Accumulation Of Bubbles Research Articles

Characteristics of ECM polishing influenced by workpiece corner feature and electrolyte flow

When polishing workpiece with inner corner feature using electrochemical machining (ECM) process, the surface quality at corner area is much poorer than that at other areas, and the surfaces of upstream area and of downstream area are much different. In this paper, the characteristics of this corner area phenomenon was investigated. Since the current density has a great influence on the surface roughness in ECM process, understanding the current density distribution on workpiece surface helps to analyze the corner area phenomenon and clarify process characteristics. Current density distribution considering the influences of several factors such as corner shape and inter electrodes gap (IEG), bubbles and electrolyte flow, were obtained with simulation and verified by experiments. ECM polishing with different fillet radius of the corner under different current densities were carried out. The observation experiments of machining by-products flowing with electrolyte flushing between machining gap, as well as the measurement of current densities at different areas, were conducted. The experimental results showed accordance with the simulation results. It was found the corner shape greatly influence the current density thus the surface roughness at the corner area, while the transport and accumulation of gas bubbles with electrolyte flow is one of the main reasons for the difference of surface roughness between the upstream area and the downstream area.

Experimental investigation of comprehensive effects of surfactant and inclined mode on spray cooling heat transfer

Spray cooling technology as a high heat flux cooling method is widely used to meet the increasingly cooling requirements, and the enhancement of spray cooling has gradually become a hot spot. In this study, three surfactants of AOS, CTAB and Tween20 with different concentrations were added into pure water to investigate their effects on spray cooling at low temperature range (≤100 °C) in a closed system. The three kinds of surfactants obviously improved the heat transfer performance, which had different optimum concentrations. The lowest mean surface temperature was obtained by CTAB with concentration of 200 ppm. The surface temperature non-uniformity of different surfactants spray was not improved, while the heat transfer deteriorated at high concentrations. In further study, the inclined mode method was used to alleviate the influence of bubble accumulation at high concentrations. The results showed that the inclined spray could significantly enhance the heat transfer capacity of the spray with surfactants and produce the optimum concentration point backward shift phenomenon. Moreover, the inclined spray mode showed a better promotion effect on spray cooling of surfactants with large surface tension.

Synergistic Mechanism of Radiation Embrittlement of Austenitic Stainless Steels under Long-Term High-Temperature Irradiation

Abstract—The results of studying the mechanisms of fracture and embrittlement of the Kh18N9 and Kh18N10T austenitic steels under long-term neutron irradiation at elevated temperatures are given. The effects of the temperature and time of irradiation and the neutron radiation dose on the fracture strain and fracture mechanisms are analyzed. On the basis of the results obtained and specially performed experiments, a synergistic mechanism of embrittlement of the material under conditions of long-term high-temperature irradiation is proposed and substantiated, which is determined by the following two factors: thermal aging and helium diffusion. The thermal aging leads to the formation of various phases at grain boundaries and, hence, to a decrease in the grain boundary strength. Helium diffusion at elevated temperatures gives rise to the accumulation and growth of helium bubbles at weakened grain boundaries.

A look into German housing markets: A bubble call?

The steep rise in German house prices in recent years raises the question of whether a speculative bubble has already emerged. Using a modified present-value model, we estimate the size of speculative house price bubbles in the German housing market. We do not find evidence for positive bubble accumulation in recent years, and interpret the current bullish run as reflecting the correction of house prices that have been undervalued for more than 10 years. With house prices close to their fair values as of 2018:Q1, our answer to the question is, ‘Not yet, but it is likely soon’.

Bubble formation effect over the potential and current during functioning of 316 L stainless steel porous electrode in a parallel volumetric cell

ABSTRACTThe effect of bubble accumulation on anodic potential and current in the conversion of Fe2+/Fe3+ under porous electrodes (316L, ε = 78.73, ae = 11 150 m2 m−3) is studied. It was observed that when the electrolyte flow is under 2 mL s−1 and the electrode thickness (L) is greater than 0.62 mm, the coalescence of bubbles blocks the electrode, thereby reducing the electroactive area and increasing the electrical resistance, which produces elevation and important oscillations in anodic potential. For higher flows between 8 and 16 mL s−1, the potential tends to be uniform over time. The average measurement of anodic potential indicated that for working conditions of I = 0.1 A, L = 0.64 mm, the flow increases from 2 to 8 mL s−1, reducing the average anodic potential value by 35%, while for I = 0.5 A, L = 1.86 mm, flow of 2 and 16 mL s−1 reduces the average anodic potential by approximately 11%.

Taming grey rhinoceros: China needs to release financial risks urgently

At present, the “grey rhinoceros” in China’s financial sector mainly exists in the real estate bubble and enterprise debt accumulation. In order to release the financial risks accumulated in the past, on the one hand, we should weaken the economic growth indicators, resolve to eliminate the zombie enterprises, neutralize the monetary policy and change the incentive mechanism of the officials’ assessment. On the other hand, we should also encourage the ideological emancipation to stabilize the expectation and further promote streamlining government and delegating authorities.

An investigation on flashing-induced natural circulation instabilities based on RELAP5 code

In this investigation, flow instability experiments carried out in a natural circulation loop at the low-pressure condition. Based on experiment data, the RELAP5 code was adopted in the analysis. The objective of this investigation is to get insight into the nature of flashing-induced natural circulation instability. Four typical oscillations observed, intermittent and periodical oscillation, double-peak oscillation, periodical sinusoidal oscillation, and staged irregular oscillation. Under low pressure and low heat flux condition, flashing plays a dominated role in natural circulation oscillation. It is suppressed in higher heat flux. The accumulation of bubbles and fluid inertia result in diverse nonlinear relationship between driving head and kinetic energy. The mechanism of flow instabilities is explored from different aspects. The simulation results of RELAP5 are presented and assessed against the experimental results. The deviation between the results for different circulation modes is discussed as well.

Comparison of normal and distributed jet array impingement boiling of HFE-7000 on smooth and pin-fin surfaces

The confined jet impingement boiling experiments are conducted with HFE-7000 as coolant to compare bubble dynamics and heat transfer characteristics of normal and distributed jet arrays on smooth surfaces and pin-fin surfaces, respectively. It is found that in single-phase regime, the two jet arrays have almost the same heat transfer coefficient (HTC) because of the negligible crossflow effect due to small jet velocities and short flow path. This is especially true on pin-fin surfaces owing to its further attenuation effect on crossflow. For each test section, the higher the flow rate is, the larger the single-phase coefficient (HTC) and critical heat flux (CHF) are, whereas the flow rate has negligible effect on HTC in fully developed nucleate boiling regimes due to the dominant role of nucleate boiling heat transfer. On smooth surfaces, nucleate boiling starts at the center of two jet rows for both jet arrays where the heat transfer is the weakest before boiling. Nevertheless, the wall superheat at the onset of nucleate boiling (ONB) for the distributed jet array is slightly lower under the same flow rate both on the smooth and pin-fin surfaces. For normal jet array, quantities of bubbles generated at high heat flux deteriorate the flow rate distribution and consequently results in the very different boiling characteristics along the streamwise direction. For distributed jet arrays, bubble characteristics are the same for each unit because of the absence of crossflow effect, and therefore its two-phase HTC and CHF are higher. This feature of distributed jet array makes it practical to large area cooling applications. Pin-fin surfaces can greatly decrease ONB, enhance HTC during whole heat transfer regimes and increase CHF for both jet arrays. Meanwhile, the boiling curves of the two jet arrays are overlapped in a wider range of two-phase regime due to the pin-fin enhancement on nucleate boiling and the attenuation of crossflow, while distributed jet arrays have much higher CHFs. For distributed jet arrays on pin-fin surfaces, boiling instability occurs at a certain range of heat flux owing to the periodical accumulation and extraction of large individual bubbles under effusion holes. The boiling instability, however, has no detrimental effect because it only leads to a slight drop in HTC and does not cause earlier occurrence of CHF.

Experimental study of subcooled flow boiling heat transfer on micro-pin-finned surfaces in short-term microgravity

The flow boiling heat transfer of subcooled air-dissolved FC-72 on micro-pin-finned surfaces was studied in microgravity by utilizing the drop tower facility in Beijing. The micro-pin-fins with the dimension of 30 × 30 × 60 μm3 (width × thickness × height), named PF30-60, were fabricated on a silicon chip by using the dry etching technique. For comparison, experiments of flow boiling heat transfer in terrestrial gravity were also conducted. The effects of inlet velocity on both flow boiling heat transfer and bubble behavior were explored. It was found that gravity has nearly no effect on flow boiling heat transfer for the departure of the inertial-force-dominant bubbles in the low and moderate heat fluxes regions. In contrast, in the high-heat-flux region, the flow boiling heat transfer deteriorates and the critical heat flux (CHF) decreases due to the bubble accumulation in the channel. For PF30-60 at V = 0.5 m/s, the CHF point can be inferred to be between 20.8 and 24.5 W/cm2, which is 63.0–74.2% of that in normal gravity. Regarding PF30-60 at V = 1.0 m/s, the CHF point can be inferred to be between 25.4 and 31.6 W/cm2, which is 67.6–84.0% of that in normal gravity. The impact of gravity on CHF is closely linked to the channel geometry parameter and surface modification. The dimensionless numbers, Ch (Channel number) and Sf (Surface number), were proposed to describe the effect of the channel geometry and surface modification on the ratio of CHF in microgravity to that in normal gravity (CHFμg/CHF1g). An empirical correlation based on We (Weber number), Ch and Sf was proposed to predict the value of CHFμg/CHF1g ratio in good agreement with the experimental data. This study provides a new perspective to determine the threshold inlet velocity of inertial-force-dominant flow boiling under different experimental conditions at different gravity levels.

The lifespan of clusters in confined bubbly liquids

A nearly monodispersed bubbly flow in a Hele–Shaw type channel was studied, for large but finite Reynolds numbers. Experiments were carried out with millimetre-sized Nitrogen bubbles rising through water-glycerin mixtures.The velocities, geometry and other flow characteristics were measured by means of a high speed camera and an image processing routine, in order to validate this experimental set-up with previous results from the literature and shed some light on the cluster formation, timespan and breakup mechanisms. The timespan of clusters was estimated by tracing the clusters trajectories. The results show that clustering occurs, but the bubble aggregates breakup rapidly due to hydrodynamic effects. Scaling arguments were used to obtain an estimate for the lifespan, which is compared to experiments. This estimate does not take into account wake effects, so it can be interpreted as a lower bound of clusters lifespan in an homogeneous bubbly flow.

A Regular Dimpled Surface Morphology for the Oxygen Evolution Reaction

Optimizing electrochemical reactions is essential to improving the energy efficiency of many renewable energy technologies, such as for increasing their competitive advantage across many sectors of the market. The oxygen evolution reaction (OER) is of particular importance for its applicability to chemical generation and energy storage, and its reliance on low cost materials, such as nickel based catalysts. Nickel electrodes with surface oxides (NiOx) are frequently used as anodes in these systems because they also offer reduced activation energies, exhibit sufficient catalytic activity, and durability under alkaline conditions. Persistent bubble accumulation on highly active electrodes can, however, result in blocked active sites and reduced system performance for these and other OER catalysts. The identification and development of surface morphologies that can effectively evolve and remove oxygen bubbles from the electrode surfaces could be a highly beneficial technology to further enhance the efficiency of the OER. In this work, regular dimpled Ni surfaces were prepared using self-assembled poly(styrene) templates with a distinct diameter (e.g., 1 μm). The electrodeposition of Ni into these assembled templates was tuned to produce four types of dimpled surface textures. The electrochemical activity of these regular features were evaluated for the OER to investigate their influence on the mass transport properties. Enhancements to the OER efficiency were demonstrated for these systems when compared to flat Ni electrodes. The wettability of the dimpled Ni electrodes was characterized by contact angle measurements acquired both before and after electrochemical cycling. Theoretical wetting models were applied to the surface morphologies to correlate the results with adhesion of the oxygen bubbles and partial wetting with the electrolyte. The regular design of these micro- and nanostructured surfaces enables further correlation of structural morphologies in Ni based electrodes to their electrochemical performance.

Deep anterior lamellar keratoplasty: dissection plane with viscoelastic and air can be different

AimsTo investigate and define the nature of big bubbles (BB) formed by injection of viscoelastic in deep anterior lamellar keratoplasty.MethodsIntrastromal injections of 0.1 and 0.3 mL of sodium hyaluronate 1.2%...

Regular Dimpled Nickel Surfaces for Improved Efficiency of the Oxygen Evolution Reaction

Persistent bubble accumulation during the oxygen evolution reaction (OER) can effectively block catalytically active surface sites and reduce overall system performance. The OER is an essential half-reaction with relevance to metal–air batteries, fuel cells, and water electrolysis for power to gas applications. The renewable energy sector could benefit from the identification of surface morphologies that can effectively reduce the accumulation of bubbles on electrocatalytic surfaces. In this work, regular dimpled nickel (Ni) features were prepared to investigate how electrode morphology and therefore its roughness and wetting properties may affect the efficiency of the OER. The dimpled Ni features were prepared using spherical poly(styrene) (PS) templates with a diameter of 1 μm. The electrodeposition against regular, self-assembled arrays of PS templates was tuned to produce four types of dimpled features each with a different depth. Enhancements to the OER efficiency were observed for some types of dimp...

Study of the Entropy as a Way to Detect Venous Air Embolism Using a Pre-cordial Doppler

Venous air embolism (VAE) is the air bubble accumulation in the right side of the heart, or in the pulmonary region. Pre-cordial Doppler allows a real-time monitoring of heart sound and is used to detect VAE episodes through changes in cardiac sound. Sometimes these changes are not detected by the operator, which reveals the importance of finding other robust methods for VAE detection. This work aims to study entropy as a feature of the heart sound that may provide useful information on VAE episodes.A clinical protocol was designed: Doppler Heart Sound (DHS) was collected at baseline, and following infusions of saline with 4 distinct volumes and 2 infusion rates, and given through 2 infusion vias, to 4 patients enrolled in the clinical study. Entropy of these segments was obtained, and relation between the extracted feature and saline infusions was studied.Entropy presents a good performance showing an increase in response to saline injections (increased blood flow turbulence).

Formation of TiO2 nanoribbons by anodization under high current density

Highly ordered TiO2 nanoribbons are obtained by anodizations in NH4F electrolyte under high current density. Although intact TiO2 nanoribbons appear, there are still nanotubes beneath nanoribbons. It is reasonable to speculate that TiO2 nanoribbons evolve from TiO2 nanotubes. While the applied current is high, the electronic current is also high, which gives rise to lots of oxygen bubbles. With the accumulation of oxygen bubbles, oxygen bubbles must expand upward and rupture finally. Therefore, gas flow from oxygen bubbles results in the generation of micro-liquid-flow. Because of micro-liquid-flow, the nanotube wall is corroded and scoured. The corrosion of upper pore wall is the most sever and thus the upper pore wall becomes the thinnest locality of nanotube wall. As a result, the nanotubes can collapse easily. Finally, one TiO2 nanotube is divided into several TiO2 nanoribbons while there is still intact tube beneath nanoribbons.

Sulfur diffusion in dacitic melt at various oxidation states: Implications for volcanic degassing

The diffusivity of S in a hydrous dacitic melt (4.5–6.0 wt.% H2O) has been investigated in the temperature (T) and pressure (P) range of 950 °C to 1100 °C and 200 to 250 MPa, respectively. Three series of experiments were conducted at relatively low oxygen fugacity (fO2) conditions [0.8 log units below fayalite-magnetite-quartz equilibrium (FMQ −0.8); referred to as “low fO2”] and high fO2 conditions (FMQ +2.5; referred to as “high fO2”) to determine if the diffusivity of S is affected by its oxidation state and speciation. Sulfur concentration profiles were measured by electron microprobe and the diffusion coefficient (D) was calculated by fitting these profiles. Sulfur diffusion is approximately one order of magnitude faster when S is dominantly present as sulfide species (low fO2) in comparison to the sulfate dominated experiments (high fO2). The following Arrhenian equations were obtained for high and low fO2 conditions at 200 MPa:highfO2:D=10-5.92±0.86∗exp-137.3±21.5kJ/molRTlowfO2:D=10-5.18±1.39∗exp-125.7±34.4kJ/molRTwhere D is the average diffusion coefficient in m2 s−1, R is the gas constant in 8.3144 J mol−1 K−1 and T is the temperature in K. Our results demonstrate for the first time in natural melts that S diffusion is strongly sensitive to fO2. Our S diffusivities under low fO2 conditions are only slightly slower of those found for H2O, suggesting that S can be rather efficiently purged from reduced dacitic melts during volcanic eruptions. However, for more oxidized systems (e.g. subduction zones), S diffusion will be much slower and will hinder equilibrium syn-eruptive degassing during rapid decompression. Therefore, we conclude that the “excess S” measured during many explosive volcanic eruptions in arcs is dominantly derived from S-rich bubble accumulation in the eruptible portion of the magma reservoir.

Hydrodynamic Interactions Among Bubbles, Drops, and Particles in Non-Newtonian Liquids

The understanding of hydrodynamic forces around particles, drops, or bubbles moving in Newtonian liquids is modestly mature. It is possible to obtain predictions of the attractive–repulsive interaction for moving ensembles of dispersed particulate objects. There is a certain intuition of what the effects of viscous, inertial, and surface tension forces should be. When the liquid is non-Newtonian, this intuition is gone. In this review, we summarize recent efforts at gaining fundamental understanding of hydrodynamic interactions in non-Newtonian liquids. Due to the complexity of the problem, most investigations rely on experimental observations. However, computations of non-Newtonian fluid flow have made increasingly significant contributions to our understanding of particle, drop, and bubble interactions. We focus on gravity-driven flows: rise or sedimentation of single spheroidal objects, pairs, and dispersions. We identify the effects of two main rheological attributes—viscoelasticity and shear-dependent viscosity—on the interaction and potential aggregation of particles, drops, and bubbles. We end by highlighting the open questions in the subject and by suggesting future directions toward the fundamental physics as well as applications.

Buoyant force-induced continuous floating and sinking of Janus micromotors.

A novel bubble-induced ultrafast floating and sinking of micromotors based on the difference between buoyant force and gravity is proposed. Asymmetric micromotors were prepared by modification with Au and Pt layers for the two faces of glassy carbon beads (GCBs) by the bipolar electrodeposition technique. After the accumulation of enough oxygen bubbles by the decomposition of H2O2 at the Pt layer, the upward net force acting on the micromotor drove its movement to the air/solution interface. In order to reverse the direction of net force for the sinking of the micromotors, sodium dodecyl sulfate (SDS) was added into the fuel solution, which could facilitate the release of bubbles and decrease the diameter of the bubbles. However, the lifetime of the bubbles was increased significantly. After the addition of a small amount of salt, the lifetime of the bubbles was obviously reduced. As a consequence, the breakup of bubbles on the micromotor changed the direction of the net force from up to down which pulled the micromotor down to the bottom of the solution. The velocity of the micromotor was dependent on the net force exerted on the micromotor, leading to an ultrafast motion of the micromotor. It still reached 1.2 cm s−1 after 3 h. Moreover, the simple asymmetric deposition technique showed great promise for the further application of the micromotors in bioanalysis and environmental remediation.

Investigation of Influence Factors of Surface Roughness at Corner Area in ECM Process

Surface quality of workpiece with sudden dimension change processed by electrochemical machining (ECM) is affected by many factors, such as the electrolyte flow pattern, gap between tool and workpiece electrodes, machining by-products such as gas bubbles, etc. In this paper, surface polishing process of a workpiece with sudden dimension change was considered. The electrolyte flow velocity distribution and vortex formed due to the sudden dimension change was discussed. Then, the transport of gas bubbles by the flow and its effect on the current density distribution in the electrolyte as well as the effect of current density distribution on the generation of bubbles were also investigated. Results show that current distribution on the electrode surface was greatly affected by the generation and accumulation of gas bubbles around the corner area due to the sudden dimension change of flow channel geometry, which leads to poor surface quality near the workpiece corner.

Induced Superaerophobicity onto a Non-superaerophobic Catalytic Surface for Enhanced Hydrogen Evolution Reaction.

Despite tremendous progress in the development of novel electrocatalysts for hydrogen evolution reaction (HER), the accumulation of hydrogen gas bubbles produced on the catalyst surface has been rather poorly addressed. The bubbles block the surface of the electrode, thus resulting in poor performance even when excellent electrocatalysts are used. In this study, we show that vertically grown graphene nanohills (VGNHs) possess an excellent capability to quickly disengage the produced hydrogen gas bubbles from the electrode surface, and thus exhibit superaerophobic properties. To compensate for the poor electrolytic properties of graphene toward HER, the graphene surface was modified with WS2 nanoparticles to accelerate the water-splitting process by using this hybrid catalyst (VGNHs-WS2). For comparison purposes, WS2 nanoparticles were also deposited on the flat graphene (FG) surface. Because of its superior superaerophobic properties, VGNHs-WS2 outperformed FG-WS2 in terms of both catalytic activity toward the HER and superaerophobicity. Furthermore, VGNHs-WS2 exhibited a low onset potential (36 mV compared to 288 mV for FG-WS2) and long-term stability in the HER over an extended period of 20 h. This study provides an efficient way to utilize highly conductive and superaerophobic VGNHs as support materials for intrinsic semiconductors, such as WS2, to simultaneously achieve superaerophobicity and high catalytic activity.