Large Number Of Small Bubbles Research Articles

Numerical study on acoustic characteristics of flow boiling in a helical tube

Helically coiled steam generators have advantages of strong heat transfer capacity, compact structure, and low thermal stress, and are widely used in many engineering fields. However, the heat transfer performance is still limited by the critical heat flux, beyond which boiling deterioration is an important cause of safety accidents. Both engineering and experimental studies have found that complex noises could be frequently generated during the occurrence of boiling crisis. In this paper, the acoustic characteristics of both subcooled and saturated flow boiling in a helical tube were numerically investigated, by using the volume of fluid scheme and acoustic model. The distribution of the vapor-liquid phase, flow pattern transition, and acoustic frequency band characteristics during boiling were analyzed under different wall temperatures (varied superheat), inlet subcooling, and flowrates. It’s found that the flow pattern under low mass flowrate was plug flow, slug flow, and wavy flow. All the three flow patterns have unique boiling acoustic features. The plug flow, consisted by a large number of small bubbles, shows the distinct high-frequency band characteristics of the boiling acoustics. The wavy flow, which is composed mainly by large bubbles, exhibits a significant low-frequency band characteristics of the boiling acoustics.

A green aqueous foam stabilized by cellulose nanofibrils and camellia saponin for geological CO2 sequestration

Geological carbon dioxide (CO2) sequestration in deep formations involves displacing resident fluids from porous media, and is considered an unstable process due to the unfavorable mobility ratio between the resident and displacing fluid. Although various chemicals have been added to improve the CO2 sweep efficiency, this often results in pollution to the environment and the formation. Here, a novel green foam was prepared to enhance CO2 sequestration. The stabilization mechanisms and rheological properties of the CO2 foam with cellulose nanofibrils (CNFs) and camellia oleifera saponin (COS) were explored. Furthermore, the pore-scale behaviors of the CO2 foams were investigated using a micromodel. The results demonstrated that the abundant hydrogen bonds between CNFs and COS molecules led to the formation of tight bubble film which could stably encapsulate CO2 and inhibit its diffusion. The interlacement and entanglement of CNFs endowed liquid phase with high viscosity, which restrained liquid drainage and improved the interfacial viscoelasticity. The pore-scale behaviors showed that the higher viscoelasticity and stability caused large numbers of small bubbles to group together, providing greater flow resistance to control the mobility of the foam. The CNF/COS foam significantly improved the CO2 saturation in aquifers and oil reservoirs.

Study on frost resistance of shotcrete by micro-nano bubble water and admixture

Shotcrete was prepared by using micro-nano bubble water as mixing concrete water in cooperation with air-entraining agent (AOS-R) and superabsorbent polymer (SAP), and its frost resistance performance was experimentally studied. The results show that the incorporation of AOS-R and SAP will reduce the 28-day strength of concrete, but MNBW can compensate for the strength loss to a certain extent. The frost resistance of concrete mixed with alkali accelerators is inferior to that of concrete mixed with the alkali-free accelerator. The incorporation of AOS-R or SAP into shotcrete can improve its frost resistance to in some degree as well. Given the frost resistance of concrete mixed with SAP is not as effective as AOS-R, MNBW can play a synergistic effect to improve the frost resistance of concrete. The main reason why the AOS-R improves the frost resistance of concrete is that it generates a large number of small bubbles to isolate the water transmission channel which manage to relieve the crystallization pressure of water. MNBW improves the frost resistance of concrete, because it refines the pore structure and increases the density of concrete.

Bubble dynamics in 2‐D gas–solid fluidized bed with Geldart A or Geldart B particles by image processing method

AbstractGas bubbles in a two‐dimensional gas–solid fluidized bed were identified using an image processing method to characterize the bubble dynamics in a binary fluidization system. The magnetite, sand (two types), and fluid catalytic cracking (FCC) catalyst particles belonging to Geldart groups A and B were used as the particulate materials. Image processing and MATLAB were applied for bubble size and velocity measurements. Bubble properties and bed expansion in fluidized beds of those four types of particles were studied. Bubble size and bubble rise velocity were found to increase with the distance above the gas distributor and excess gas velocity. Geldart A particles have a higher bed expansion than Geldart B particles due to the large number of small bubbles in the bed. A new empirical correlation for estimating bubble diameter along the bed height was proposed, giving good agreement with the experimental data.

Promoting Effect of Ultra-Fine Bubbles on CO2 Hydrate Formation

When gas hydrates dissociate into gas and liquid water, many gas bubbles form in the water. The large bubbles disappear after several minutes due to their buoyancy, while a large number of small bubbles (particularly sub-micron-order bubbles known as ultra-fine bubbles (UFBs)) remain in the water for a long time. In our previous studies, we demonstrated that the existence of UFBs is a major factor promoting gas hydrate formation. We then extended our research on this issue to carbon dioxide (CO2) as it forms structure-I hydrates, similar to methane and ethane hydrates explored in previous studies; however, CO2 saturated solutions present severe conditions for the survival of UFBs. The distribution measurements of CO2 UFBs revealed that their average size was larger and number density was smaller than those of other hydrocarbon UFBs. Despite these conditions, the CO2 hydrate formation tests confirmed that CO2 UFBs played important roles in the expression of the promoting effect. The analysis showed that different UFB preparation processes resulted in different promoting effects. These findings can aid in better understanding the mechanism of the promoting (or memory) effect of gas hydrate formation.

Experimental study of horizontal air-water plug-to-slug transition flow in different pipe sizes

The current work investigates the plug-to-slug transition in horizontal air–water two-phase flow in small (38.1 mm) and large (101.6 mm) diameter pipes. An extensive database is established to study the local interfacial structure in plug-to-slug transition flow. Detailed measurements across the flow area are performed for nine and six test conditions in small and large pipes, respectively, at three different axial locations downstream of the inlet using the local four-sensor conductivity probe. The effects of jg, jf, development length and pipe size are investigated. It is found that the number of small bubbles in the liquid plug/slug increases significantly in plug-to-slug transition with increasing jg, which are generated by the strong shear between the gas slug and liquid film. Due to the relative motion, these small bubbles either coalesce with the nose of the following plug/slug bubble, or slide between the plug/slug bubble and the wall, and then travel around the pipe circumference to reside beneath the large bubbles. This explains the large number of small bubbles observed at the top of the liquid film for the conditions at high gas flow rates. In the process of traveling downwards, some of the small bubbles coalesce with the slug bubbles. It is also found that increasing jg or jf decreases the size of the small bubbles. While shearing-off is believed to dominate as jg increases, turbulent-impact is enhanced as jf increases due to the increasing turbulence level in the liquid phase. Increasing jg, development length, or decreasing jf slightly increases the depths of the plug/slug bubbles; however, significant growth of plug/slug bubbles is observed in the axial direction. For the same condition, the contribution from large bubbles to total void fraction increases as pipe size increases, while the distribution of total void fraction is similar. The size of both small and large bubbles is found to be larger in the large diameter pipe. Due to the current bubble injection mechanism, small bubbles are generated at the inlet; they coalesce into large bubbles as the flow develops. The large bubble is found to accelerate as it grows along the axial direction, which can lead to a decreasing void fraction although pressure keeps decreasing.

Experimental investigation of horizontal air–water bubbly-to-plug and bubbly-to-slug transition flows in a 3.81 cm ID pipe

The present study seeks to investigate horizontal bubbly-to-plug and bubbly-to-slug transition flows. The two-phase flow structures and transition mechanisms in these transition flows are studied based on experimental database established using the local four-sensor conductivity probe in a 3.81cm inner diameter pipe. While slug flow needs to be distinguished from plug flow due to the presence of large number of small bubbles (and thus, large interfacial area concentration), both differences and similarities are observed in the evolution of interfacial structures in bubbly-to-plug and bubbly-to-slug transitions. The bubbly-to-plug transition is studied by decreasing the liquid flow rate at a fixed gas flow rate. It is found that as the liquid flow rate is lowered, bubbles pack near the top wall of the pipe due to the diminished role of turbulent mixing. As the flow rate is lowered further, bubbles begin to coalesce and form the large bubbles characteristic of plug flow. Bubble size increases while bubble velocity decreases as liquid flow rate decreases, and the profile of the bubble velocity changes its shape due to the changing interfacial structure. The bubbly-to-slug transition is investigated by increasing the gas flow rate at a fixed liquid flow rate. In this transition, gas phase becomes more uniformly distributed throughout the cross-section due to the formation of large bubbles and the increasing bubble-induced turbulence. The size of small bubbles decreases while bubble velocity increases as gas flow rate increases. The distributions of bubble size and bubble velocity become more symmetric in this transition. While differences are observed in these two transitions, similarities are also noticed. As bubbly-to-plug or bubbly-to-slug transition occurs, the formation of large elongated bubbles is observed not in the uppermost region of bubble layer, but in a lower region. At the beginning of transitions, relative differences in phase velocities near the top of the pipe cross-section to those near the pipe center become larger for both gas and liquid phases, because more densely packed bubbles introduce more resistance to both phases.

Evolution of cake batter bubble structure and rheology during planetary mixing

The incorporation of air into a high ratio cake batter by planetary mixing was studied using two bench mixers, a Kenwood KM250 and a Hobart-N50. Power draw (expressed as specific mechanical power input) and air volume fraction, ϕ, were monitored over time for batters prepared in each mixer for heat-treated and un-treated cake flours. The two flour types gave very similar results, indicating that the benefit of heat treatment for cake manufacture is manifested during the baking step. Both mixers gave a rapid initial increase in ϕ up to ∼0.50 followed by a gradual decay. Bubble size distributions of batters prepared in the Hobart showed the initial aeration stage to be accompanied by the formation of a large number of small bubbles, with diameters around 5μm: the average bubble size increased and number of bubbles decreased with extended mixing. Batters prepared without emulsifier exhibited a steady increase in ϕ to a plateau at ∼0.2 and contained larger bubbles which did not change in average size significantly over time. All batters exhibited power-law shear-thinning behaviour with power law indices and consistency strongly dependent on ϕ. At low shear rates, the apparent viscosity showed the non-linear dependency on ϕ expected for dense suspensions. At higher shear rates, elastic forces generated by the bubble phase became significant. The non-Newtonian nature of the liquid phase and the elasticity generated by the bubbles meant that shear-thinning and bubble break-up could not be predicted by analyses based on the capillary number. The results support a qualitative model of bubble formation and break-up caused by extensional deformation generated by shearing these bubbly liquids: the bubbles cream out slowly owing to the high viscosity of the continuous phase at rest.

An experimental study of Mesler entrainment on a surfactant‐covered interface: The effect of drop shape and Weber number

AbstractMesler entrainment is the formation of large numbers of small bubbles which occurs when a drop strikes a liquid reservoir at a relatively low velocity. Existing studies of Mesler entrainment have focused almost exclusively on water as the working fluid in a nominally clean state, where even very small levels of contamination can cause significant changes in surface tension that affect the repeatability of the results. Herein water combined with the soluble surfactant Triton X‐100 is used as the working fluid in an attempt to stabilize the state of the water surface. Despite this approach, nominally identical drops did not always result in the same bubble formation event. Accordingly, Mesler entrainment was quantified by its frequency of occurrence for drops having the same nominal diameter and impact velocity. This frequency of occurrence was found to be well correlated to both the Weber number and the shape of the drop on impact. © 2011 American Institute of Chemical Engineers AIChE J, 2012

A-B Phase Conversion and Coexistence of Superfluid 3He in Aerogel

We have studied phase transition of superfluid 3He at 2.4 MPa in cylindrical aerogel by NMR method. When the liquid is cooled down from the normal state, the A-like phase appears below superfluid transition temperature T which is suppressed in comparison with the transition temperature of the bulk liquid. With further cooling below the certain temperature T , , the A-like phase is converted into the B-like phase gradually. Both phases stably coexist within about 90 μK. When you keep the temperature constant in which both phases coexist, the A-B phase conversion stops. With furthermore cooling, the whole liquid becomes the B-like phase. The cwNMR spectra at the coexistence state suggest that the B-like phase is not uniformly distributed in the A-like phase like a large number of small bubbles in a liquid, but separated as a whole from the A-like phase. By applying a field gradient which changes as a function of square of radius, we found that the A-like phase is in the edge part with a cylindrical shape and the B-like phase is in the central part with a columnar shape.

Synthesis method for amorphous metallic foam

A synthesis method for the production of amorphous metallic foam is introduced. This method utilizes the thermodynamic stability and thermoplastic formability of the supercooled liquid state to produce low-density amorphous metallic foams in dimensions that are not limited to the critical casting thickness. The method consists of three stages: the prefoaming stage, in which a large number of small bubbles are created in the equilibrium liquid under pressure; the quenching stage, in which the liquid prefoam is quenched to its amorphous state; the foam expansion stage, in which the amorphous prefoam is reheated to the supercooled liquid region and is processed under pressures substantially lower than those applied in the prefoaming step. Results from a dynamic model suggest that the foam expansion process is feasible, as the kinetics of bubble expansion in the supercooled liquid region are faster than the kinetics of crystallization. Within the proposed synthesis method, bulk amorphous foam products characterized by bubble volume fractions of as high as 85% are successfully produced.

Regulation of Cell Structure in Water Blown Rigid Polyurethane Foam

AbstractRigid polyurethane foams are generally “closed celled” with a gas entrapped in each cell. The properties of such foams are significantly affected by the cell size distribution and morphology of the cells. For example, in thermal insulation foams, small cells lead to lower thermal conductivity and in foams for buoyancy applications, stronger cell windows are resistant to ingress of water until higher hydraulic pressures. The effect of formulation parameters on the size and structure of rigid polyurethane foams was investigated. Foams were made with different surfactants, with variation of surfactant concentration at different blowing agent concentration, with variation of nucleating agent concentration and with variation of catalyst concentration. Micron sized silica particles were used as nucleating agent. The densities of the foams were in the range of 140 to 165 kg/m3. The cell window areas of different foams have been measured. The bubble size in the polyol depends on the surface tension lowering ability of a surfactant, and the entrainment of a large number of small bubbles during mixing leads to a foam with a small cell size and a narrow cell size distribution. Increasing the concentration of the surfactant reduces the cell size and narrows the distribution. Increasing the proportion of organometallic catalyst in the amount of total catalyst used, results in smaller cell sizes, which are narrowly distributed. Hydraulic resistance of the foams was monitored by measuring buoyancy losses at different hydraulic pressures. Hydraulic resistance of the foams can be improved by decreasing the area and increasing the thickness of the cell windows.

Kinetics of bubble nucleation in a rhyolitic melt: an experimental study of the effect of ascent rate

In order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma, we performed three series of experiments decompressed at rates of either 1000, 167, or 27.8 kPa/s. The experiments were carried out in an externally heated pressure vessel at 800°C and in the pressure range 260–59 MPa; the starting material was a crystal-free and bubble-free rhyolitic glass containing 7.0 wt% dissolved H 2O. In all the decompression experiments, homogeneous bubble nucleation began at 90±2 MPa, that is, ≈150 MPa below the water saturation pressure of the silicate liquid, 240 MPa. The degree of supersaturation Δ P HoN required to trigger homogeneous bubble nucleation was almost independent of decompression rate (Δ P HoN is the difference between the saturation pressure and the nucleation pressure): nucleation pressure decreased by ≤3 MPa for a 36-fold increase in decompression rate. These results are in good agreement with the classical theory of nucleation assuming a rhyolite–H 2O surface tension of 0.106 N m −1. Our major experimental finding is that, after a short nucleation event, the nucleation rate dropped and the bubble number density N reached a stationary value that was strongly sensitive to decompression rate: 6.8 mm −3 at 27.8 kPa/s, 470 mm −3 at 167 kPa/s, and 5800 mm −3 at 1000 kPa/s. The smaller value of N at low decompression rate was compensated by a larger mean bubble size, so that, at a given pressure, vesicularity was almost independent of decompression rate. The experimental values of N can be reproduced within a factor 0.3–1.4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas. The nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth, which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation. Once a critical value of N is attained, diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure. The strong correlation between bubble number density and decompression rate has fundamental volcanological implications. If we extrapolate the experimental data to the typical ascent rates of silicic magmas, we obtain bubble number densities (10 −3–10 1 mm −3 for homogeneous nucleation; ≈10 −1–10 3 mm −3 for heterogeneous nucleation) that are orders of magnitude smaller than those measured in most natural pumices. We therefore propose that the large values of N in silicic pumices may be due to two successive nucleation events: (1) a first event, which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles; and (2) a second nucleation event, yielding a very large number of small bubbles, and presumably related to the dramatic increase of decompression rate that precedes fragmentation. The small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates.

Gas bubble nucleation kinetics in a live heavy oil

The current primary production of heavy oil and bitumen is highly dependent on the development of ‘foamy oil’, heavy oils containing dispersions of long-lived bubbles of gas. The nucleation and initial growth of these bubbles are key elements of the behavior of foamy oils. The tremendous levels of supersaturation associated with the expansion of live heavy oils within a porous medium can generate large numbers of small bubbles. Colloidal and interfacial science techniques were used to develop a kinetic model of the nucleation and initial coalescence of gas bubbles from the live oil. Expanding a live, heavy oil within a porous medium in laboratory experiments allowed the pressure deficit driving bubble formation to be quantified. The kinetic model and experimental data were used to calculate critical bubble radii, spacing and saturation. The large number of small bubbles predicted by the kinetic model provides an explanation for the pressure deficit observed in the volume expansion experiments. A significant amount of energy is spent on the creation of new interfaces.

Morphological transition in compressible foam

A theory is constructed to describe the morphological transition that occurs in a compressible foam when its volume is increased. The foam is observed to separate into two bubble populations or “phases,” one consisting of a large number of small bubbles, the “liquid” phase, the other consisting of a small number of large bubbles, the “gaseous” phase. First, working along lines similar to the van der Waals theory for a fluid system, approximate forms of the equation of state of the foam are derived and explored. These describe the weakly compressible range well, but fail to capture the nature of the transition. Taking a clue from the phenomenology, a theory of the “phase-separated” regime is then formulated working with the approximation that the two phases into which the foam separates are each relatively homogeneous. The successful single-phase formulas are applied to each phase, introducing an additional “order parameter” which gives the ratio of the average size of bubbles in the two phases. Approximate expressions are written for the Helmholtz free energy and the equilibria of the foam derived from these. The theory is compared to numerical simulations using SURFACE EVOLVER with very satisfying results. The transition from a uniform to a nonuniform configuration consists of avalanches of reconnections with smooth evolution between them. Hysteresis is observed as the foam is first expanded and then compressed.

The Connection between Bubble Size Spectra and Energy Dissipation Rates in the Upper Ocean

Abstract A formula for the maximum size of a bubble for which surface tension forces can prevent bubble breakup by inertial forces, combined with the observed sizes of air bubbles in breaking waves, implies an energy dissipation rate. One dataset from the surf zone gives a dissipation rate of the order of 0.1 W kg−1, but the large number of small bubbles, and the bubble size spectrum generally, are puzzling. A simple dimensional cascade argument suggests that injected air beneath a breaking wave is rapidly broken up by turbulence, producing an initial size spectrum proportional to (radius)−10/3 before modification by dissolution and rising under buoyancy. This spectral slope is comparable with data from the surf zone. The cascade argument does, however, predict that for a constant dissipation rate there is a rapid accumulation of a large number of bubbles at the scale at which surface tension prevents further breakup; it is possible that the observed size spectrum reflects the range of turbulent energy di...

The equation of state of a foam

The equation of state of a dry foam with ideal gas in the bubbles, proposed by Ross, is proved. This equation suggests that a foam with a free boundary will expand to a maximum volume if the external pressure is lowered at constant temperature. The same foam enclosed in a container, on the other hand, can be expanded further but, as the volume is increased, will eventually become unstable. We speculate that this instability leads to a “bubble differentiation” transition of the kind observed by Herdtle in numerical simulations of two-dimensional foam with ideal gas in the bubbles. In the simulations the foam separated into two “phases,” one with a large number of small bubbles, another with a small number of very large bubbles.

Opaque Silica Glass Fabricated by Slip Casting and Sintering. (Part 2). Bubble Formation.

Bubbles are essential in the formation of opaque silica glass, because opacity results from light scattering by bubbles dispersed throughout the glass matrix. In this sintering study of slip casting of a silica compact, the formation of bubbles was observed. In the final densification stage, a large number of small bubbles formed at around 1600°C. With increasing sintering temperature, isolated bubbles became larger and, concurrently, the number of bubbles decreased. Opacity increased with increasing sintering temperature up to 1800°C. These results are discussed from the viewpoint of the critical bubble size which determines whether bubbles expand or shrink.

Opaque Silica Glass Fabricated by Slip Casting and Sintering. Part 3. Evaluation of Physical, Chemical and Mechanical Properties.

Physical, chemical and mechanical properties of opaque silica glass fabricated by the slip casting method were evaluated and compared with commercial products manufactured by the fusion method. The slip casting sample contained a large number of small bubbles, and exhibited a higher light scattering coefficient and higher ability as a thermal radiation barrier. These features were well described in terms of the variation of the total surface area of bubbles per unit volume. Due to the smaller size of the bubbles, other properties such as bending strength, surface roughness and chemical durability, were considerably improved as compared to those of the fusion samples.

Various Types of Bubble-Containing Silica Glasses Fabricated by Sintering Powder Mixtures of Silica with Silicon Nitride.

Bubble-containing silica glasses were fabricated by sintering amorphous silica powders with small amount of silicon nitride powders. Sintering was performed at 1800°C in N2 gas atmosphere. It was found that the addition of Si3N4 is effective for forming bubbles. By controlling the Si3N4 content, various types of bubble-containing silica glasses, such as the so-called the opaque silica glass and the foamed silica glass, could be obtained. Addition of 0.004mol% Si3N4 produced highly opaque silica glasses containing a large number of small bubbles, while 0.431mol% Si3N4 resulted in low-density foamed silica glasses containing large 1mm-sized bubbles. Wide variations of bubble size and density were observed on these glasses. Thermal conductivities were estimated and compared with several literature values. The relation between thermal conductivity and bubble density was discussed based on a model simulation.