Bubble Formation Characteristics Research Articles

Influence of wettability and surface tension on bubble formation from a needle

AbstractThe multiphase flow accompanied by contact line motion is a common flow phenomenon in nature and industry. When investigating the bubble formation from a needle, it is generally assumed that the contact line is fixed at the inner diameter of a needle. Namely, the influence of contact line motion on bubble formation is ignored. In this paper, the effects of wettability and surface tension on the characteristics of bubble formation were studied using volume of fluid method. The changes of bubble volume, instantaneous contact angle, contact line radius, and the motion mechanism of contact line were analyzed in detail. It is found that when surface wettability and/or surface tension are different, the motion type of contact line during bubble formation is different, and the stage of bubble formation shows a great difference too. The motion of contact line is mainly affected by the pressure inside a bubble, the adhesion force, and the hydrostatic pressure of liquid outside a bubble. The sum of all forces driving contact line to move inward or outward controls the motion type of contact line.

Numerical investigation on boiling heat transfer in a rectangular channel under sub-atmospheric environment

Water evaporators are often used as heat sinks in spacecraft. The aim of this study is to investigate the heat transfer characteristics and the spatial and temporal characteristics of bubble formation in an airborne water evaporator at an absolute pressure of 7 KPa. In order to achieve this, a rectangular single-channel model based on the structure of the water evaporator is established. Since the temperature change of the glycol in the water evaporator is not obvious when it flows through the rectangular single-channel, the inner wall surface is set to be 333.15 K in order to reduce the computational cost in the simulation. In this paper, a VOF method is adopted to numerically investigate the heat transfer characteristics of the boiling in the upper pool of the rectangular channel under a low-pressure(7 kPa) environment. Good agreement was obtained by comparing the numerical results with the experimental data with the error within 6 %. The numerical results show that bubbles rise along the wall, combine with nearby small bubbles, grow and attach to the wall, and slide along the wall until they are discharged from the top of the wall. The perturbations generated by the bubbles cause the heat transfer coefficient and wall temperature to fluctuate, with the maximum fluctuation of the wall temperature being around 5 K. In addition, the effects of different rectangular channel structure sizes (0.5 mm, 1.5 mm, and 2.5 mm) on the boiling heat transfer and bubble characteristics were investigated. A channel size of 0.5 mm leads to severe bubble coalescence, which increases the wall temperature and deteriorates the heat transfer performance. When the channel spacing is increased, bubble coalescence is improved, but a channel spacing of 2.5 mm results in less bubble turbulence and weaker heat transfer performance.

Influence of the electrolyte conductivity on the critical current density and the breakdown voltage

The work investigates influence of the electrolyte conductivity on the onset of partial contact glow discharge electrolysis (CGDE) in a water electrolysis. Critical current density (CCD) and breakdown voltage were measured together with in situ observation of hydrogen bubble behavior, whose influence has not been focused on. For a fixed current during normal electrolysis, hydrogen coalescence adjacent to cathode surface was invigorated at a lower conductivity. Photographic analyses elucidated the hydrogen coalescence characteristics by quantifying size and population of detached hydrogen bubbles. The CCD increased about 104% within given range of conductivity (11.50–127.48 mS·cm–1) due to impaired bubble coalescence, which delays hydrogen film formation on the cathode. Meanwhile, decreasing trend of breakdown voltage was measured with increased conductivity showing maximum drop of 74%. It is concluded that onset of partial CGDE is directly affected by hydrodynamic bubble behaviors, whereas the electrolyte conductivity affects the bubble formation characteristics adjacent to cathode electrode.

The bubble formation characteristics of oil‐paper insulation and its influence on insulation performance

AbstractEngineering experience shows that for transformers with long service life and serious moisture exposure, the inter‐turn paper insulation inside the winding will precipitate bubbles under the condition of electric field and conductor heating, which will endanger the equipment insulation. In order to explore the influence factors of bubbles formation in the moist insulating paper of transformers, based on the existing research on the influence of moisture content and gas dissolution in transformer oil, this study further considers the influence of air pressure (AP) and electric field and establishes an experimental platform to change the air pressure and electric field, thereby studying the formation characteristics of bubbles in oil‐paper insulation. The results show that air pressure and electric field can affect the initial temperature, volume, and shape of bubble formation. From 0 to −0.05 MPa for air pressure, the initial temperature of bubble generation decreased by 41°C, its maximum two‐dimensional projected area increased by 418.4% with more severe distortion, and the breakdown voltage decreased. When the AC voltage applied on the needle plate electrode increased from 0 to 10 kV, the initial temperature of bubble formation increased by 29°C, and the maximum two‐dimensional projection area increased by 336.6%.

Amelioration of pool boiling thermal performance utilizing graphene-silver hybrid nanofluids

Graphene nanoplatelets (GNP) have natural hydrophobicity and are not easy to disperse in deionized (DI) water. In this study, Ag modified functionalized GNP (f-GNP) was used to prepare novel hybrid nanoparticles. Through phase and structural analysis, it is found that the material has high purity, and is dispersed in DI water to produce the nanofluids that are stable. The boiling heat transfer properties of GNP nanofluids, GNP/Ag mixed nanofluids and GNP-Ag hybrid nanofluids of three weight concentrations (wt% = 0.001, 0.002, 0.003) were investigated. Among the nine samples, the GNP-Ag hybrid nanofluids with weight concentration of 0.001% has the best heat transfer property, with a critical heat flux (CHF) of 170.74 W/cm2, which is 52.31% more than the one of DI water. After the boiling experiment, the heating surface was analyzed by scanning electron microscope (SEM) to observe the deposition structure of nanoparticles. It is found that the self-assembled structure of the hybrid nanoparticle deposition is more complex. Under the combined action of the oxygen-containing functional groups of the nanoparticles, the lateral heat transfer of the working medium is strengthened, and the separation of the vapor bubbles is promoted. In the wettability analysis, the contact angle of the nanofluids were measured, and it was found that the hybrid nanofluids was hydrophilic, which improved the liquid supply mechanism on the heating surface and was beneficial to increase the CHF. The bubble formation characteristics of nanofluids were observed by high-speed camera, and it was found that the bubble formation period of hybrid nanofluids was short and the bubble separation diameter was small. Results from tests and experiments indicate that covalent modified hybrid nanofluids have good heat transfer performances.

Experimental study of boiling heat transfer for a novel type of GNP-Fe3O4 hybrid nanofluids blended with different nanoparticles

This work is based on a new research direction to ameliorate the hydrophobicity of graphene nanoplatelets (GNP) by functionalizing it without affecting its good thermal properties and to prepare novel hybrid nanofluids for enhancing heat transfer. In this work, a novel type of GNP-Fe3O4 hybrid nanoparticles is prepared, and the crystal structure test proves that it has high purity, contains a variety of hydrophilic functional groups, and has a strong binding force between elements. The blending of metal or non-metal nanoparticles in the GNP-Fe3O4 hybrid nanofluids is beneficial to improve the deposition effect of the nanofluids and reduces the heat transfer resistance of the heating surface. Therefore, the boiling heat transfer characteristics of GNP nanofluids, GNP-Fe3O4 hybrid nanofluids, GNP-Fe3O4 + Al2O3 hybrid nanofluids, and GNP-Fe3O4 + SiO2 hybrid nanofluids with different mass concentrations (0.0005 wt%, 0.001 wt%, 0.002 wt%, and 0.003 wt%) were studied experimentally. The heat transfer effects of different nanofluids at the same concentration were also compared. The experimental results show that the heat transfer characteristics of nanofluids are significantly enhanced compared with the deionized water (DI water). Among them, GNP-Fe3O4 + Al2O3 hybrid nanofluids and GNP-Fe3O4 + SiO2 hybrid nanofluids have the best heat transfer effect, when the mass concentration is 0.003 wt% critical heat flux (CHF) is 180.23 W/cm2 and 185.29 W/cm2, which are 60.61% and 65.12% higher than DI water. Also, the maximum heat transfer coefficient (HTC) is 9.73 W/cm2K and 10.23 W/cm2K, which are 82.74% and 92.13% higher than DI water. Both of them have similar heat transfer intensity at the same mass concentration. Through the observation of the macroscopic morphology and microstructure of the deposition surface, the wettability test and the study of the bubble generation characteristics, the boiling heat transfer mechanism of the new hybrid nanofluid is qualitatively analyzed. The results show that the wettability and deposition characteristics of GNP-Fe3O4 hybrid nanofluids are significantly improved after blended with other nanoparticles, the microscopic deposition structure is more complex and porous, the macroscopic deposition amount is significantly reduced, and the heat transfer resistance is reduced; the characteristics of bubble formation show that the fluid is violently disturbed and the heat transfer intensity is greatly improved. At the same time, the functional groups introduced by acid mixing treatment and metal modification in the preparation of the new hybrid nanoparticles also plays a great role in promoting the heat transfer of the fluid.

Numerical Simulation and Experimental Verification of Bubble Formation Characteristics in Coaxial Microchannels

Numerical Simulation and Experimental Verification of Bubble Formation Characteristics in Coaxial Microchannels

Three dimensional transient analysis of bubble dynamics in centrifugal bubble singlet oxygen generator for scalable COIL

Chemical Oxygen Iodine Laser (COIL) is the shortest wavelength (1.315 μm) chemical laser employing singlet oxygen molecules as the pumping species and iodine atoms as lasing species. Singlet Oxygen Generator (SOG) is essentially the heart of this laser device which is basically a gas–liquid phase chemical reactor. SOG facilitates the reaction between a basic hydrogen peroxide solution (BHP, H2O2 + KOH) and chlorine gas to produce singlet oxygen molecules. Centrifugal Bubble Single Oxygen Generator (CBSOG) is a potential candidate for an alternative SOG with better operating conditions for high throughput and high power COIL systems. In CBSOG Chlorine gas is bubbled through a layer of BHP solution and the required reaction takes place at the gas–liquid interface of the formed bubbles under high speed rotation of the chamber for gravity independent operation. The performance of this device largely depends on the bubble formation characteristics and their traverse inside the BHP layer. The aim of this paper is to present the numerical analysis of the mechanics of bubble formation, detachment and movement in the BHP layer for different operating parameters as applicable to COIL. The determination of conditions for the transition from bubbling to the jetting regime at different centrifugal accelerations of high magnitude (G ∼ 204 g, 306 g & 387 g) with low liquid column thickness (∼6–8 mm) operating under low pressure (∼100 torr) is another point of focus. The Volume of Fluid (VOF) method has been used for the 3-D transient interface tracking between the phases. This paper also compares the simulation results for various gas flow rates and centrifugal accelerations with select experimental results reported elsewhere showing reasonably good agreement.

Effects of liquid viscosity on bubble formation characteristics in a typical membrane bioreactor

The generation and formation characteristics of air bubbles have significant impact on the performance of the membrane bioreactor system. Experiments were conducted in a tank filled with Glycerine-Water solutions so as to investigate the impacts of liquid viscosity on the bubble formation characteristics. Nozzle size ranging from 2 to 10 mm, air inlet flow rate ranging from 300 to 1500 cc/min and liquid viscosity ranging from 6 to 850 cP have been investigated. Methods to compute the non-linear characteristics such as bubble diameter and bubble rising velocity have been identified. The nonlinear behaviour of the air bubbles were further correlated to the dimensionless numbers such as Reynolds number, Eötvös number and Galilei number. A bubble shape regime was constructed so as to classify the distinct regions of the air bubbles generated from different viscous solutions. Coalescence of bubbles were studied extensively and the impact of liquid viscosity on the bubble flow regimes were presented.

A comparative study of bubble formation characteristics in non-Newtonian and high-viscosity Newtonian fluids by a laser image technique

A laser image measurement system has been established to study the problem of dynamic bubble formation from a submerged orifice in two kinds of high-viscous fluids, namely Newtonian glycerol aqueous solution and non-Newtonian carboxymethylcellulose (CMC) aqueous solutions. The bubble generation process was measured accurately by means of He-Ne laser as light source using the beam expanding and light amplification technology. The whole formation process, including bubble expansion, stretching and detachment three stages, and the dynamic characteristics of each stage, were investigated experimentally by analyzing comparatively the evolution of the instantaneous shape, volume, surface area, and deformation rate in both fluids. The influences of solution mass concentration, gas chamber volume and orifice diameter on bubble detached volume were discussed respectively, and the results reveals that bubble begins to grow spherically due to the predominant role of surface tension, and then is elongated gradually owing to the increasing buoyancy force. Compared with Newtonian fluid, bubble formed in non-Newtonian fluid presents a slim shape as a result of the shear-thinning effect of CMC solution. Bubble detachment volume in both kinds of fluids increases with solution mass concentration and gas chamber volume, but declines with orifice inner diameter, respectively.

Experimental Study on Hydraulic Characteristics of Pneumatic Lifting Pump

A pneumatic lifting pump is used in sewage treatment, offshore oil production, and other fields because of its simple structure and strong practicability. In order to study its internal hydraulic characteristics and gas-liquid two-phase flow, this paper carries out experimental research on the influence of different air intake modes and riser diameters on the performance of a pneumatic lifting pump. The air-water two-phase flow pattern in the riser and motion characteristics of bubble formation at the nozzle are obtained by a high-speed camera. Through theoretical analysis, the theoretical model of a pneumatic lifting pump is established, and experimental results verify the theoretical model well. The results show that when the submergence ratio is constant, the lifting efficiency decreases with the smaller intake area under different intake areas; and the influences of different holes distributions on liquid flow rate and lifting efficiency are not significant under the same intake area. At the same submergence ratio, the smaller the riser diameter, the smaller the final lifting liquid flow rate and the larger the lifting efficiency peak value.

Bubble Formation Characteristic of Submerged Single-Hole Orifice in Aerosol Suspension

Bubble column is regarded as an effective containment filtration method. A large amount of radioactive aerosol will retain in liquid pool and generate decay heat. And the surface tension, viscosity and other parameters of liquid phase are affected by increasing of the temperature of liquid pool and the presence of aerosol in pool, which in turn affects the formation characteristics of bubbles. Another important factor affecting bubble generation characteristics is the proportion of steam in gas phase, that is, the exchange of mass and energy of gas phase. This paper will investigate the effects of liquid temperature, aerosol and gas phase composition on bubble formation characteristics by analyzing the variation of bubble detachment volume of the orifice in liquid pool.

Effect of orifice size and bond number on bubble formation characteristics: A CFD study

The volume of fluid with the continuum surface force (VOF‐CSF) model has been used in the present numerical study to investigate bubble formation and shapes in a bubble column. The effects of orifice sizes ranging from 0.5–1.5 mm on the bubble formation stages (i.e., expansion, elongation, and pinch‐off), bubble contact angle, departure diameter, time, and shape of bubble are investigated under a constant inlet velocity (0.2 m/s) boundary condition (BC). It is found that the initial formation of a bubble is dependent on the orifice diameter. Consequently, the formation of the bubble's hemispherical shape at the orifice is faster for a smaller orifice diameter than for a larger orifice diameter which forms a bigger bubble. A leading bubble requires a longer time to detach itself from an orifice in comparison to the next bubble (the second bubble), but interestingly the third bubble detaches much faster than the second. This model was also used to investigate the effect of the Bond number (Boσ). An increase of the Bond number (Boσ) from 0.047 to 0.16 speeds up the bubble pinch‐off, but a further increase in the Boσ (e.g. 0.17–0.47) slows it down. Based on the simulated cases used in this study, the findings demonstrate the capacity and accuracy of the CFD method in predicting bubble formation characteristics. The findings may be useful in the design of spargers for bubble column reactors.

Bubble growth rate from stainless steel substrate and needle nozzles

The formation of bubbles from a substrate nozzle immersed in water is studied experimentally and compared with bubble formation from needle nozzles under the same conditions. Nozzles with different internal diameter sizes in the range of 0.11–0.84 mm and low gas flow rates from 0.015 to 0.85 ml/min are used. The bubble formation is recorded by a high speed video camera and detailed characteristics of bubble formation are obtained. Interestingly, it is realized that the bubble volume expansion rate follows a cyclic behavior for the substrate nozzles while it shows a smooth decrease after an initial increase for the needle nozzles. Force balance analysis suggests that fluctuations in the substrate are related to the rapid increase of the attaching inertial force at the initial stages of bubble formation. In contrast, the inertial force in the case of the needle is smaller and takes longer to acquire its maximum value. The results also show that bubbles emerge earlier on top of the substrate than on the needle but the initial bubble volume is smaller in the former case. However, the bubble expansion rate is larger for the substrate than for the needle and as a result the final bubble volume is similar for both cases. By using experimentally captured values of bubble height and radius of contact line, the Young–Laplace equation predicts well the bubble evolution until just before the bubble detachment. Other bubble characteristics follow similar trends for both inlets with time and volume.

Fabrication of titanium microchannels using laser-assisted thermochemical wet etching

A fabrication method of titanium microchannels of an aspect ratio up to 16 and channel width of 10–30 μm is developed using laser-assisted thermochemical wet etching. For the fabrication of deep channel, the workpiece was repeatedly etched over the same path with the control of process parameters. The cross-sectional profile of the microchannel varied between a V shape and a U shape, depending largely upon the laser beam power and etchant concentration. Bubble formation characteristics that influence significantly on the surface quality and process stability are investigated. It is demonstrated that high-aspect-ratio metallic microchannels with different cross-sectional profiles and sizes can be fabricated by properly controlling the process parameters.

A study of non-symmetric air water flow using wire mesh sensor

This study investigates the characteristics of upward air–water flow for non-symmetric initial condition in the 50 mm vertical tube. Wire mesh sensor is utilized to obtain the local properties of the gas phase i.e. void fraction, gas velocity and bubble size. To generate the non-symmetric initial flow condition, air is supplied from a single injecting hole and mixed into the liquid flow inside the pipe through a narrow-ring shaped exit. In this case, large bubble can be generated at the inlet. In this experiment, the characteristics of bubble formation at the inlet and flow evolution along the pipe are emphasized. The results show the capability of the measurement technique to analyze the non-symmetric distribution of the gas phase. The effects of superficial gas and liquid velocity on the bubble distributions and on the local void fraction spreading at the inlet are clarified. The flow evolution dominated by bubble coalescence and break up is discussed in term of the development of Sauter mean diameter over the cross-section of the pipe. The results also show an inconsistency with uniform flow conditions at low gas flux. Hence, the effect of phase distribution spreading is proposed. In addition, the development of rising gas velocity in term of mean bubble size during coalescence and break up is revealed.

Computer simulation of bubbles in large-particle fluidized beds

A distinct element model is used to study the hydrodynamics of large-particle fluidized beds. The computed bubble rise velocity, voidage variations, averaged particle/particulate and fluid velocities are compared with the other continuum theory based on two fluid model. Based on the averaged particle/particulate velocities in a grid cell, deformation of the particle layers predicted by the two fluid model and the distinct element method are also compared. The predicted characteristics of bubble formation, motion, and eruption at the bed surface are in good qualitative agreement with the experimental observations. The quantitative differences in predicting the above parameters along with the advantages and limitations of two approaches for the case of a single isolated bubble rising in a two-dimensional fluidized bed are discussed.

金属精錬プロセスにおける吹込み操作

The present paper is concerned with physical phenomena associated with fluid flow in gas and powder injection in refining processes. The characteristics of bubble formation from a nozzle and an orifice (or porous plug) are described. The critical condition is discussed for jetting, in which the injected gas leaves the nozzle as a continuous jet. Dispersion behavior of the injected gas in the bath is investigated to clarify the structure of jet and plume. The stirring power of the injected gas is formulated, and the mixing time and circulating flow in the bath are also mentioned.

ChemInform Abstract: ELECTROCATALYSIS AND THE CHLORINE EVOLUTION REACTION

AbstractZur Untersuchung der Reaktion der Cl2‐Entwicklung an D.S.A.‐Typ‐Elektroden ( ;z.B. RuO2/TiO2) in 5.13 M NaCl wurden potentiostatische Strom‐Spannungs‐ und Impedanz‐Spannungs‐Daten analysiert und auf eine Reihe von Parameterkurven zurückgeführt.

Electrocatalysis and the chlorine evolution reaction

The chlorine evolution reaction has been investigated on D.S.A. type electrodes in 5.13 M NaCl solution. A large amount of potentiostatic current—potential and impedance—potential data has been collected over a wide potential and frequency range. The current—potential and impedance—potential data has been reduced by curve fitting to a series of a parameter curves. The impedance—potential data were analysed by two methods: an equivalent circuit and the parameter curves C dl E, R ct E, R ω E reported, and a reaction mechanism and the parameter curves C dl E, kSH E, R ω E reported. The two ways of analysing the impedance data are equivalent. However the analysis into a reaction parameter has the advantage that all the data, current—potential and impedance—potential, can be tested using a single set of parameters ( kSH, R ω, a, b, D A, D B, δ).In this work the data is compared to that expected for a redox reaction. Any deviation from the expected behaviour of a perfect redox reaction is ascribed to Cl 2 bubble formation and to electrode structure effects. It is suggested that the parameter curves are related also to bubble formation characteristics and the ‘real’ area of the electrode.