Critical Bubble Diameter Research Articles

Study on flow regime characteristics in 3×3 rod bundle channels based on wire mesh sensor

Abstract The characteristics and evolution of two-phase flow are of critical importance to the safety of pressurized water reactors. Based on the double-layer wire mesh sensors in this paper, the air-water two-phase flow experiment of the 3×3 rod bundle channels is carried out at room temperature and pressure. The results show that the critical bubble diameter range for the reversal of lateral lift direction is 4 to 5.8 mm. In addition, the time-averaged void fraction for bubbly flow reveals a wall peak distribution at lower superficial gas velocities and shifts to a core peak as these velocities increase. For cap flow, the cross-distribution of cap bubbles within adjacent subchannels triggers large-scale mixing of the liquid phase between adjacent subchannels. For slug flow, large-sized bubbles develop along the axis and cross subchannel gaps to aggregate into slug-shaped bubbles, with a more pronounced distribution of the central peak of void fraction. The drift-flux models are evaluated against the experimental data, and the Ozaki model demonstrates higher precision in estimating void fraction, exhibiting a deviation of 9.8% on average.

Microfluidic nanobubbles: observations of a sudden contraction of microbubbles into nanobubbles.

Microfluidic devices are often utilized to generate uniform-size microbubbles. In most microfluidic bubble generation experiments, once the bubbles are formed the gas inside the bubbles begin to dissolve into the surrounding aqueous environment. The bubbles shrink until they attain an equilibrium size dictated by the concentration and type of amphiphilic molecules stabilizing the gas-liquid interface. Here, we exploit this shrinkage mechanism, and control the solution lipid concentration and microfluidic geometry, to make monodisperse bulk nanobubbles. Interestingly, we make the surprising observation of a critical microbubble diameter above and below which the scale of bubble shrinkage dramatically changes. Namely, microbubbles generated with an initial diameter larger than the critical diameter shrinks to a stable diameter that is consistent with previous literature. However, microbubbles that are initially smaller than the critical diameter experience a sudden contraction into nanobubbles whose size is at least an order-of-magnitude below expectations. We apply electron microscopy and resonance mass measurement methods to quantify the size and uniformity of the nanobubbles, and probe the dependence of the critical bubble diameter on the lipid concentration. We anticipate that further analysis of this unexpected microbubble sudden contraction regime can lead to more robust technologies for making monodisperse nanobubbles.

Mono- and multi-diameter approaches to predict stratified flow structure by means of CFD simulations in DAF systems

This paper presents a Computational Fluid Dynamics (CFD) model of a pilot scale dissolved air flotation (DAF) tank. A Multiphase Mixture model was used to analyse the influence of bubble sizes on the formation of a stratified flow structure. Critical bubble diameter is defined as the size of the bubble that implies the equality of the bubble rising velocity and flow downward velocity in the separation zone (SZ). The fact as to whether using air bubble sizes which are greater or less than the critical diameter value significantly affects the air content, flow structure and the limit of the whitewater blanket inside the SZ is assessed. The study was carried out using two approaches, namely, mono- and multi-diameter. The results obtained via the mono-diameter approach proved to be closely in line with experimental data when air concentration in the SZ had almost, but not quite, a constant value. However, it failed to predict the case of the progressive decrease in air below half of SZ height. A combined effect of bubbles with different rising speed was required to reproduce a smooth air profile curve, as measured experimentally. In this context, a multi-diameter approach is deemed to be a suitable method for reproducing the stratified structure. In addition, this approach offers the chance to study bubble size distribution inside the SZ domain.

Salt effects on dynamic bubble nucleation on hydrophobic surfaces in air super‐saturated water

AbstractEffect of salts (NaCl and CaCl2) on dynamic bubble nucleation on hydrophobic bitumen and silanated glass in air‐supersaturated water was examined using an autoclave at air saturation pressure of 700 kPa and temperature of 80°C. The results showed that at a low salt concentration, there was virtually no difference in the sizes of bubbles nucleated on the surface (0.06‐0.2 cm in diameter), as compared to the cases without salt addition. At a higher salt concentration (0.2 M CaCl2, or 0.3 M NaCl), the size of bubbles nucleated on the surfaces reduced drastically (more than 10‐fold smaller), with the surface being entirely frosted with small bubbles lined up neatly. The observed sizes of surface bubbles were up to more than 3600 times larger for the cases without salts, and 600 times larger at the high salt concentrations, than the critical equilibrium bubble diameter under given test conditions (~328 nm). Possible reasons for the observed phenomena were briefly discussed.

Multiphase numerical modeling of a pilot-scale bubble column with a fixed poly-dispersity approach

A three-dimensional numerical study of air/water bubbly flow in a cylindrical large-scale bubble column is performed using Euler-Euler approach. The main objective is to investigate the influence of different boundary conditions such as bubble size distribution (BSD), polydisperse effects (mono and bi-dispersed approach), lift force modelling and flow rate distribution at sparger using a computationally affordable approach. In the bi-dispersed approach the population of bubbles are divided into two groups of small and large bubbles and a mean diameter is considered for each group. The division is based on the critical bubble diameter, for which the lift coefficient changes its sign from positive to negative. For air/water system Tomiyama lift coefficient model is widely used and the critical bubble diameter is equal to 5.8 mm. An alternative critical bubble diameter and lift force coefficient correlation is used in this study and compared with the well-known Tomiyama model. The numerical predictions are compared against the experimental data and the effect of different conditions is assessed on basis of comparison of axial gas fraction (local holdup) and global holdup. Better predictions are obtained by taking into account polydisperse flow with new critical bubble diameter and new lift coefficient model. Also, it was found that mass flow-rate distribution at the sparger does not affect numerical results for global and local holdup, however a different flow pattern is observed near the sparger region.

High-Viscosity Liquid/Gas Flow Pattern Transitions in Upward Vertical Pipe Flow

Summary High-viscosity liquid two-phase upward vertical flow in wells and risers presents a new challenge for predicting pressure gradient and liquid holdup due to the poor understanding and prediction of flow pattern. The objective of this study is to investigate the effect of liquid viscosity on two-phase flow pattern in vertical pipe flow. Further objective is to develop new/improve existing mechanistic flow-pattern transition models for high-viscosity liquid two-phase-flow vertical pipes. High-viscosity liquid flow pattern two-phase flow data were collected from open literature, against which existing flow-pattern transition models were evaluated to identify discrepancies and potential improvements. The evaluation revealed that existing flow transition models do not capture the effect of liquid viscosity, resulting in poor prediction. Therefore, two bubble flow (BL)/dispersed bubble flow (DB) pattern transitions are proposed in this study for two different ranges of liquid viscosity. The first proposed transition model modifies Brodkey's critical bubble diameter (Brodkey 1967) by including liquid viscosity, which is applicable for liquid viscosity up to 100 mPa·s. The second model, which is applicable for liquid viscosities above 100 mPa·s, proposes a new critical bubble diameter on the basis of Galileo's dimensionless number. Furthermore, the existing bubbly/intermittent flow (INT) transition model on the basis of a critical gas void fraction of 0.25 (Taitel et al. 1980) is modified to account for liquid viscosity. For the INT/annular flow (AN) transition, the Wallis transition model (Wallis 1969) was evaluated and found to be able to predict the high-viscosity liquid flow pattern data more accurately than the existing models. A validation study of the proposed transition models against the entire high-viscosity liquid experimental data set revealed a significant improvement with an average error of 22.6%. Specifically, the model over-performed existing models in BL/INT and INT/AN pattern transitions.

Simulation Study on Gas Holdup of Large and Small Bubbles in a High Pressure Gas–Liquid Bubble Column

The computational fluid dynamics-population balance model (CFD-PBM) has been presented and used to evaluate the bubble behavior in a large-scale high pressure bubble column with an inner diameter of 300 mm and a height of 6600 mm. In the heterogeneous flow regime, bubbles can be divided into “large bubbles” and “small bubbles” by a critical bubble diameter dc. In this study, large and small bubbles were classified according to different slopes in the experiment only by the method of dynamic gas disengagement, the critical bubble diameter was determined to be 7 mm by the experimental results and the simulation values. In addition, the effects of superficial gas velocity, operating pressure, surface tension and viscosity on gas holdup of large and small bubbles in gas–liquid two-phase flow were investigated using a CFD-PBM coupling model. The results show that the gas holdup of small and large bubbles increases rapidly with the increase of superficial gas velocity. With the increase of pressure, the gas holdup of small bubbles increases significantly, and the gas holdup of large bubbles increase slightly. Under the same superficial gas velocity, the gas holdup of large bubbles increases with the decrease of viscosity and the decrease of surface tension, but the gas holdup of small bubbles increases significantly. The simulated values of the coupled model have a good agreement with the experimental values, which can be applied to the parameter estimation of the high pressure bubble column system.

The critical bubble diameter of the lift force in technical and environmental, buoyancy-driven bubbly flows

The lift force acting on particles, drops, and bubbles in a shear field is a well know effect that was extensively investigated since the 1980s. Experiments with single bubbles in a linear shear field reveal that the lift force coefficient has at a specific bubble size a zero where the coefficient switches its sign from positive to negative. Solving the lateral force balance for a polydisperse bubbly flow with the liquid flow field usually found in a bubble column, the lift force causes a spatial separation of small, which tend to the wall, and big bubbles, which tend to the center. Bubbles with a zero lift force coefficient on the other hand are equally distributed over the cross section. In order to investigate the influence of the flow field on the force balance, simulations with the Euler-Euler two-fluid model were conducted. The simulations are in good agreement with experiments and showed that the liquid flow field found in the used bubble column has a minor influence on the steady state distribution of the bubbles. From evaluating six different bubble column experiments conducted in air/water, the critical diameter at which the lift force coefficient is zero is determined between 5.1 and 5.2 mm. This result is very close to the critical diameter that we were able to determine in previous, single-bubble experiments. Therefore, the conclusion is possible that the lift force coefficients that are determined with single bubble experiments are applicable to the complex flow field found in polydisperse bubbly flows at low void fractions and low turbulence levels. Moreover, a very simple experimental setup is described to determine the critical diameter in complex flow situations, which is the most important point when it comes to modelling the lift force. Such a setup is therefore beneficial when complex substance mixtures used in a specific industrial or environmental application needs to be investigated since lift force correlations are only available for clean, ideal substances.

Experimental investigation of the dissipation rate in a chute aerator flow

Although chute aerator flows have been experimentally investigated in several studies, there is a paucity of information on the air–water flow properties such as bubble size distribution, dissipation rate, and bubble breakup frequency. Herein, experiments are conducted to investigate the aforementioned microcosmic characteristics for a wide range of Froude numbers (3.3 < F0 < 7.4) at relatively large Reynolds numbers (5 × 105 < R < 10.4 × 105). Observations indicate the development of bubble variation processes in terms of the range of probability for bubble chord lengths along the chute. The dissipation rate increases from the lower surface to the inner flow in the cavity zone and decreases from the bottom to the upper surface in the impact and equilibrium zones. The critical bubble diameter, dc = (σ/2ρ)3/5ε−2/5, exhibits good agreement with the observed change in power–law scaling of the bubble distribution and exceeds the corresponding mean bubble size. With respect to bubbles exceeding the critical bubble diameter that are subject to fragmentation, they exhibit –5/3 power–law scaling with respect to the diameter, and the breakup frequency increases and reaches a maximum when dab = 1.6dc. However, for bubbles with sizes exceeding 1.6dc, the frequency monotonically decreases relative to the bubble diameter.

Measurements of internal flow regime and bubble size in effervescent atomizer

The internal flow in an effervescent atomizer was experimentally observed for a wide range of gas–liquid flow rates with various atomizer geometries. The effects of atomizer geometry, gas and liquid flow rates on the flow regime and bubble size were investigated. The results show that the internal flow is significantly affected by exit orifice geometry, aerator location, and fluids flow rates, but less affected by aerator configuration. A high velocity of the liquid, small exit orifice diameter, and large distance between the aerators and the exit orifice are beneficial for the formation of bubbly flow. In bubbly flow, the bubble size decreases with liquid flow rate and gas-to-liquid mass ratio (GLR); it follows a normal distribution, and is less related to the atomizer geometry. At a certain liquid flow rate and GLR, small bubbles are readily formed under the exit orifice with a small diameter and proper length. It is found that there are critical values of GLR and critical mean bubble diameters related to gas–liquid mixing; values above the critical GLR and below the critical mean bubble diameter are beneficial for internal gas–liquid mixing. Based on the results of comparison of internal and external flow, it is possible to improve the spray performance by optimizing the internal flow parameters.

Comment on “Aqueous dispersions of nanobubbles: Generation, properties and features“ by A. Azevedo, R. Etchepare, S. Calgaroto, J. Rubio [Miner. Eng. 94 (2016) 29–37

• Much smaller critical bubble diameter by air supersaturation than prediction. • Nanobubble population from air supersaturation is <1% of that from the theoretical prediction. • Spontaneous bubble nucleation on hydrophobic solids is fastest for particle-bubble attachment.

Experimental study and mechanistic modeling of pressure surging in electrical submersible pump

Gas entrainment is frequently encountered in crude oil production with electrical submersible pumps (ESP). Previous studies revealed that the increase of gas entrainment rate in ESPs results in mild degradation of boosting pressure followed by a drastic drop. This critical condition, termed as pressure surging, significantly affects ESP's operational stability and run-life. In this paper, the pressure surging phenomenon in ESPs is studied through experimental measurements and mechanistic modeling. A 7.62-cm two-phase flow loop with a 14-stage radial-type ESP is used for testing pump performance under single- and two-phase flow conditions. The stage-by-stage boosting pressure with different gas entrainment rates is measured. Effects of intake pressure, gas volumetric fraction (GVF) and rotational speeds on the ESP two-phase pressure increment are investigated. Experimental results show that the boosting pressure of ESP under gassy flow conditions varies significantly with inlet GVFs and fluid properties. For low GVFs (<6%), the ESP pressure increment deteriorates gradually with the increase of gas flow rate. However, severe degradation of pump boosting pressure is observed if the inlet GVF exceeds a certain value (>7%), which triggers ESP's unstable operations. A mechanistic model based on the critical bubble diameter in rotating multiphase flow field is developed to predict the surging initiation in ESPs. Compared with experimental results, the model predictions demonstrate good agreement.

The lateral migration of relative large bubble in simple shear flow in water

The lateral migration of relative large bubble in shear flow was essential for two phase flow modeling when relative large bubble was presented. To better understand the lift force for relative large bubble, the single air bubble motions with diameter ranged from 10mm to 20mm in shear flow in water were captured by high speed camera. The linear shear flow with low turbulence intensity was achieved by curved screen. The bubbles were collected in an inverted semi-spherical cup and released by rotating the cup. The lateral migration velocity and quasi-steady lift force were obtained by linear fitting of bubble trajectories. Reproducible results were obtained by measurements of many bubbles. The results showed that bubble shape parameters, rising velocity and drag force were stayed at same level in shear flow as in still water. The relative large bubble migrated to left side and the lift force was negative. A critical bubble diameter of 15mm was observed which was near the boundary between oblate bubble and spherical cap bubble. When the bubble equivalent diameter was lower than the critical value, the lift coefficient was decreasing with the increasing of shear stress magnitude. When the bubble diameter was larger than the critical diameter, the lift coefficient was monotonically decreased with the bubble equivalent diameter and the shear stress magnitude had no effect in the present experiment condition. The comparison with existing study revealed that the present lift correlations were not able to predict the values and new models were needed.

Helium embrittlement of a lamellar titanium aluminide

Embrittlement by helium was investigated in a lamellar TiAl alloy under two conditions:(a)Specimens were implanted to various amounts of helium up to 762appm at temperatures from 630°C to 1000°C and some of them subsequently creep-tested at the same temperature under stresses from 150 to 300MPa. The microstructure and fracture surfaces of creep-deformed and non-creep-deformed specimens were then studied by transmission electron microscopy (TEM) and by scanning electron microscopy (SEM), respectively.(b)Specimens were implanted to various amounts of helium at a low temperature (150°C) and post-implantation annealed at elevated temperatures for TEM studies.Embrittlement was revealed by reduction in time- and strain-to-rupture and by a transition in fracture surface from ductile to an inter-lamellar appearance. Embrittlement occurred above a critical He concentration, which decreased from about 10appm at 700°C to below 6appm at 900°C. TEM showed that embrittlement could be associated to reaching a critical bubble diameter of about 5nm. Bubble diameters increased with increasing temperature ranging in high-temperature implanted specimens from about 3nm (630°C) to 20nm (1000°C) and in post-implantation annealed ones from 1.2nm (600°C) to 2.2nm (900°C), respectively. With increasing temperature, the bubble distribution grew less homogenous with a lower density of larger bubbles situated preferentially at interfaces and sinks. This was ascribed to a change in bubble nucleation mode from homogeneous di-atomic nucleation at lower temperatures to multi-atomic nucleation at sinks at higher temperature.

On the role of the lateral lift force in poly-dispersed bubbly flows

An extensive experimental database comprising air–water as well as steam-water upwards vertical pipe flows for a pressure up to 6.5 MPa was used to investigate the effect of the lateral lift force on turbulent poly-dispersed flows with medium or high gas volume fraction. It was clearly shown that the lift force plays an important role also in such flows. Several effects such as bubble coalescence and breakup as well as fast rising large bubbles which push small bubbles towards the pipe wall superpose the effect of the lift force but can be separated from this effect. The critical bubble diameter, at which the lift force changes its sign, predicted by using Tomiyama’s correlation agrees well with experimental data obtained for turbulent air–water and steam-water flows with medium and high void fraction and a broad spectrum of bubbles sizes. The values for this critical bubble diameter are confirmed by the experimental data within the frame of the uncertainty of the data. Consequences of the action of the lateral lift force on flow structures in different flow situations are discussed. From the investigations it can be concluded that the lift force including the bubble size dependent change of its sign should be considered in a proper numerical 2D or 3D-simulation on flows in which bubbles in the range of several millimeters are present.

Boiling heat transfer in a narrow space controlled by punched interference plate

AbstractAn experiment on pool boiling in methanol was performed for a case in which the boiling space was controlled by an interference plate with many holes. The narrow space, 0.12 mm in thickness, between the heat transfer surface and the interference plate was hermetically sealed at the perimeter. Therefore, the vapor and liquid were only exchanged through the holes in the interference plate. The degree of superheat at the onset of boiling was 0.7 K without overshoot at 10‐mm plate thickness, 1‐mm hole diameter, and 3.85‐mm hole pitch. The critical heat flux obtained was the same value without the interference plate mentioned above. The interference plate disturbed free convection and a superheat layer was provided under small heat flux on the heat transfer surface. The critical bubble diameter for the onset of boiling was decreased as the temperature of the superheat layer was increased. Thus, the degree of superheat at the onset of boiling was decreased. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 462–471, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20028

Boiling Heat Transfer in a Narrow Space Controlled by Punched Interference Plate

An experiment on pool boiling in methanol was performed for a case in which the boiling space was controlled by an interference plate with many holes. The narrow space, 0.12 mm in thickness, between the heat transfer surface and the interference plate was hermetically sealed at the perimeter. Therefore, the vapor and liquid were only exchanged through the holes in the interference plate. Degree of super heat at onset of boiling was 0.7 K without overshoot at 10 mm in plate thickness, 1 mm in diameter of holes and 3.85 mm in pitch of holes. In this case of interference parameter, the critical heat flux was obtained the same value without interference plate. Interference plate was disturbed free convection and superheat layer was provided under small heat flux on the heat transfer surface. And the critical bubble diameter for the onset of boiling was decreased as the temperature of the superheat layer was increased. For this reason, the degree of superheat of onset of boiling was decreased.

Does co burn in a fluidized bed?—a detailed chemical kinetic modeling study

One important issue in fluidized bed combustion is the effect of the fluidized particles on the homogeneous oxidation of both the volatiles and CO from char combustion. These surfaces are thought to quench the radicals and thus lower the rate of combustion significantly. In this work the quenching of the radicals is introduced in a detailed chemical kinetic mechanism to model the effect of the solids on the oxidation of CO in the particulate and bubble phases. In this way the effect of particle concentration, particle diameter, temperature, air-to-fuel ratio, and water concentration has been studied. The results do confirm that oxidation rates are significantly reduced in the particulate phase, although no complete suppression was found. In the bubble phase the effect of heterogeneous radical quenching following mass transfer into the particulate phase and also quenching on the solids at a bubble’s surface is small. No critical (minimum) bubble diameter for the ignition of CO in the bubble phase was found under the investigated conditions. A comparison with the literature on the oxidation of CH 4 in an incipiently fluidized bed confirms the predictions for the oxidation of CO.

Transverse migration of single bubbles in simple shear flows

Trajectories of single air bubbles in simple shear flows of glycerol–water solution were measured to evaluate transverse lift force acting on single bubbles. Experiments were conducted under the conditions of −5.5⩽ log 10 M⩽−2.8 , 1.39⩽ Eo⩽5.74 and 0⩽| dV L/ dy|⩽8.3 s −1 , where M is the Morton number, Eo the Eötvös number and d V L /d y the velocity gradient of the shear flow. A net transverse lift coefficient C T was evaluated by making use of all the measured trajectories and an equation of bubble motion. It was confirmed that C T for small bubbles is a function of the bubble Reynolds number Re, whereas C T for larger bubbles is well correlated with a modified Eötvös number Eo d which employs the maximum horizontal dimension of a deformed bubble as a characteristic length. An empirical correlation of C T was therefore summarized as a function of Re and Eo d . The critical bubble diameter causing the radial void profile transition from wall peaking to core peaking in an air–water bubbly flow evaluated by the proposed C T correlation coincided with available experimental data.

Formation of metal droplets from gas bubbles bursting on iron melt

The bursting of gas bubbles on the free surface of liquid iron produces iron droplets that are ejected into the surrounding atmosphere. The bubble bursting phenomenon was detected in situ by applying X‐ray transmission techniques and by collecting and measuring the metal droplets formed. The effect of equivalent bubble diameter on the mass of ejections was studied. A critical bubble diameter was found which produces the maximum quantity of droplets. The critical diameter was about 9 mm for argon/liquid iron system with a surface tension of around 1.4 N/m. The effect of an equivalent bubble diameter on the size and the number of jet droplets formed was also investigated.