Incipient Bubble Research Articles

Effect of bubble size and spatial distribution on the flow and mixing of steel

ABSTRACT The size and spatial distribution of the argon bubbles have a significant impact on the refining effect when refining steel by blowing argon gas. Based on the VOF model, this paper simulated the rising behaviour of incipient bubbles in steel during ladle argon blowing. The effects of the bubble rising trajectory, velocity, offset coefficient, and active zone coefficient for steel on the effect of steel mixing were analysed. It was found that when the distance between the two incipient bubbles in the horizontal direction is greater than four bubble diameters, it will produce the best stirring effect. Moreover, when the same amount of argon gas generates several small bubbles, it provides more stirring power to the steel than when a single large bubble of equal volume is generated, and the stirring effect is superior.

In-situ TEM investigation of nano-scale helium bubble evolution in tantalum-doped tungsten at 800°C

The aim of this work is to probe the helium-induced defect production and accumulation in 40 keV He+ irradiated polycrystalline W and its alternative alloy W-5wt.%Ta using transmission electron microscopy (TEM) combined with in-situ helium irradiation at 800°С. A maximum damage level of 1 dpa with a maximum He-to-dpa ratio of 5.5 at%/dpa has been reached in this work for both materials, which corresponds to an ion fluence of 7.33 × 1016 He+/cm2. The presence of radiation-induced dislocation loops was not observed at this temperature. The low density of the incipient bubbles in W has been already detected at 0.004 dpa, which corresponds to a fluence of 3.3 × 1014He+/cm2. The experiments conducted at 800°C have shown that the addition of 5wt.% of tantalum into tungsten may diminish the binding of He ions with vacancies into complexes, which serve as the core of the bubble, thus hindering helium bubble formation below 0.02 dpa and their further growth and population at higher damage levels. By exceeding the damage dose ≥0.3 dpa, a progressive transition from a spherical to a faceted shape of the bubbles has been observed in W but not in the W-5Ta alloy. At 1 dpa, >80% of the bubbles in W were of the faceted type with the facet planes of {110}.

Start-up characteristics of MEMS-based micro oscillating heat pipe with and without bubble nucleation

The startup characteristics of micro oscillating heat pipe (micro-OHP) have been experimentally investigated using a high-speed CCD camera in conjunction with the temperature measurement. The micro-OHP was fabricated on a silicon wafer by the MEMS technology, having trapezoidal channels with a hydraulic diameter of 357 μm. HFE-7100 was used as the working fluid with volumetric filling ratios ranging from 31% to 72%. The effects of filling ratio and heating power input on the start-up process were presented. Two different start-up behaviors, so-called start-up with and without bubble nucleation (STWBN/STWOBN), subject to different heat input levels were observed, and the underlying mechanisms were elaborately analyzed. The occurrence of STWBN or STWOBN depends mostly upon the spatial distribution of slugs/plugs in the micro-OHP. For the STWOBN mode, the fluid movement could be triggered by intense liquid film evaporation without nucleation with evaporator temperature less than 80 °C. However, the STWBN mode is sometimes indispensable to excite the two-phase oscillations in the micro-OHP due to the non-desirable slug/plug distribution, leading to much higher evaporator temperatures even greater than 140 °C to initiate the bubble generation. To predict the startup temperature at the STWBN mode, a theoretical model to describe the wall superheat required for incipient bubble nucleation in microchannels was propounded and largely agreed with the present experimental data. Micro-OHPs worked at the STWBN mode may greatly hinder their applications in electronics cooling, and artificial cavities fabricated or embedded on silicon channel wall surfaces are proposed to facilitate bubble nucleation and then the startup at low temperatures.

Liquid flow in a gap between a cylinder and a wall in motion

The results of an experimental investigation and calculations of the location of the minimum pressure point are presented for the case, when a cylindrical body moves along a wall in the presence of a small gap. The pressure on the cylindrical body surface is measured in the confusor and diffuser regions. It is shown that with decrease in the gap the minimum pressure point is displaced toward the minimum gap line, with increase in the pressure drop. An increase in the velocity of the motion at a constant gap leads only to a pressure increase in the diffuser region, while the location of the minimum pressure point remains the same. It is established that an increase in the inner cylinder radius moves the minimum pressure location away from the minimum gap line. The formation of two return flow regions in the confusor and diffuser regions near the cylindrical surface is detected. It is shown that the return flow in the pressure drop region reaches the stage of incipient cavitation bubbles. The results obtained can be useful in lubrication theory, as well as in medicine and biology.

Investigation of the source of histotripsy acoustic backscatter signals

Recent work has demonstrated that acoustic backscatter signals from histotripsy-generated bubble clouds may be used to localize generated bubble clouds and perform non-invasive aberration correction transcranially. However, the primary source of the measured signals, whether from emissions generated during bubble expansion, or scattering of the incoming pulses off of the incipient bubble clouds, remains to be determined and may have important implications for how the acquired signals may be used. Here, we present results from experiments comparing the acoustic emissions and growth-collapse curves of single bubbles generated optically to those generated via histotripsy. Histotripsy bubbles were generated using a 32-element, 1.5 MHz spherical transducer with pulse durations <2-cycles; optical bubbles were nucleated using a pulsed Nd:YAG laser focused at the center of the histotripsy transducer. Optical imaging was used to capture the time evolution of the generated bubbles from inception to collapse. Acoustic emissions from the generated bubbles were captured using the receive-capable histotripsy transducer elements as well as with a commercial hydrophone mounted within. Imaging results indicated that optically nucleated bubbles experienced more rapid growth than histotripsy generated bubbles. Acoustic emissions from both sets of bubbles were comparable, however, suggesting the primary component of the measured histotripsy “backscatter” signal is an emission generated during bubble expansion.

Effect of back pressure on nozzle inner flow in fuel injector

The internal nozzle flow has great influence on fuel injection and spray. This work investigates the impacts of the injection back pressure on the nozzle inner cavitation developing, especially the flow characteristic during choking process. Based on the theoretical analysis, a three-dimension numerical model is developed to investigate the details of the inner nozzle cavitation flow. The numerical model is verified by experiments on a diesel fuel system test rig under different boundary pressures, including varied injection pressure and back pressure. The investigation shows that for a given injection pressure, cavitation occurs with the drop in the back pressure. The incipient bubbles appear just at the upper corner of nozzle inlet hole, and the cavitation area continues to expand until it reaches to the nozzle outlet. The nozzle inner cavitation is divided into three periods: no cavitation period; local cavitation developing period and super cavitation period. Before the super cavitation, as the back pressure drops, bubbles at the entrance of the nozzle hole cover more area, but the average liquid velocity increases and as a result the mass flow increases. Once the liquid velocity reaches to its maximum, the cavitation of the nozzle inlet hole maintains stable, so does the vapor–liquid mixture cross section, and the effective cross section of the liquid phase is minimum; by now the super cavitation forms. Under the condition of the super cavitation, the nozzle inner flow becomes choking and the back pressure has little influence on the mass flow, but the discharge coefficient declines as the back pressure decreases. During the choking flow period, when the back pressure declines, along the nozzle hole central axis from the inlet to the outlet, the effective flow cross section maintains stable in sequence and the liquid velocity also reaches its maximum in order. Moreover, at the outlet of the nozzle hole, super cavitation induces the increase of the turbulence kinetic energy and the outlet velocity. This increased outlet velocity makes up for the loss of the contracting effective flow passage due to super cavitation, thus the total mass flow maintains the same value after choke even the back pressure decreases.

Heterogeneous bubble nucleation on ideally-smooth horizontal heated surface

Our experiments and analyses of the bubble incipient on ideally-smooth, horizontal heated surface confirm the observed low superheat and the weak surface-wettability dependence. These are contrary to the previous heterogeneous nucleation predictions, so to clarify this difference and the experimental results not explained by previous nucleation theories we adapt a new model based the thermal boundary layer. The model includes the kinetic dynamics of the superheated liquid and the thermodynamic stability of the generated vapors. The fluid particle transfer rate is estimated with the Smoluchowski equation. Consequently, the incipient bubble nucleation on the ideally-smooth horizontal surface, with different wettability, is described and the predictions match the experiments fairly well.

Microlayer structure in nucleate boiling of water and ethanol at atmospheric pressure

This study investigates the structure of the microlayer that forms between growing bubbles and the heat transfer surface for nucleate pool boiling in the isolated bubble region. The microlayer thickness was measured for water and ethanol by a specially designed measurement system that employs the laser extinction method. A thin (diameter: 94μm) optical fiber in a thin stainless-steel tube was inserted in a liquid pool. Gas flow was used to prevent the fiber, which was made of fluorinated glass, from becoming wet and to remove the pool liquid from the light path. A method for determining the initial microlayer thickness was developed based on numerical calculations. The microlayer thickness decreased over time due to evaporation. The thickness of the initially formed microlayer was found to be uniquely determined. For water, it was in the approximate range of 0–9μm and it increased linearly with increasing distance between the incipient bubble site and the measurement position; this relation was independent of the heat flux. The initial microlayer structure for ethanol was similar to that for water, but its thickness was about twice that for water.

Investigation of transient processes at liquid boiling under nonstationary heat generation conditions

The paper presents results on experimentally investigated dynamics of boiling development and formation of film boiling zones under stepwise heat generation on a horizontally and vertically oriented cylindrical surface in a large volume of Freon-21. Experimental data on the expectation time and boiling temperature, the propagation velocity and structure of evaporation and boiling fronts for different heat flux density both in saturated liquid and in subcooling conditions are obtained. Results of experiments on investigating the nucleation forms under development of nonstationary heat release crisis caused by heat loading on the vertical heater immersed into the volume of liquid (water, ethanol) subcooled to saturation temperature are presented. A calculation ratio for determining the expectation time to the beginning of intense vaporization in water is proposed and compared with experimental data obtained on surfaces with different-size roughness. Peculiarities of evolution of evaporation fronts from incipient bubbles are investigated in the experiments with ethanol. Data on the evaporation front velocity as a function of wall overheating are obtained. The obtained experimental data on the propagation velocity of self-sustained evaporation fronts are compared with the known calculated data.

ICONE19-43556 Bubble behavior in subcooled flow boiling in a vertical rectangular channel

In this study bubble behavior in subcooled flow boiling in an upward rectangular channel was discussed. Filtrated and deionized tap water was used as a working fluid. Heated surface is produced by cupper with high wetability or low contact angle (18°). Subcooled flow boiling is beginning with onset of nucleate boiling (ONB) which surface temperature is higher than saturation temperature while liquid temperature is less than saturation temperature. At various pressure, mass fluxes and inlet temperatures ONB heat flux is detected within observation incipient nucleate bubbles through high speed video camera. Comparing with available ONB correlations, it is revealed that surface wettability is a significant parameter. Among ΔT_<w,ONB> correlation only Basu et al. (2002)'s correlation has a good consistency with experimental data, and other correlations are most underpredicted. Nucleation rate of bubbles after ONB remain in very small amount until a specific condition nominated onset of significant void (OSV) or incipient point of net vapor generation (IPNVG) that void fraction drastically increased. Levy (1967) was the first to postulate bubble departure mechanism for OSV and many researchers believed this assumption and analytically and empirically proposed OSV correlation. However, in all the experimental conditions even at incipient bubble nucleation, bubbles are observed depart or lift-off from activated nucleation site on the heated surface. Bubble nucleation behavior as whether bubble Lift-off from hot surface or sliding along it are investigated. It is performed for low void fraction condition. It is concluded that lifting-off condition occurred when bubble size and subcooling temperature is high and sliding phenomena in the opposite condition happened. Bubble size dramatically drop down after atmospheric pressure. This categorization is useful for developing new OSV mechanism regarded to actual situation.

Incipient bubble formation during bulk polymerization of methyl methacrylate under near‐isothermal conditions using a ribbon agitator

AbstractThe polymerization of methyl methacrylate (MMA) is carried out under near‐isothermal conditions in a 1‐liter stainless steel stirred batch reactor with a glass bottom. A ribbon agitator is used to study the incipient stable‐bubble formation point, Bi, during the bulk polymerization of MMA. A camcorder has been used to get images of the bubbles from the beginning of the polymerization till just beyond the incipient stable‐bubble formation point. Experimental data on the monomer conversion, xm,Bi, the weight average molecular weight, Mw,Bi, and the viscosity, ηBi, of the reaction mass have been generated at this important point. Data have been taken at several values of the (near‐isothermal) temperature, initiator loading, reactor pressure, and the RPM of the ribbon stirrer. No significant effect of pressure and the RPM of the stirrer were observed over the ranges of these variables studied. Correlations have been developed for xm,Bi, Mw,Bi, and ηBi, relating these to the initiator loading and the temperature. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Configuration of the micro-layer and characteristics of heat transfer in a narrow gap mini/micro-channel boiling system

Abstract Heat transfer measurements were performed in a mini/micro-channel boiling system for water, and the thickness of the micro-layer that formed between the heating surface and the generated vapor under vapor growth was measured by application of the laser extinction method for narrow gaps of 0.5, 0.3 and 0.15 mm. The process of bubble growth was recorded by using a high-speed camera simultaneously. The effects of gap size, the velocity of vapor bubble forefront and the distance from the incipient bubble site were investigated on the micro-layer thickness in a narrow gap mini/micro-channel boiling system and the configuration of the thin liquid micro-layer distributions on the heat transfer surface was clarified. Furthermore, factors that would possibly affect the mechanism and characteristics of heat transfer, such as the position of the generated vapor bubble, the velocity of the vapor forefront, the micro-layer dominant period and the liquid saturation period in the boiling cycle, and so forth, were quantitatively investigated and analyzed on the basis of the measured data. The heat transfer characteristics were analyzed and the data calculated were coincided with the measured data in the boiling curve for the gap size of 0.5 and 0.25 mm measured in the previous report. It was shown that the heat transfer was enhanced due to the micro-layer evaporation.

Configuration of Micro-Layer in Nucleate Boiling

The objective of the study is to clarify the configuration of micro-layer formed between growing bubble and the heat transfer surface in nucleate pool boiling of region of isolated bubble. Micro-layer thickness was measured by specially devised measuring system applying the laser extinction method. The thin optic fiber of 94 μm in diameter arranged in a thin stainless tube was inserted in the liquid pool with the gas blowing both for preventing the getting wet of fiber made of fluorinated glass and removing the pool water in the path of laser light. The precision of the measurement was improved by using the two high speed cameras to decide the distance from bubble incipient site. It was elucidated that the thickness of the initially formed micro-layer was decided uniquely. The thickness was between 1 μm and 8 μm and was increased with the increase of distance between incipient bubble site and measuring position independently upon heat flux in this experiment.

Structure of Micro-Layer and Characteristics of Boiling Heat Transfer in Narrow Gap Mini-Channel System

In the previous reports, the heat transfer measurement was performed in a mini-channel boiling system and the thickness of micro-layer that formed between the heating surface and generated vapor with the vapor growth was measured applying the laser extinction method for the narrow gaps and the process of bubble growth was recorded by using a high-speed camera simultaneously. The effects of gap size, the velocity of vapor bubble forefront and the distance from the incipient bubble site were investigated on the micro-layer thickness in a narrow gap mini-channel boiling system and the configuration of the thin liquid micro-layer distributions on the heat transfer surface was clarified. In this study, the mechanism and characteristics of heat transfer were investigated quantitatively by analyzing the affecting factors such as the position of vapor bubble generated, the velocity of vapor forefront, the periods of the micro-layer dominant and the liquid saturation in a boiling cycle and so forth on the basis of the measured data in the previous researches. The heat transfer characteristics were analyzed and the data calculated were coincided with the measured data in the boiling curve for the gap size of 0.5 mm measured in the previous report. Therefore, it was concluded that the boiling phenomena in a narrow gap mini-channel were quantitatively determined by the two kinds of characteristic periods of the micro-layer dominant and the liquid saturated and the characteristic factors shown above.

A new calculation of the work of formation of bubbles and drops

The thermodynamic basis of surface tension for bubbles and drops is reformulated to take into account the variation in surface tension for very small, growing or incipient bubbles and drops. After assessing the magnitudes involved by means of conventional theory, a model is put forward for this variation that avoids the supposition of extreme pressures at small sizes in the simple model. The parameter of this new model is then set by consideration of the properties of real gases, determined by analysis of the incipient bubble and then extended, it is argued, to the incipient drop. van der Waals' model of a real gas is used to show how this combines with the Lewins model in a self-consistent approach to the thermodynamics of surface tension for bubbles and drops. The changes from the simple theory will then allow a revision of the dynamic statistical theory of auto-nucleation and a reassessment of the critical sizes for imposed or heterogeneous nucleation, based on a simple and self-consistent thermodynamic model.

Behaviors of micro-layer in micro-channel boiling system applying laser extinction method

To elucidate the mechanism and characteristics of boiling heat transfer in a micro-channel vaporizer, the experimental investigation of the micro-layer thickness that formed between the heating surface and vapor generated was important. The micro-layer thickness was measured applying the laser extinction method for channel gap sizes of 0.5, 0.3, and 0.15 mm. It was clarified that the gap size, the rate of bubble growth, and the distance from the incipient bubble site have an effect on the micro-layer thickness in a micro-channel boiling system. The initial micro-layer thickness grew with an increase of the velocity of bubble forefront to moderate the value of the velocity. In the region of greater velocity, the thickness was constant for each gap. The distributions of the initial thickness of micro-layer on the heat transfer surface were shown. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 35–46, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20096

Behaviours of Micro-Layer in Micro-channel Boiling System Applying Laser Extinction Method

To elucidate the mechanism and characteristics of boiling heat transfer in a micro-channel vaporizer, the experimental investigation of the micro-layer thickness that formed between the heating surface and vapor generated was important. The micro-layer thickness was measured applying the laser extinction method for the channel gap sizes of 0.5, 0.3 and 0.15 mm. It was clarified that the gap size, the rate of bubble growth and the distance from the incipient bubble site have an effect on the micro-layer thickness in a micro-channel boiling system. The initial micro-layer thickness increased with increase of the velocity of bubble forefront to moderate value of the velocity. In the region of larger velocity, the thickness was constant for each gap. The distributions of the initial thickness of micro-layer on the heat transfer surface were shown.

Cavitation in flow through a micro-orifice inside a silicon microchannel

Hydrodynamic cavitation in flows through a micro-orifice entrenched in a microchannel has been detected and experimentally investigated. Microfabrication techniques have been employed to design and develop a microfluidic device containing an 11.5μm wide micro-orifice inside a 100.2μm wide and 101.3μm deep microchannel. The flow of de-ionized water through the micro-orifice reveals the presence of multifarious cavitating flow regimes. This investigation divulges both similarities and differences between cavitation in micro-orifices and cavitation in their macroscale counterparts. The low incipient cavitation number obtained from the current experiments suggests a dominant size scale effect. Choking cavitation is observed to be independent of any pressure or velocity scale effects. However, choking is significantly influenced by the small stream nuclei residence time at such scales. Flow rate choking leads to the establishment of a stationary cavity. Large flow and cavitation hysteresis have been detected at the microscale leading to very high desinent cavitation numbers. The rapid transition from incipient bubbles to choking cavitation and subsequent supercavitation suggests the presence of radically different flow patterns at the microscale. Supercavitation results in a thick cavity, which extends throughout the microchannel, and is encompassed by the liquid. Cavitation at the microscale is expected to considerably influence the design of innovative high-speed microfluidic systems.

Micro bubble dynamics in DNA solutions

Micro bubble generation and its subsequent dynamic behavior in single-stranded DNA (ssDNA) solutions are presented in this paper. A micro vapor bubble was generated in ssDNA using a micro bubble actuator, which is capable of producing periodic and stable single vapor bubbles under pulse heating. The growth and collapse of the micro vapor bubble were visualized by a high-speed CCD camera, and the bubble dynamics was investigated at different ssDNA concentrations and under various pulse widths. It was observed that an increase in the ssDNA concentration led to an increase of the electric power required for incipient bubble nucleation. Based on thermodynamics considerations and a simple model for nucleus formation in ssDNA solution, an analysis of bubble nucleation work was carried out and the results are consistent with experimental data. It is found that the bubble dynamics in ssDNA solutions is different from that in DI water, and an obvious retardation effect on the motion of the micro bubble was observed at high ssDNA concentrations. Based on Zimm's model, the effect of ssDNA macromolecules on the total viscosity of the solution is revealed. The present study indicates that polymer properties can significantly influence bubble nucleation and the subsequent evolution of bubble dynamics, owing to hydrodynamic intermolecular interactions of polymer macromolecules.

Dynamics of Excess Electron Localization in Liquid Helium and Neon

In this paper, we address the relations between the structure, electronic level structure, energetics, and localization dynamics of an excess electron in a bubble in liquid 4He, 3He, and Ne. Our treatment of the dynamics of formation for the electron bubble rests on a quantum mechanical Wigner−Seitz description of the excess electron in conjunction with a hydrodynamic picture for the liquid. The dynamics of electron localization is described in terms of the initial formation of an incipient bubble of radius 3.3−3.5 Å followed by adiabatic bubble expansion in the ground electronic state. The hydrodynamic model for bubble expansion considers the expansion of a spherical cavity in an incompressible liquid with the energy dissipation being due to the emission of sound waves. This model predicts the bubble expansion time ( ) in liquid 4He to be = 8.5 ps at P = 0, exhibiting a marked pressure dependence (decreasing by a numerical factor of 4 at P = 16 atm) and revealing a small isotope effect of (4He)/ (3He) = 0.83 in liquid 3He, while for liquid Ne, ≅ 1 ps. The collapse times of the empty bubble formed by vertical photoionization of the electron bubble are τc = 19.8 ps for 4He and τc = 34 ps for 3He, with τc being considerably longer than , reflecting the effect of the kinetic electron energy on the fast electron bubble expansion. The interrogation of the electron bubble localization dynamics by femtosecond absorption spectroscopy is explored by the analysis of the temporal evolution of the electronic excitation energies.