) for bubbles. At many locations in the column the radial flux of the gas phase by turbulent dispersion and the radial slip were estimated. The radially inward movement of bubbles from low to high axial velocity (from column wall to center, i.e., C L < 0 ) was observed at most of the measurement locations. The local lift coefficient was estimated using the transverse drag force and the values support the results from the material balance approach. The estimated C L values showed a wide variation over the column cross-section.

Prediction of bubble behaviour in fluidised beds based on solid motion and flow structure

Abstract It has been demonstrated that the non-intrusive positron emission particle tracking (PEPT) could be a potential technique for observing bubble flow pattern, measuring bubble size and rise velocity in bubbling fluidised beds according to the solid motion in bubble and its wake. The results indicate that the behaviour of air bubbles varies greatly with the bed materials and superficial gas velocity. Three types of bubbling patterns (namely A, B and C) have been reported in this study, in which the pattern C is observed when the polyethylene fluidised bed is operated at the superficial gas velocity ( U − U mf ) of 0.25–0.5 m/s and the ratio of bed height to bed diameter is unity. After the comparison of the results measured by the PEPT technique with the values calculated by using a number of empirical correlations, two modified correlations are recommended to calculate the bubble size based on the PEPT data.

Bubble size in a forced circulation loop reactor

AbstractBACKGROUND:The bubble size distribution in gas‐liquid reactors influences gas holdup, residence time distribution, and gas‐liquid interfacial area for mass transfer. This work reports on the effects of independently varied gas and liquid flow rates on steady‐state bubble size distributions in a new design of forced circulation loop reactor operated with an air–water system. The reactor consisted of a cylindrical vessel (∼26 L nominal volume, gas‐free aspect ratio ≈ 6, downcomer‐to‐riser cross‐sectional area ratio of 0.493) with a concentric draft tube and an annular riser zone. Both gas and liquid were in forced flow through a sparger that had been designed for minimizing the bubble size.RESULTS:Photographically measured bubble size distributions in the riser zone could be approximated as normal distributions for the combinations of gas and liquid flow rates used. This contrasted with other kinds of size distributions (e.g. bimodal, Gaussian) that have been reported for other types of gas‐liquid reactors. Most of the bubbles were in the 3 to 5 mm diameter range. At any fixed low value of aeration rate (≤1.8 × 10−4 m3s−1), increase in the liquid flow rate caused earlier detachment of bubbles from the sparger holes to reduce the Sauter mean bubble size in the riser region.CONCLUSION:Unlike in conventional bubble columns where bimodal and Gaussian bubble size distributions have been reported, a normal bubble size distribution is attained in forced circulation loop reactors with an air–water system over the entire range of operation. Copyright © 2007 Society of Chemical Industry

Modeling and measurement of bubble size in a rotating fluidized bed

AbstractModeling and measurement of bubble size in a rotating fluidized bed (RFB) is described. A novel model of bubble growth has been proposed based on a bubble coalescence model by Darton et al. We modified the model according to the following concepts: (i) local centrifugal acceleration and excess gas velocity are considered as the parameters in the radial direction and (ii) bubble volume flow rate locally changes depending on the radial distance. Bubbling behavior in a two‐dimensional RFB was experimentally observed by means of a high‐speed video camera. Bubble sizes were measured using an image analysis technique. The estimated bubble sizes by our proposed model showed good agreement with the experimental results, while the estimated values by the model for conventional fluidized bed underestimated. The bubble growth mechanism in an RFB was also discussed and then concluded that the initial bubble size decreases, and the rate of bubble growth increases with an increase in the centrifugal acceleration. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Planar shadow image velocimetry for the analysis of the hydrodynamics in bubbly flows

In order to allow for more reliable modelling of microscale processes in turbulent bubbly flows, detailed experiments in a laboratory-scale double loop facility and a bubble column with a diameter of 140 mm were performed. The range of bubble mean diameters considered in the measurements was between 2 and 4 mm. The average gas volume fraction was in the range between 0.5 and 5% for different experiments. To allow simultaneous measurements of bubble size, bubble velocity and liquid velocity field, a combined system of planar shadow imaging, particle tracking velocimetry (PTV) and particle image velocimetry (PIV) for online measurements was developed and applied. The measurements of the liquid phase velocities were realized by seeding the flow with polyamide tracer particles having a mean diameter of 65 μm. A background illumination was established by using an array of 551 LEDs with a size of 160 mm by 100 mm. A double image CCD camera with macro optics was used to record the images of tracers and bubbles simultaneously. Because of the small depth of field of the macro camera optics (<4 mm for the bubbles), it was possible to discriminate between bubbles and tracer particles inside and outside the camera's focal plane using the gradient of grey values. A set of digital image filters was applied to perform phase discrimination between bubbles and tracer particles, i.e. to obtain separate double images for bubbles and tracer, respectively. The contour of in-focus bubbles was determined using an edge detecting Sobel filter and a spline interpolation technique. Thereby, the bubble size, shape and orientation could be derived. The bubble velocity was obtained by applying PTV and the continuous phase velocity field was determined by PIV, using a successive refinement of the interrogation area. By recording and evaluating at least 500 double images, it was possible to determine bubble size distributions and mean as well as fluctuating velocities for both phases. Thereby, detailed data on the hydrodynamics of bubble-driven flows are provided.

Use of models for lift, wall and turbulent dispersion forces acting on bubbles for poly-disperse flows

Closure laws are needed for the qualification of CFD codes for two-phase flows. In case of bubbly and slug flow, forces acting on the bubbles usually model the momentum transfer between the phases. Several models for such forces can be found in Literature. They show, that these forces depend on the liquid flow field as well as on the size and the shape of the bubbles. A validation of consistent sets of bubble force models for poly-disperse flows is given, basing on a detailed experimental database for vertical pipe flows, which contains data on the radial distribution of bubbles of different size as well as local bubble size distributions. A one-dimensional (1D) solver provides velocity profiles and bubble distributions in radial direction. It considers a large number of bubble size classes and is used for the comparison with the experiments. The simplified model was checked against the results of full 3D simulations done by the commercial code CFX-5.7 for simplified monodisperse cases. The effects of the number of bubbles classes as well as the effect of the lateral extension of the bubbles were analyzed. For the validation of bubble force models measured bubble size distributions were taken as an input for the calculation. On basis of the assumption of an equilibrium of the lateral bubble forces, radial volume fraction profiles were calculated separately for each bubble class. In the result of the validation of different models for the bubble forces, a set of Tomiyama lift and wall force, deformation force and Favre averaged turbulent dispersion force was found to provide the best agreement with the experimental data. Some discrepancies remain at high liquid superficial velocities.

Experimental study of bubble formation at metal porous spargers: Effect of liquid properties and sparger characteristics on the initial bubble size distribution

This work is a study of the effect of liquid properties and pore size on the initial bubble size distribution of a bubble column equipped with fine pore sparger. Various liquids covering a wide range of surface tension and viscosity values are employed, while the gas phase is atmospheric air. A fast video recording technique is used for both the visual observations of the phenomena occurring onto the sparger and the bubble size measurements. A new correlation regarding the prediction of the initial mean Sauter diameter of bubbles formed from porous spargers at the homogeneous regime has been formulated and found to be in good agreement with available data. The experiments show that the mechanisms of bubble formation as well as the initial bubble size distribution depend strongly on the liquid properties, the sparger design and the gas flow rate.

Measurement of bubble size distribution using a flying optical probe technique: Application in the highly turbulent region above a distillation plate

A flying optical probe technique has been developed and employed to measure bubble size above sieve trays in an air–water facility simulating cross-flow distillation (CRODIS). Despite the highly turbulent conditions occurring in the flows investigated, successful measurements were made of bubble size distributions at a series of levels above the trays. Using this technique, experiments are carried out on the CRODIS facility covering a range of gas and liquid flowrates and several tray configurations and the results are presented in this paper. Gas flowrate, sieve hole size and weir height were found to exhibit a significant effect on bubble size distribution, whereas liquid flowrate was shown to have negligible influence.

Stabilization of aerated sugar particle systems at high sugar particle concentrations

We report on the stability of aerated sugar dispersions containing a high concentration of sugar particles and low air phase volume with a composition typical of fondant confectionery. The main instability mechanisms are creaming of the air phase and sedimentation of the solid continuous phase. The influences of particle concentration, mean particle size and the type of particles (sugar/silica) on quiescent storage stability have been investigated. It has been found that finer particles give a lower particle settling rate and a greater sediment volume due to more attractive particle–particle interactions. The creaming behaviour is mainly affected by the concentration-dependent viscosity of the dispersion, the particle settling rate, and the time-dependent network formation at high solid concentrations. Stability differences between systems containing sugar particles and silica particles of different sizes can be attributed to differences in densities and shapes of the two kinds of particles. The minimum sugar particle concentration required for creaming stability has been estimated from the apparent yield stress of the system, the measured bubble sizes, and the density differences between the gaseous and continuous phases. Due to time-dependent network formation of sugar aggregates during ageing, the sugar particle concentration required for long-term stability is lower than that estimated on the basis of the initially measured apparent yield stress.

911 マイクロバブルの計測と特性評価

This study focused on the characteristics of water clarification by microscopic bubble. The two parameters which influence the particle size of bubbles, i.e. fluid flow and water temperatures, were investigated toward the depolution of water in hydrosphere environment. Measurements of bubble size were carried out with changing fluid flow at generator and water temperatures. After the generation of bubbles, the particle size and the number of bubbles were measured from 15 to 80 seconds. From the result, the effect of fluid flow on the particle size seemed to be small. On the other hand, at a constant flow, the higher the water temperature was, the smaller the particle sizes of bubble were. The size also became to be small in the course of time

Ｒａｓｃｈｉｇ ｒｉｎｇ充填層通過の液流れにより生成する分裂気泡群の気泡径分布

Experiments were performed to investigate splitting bubbles formed by liquid flow through a Raschig ring packed bed. A liquid was discharged vertically upwards through a packed bed into a large volume of the same liquid. Shear layers were thus formed both in the packed bed and around the jet. A bubble rising through the packed bed and the jet splits in each layer. The measured bubble size distribution can be well represented by an upper limit log-normal probability function with two constant parameters and only one variable, the maximum stable bubble diameter. In this operation, micro-bubble, the diameter of which is less than 1 mm, was confirmed with increasing liquid velocity through a packed bed. Furthermore, we need mean bubble diameter to give the distribution of splitting bubbles the full picture. Therefore, a correlation equation for volumetric mean bubble diameter of splitting bubbles is presented.

Air entrainment processes in a full-scale rectangular dropshaft at large flows

Rectangular dropshafts, commonly used in sewers and storm water systems, are characterized by significant flow aeration. New detailed air–water flow measurements were conducted in a near-full-scale dropshaft at large discharges. In the shaft pool and outflow channel, the results demonstrated the complexity of different competitive air entrainment mechanisms. Bubble size measurements showed a broad range of entrained bubble sizes. Analysis of streamwise distributions of bubbles suggested further some clustering process in the bubbly flow although, in the outflow channel, bubble chords were in average smaller than in the shaft pool. A robust hydrophone was tested to measure bubble acoustic spectra and to assess its field application potential. The acoustic results characterized accurately the order of magnitude of entrained bubble sizes, but the transformation from acoustic frequencies to bubble radii did not predict correctly the probability distribution functions of bubble sizes

Trapped Gas Fraction During Steady-State Foam Flow

Trapped or stationary gas contributes significantly to the extent of gas mobility reduction for aqueous foams. Simultaneous measurements of effluent bubble sizes and trapped gas saturation in sandstone are reported for the first time. Roughly 80% of the gas saturation in an aqueous foam is stationary at steady state in this permeable porous medium. The experiments show that as gas velocity increases, the trapped gas fraction decreases. Similarly, as injected gas–liquid ratio increases, the trapped gas fraction decreases. Hence, the absolute velocities of gas and aqueous surfactant solution are fundamental to foamed-gas mobility reduction for they help determine in situ foam texture. Effluent foam bubbles range in size from 60 to 120 μm in diameter. The smaller the effluent bubble, the smaller is the fraction of mobile gas. Scaling laws from network percolation theory are used to engender a mechanistic understanding of the various parameters identified as important in the experimental program. The closed form approimation predicts that the trapped gas fraction is a weak function of pressure gradient, foam-bubble size, and the permeability of the porous medium. Moreover, the theory reproduces well the newly obtained experimental data.