Secondary Bubble Research Articles

Study on heat transfer performance of spray cooling: model and analysis

In consideration of droplet–film impaction, film formation, film motion, bubble boiling (both wall nucleation bubbles and secondary nucleation bubbles), droplet–bubble interaction, bulk air convection and radiation, a model to predict the heat and mass transfer in spray cooling was presented in this paper. The droplet–film impaction was modeled based on an empirical correlation related with droplet Weber number. The film formation, film motion, bubble growth, and bubble motion were modeled based on dynamics fundamentals. The model was validated by the experimental results provided in this paper, and a favorable comparison was demonstrated with a deviation below 10%. The film thickness, film velocity, and non-uniform surface temperature distribution were obtained numerically, and then analyzed. A parameters sensitivity analysis was made to obtain the influence of spray angle, surface heat flux density, and spray flow rate on the surface temperature distribution, respectively. It can be concluded that the heat transfer induced by droplet–film impaction and film-surface convection is dominant in spray cooling under conditions that the heated surface is not superheated. However, the effect of boiling bubbles increases rapidly while the heated surface becomes superheated.

Development of bubble microstructure in ErT2 films during aging

Helium bubbles form in metal tritide films as tritium decays into H3e, influencing mechanical properties and long-term film stability. The bubble nucleation and growth mechanisms comprise an active research area, but there has been only one previous systematic experimental study of helium bubble growth in metal tritides, on zirconium tritides. There have been no such studies on tritides such as ErT2 that form platelike bubbles and lack a secondary bubble population on a network of line dislocations, and yet such a study is needed to inform the modeling of helium bubble microstructure development in a broader range of metal tritides. Transmission electron microscopy has been used to study the growth and evolution of helium bubbles in ErT2 films over a four-year period. The results have been used to test the present models of helium bubble nucleation and growth in metal tritides, particularly those forming platelike bubbles. The results support the models of Trinkaus and Cowgill. The observations of nonuniform bubble thicknesses and the pattern of grain-boundary bubble formation, however, indicate that these models could be strengthened by closer attention to details of interfacial energy. It is strongly recommended that efforts be made (either experimentally or by calculation) to determine anisotropy of tritide/helium interfacial energy, both for clean, stoichiometric interfaces, and also allowing for such factors as nonstoichiometry and segregation.

The effect of gas-injector location on bubble formation in liquid cross flow

Liquid flows incorporating small-size bubbles play a vital role in many industrial applications. In this work, an experimental investigation is conducted on bubble formation during gas injection from a microtube into the channel of a downward liquid cross flow. The tip of the air injector has been located at the wall (wall orifice) and also at several locations from the wall to channel centerline (nozzle injection). The size, shape, and velocity of the bubbles along with liquid velocity field are measured using a shadow-particle image velocimetry/particle tracking velocimetry system. The process of bubble formation for the wall orifice and the nozzle injection configurations is physically explained. The effect of variation in water and air flow rates on the observed phenomena is also investigated by considering water average velocities of 0.46, 0.65, and 0.83 m/s and also air average velocities of 1.32, 1.97, 2.63, and 3.29 m/s. It was observed that shifting the air injector tip toward the center of the channel resulted in the coalescence of some of the preliminary bubbles and the formation of larger bubbles termed secondary and multiple bubbles. Increase in air flow rate and reduction in water flow rate also intensify the rate of bubble coalescence. A correlation-based model is also suggested to overcome the shortcoming of the available models in the literature which are developed to only estimate the size of the preliminary bubbles. The model predicts the percent of the preliminary, secondary, and multiple bubbles along with the average size of secondary and multiple bubbles as a function of nozzle position within a cross flow.

Nonspherical laser-induced cavitation bubbles

The generation of arbitrarily shaped nonspherical laser-induced cavitation bubbles is demonstrated with a optical technique. The nonspherical bubbles are formed using laser intensity patterns shaped by a spatial light modulator using linear absorption inside a liquid gap with a thickness of 40 microm. In particular we demonstrate the dynamics of elliptic, toroidal, square, and V-shaped bubbles. The bubble dynamics is recorded with a high-speed camera at framing rates of up to 300,000 frames per second. The observed bubble evolution is compared to predictions from an axisymmetric boundary element simulation which provides good qualitative agreement. Interesting dynamic features that are observed in both the experiment and simulation include the inversion of the major and minor axis for elliptical bubbles, the rotation of the shape for square bubbles, and the formation of a unidirectional jet for V-shaped bubbles. Further we demonstrate that specific bubble shapes can either be formed directly through the intensity distribution of a single laser focus, or indirectly using secondary bubbles that either confine the central bubble or coalesce with the main bubble. The former approach provides the ability to generate in principle any complex bubble geometry.

Heat Transfer and Bubble Detachment in Subcooled Pool Boiling from a Downward‐Facing Microheater Array in a Nonuniform Electric Field

The effects of a nonuniform electric field on vapor bubble detachment and heat transfer in subcooled pool boiling from a microheater array are investigated. The heater array faced downward to simulate a -1 g gravity condition and to eliminate the dominant masking effect of the buoyancy force. Experiments were conducted at different subcooling levels for various wall temperatures and electric field magnitudes. A dielectric fluid, FC-72, was used as the working fluid at ambient pressure. The array of 3 x 3 independently controlled microheaters was maintained at constant temperature and the rate of heat transfer from each heater was measured. Bubble images were recorded using a high-speed camera. The electric field was applied between the horizontal downward-facing microheater array, which was grounded, and a spherical, off-axis electrode beneath it. Boiling heat transfer results with and without the electric field are presented in this study. In the absence of the nonuniform electric field, compared to the same bulk fluid temperature and wall superheat settings in the +1 g situation, a much larger primary bubble was formed on the heater array, due to the coalescence of the secondary bubbles that nucleated on the heater array. The vapor bubble remained on the heater array surface and no bubble detachment was observed. With the nonuniform electric field applied, bubbles were lifted and sheared off from the heater array surface. The electric field was able to break up the primary bubble into several smaller bubbles--considerably greater heat transfer enhancement was measured than under similar conditions in +1 g.

The dynamics of the transitional flow over a backward-facing step

The internal flow over a backward-facing step in the transitional regime (ReD = 6000) was studied based on direct numerical simulations. The predictions were carried out with the help of a finite-volume Navier–Stokes solver equipped with a co-visualization facility which allows one to investigate the flow dynamics at high temporal resolution. First, grid-induced oscillations were precluded by a careful grid design. Second, the strong influence of the velocity profile approaching the step was studied and outlined. The main objective, however, was to provide a comprehensive insight into the dynamic flow behaviour, especially oscillations of the reattachment length of the primary recirculation region. The origin of this well-known flapping behaviour of the reattachment line is not yet completely understood. In the present work, the mechanisms leading to the oscillations of the reattachment length were extensively investigated by analysing the time-dependent flow. Besides the oscillations of the primary recirculation region, oscillations of the separation and the reattachment line of the secondary recirculation bubble at the upper channel wall were also observed. The results clearly show that in the present flow case the flapping of the primary reattachment and the secondary separation line is due to vortical structures in the unstable shear layers between the main flow and the recirculation bubbles. Vortices emerging in the shear layers and sweeping downstream convectively induce small zones of backward-flowing fluid at the channel walls while passing the recirculation regions. In the case of the primary recirculation region, the rotational movement of the shear-layer vortices impinging on the lower channel wall was found to cause zones of negative fluid velocity at the end of the recirculation bubble and thus flapping of the reattachment line. In contrast, in the case of the secondary recirculation region, the shear-layer vortices moved away from the upper channel wall so that their rotational movement did not reach the boundary. In this case, the pressure gradients originating from local pressure minima located in the shear-layer vortices were identified as being responsible for the oscillations of the separation line at the upper channel wall. While moving downstream with the shear-layer vortices, the pressure gradients were found to influence the top boundary of the channel and create alternating zones of forward- and backward-flowing fluid along the wall. All of these unsteady processes can best be seen from animations which are provided on the Journal of Fluid Mechanics website: journals.cambridge.org/FLM.

Sound generation on bubble coalescence following detachment

A system in which bubbles coalesced on formation was used to probe one mechanism by which bubbles create sound. The aim was to determine in which situations sound is produced and to predict its amplitude. A set of carefully co-ordinated high-speed video and acoustic timeseries showed that needle-formed bubbles generated loud bubble-acoustic emissions at the instant of coalescence of secondary bubbles with the primary bubble. As the air flow rate increased, the size and number of secondary bubbles increased, and the sound amplitude also increased. On coalescence, the sound pressure always rose initially. A dimensionless scaling found that the sound amplitude emitted scaled with the volume of the secondary bubble. This scaling was shown to be consistent with the sound-emission mechanism being the equalization of pressures in the coalescing bubbles. The trend in amplitude with bubble production rate was well predicted by the scaling.

The Bubbling Galactic Disk. II. The Inner 20o

We report 269 mid-infrared bubbles within 10° of the Galactic center from visual inspection of the Spitzer GLIMPSE II Legacy Science program images. The surface density of bubbles is ~5 deg-2 or about 3 times that detected in longitudes |l| = 10°-65°, because the inner 10° of longitude were more thoroughly searched for small bubbles. There is a gradient in the number of bubbles with longitude with an increase of about a factor of 2 from 2° to 10°; this is probably the result of several factors, including decreasing diffuse background brightness, confusion, and opacity with longitude. Bubble eccentricities are typically between 0.6 and 0.8, and >50% show evidence for blowouts, which we suggest result from local density fluctuations of the ISM and/or anisotropic stellar winds and radiation fields. The fraction of bubbles identified with H II regions and clusters is only about half that found at |l| > 10°. This is largely a result of the much smaller angular diameter of bubbles cataloged in the inner Galaxy than in the outer Galaxy. At least 12% of the bubbles have morphologies suggestive of triggered star formation. Most of the bubbles that show evidence for triggered star formation in the inner Galaxy have not formed secondary bubbles; it is postulated that this may be because they are too young to have had time for this to occur.

Heat Transfer and Fluid Flow Characteristics of Separated Flows Encountered in a Backward-Facing Step Under the Effect of Suction and Blowing

Abstract Numerical investigation of heat transfer and fluid flow over a backward-facing step (BFS), under the effect of suction and blowing, is presented. Here, suction/blowing is implemented on the bottom wall (adjacent to the step). The finite volume technique is used. The distribution of the modified coefficient of friction and Nusselt number at the top and bottom walls of the BFS are obtained. On the bottom wall, and inside the primary recirculation bubble, suction increases the modified coefficient of friction and blowing reduces it. However, after the point of reattachment, mass augmentation causes an increase in the modified coefficient of friction and mass reduction causes a decrease in modified coefficient of friction. On the top wall, suction decreases the modified coefficient of friction and blowing increases it. Local Nusselt number on the bottom wall is increased by suction and is decreased by blowing, and the contrary occurs on the top wall. The maximum local Nusselt number on the bottom wall coincides with the point of reattachment. High values of average Nusselt number on the bottom wall are identified at high Reynolds numbers and high suction bleed rates. However, the low values correspond to high blowing rates. The reattachment length and the length of the top secondary recirculation bubble are computed under the effect of suction and blowing. The reattachment length is increased by increasing blowing bleed rate and is decreased by increasing suction bleed rate. The spots of high Nusselt number, and low coefficient of friction, are identified by using contour maps.

Modulation of surface flow and vortex shedding of a circular cylinder in the subcritical regime by a self-excited vibrating rod

The surface flow, pressure distributions, and vortex-shedding properties of the circular cylinder flow in the subcritical regime modulated by a self-excited vibrating rod were experimentally studied in a closed-circuit wind tunnel for Reynolds numbers between . In the low-Reynolds-number subrange, the laminar separation mode, which was similar to the result reported by previous investigators, was observed. While in the high-Reynolds-number subrange, the separation bubble mode, which was rarely discussed previously, was found. The term separation bubble mode was used to denote the existence of secondary recirculation bubbles induced by the reattachment of separated boundary layers. The surface flow modulated by the vibrating rod presented complex variations in these two subranges. Depending on the position of the control rod, the separation bubble mode or the turbulent separation mode might appear because of the increase of the turbulence kinetic energy. Modulating the surface flow patterns could significantly influence the surface pressure distributions on the main cylinder and the vortex shedding in the wake. Forcing the boundary layer at the positions upstream of the natural separation point would drastically lower the values of the minimum pressure coefficient and the base pressure coefficient, and therefore increase the lift. The drag coefficient, however, would not be apparently decreased. The frequency of the vortex shedding in the wake could ‘lock’ to the rod vibration frequency if the position of the vibrating rod was properly adjusted.

Modelling Particle Contacts in Distinct Element Simulations: Linear and Non-Linear Approach

In numerical simulations using the Distinct Element Method (DEM) a major role in determining particles’ trajectories is played by contact forces. In the present paper we thoroughly compare a commonly used linear force-displacement model and a recently proposed non-linear model. A discussion of the two mathematical formulations and the importance of the material parameters is presented first. Then, numerical simulations of a particle impacting on a flat wall at various angles using the two approaches are compared. Macroscopically, experimental results are used as validation means. The evolution of the forces and velocities during the collisions are compared against rigourous results. Finally, the influence of choosing a contact model and the parameters is discussed for a bubble growth in a fluidized bed. The results show the degree of accuracy gain achievable using the non-linear model, in terms of the realistic material properties that can be used and correct reproduction of the force-displacement interaction. On the other hand, using the correct parameters and despite the strong assumptions, the linear model is capable of representing the kinematics of a collision as well as the macroscopic patterns in multi-particle simulations. Differences in the bubble dynamics are confined to a slight delay in the bed surface rise and evolution of secondary bubbles.

Dynamics of a Gas Bubble during Its Interaction with Compression and Rarefaction Waves

The problem of the passage of acoustic waves in the neighborhood of a gas bubble in a tube is formulated and solved numerically. The main parameters determining the bubble dynamics in a non-stationary field are determined. The mechanism of jet deformation of the bubble followed by jet fragmentation and formation of a secondary small-size bubble fraction is studied. A possible explanation of the nature of local sonoluminiscence is proposed.

Mineralogical and physical characterization of the bricks used in the construction of the “Triangul Bastion”, Riga (Latvia)

Bricks proceeding from one of the bastions of the Riga medieval defence wall were studied. Two main type of bricks were identified: one rich in quartz and red coloured and the other with a calcium silicate phase and yellow coloured. The survey of the bastion reveals a partial deterioration of the bricks. It was observed that differences in mineralogy and texture of calcareous and non-calcareous bricks were correlated with their behaviour in hydric and weathering tests. Data from powder X-ray diffraction (presence of high-temperature mineral phase in yellow bricks), optical microscopy (vitrification in yellow bricks), Field Emission Scanning Electron Microscope (development of secondary bubbles due to extensive melting in yellow bricks) and ultrasounds measurements (lower velocity in red bricks) allow to deduce that yellow bricks were fired at a higher temperature than the red ones. The study of the pore system revealed a bad interconnection among pores in yellow bricks (hydric data) since melt isolated the pores and filled the smallest, whereas large pores increased in size (porosimetric data). As a consequence, considering the climatic conditions of the region, yellow bricks are the materials most subjected to environmental alteration. It is in agreement with the freeze–thaw test results. Finally, low contamination of the bricks with water-soluble salts in situ leads to exclude salt-caused decay process. The relevance of these results in the conservation of Cultural Heritage is discussed.

Quantification of optison bubble size and lifetime during sonication dominant role of secondary cavitation bubbles causing acoustic bioeffects.

Acoustic cavitation has been shown to deliver molecules into viable cells, which is of interest for drug and gene delivery applications. To address mechanisms of these acoustic bioeffects, this work measured the lifetime of albumin-stabilized cavitation bubbles (Optison) and correlated it with desirable (intracellular uptake of molecules) and undesirable (loss of cell viability) bioeffects. Optison was exposed to 500 kHz ultrasound (acoustic pressures of 0.6-3.0 MPa and energy exposures of 0.2-200 J/cm2) either with or without the presence of DU145 prostate cancer cells (10(6) cells/ml) bathed in calcein, a cell-impermeant tracer molecule. Bubble lifetime was determined using a Coulter counter and flow cytometer, while bioeffects were evaluated by flow cytometry. The lifetime of Optison cavitation nuclei was found to decrease and bioeffects (molecular uptake and loss of cell viability) were found to increase with increasing acoustic energy exposure. These bioeffects correlated well with the disappearance of bubbles, suggesting that contrast agent destruction either directly or indirectly affected cells, probably involving unstabilized cavitation nuclei created upon the destruction of Optison. Because Optison solutions presonicated to destroy all detectable bubbles also caused significant bioeffects, the indirect mechanism involving secondary cavitation bubbles is more likely.

Bubble formation with pool boiling on tubes with or without basic surface modifications for enhancement

Bubble formation is investigated on a single, horizontal copper tube ( D=25.4 mm) before and after applying surface modifications in the form of macrocavities with comparatively simple shapes (size: 200 × 100 μm on the tube surface and 50 μm in depth) in order to link bubble formation and heat transfer to the geometric features of the cavities. Two kinds of cavities exist in parallel, one with reduced mouth width (so-called “main structure”) and one without this reduction (“secondary structure”). Pool boiling experiments with different organic liquids have been performed with variation of heat flux and saturation pressure. The resulting bubble formation on the tubes with and without surface modifications is discussed in terms of characteristic parameters such as local distribution and temporal activation behavior of the nucleation sites for bubble formation at active nucleation sites (“primary bubbles”) or for bubbles released from the macrocavities (“secondary bubbles”).

Growth of Secondary Bubbles on the Wall of a Primary Bubble in Superheated Liquid

Growth of Secondary Bubbles on the Wall of a Primary Bubble in Superheated Liquid

Secondary bubble production from breaking waves: The bubble burst mechanism

Bubble mediated processes such as gas transfer and bubble‐burst aerosol production have long been of geophysical interest. In contrast, the bubble generation by bubble bursting remains unquantified. Based on a series of laboratory experiments, evidence for bubble production by bursting bubbles is presented as well as the bubble distribution produced by this mechanism for salt and fresh water.

Low Reynolds number two-dimensional separated and reattaching turbulent shear flow

Pulsed-wire measurements of the streamwise mean velocity and Reynolds stress, mean and fluctuating surface shear stress, and other statistics have been made in a sharp- edged turbulent separation bubble formed behind a normal flat plate mounted on the front of a long splitter plate, covering nearly a decade range of low Reynolds number. The streamwise Reynolds stress increases appreciably with Reynolds number, from a level comparable with that in an isolated plane mixing layer to more than twice that level, while the change in the mean flow is at most slight. It is inferred using previous measurements that the other stresses do not change as much, thereby leaving the mean flow relatively unaffected. The mean wall shear stress and the r.m.s. of the (streamwise) fluctuations decrease in fixed proportion with increasing Reynolds number. Normalized p.d.f. distributions of velocity and wall shear stress do not change, except in the vicinity of the secondary separation bubble. At low Reynolds number the development length of the overlying mixing layer is an appreciable proportion of the bubble length, the development length being a function of the momentum thickness at separation. It is argued that the observed changes with Reynolds number in the bulk of the flow arise primarily as a result of a change in response of this layer to the fluctuating rates of strain imposed by the recirculating flow; at a low Reynolds number the response of the shear layer structures to the fluctuating rates of strain is less than it is at a high Reynolds number. As a consequence of the sensitivity to fluctuating strain rates the structure of the layer differs fundamentally from that of an isolated mixing layer.The measurements help to reconcile previous measurements in this flow geometry which otherwise appear to be inconsistent. Moreover, in most of the previous measurements the flow width was not sufficient for end effects to have been negligible, most noticeably in the near-wall flow, where such effects are largest.

Investigation of the separation bubble formed behind the sharp leading edge of a flat plate at incidence

Detailed measurements of the separation bubble formed behind the sharp leading edge of a flat plate at low speeds and incidence are reported. The Reynolds number based on chord length ranged from 0.1 × 105 to 5.5 × 105. Extensive use of laser Doppler anemometry allowed detailed velocity measurements throughout the bubble. The particular advantages of laser Doppler anemometry in this application were its ability to define flow direction without ambiguity and its non-intrusiveness. It allowed the mean reattachment point to be accurately determined. The static pressure distribution along the plate was also measured. The length of the separation bubble was primarily determined by the plate incidence, although small variations occurred with Reynolds number because of its influence on the rate of entrainment and growth of the shear layer. Above about 105, the Reynolds number effect was no longer evident. The reverse flow boundary layer in the bubble exhibited signs of periodic stabilization before separating close to the leading edge, forming a small secondary bubble rotating in the opposite sense to the main bubble.

Low-frequency acoustic resonances in the bubble plume formed by a plunging water jet

As a jet of water penetrates a free water surface, air is entrained in the form of bubbles which produce sound by various mechanisms, including both individual and collective oscillations. The low-frequency acoustic emissions of the bubble clouds entrained and dispersed by vertical continuous and transient fresh water jets impinging from varying heights into a fresh water receiving pool in a large laboratory tank were studied both theoretically and experimentally for jet velocities ranging to about 10 m/s, and jet diameters of millimeter order. The acoustic signals were measured with hydrophones in different positions, amplified, and digitally sampled. The measured power spectra revealed well-defined, nonuniformly spaced resonance peaks at frequencies below about 1 kHz. A theory based on a conical bubble plume, with a nonuniform sound speed profile, acting as a resonant cavity predicts the frequencies of the resonance peaks and shows remarkable agreement with the experimentally measured values. Further, time-frequency analysis gives bubble size distributions and shows the driving source for the resonant cavity to be large secondary bubbles formed through coalescence, which become acoustically active through deformation or break-up as they are re-entrained by the downstream flow in a failed attempt to rise to the surface.