Single Bubble Behavior in Direct Current Electric Field

The investigation on bubble behavior in electric field helps to analyze the mechanism of electric enhancement of boiling heat transfer. Experiments were performed to investigate the bubble deformation in direct current （DC） electric field with bubbles attached to the orifice. The air bubbles were slowly generated in the transformer oil pool at different orifices, so that the effect of flow on bubble shape was eliminated. The results showed that the bubbles were elongated and the departure volume decreased when the electric field was intensified. The major and minor axes, aspect ratio and departure volume increased with increasing the orifice diameter. Both the electric field and orifice size have great influence on bubble behavior. The bubble deformation was also simulated to compare with the experimental results. The numerical and experimental data qualitatively agree with each other.

Mixing Characteristics and Bubble Behavior in an Airlift Internal Loop Reactor with Low Aspect Ratio

The present study summarizes the results of macro-and micro-mixing characteristics in an airlift internal loop reactor with low aspect ratio(H/D≤5) using the electrolytic tracer response technique and the method of parallel competing reactions respectively. The micro-mixing has never been investigated in airlift loop reactors. The dual-tip electrical conductivity probe technique is used for measurement of local bubble behavior in the reactor. The effects of several operating parameters and geometric variables are investigated. It is found that the increase in superficial gas velocity corresponds to the increase in energy input, liquid circulation velocity and shear rate, decreasing the macro-mixing time and segregation index. Moreover, it is shown that top clearance and draft diameter affect flow resistance. However, the bubble redistribution with a screen mesh on the perforated plate distributor for macro-mixing is insignificant. The top region with a high energy dissipation rate is a suitable location for feeding reactants. The analysis of present experimental data provides a valuable insight into the interaction between gas and liquid phases for mixing and improves the understanding of intrinsic roles of hydrodynamics upon the reactor design and operating parameter selection.

Measurements of Laser Induced Bubble Behavior in Elastic Tube and Temperature around Bubble in TUL Treatment

Transurethral ureteral lithotripsy (TUL) is a treatment that breaks stones by irradiating a pulsed laser through an optical fiber. Heat and impulsive force of the laser may affect nearby tissues during treatment. A bubble induced by the pulsed laser plays an important role in laser lithotripsy. It is important to understand effects of the bubble on the surroundings by simulating treatment in a narrow space such as in a ureter. In this study, we observe behaviors of the bubble in the narrow space inside a soft material simulating under in vivo condition. The bubble formed under various laser irradiation conditions exhibits characteristic behavior, and the surrounding elastic wall is compressed and bulged when the bubble grows and collapses. In the case of bubble formed near the elastic wall, the bubble contacts with the elastic wall during growth, and severe large deformation of the elastic wall is observed at bubble collapse. According to the temperature measurement, a temperature rise of 25℃ - 30℃ occurs in the area where the bubbles are in contact. From the above, by presenting the deformation of the elastic wall and temperature increase, we can show useful information to improve the safety for treatment at narrow space.

Bubble behaviors of geldart B particle in a pseudo two-dimensional pressurized fluidized bed

Pressurized fluidized beds have been developed in quite a few industrial applications because of intensified heat and mass transfer and chemical reaction.The bubble behaviors under elevated pressure,strongly influencing the fluidization and reaction conversion of the whole system,are of great research significance.In this work,the bubble behaviors of Geldart B particle in a pseudo two-dimensional(2D)pressurized fluidized bed were experimentally studied based on digital image analysis technique.The effects of pressure and fluidization gas velocity on the general bubble behaviors(i.e.,size,shape and spatial distribution)and the dynamic characteristics,such as the time-evolution of voidage distribution and local flow regimes,were comprehensively investigated.Results show that increasing pressure re-duces the stability of bubbles and facilitates gas passing through the emulsion phase,resulting in the"smoother"fluidization state with smaller bubbles and declined bubble fraction and standard deviation.The equivalent bubble diameter and bubble aspect ratio increase with the increasing gas velocity while decrease as pressure rises.The elevated pressure reduces bubbles extension in the vertical direction,prohibits the"short pass"of fluidization gas in large oblong bubbles/slugs and benefits the gas-solid interaction.The flow regimes variation with gas velocity is affected by the elevated pressure,and demonstrates different features in different local positions of the bed.

Experimental Investigation on Flow Pattern and Bubble Behavior during Subcooled Flow Boiling of R1233zd(E)in Parallel Channels

In this study,the flow pattern and bubble behavior of R1233zd(E)during subcooled flow boiling in parallel channels are experimentally investigated with visualization and thermal measurement.The test section is composed of 21 rectangular mini channels with the hydraulic diameter of 1.5 mm and the length of 140 mm.Bubbly flow,slug flow,churn flow and wavy-annular flow occur in sequence with the increase of vapor quality,while transient flow pattern transition process involving multiple flow patterns are also captured.The distribution of flow pattern is non-synchronized and axial-asymmetric,with earlier flow pattern transitions observed in peripheral channels away from the center axis.The initial nucleate site in each channel also show a random and axial-asymmetric distribution,while faster bubble growth can be noted in some channel under the comprehensive effects of liquid evaporation and bubble coalescence.The variation of heat transfer coefficient is correspondence to the flow pattern transition,showing different trends along the flow direction.The increase of mass flux can lead to delayed flow pattern transition and variation of heat transfer coefficient.In addition,higher heat transfer coefficient can be noted in channels away from the center axis.

Numerical investigation of heat transfer performance and bubble behavior of nucleate pool boiling in a confined space under rolling conditions

Nucleate pool boiling process is widely used in heat exchangers because of its excellent heat transfer performance.With the gradual increase of applications,more and more equipments work in a non-static state,but there is little research under rolling conditions.Therefore,it is necessary to investigate the influence of rolling motion on the nucleate pool boiling process.In this study,a numerical investigation of the nucleate pool boiling process under static and rolling conditions is performed based on the volume-of-fluid(VOF)method.Physical fields and phase distribution under static state and rolling motion are compared to investigate the effect of rolling motion on the nucleate pool boiling process.The results show that rolling motion greatly influences the bubble behavior and void fraction owing to the differences between flow fields.The void fraction decreased by 11.84%,48.82%,and 56.87%as the maximum rolling angle increased from 15°to 45°,and by 11.84%,22.27%,and 21.81%as the rolling period increased from 1 s to 3 s.The void fraction decreased by 11.84%,48.82%,and 56.87%as the maximum rolling angle increased from 15°to 45°.The heat transfer coefficients of different cases are compared,and it is found that the effects of rolling motion on heat transfer coefficients can be ignored.

Effect of baffles on bubble behavior in a bubbling fluidized bed for chemical looping processes

A bubbling fluidized bed is generally used as the fuel reactor in chemical looping processes.However,in sufficient gas-solid contact remains a steadfast problem.As such,we propose the use of in ternal baffles to split bubbles at a high gas velocity and to restrict the movement of large-scale solids in the fluidized bed.We investigated the effect of baffle arrangement on the bubble size and energy,pressure gradient,particle distribution,and particle velocity in a fluidized bed using a computational particle fluid dynamics simulation based on our experimental results.We discuss the main influenci ng factors,which in elude presence of caps,number of baffles,and the baffle opening ratio.The baffle structure with caps efficiently broke the large bubbles without creating jet flow.Three baffles were deemed suitable in the bed owing to the small bubble size and uniform gas-solid distribution in each compartment.We selected a baffle opening ratio of 20.5%,which improved the flow pattern without causing a significant increase in the pressure gradient and particle velocity through the baffles.We also discussed the potential for sea ling up this baffled fluidized bed for industrial applications.

Modeling of Liquid Level and Bubble Behavior in Vacuum Chamber of RH Process

In the Ruhrstahl-Heraeus （RH） refining process, liquid steel flow pattern in a ladle is controlled by the fluid flow behavior in the vacuum chamber. Potassium chloride solution and NaOH solution saturated with CO2 were respectively used as a tracer to investigate the liquid and gas flow behaviors in the vacuum chamber. Principal compo nent and comparative analysis were made to show the factors controlling mixing and circulation flow rate. The liquid level and bubble behavior in the vacuum chamber greatly affect fluid flow in RH process. Experiments were performed to investigate the effects of liquid steel level, gas flow rate, bubble residence time, and gas injection mode on mixing, decarburization, and void fraction. The results indicate that the mixing process can be divided into three regions： the flow rate affected zone, the concentration gradient-affected zone, and their combination. The liquid steel level in the vacuum chamber of 300 mm is a critical point in the decarburization transition. For liquid level lower than 300 mm, liquid steel circulation controls decarburization, while for liquid level higher than 300mm, bubble behavior is the main controlling factor. During the RH process, it is recommended to use the concentrated bubble injection mode for low gas flow rates and the uniform bubble injection mode for high gas flow rates.

Experimental Investigation of Bubble Behaviors in Subcooled Flow Boiling

The experimental investigation on vapor bubble growth is performed for analyzing subcooled boiling in a vertical annular channel with inner heating surface and upward water flow under atmospheric pressure. Bulk liquid mass flux ranges from 79 kg/m2s to 316 kg/m2s, and subcooling is from 40 K to 60 K. The bubble behaviors from inception to collapse are captured by High-speed photography. The performance of bubble growth recorded by the high-speed photography is given in this paper. The bubble behaviors, effect of the bubble slippage on the heat transfer, and various forces acting on the bubble are discussed.

Bubble behavior characteristics based on virtual binocular stereo vision

The three-dimensional(3D) behavior characteristics of bubble rising in gas-liquid two-phase flow are of great importance to study bubbly flow mechanism and guide engineering practice. Based on the dual-perspective imaging of virtual binocular stereo vision, the 3D behavior characteristics of bubbles in gas-liquid two-phase flow are studied in detail, which effectively increases the projection information of bubbles to acquire more accurate behavior features. In this paper, the variations of bubble equivalent diameter, volume, velocity and trajectory in the rising process are estimated, and the factors affecting bubble behavior characteristics are analyzed. It is shown that the method is real-time and valid, the equivalent diameter of the rising bubble in the stagnant water is periodically changed, and the crests and troughs in the equivalent diameter curve appear alternately. The bubble behavior characteristics as well as the spiral amplitude are affected by the orifice diameter and the gas volume flow.

Numerical simulation and experimental verification of bubble size distribution in an air dense medium fluidized bed

Bubble size distribution is the basic apparent performance and obvious characteristics in the air dense medium fluidized bed (ADMFB). The approaches of numerical simulation and experimental verification were combined to conduct the further research on the bubble generation and movement behavior. The results show that ADMFB could display favorable expanded characteristics after steady fluidization. With different particle size distributions of magnetite powder as medium solids, we selected an appropriate prediction model for the mean bubble diameter in ADMFB. The comparison results indicate that the mean bubble diameters along the bed heights are 35 mm < D b < 66 mm and 40 mm < D b < 69 mm with the magnetite powder of 0.3 mm+0.15mm and 0.15mm+0.074mm, respectively. The prediction model provides good agreements with the experimental and simulation data. Based on the optimal operating gas velocity distribution, the mixture of magnetite powder and <1mm fine coal as medium solids were utilized to carry out the separation experiment on 6-50mm raw coal. The results show that an optimal separation density d P of 1.73g/cm 3 with a probable error E of 0.07g/cm 3 and a recovery efficiency of 99.97% is achieved, which indicates good separation performance by applying ADMFB.

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.

Space Materials Science in China:I.Experiment Studies under Microgravity

The virtual absence of gravity-dependent phenomena in microgravity allows an in-depth understanding of fundamental events that are normally obscured and therefore are difficult to study quantitatively on Earth.Of particular interest is that the low-gravity environment aboard space provides a unique platform to synthesize alloys of semiconductors with homogeneous composition distributions,on both the macroscopic and microscopic scales,due to the much reduced buoyancy-driven convection.On the other hand,the easy realization of detached solidification in microgravity suppresses the formation of defects such as dislocations and twins,and thereby the crystallographic perfection is greatly increased.Moreover,the microgravity condition offers the possibilities to elucidate the liquid/solid interfacial structures,as well as clarify the microstructure evolution path of the metal alloys(or composites)during the solidification process.Motivated by these facts,growths of compound semiconductors and metal alloys were carried out under microgravity by using the drop tube,or on the scientific platforms of Tiangong-2 and SJ-10.The following illustrates the main results.

CFD simulation of bubbling and collapsing characteristics in a gas-solid fluidized bed

Computational Fluid Dynamics （CFD） has become an alternative method to experiments for understanding the fluid dynamics of multiphase flow. A two-fluid model, which contains additional terms in both the gas- and solid-phase momentum equations, is used to investigate the fluidization quality in a fluidized bed. A case study for quartz sand with a density of 2,660 kg/m^3 and a diameter of 500 μm, whose physical property is similar to a new kind of catalyst for producing clean fuels through the residue fluid catalytic cracking process, is simulated in a two-dimensional fluidized bed with 0.57 m width and 1.00 m height. Transient bubbling and collapsing characteristics are numerically investigated in the platform of CFX 4.4 by integrating user-defined Fortran subroutines. The results show that the fluidization and collapse process is in fair agreement with the classical theory of Geldart B classification, but the collapse time is affected by bubbles at the interface between the dense phase and freeboard.

Dynamic behavior of gas bubble in single bubble sonoluminescence—vibrator model

Single bubble sonoluminescence is a process of energy transformation from sound to light. Therefore the motion equations of near spherical vibration of a gas bubble in an incompressible and viscous liquid can be deduced by Lagrangian Equation with dissipation function when the bubble is considered as a vibrator surrounded by liquid. The analytical solutions in the bubble expanding, collapsing and rebounding stages can be obtained by solving these motion equations when some approximations are adopted. And the dynamic behaviors of the bubble in these three stages are discussed.

NUMERICAL CALCULATION OF SINGLE BUBBLE MOVEMENT AND ITS CAVITATION BEHAVIOR

This paper presents analyses and numerical calculations of a single bubble movement and its cavitation behav- ior where the influence of air diffusion,pressure fluctuation,relative velocity between the bubble and fluid and the growing bubble burst effect are considered.The calculational resuits provides rather detailed description of a single bubble movement and its cavitation behavior.

Bubbling behavior of cohesive particles in a two-dimensional fluidized bed with immersed tubes

Fluidization hydrodynamics are greatly influenced by inter-particle cohesive forces. This paper studies the fluidization of large cohesive particles in a two-dimensional fluidized bed with immersed tubes using “polymer coating” to introduce cohesive force, to gain better understanding of bubbling behavior when particles become cohesive and its effect on chemical processes. The results show that the cohesive force promotes bubble splitting in the tube bank region, thereby causing an increase in the number and a decline in the aspect ratio of the bubbles. As the cohesive force increases within a low level, the bubble number increases and the bubble diameter decreases, while the aspect ratio exhibits different trends at different fluidization gas velocities. The difference in the evolution of bubble size under various cohesive forces mainly takes place in the region without tubes. When the cohesive force is large enough to generate stable agglomerates on the side walls of the bed, the bubble number and the bed expansion sharply decrease. The tubes serve as a framework that promotes the agglomeration, thus accelerating defluidization. Finally, the bubble profile around tubes was studied and found to greatly depend both on the cohesive forces and the location of tubes.