Bubble Separation Research Articles

The thermal Marangoni effect during photoelectrochemical conversion

Marangoni effect is a convective effect produced by surface tension gradient and has an important influence on the efficiency of hydrogen production by photoelectrochemical conversion. In this paper, COMSOL Multiphysics was used for numerical simulation to build a Marangoni effect model at the bubble interface in the photoelectrochemical conversion process, to clarify the mechanism of Marangoni effect of bubbles on the surface of photoelectrodes, and to discuss the influence of Marangoni effect on the process of bubble growth and separation. The results show that the thermal effect of light will cause the Marangoni convection around the bubbles and form a negative pressure zone at the root of the bubbles, which will attract the surrounding bubbles. Gravity has little effect on Marangoni convection, and an increase in the intensity of incoming light enhances the Marangoni effect. At the same time, Marangoni convection leads to a Marangoni force on the surface of the bubble, which prevents the bubble from leaving the surface of the photoelectrode.

Analysis of Bubble Flow Mechanism and Characteristics in Gas–Liquid Cyclone Separator

The separation of bubbles in a gas–liquid cyclone is complicated. A combination of numerical simulation and visual experimentation was considered apt to reveal the microscopic mechanisms of bubble flow. First of all, cyclones with different structures were numerically simulated. The calculation results show that the larger the diameter of the exhaust port, the better the bubble flow effect. When the exhaust port diameter was 24 mm, the gas discharge efficiency was 8% higher than that with an exhaust port diameter of 16 mm. The sequence of the bubble flow effect of a four-structure cyclone was obtained, and the gas discharge efficiency of the cyclone with a rectangular inlet was 7% higher than that of the trapezoidal inlet. Finally, a visual experimental platform was built to compare the rectangular inlet cyclone and spiral inlet cyclone with the best bubble flow effect. In accordance with the simulation numerical calculations, the bubble flow effect of the rectangular inlet cyclone was better than that of the spiral and trapezoid inlet cyclones, and the rectangular inlet in the middle was better. This article provides a specific theory and experience to guide further research on the separation mechanism, flow field characteristics and structurally optimal design of gas–liquid cyclones.

Foam fractionation for the separation of SDBS from its aqueous solution: Process optimization and property test

Foam fractionation is an adsorption bubble separation technique allowing recovery and immediate reuse of surfactants. This work was aimed at investigating the variations in recovery efficiency and physicochemical property of sodium dodecyl benzene sulfonate (SDBS) through multiple foam fractionation operation. First, a suitable SDBS concentration of 0.05 g/L was determined for preparing feeding solution. Subsequently, effects of volumetric air flow rate, height ratio of liquid phase and foam phase, and pore diameter of gas distributor on recovery efficiency of SDBS were investigated, respectively. Response surface methodology (RSM) was used to optimize the operating conditions of foam fractionation for separating SDBS. Under the conditions of volumetric air flow rate 80 mL/min, height ratio of liquid phase and foam phase 1:2, and pore diameter of gas distributor 0.180 mm, enrichment ratio and recovery percentage of SDBS were 134.83 ± 6.74 and 63.43 ± 3.17%, respectively. In repetitive separation experiments, the maximum separation number of recovered SDBS by foam fractionation was 3. Experimental results of surface activity and foam properties of recovered SDBS solution suggested that the interfacial adsorption and desorption processes had induced the structural transition and irreversible aggregation of SDBS molecules after multiple foam fractionation operation.

Effect of Alcohol on Adsorptive Bubble Separation of Basic Dyes for Their Rapid Removal from Water

Effect of Alcohol on Adsorptive Bubble Separation of Basic Dyes for Their Rapid Removal from Water

Phase transitions in an expanding universe: stochastic gravitational waves in standard and non-standard histories

We undertake a careful analysis of stochastic gravitational wave production from cosmological phase transitions in an expanding universe, studying both a standard radiation as well as a matter dominated history. We analyze in detail the dynamics of the phase transition, including the false vacuum fraction, bubble lifetime distribution, bubble number density, mean bubble separation, etc., for an expanding universe. We also study the full set of differential equations governing the evolution of plasma and the scalar field during the phase transition and generalize results obtained in Minkowski spacetime. In particular, we generalize the sound shell model to the expanding universe and determine the velocity field power spectrum. This ultimately provides an accurate calculation of the gravitational wave spectrum seen today for the dominant source of sound waves. For the amplitude of the gravitational wave spectrum visible today, we find a suppression factor arising from the finite lifetime of the sound waves and compare with the commonly used result in the literature, which corresponds to the asymptotic value of our suppression factor. We point out that the asymptotic value is only applicable for a very long lifetime of the sound waves, which is highly unlikely due to the onset of shocks, turbulence and other damping processes. We also point out that features of the gravitational wave spectral form may hold the tantalizing possibility of distinguishing between different expansion histories using phase transitions.

Enhanced pool boiling heat transfer on soft liquid metal surface

Pool boiling is a high-efficient energy transfer method through the gas-liquid phase transition. It has the characteristics of small heat transfer temperature difference and high heat flux density. The current enhancement of boiling heat transfer is realized mainly through modifying the solid heating surface. So far, there has been no report on the study of pool boiling heat transfer on soft surfaces. Therefore, in this work the pool boiling heat transfer performances of ethanol on the copper surface and soft liquid metal surface are investigated experimentally. The experimental results indicate that soft surface can effectively reduce the wall superheat corresponding to the onset of boiling (ONB). In saturation boiling, the superheat of the wall surface at ONB is reduced by nearly 12 ℃, while the heat transfer coefficient is improved by 149%. It is found that soft surface enhances pool boiling heat transfer performance significantly by increasing nucleate site density, reducing the bubble departure diameter, and increasing bubble departure frequency. Unlike the copper surface, the soft surface deforms elastically under the action of the vertical component of surface tension <i>γ</i><sub>lv</sub>sin<i>θ</i> at the three-phase contact line of the vapor bubble. From the perspective of surface energy analysis, the difference in surface energy Δ<i>E</i><sub>LM</sub> between before and after bubble departure on soft liquid metal surface is smaller than in smooth surface Δ<i>E</i><sub>CS</sub>. The potential barrier of the soft surface is smaller than of the copper surface, and the buoyancy required for bubble separation is small, and the bubble is easy to separate. Elastocapillary wave and bubble jet phenomenon on the soft surface are observed particularly, which are generated on liquid metal under the action of elastic restoring force. The fluctuation of elastocapillary wave contributes to the enhancement of heat and mass transfer in thermal boundary layer and the generation of residual nucleation site. The residual bubble grows up rapidly and coalesces with the rising large bubble, forming bubble jet phenomenon. Elastocapillary wave and bubble jet contribute to the enhancement of pool boiling heat transfer on soft liquid metal surface.

Magnetically steerable iron oxides-manganese dioxide core–shell micromotors for organic and microplastic removals

Because of micro/nanoscale manipulation and task-performing capability, micro/nanomotors (MNMs) have attracted lots of research interests for potential applications in biomedical and environmental applications. Owing to the low-cost, good motion behavior, and environmental friendliness, various low-cost metal oxides based MNMs become promising alternatives to the precious metal based MNMs, in particular for environmental remediation applications. Hereby, we demonstrate the facile and scalable fabrication of two types of bubble-propelled iron oxides-MnO2 core–shell micromotors (Fe3O4-MnO2 and Fe2O3-MnO2) for pollutant removal. The Fe2O3-MnO2 micromotor exhibits efficient removals of both aqueous organics and suspended microplastics via the synergy of catalytic degradation, surface adsorption, and adsorptive bubbles separations mechanisms. The adsorptive bubbles separation achieved more than 10% removal of the suspended microplastics from the polluted water in 2 h. We clarified the major contributions of different remediation mechanisms in pollutants removals, and the findings may be beneficial to a wide range of environmental applications of MNMs.

Experimental investigation on flow regimes and transitions of steam-water two-phase flow in narrow rectangular horizontal channels

Experimental investigations on atmospheric steam-water two-phase flow regimes and transitions in two transparent narrow rectangular channels with cross-section areas of 1 mm × 20 mm and 2 mm × 20 mm were carried out under conditions of 0.007 ≤ Jf ≤ 0.27 m s−1, 0.05 ≤ Jg ≤ 65.2 m s−1, 0 < q ≤ 18 kW m−2. Four flow patterns were observed in narrow rectangular horizontal channels under heated condition, i.e., bubbly flow, slug flow, churn flow and annular flow. New dryout phenomenon of annular flow was observed under high wall heat flux conditions. The liquid phase occupies half of the middle of narrow channel above the bottom rectangular plate. Below the top plate, liquid phase is concentrated to a slender streamline in the middle of upper half narrow channel by high speed steam flow. For the flow regime transitions, the critical values of void fraction for transitions from bubbly flow to slug flow, slug flow to churn flow, and churn flow to annular flow in narrow rectangular horizontal channels under heated condition are 0.7, 0.8 and 0.9, respectively. The heat flux decays flow regime transitions because the generated bubbles from heated wall surfaces promote the separation of bubbles in the upstream and downstream. Flow regime transitions under larger surface tension condition need greater vapor phase velocities to disturb the mainflow. In a general, new flow regime transition criteria were developed and can present excellent predictions for the present data of flow regimes and transitions in narrow rectangular horizontal channels under heated condition.

Aerodynamic Characteristics and Laminar Bubble Separation Study on a Generic Light Aircraft Model

This paper presents the aerodynamic characteristics, flow separation and laminar bubble analysis of a generic UTM-LST light aircraft model at low Reynolds number. The complex interaction between flow separation and laminar bubble is unclear to date. The model has overall length of 1.3m and wingspan of 1.5m and has been designed for wind tunnel experiments in Universiti Teknologi Malaysia Low Speed Wind Tunnel, Aerolab. The aircraft model is equipped with several control surfaces such as ailerons, rudder, elevators and flaps. The experiments were conducted at the speed of 35 m/s corresponding to Reynolds number of 0.515 x 106 and at angles of attack ranging from 0° to 16°. The experiments were performed at several pitching and yawing configurations. In order to investigate the effects of control surfaces, several control surfaces were changed during the experiments; for this paper, however, only elevator changes will be highlighted. Three measurement techniques were employed during the experiments; the first one was the Steady balance, the second was the surface pressure while the last one was the tuft flow experiment. The main observation from steady balance data was that the aircraft possesses longitudinal static stability for all test cases. The main observation from the surface pressure measurement and tuft experiments is that the laminar bubble separation occurred at lower angles of attack of the wing. This separation is seen to be travelling towards the leading edge as the angle of attack is increased and eventually results in flow separation.

Characterisation of a Nickel-iron Battolyser, an Integrated Battery and Electrolyser

Electricity systems require energy storage on all time scales to accommodate the variations in output of solar and wind power when those sources of electricity constitute most, or all, of the generation on the system. This paper builds on recent research into nickel-iron battery-electrolysers or “battolysers” as both short-term and long-term energy storage. For short-term cycling as a battery, the internal resistances and time constants have been measured, including the component values of resistors and capacitors in equivalent circuits. The dependence of these values on state-of-charge and temperature have also been measured. The results confirm that a nickel-iron cell can hold 25% more than its nominal charge. However, this increased capacity disappears at temperatures of 60°C and may be dissipated quickly by self-discharge. When operating as an electrolyser for long-term energy storage, the experiments have established the importance of a separation gap between each electrode and the membrane for gas evolution and established the optimum size of this gap as approximately 1.25 mm. The nickel-iron cell has acceptable performance as an electrolyser for Power-to-X energy conversion but its large internal resistance limits voltage efficiency to 75% at 5-h charge and discharge rate, with or without a bubble separation membrane.

Experimental and Numerical Study of Two Dimensional Flow of Bubble Separation over the Leading of Thickness Plate

Various design modifications made by transportation equipment companies aim to increase the efficiency of fuel consumption. One of them is by reducing the drag force. This can be done by controlling passive turbulent boundary layers. The addition of rounded shape on the leading edge which is correlated with the length of the plane is one way of controlling the passive turbulent boundary layer which can accelerate the transition from the laminar boundary layer to the turbulent boundary layer. Therefore, this study aims to determine the effect of the Reynolds number and flow support length on the flow characteristics so that the downstream separation can be delayed. This research was conducted by experimental and numerical methods. The Reynolds number used is Ret = 5.08 × 104 and Ret = 8.46 × 104 . The test part of this research is the rounded leading edge of thick plate with a plane length of c/t = 6.5 and c/t = 10. The amount of rounded on the leading edge, the height of the test model and the length of the trailing edge are 10 mm, 100 mm, and 300 mm, respectively. The parameters varied in this study were the flow bearing field length (c/t) and the Reynolds number (Ret). Two-dimensional, steady numerical simulation was carried out using ANSYS Fluent software. The turbulence model used is k − kl − ω. The results obtained in this study are the longer the flow bearing plane and the greater the Reynolds number can delay the separation on the upper side of the circular front edge of the thick plate. The optimal length of the separation delay time is c/t = 10 with Ret = 8.46 × 104 .The separation bubble profile at c/t = 10 with Ret = 8.46 × 104 has length (x/c) = 0.129, thickness (y/t) = 0.1363, and angle (Θ) = 30.3 ◦ with the form factor (H) at point O 1.424. Overall the optimal variation is at c/t = 10 with Ret = 8.46 × 104 where the point of separation occurs when x/c = 0.945.

Foam Fractionation Technology for Enrichment and Recovery of Cheese Whey Proteins

Background: Foam fractionation technology works on the adsorptive bubble separation principle. This technique involves adsorption of the surface-active substances on to a gas-liquid interface and separation of these components from the liquid along with bubbles as foam. The foam separation technology has been successfully utilized in the recovery of proteins from solutions containing either a single protein or binary mixtures. To develop a foam fractionation technology for selective enrichment and recovery of whey proteins, it is essential to investigate the effect of different feed and process variables that affect the foam fractionation process. The aim of the current study was to investigate the effect of two important feed variables, such as pH and initial protein concentration on recovery and enrichment of total whey proteins as well as α-lactalbumin and β-lactoglobulin. Methods: All the experiments were conducted in Agriculture and biosystems engineering lab and Alfred Dairy Science lab at South Dakota State University, Brookings, South Dakota during 2011-2013. The experiments used four levels of initial protein concentration and five levels of feed pH. Yield and enrichment ratios were determined for total whey proteins, α-Lactalbumin (α-La) and β-Lactoglobulin (β-Lg). Result: Whey protein yields ranged from 51.58 to 90.92%, while the enrichment ratios were between 1.2 to 5. The yield of α-La varied from 59 to 94% and the highest enrichment ratio of 8.45 was obtained with the treatment combination of initial protein concentration of 109 mg/L and pH of 5.1. Selective enrichment of α-La over β-Lg was observed at a pH of 4.65 with α-La to β-Lg ratio of 0.49. These findings will be helpful in selective enrichment and recovery of valuable proteins from Cheddar cheese whey using the foam fractionation process.

Numerical Computation of Low Reynolds Number Viscous Flow Past Bluff Bodies

This article presents a two-dimensional steady viscous flow simulation past circular and square cylinders at low Reynolds numbers (based on the diameter) by the finite volume method with a non-orthogonal body-fitted grid. Diffusive fluxes are discretized using central differencing scheme, and for convective fluxes upwind and central differencing schemes are blended using a ‘deferred correction’ approach. A simplified pressure correction equation is derived, and proper under-relaxation factors are used so that computational cost is reduced without adversely affecting the convergence rate. The governing equations are expressed in Cartesian velocity components and solution is carried out using the SIMPLE algorithm for collocated arrangement of variables. The mesh yielding grid-independent solution is then utilized to study, for the very first time, the effect of the Reynolds number on the separation bubble length, separation angle, and drag coefficients for both circular and square cylinders. Finally, functional relationships between the computed quantities and Reynolds number (Re) are proposed up to Re = &lt;i&gt;40&lt;/i&gt;. It is found that circular cylinder separation commences between Re= &lt;i&gt;6.5-6.6&lt;/i&gt;, and the bubble length, separation angle, total drag vary as Re, Re&lt;i&gt;−0.5&lt;/i&gt;, Re&lt;i&gt;−0.5&lt;/i&gt; respectively. Extrapolated results obtained from the empirical relations for the circular cylinder show an excellent agreement with established data from the literature. For a square cylinder, the bubble length and total drag are found to vary as Re and Re&lt;i&gt;−0.666&lt;/i&gt;, and are greater than these for a circular cylinder at a given Reynolds number. The numerical results substantiate that a square shaped cylinder is more bluff than a circular one.

Immersion cooling effect of dielectric liquid and self-rewetting fluid on smooth and porous surface

Dielectric liquid Novec 7300 and 5% 1-butanol aqueous solution are applied for immersion cooling of heater in order to investigate effect of nucleate boiling and self-rewetting on heat transfer. Experimental results are compared with water as common liquid for immersion cooling. In first part of study, bare heater is heated until temperature arrives at certain levels (from 100 °C to 500 °C), and then immersed in liquid for observing rapid cooling performance of these three solutions. In 5% 1-butanol binary solution, heater temperature decreases the fastest and stabilizes at the lowest level. Inversed thermal Marangoni flow induced by vertical temperature difference along heat surface is supposed to assist vapor bubbles separation and improve convective heat dissipation. From 200 °C, vapor film appears on the surface of heater immersed in Novec 7300 and water, whose thickness is dependent of surface temperature but independent of heat flux. In second part of study, immersion cooling performances of three testing liquids on heater with or without porous shells of different porosity are explored with increasing heating power. At different heat flux input, shell porosity, capillary effect, inverse thermal Marangoni effect and pool boiling stages influence heat transfer efficiency of each fluid-heater combination. A curve of optimal heat transfer coefficient is established, which is beneficial for selection of suitable fluid-heater combination.

3D Transient heat transfer analysis and flow visualization study in diverging microchannel for instability mitigated two-phase flow: A numerical study

This paper presents a 3D conjugate numerical simulation of the diverging microchannel heat sink, which is performed using the Volume of Fluid (VOF) model coupled with the phase change model. The modeling approach and simulation results are validated with standard correlations and published experimental results. The model is used to analyze the flow boiling and transient heat transfer mechanism through simulation of bubble nucleation, bubble growth pattern, pressure, temperature, and vapor quality variations. Moreover, local and transient Heat Transfer Coefficient (HTC) is calculated and then correlated with the simulated bubble dynamics. Herein, simulations are performed on the expanding microchannel with a varying outlet to inlet width ratio for uniform heat flux 150 kW/m2 at two different mass fluxes 240 kg/m2s and 710 kg/m2s. Further, to understand the effect of wall contact angle, simulations are performed for two different values of wall contact angle of 140° and 65°. The results indicate that the channels with low divergence angle (θ) and low outlet to inlet width ratio (δ) have low nucleation time. Rapid bubble growth and elongated flow due to channel confinement were also observed. Higher values of the heat transfer coefficient are observed in the microchannels with small hydraulic diameter, and as the diameter is increased, a reduction in the HTC values is observed. The transient heat transfer coefficient fluctuation in the channel with time has been studied along with the corresponding flow pattern. Due to the smaller wall contact angle, the bubble separation rate from the wall is low, thereby creating localized hotspots. A comparative study between straight channel and diverging channel has showed that diverging microchannels have a better performance in terms of instability mitigated flow boiling. This study has considerable relevance in electronic chip cooling using expanding microchannel with instability mitigated two-phase flow.

Fabrication of Hydrophobic Coated Tubes for Boiling Heat Transfer Enhancement

The hot wire chemical vapor deposition method was applied for fabrication of hydrophobic fluoropolymer coatings on the surface of tubular heaters. The effect of hydrophobic surfaces (with static contact angle of 125°) on the local and integral heat transfer characteristics was studied during boiling of saturated water at atmospheric pressure, including high speed visualization. The heat transfer enhancement by 30% was observed in the range of low heat fluxes during boiling on the hydrophobic coated tubes. It was shown that the value of the bubble departure diameter and the nucleation site density significantly increased, and the bubble separation frequency was significantly reduced in comparison with the data obtained on the uncoated tube. The analysis of the operational characteristics of the synthesized coatings was carried out.

Behavior of microbubbles in homogeneous stratified turbulence

Dynamics behavior of rising microbubbles in homogeneous stratified turbulence is investigated by direct numerical simulation. Stratification creates a predominantly horizontal oscillatory motion in a fluid, inducing bubbles' zigzag pattern. Despite this oscillating motion, the horizontal dispersion of a single bubble is rather suppressed. The horizontal separation of paired bubbles displays a power-law growth in the Batchelor and Richardson regimes.

Crucial role of an aerophobic substrate in bubble-propelled nanomotor aggregation

A bubble-propelled autonomous micro/nanomotor (MNM) is a device driven by a catalytic reaction that involves a solid-liquid-gas interface, which in turn is a key factor in achieving effective propulsion. Generally, modifying the liquid phase by adding surfactants can improve propulsion, but it has several disadvantages. It is reported that the rapid separation of bubbles will accelerate the movement of MNMs. Our focus is on methods to drive the motor efficiently by controlling the wettability of the solid phase, accelerating bubble separation without compromising the activity of the catalyst. In this study, different from most of the previous studies on moving MNMs, a static Pt loaded TiO2 nanowire aggregation was utilized as a nanomotor aggregation to investigate the wettability of the solid phase on bubble release. In comparison to an underwater aerophilic solid phase, in which bubbles are strongly held on the surface, the nanomotor’s aggregation showed good aerophobicity. In particular, after UV illumination for 30 s, the nanomotor’s aggregation became superaerophobic, which significantly promoted the release of O2 bubbles. The results of this study reveal how to modify the detachment behaviour of bubbles by controlling the aerophobic behaviour of solid surfaces of autonomous MNMs in an aqueous medium.

Separation of chemically modified carbon nanotubes by surfactant free microbubble generation

Bubble separation is a widely used and effective method for separating solids from liquids. In this study, we propose a method to generate roughly ∼104 microbubbles/μl in an ethanol–water solution (surfactant-free) by subjecting the container of the solution to impact. Hydrophobic carbon nanotubes (CNTs) and hydrophilic carboxylated CNTs can be rapidly separated by the process of attachment of CNTs to rising microbubbles. More than 90% of hydrophobic CNTs can be collected on the top layer of the solution by microbubbles generated in the ethanol solution. The proposed mechanism of bubble enhancement material separation in ethanol solution is because of the effect of hydrophobic surfaces of surfactant-free microbubbles, while normal microbubbles in a surfactant solution have hydrophilic surfaces. CNTs can be separated from the solution by microbubbles with hydrophobic surfaces. The results show that the use of surfactant-free microbubbles can achieve high efficiency of recovery of CNTs in a short time, which provides a potential method for the separation of different materials based on their surface properties.

Erratum: Shape of the acoustic gravitational wave power spectrum from a first order phase transition [Phys. Rev. D 96 , 103520 (2017)

We present results from large-scale numerical simulations of a first order thermal phase transition in the early universe, in order to explore the shape of the acoustic gravitational wave and the velocity power spectra. We compare the results with the predictions of the recently proposed sound shell model. For the gravitational wave power spectrum, we find that the predicted $k^{-3}$ behaviour, where $k$ is the wavenumber, emerges clearly for detonations. The power spectra from deflagrations show similar features, but exhibit a steeper high-$k$ decay and an extra feature not accounted for in the model. There are two independent length scales: the mean bubble separation and the thickness of the sound shell around the expanding bubble of the low temperature phase. It is the sound shell thickness which sets the position of the peak of the power spectrum. The low wavenumber behaviour of the velocity power spectrum is consistent with a causal $k^{3}$, except for the thinnest sound shell, where it is steeper. We present parameters for a simple broken power law fit to the gravitational wave power spectrum for wall speeds well away from the speed of sound where this form can be usefully applied. We examine the prospects for the detection, showing that a LISA-like mission has the sensitivity to detect a gravitational wave signal from sound waves with an RMS fluid velocity of about $0.05c$, produced from bubbles with a mean separation of about $10^{-2}$ of the Hubble radius. The shape of the gravitational wave power spectrum depends on the bubble wall speed, and it may be possible to estimate the wall speed, and constrain other phase transition parameters, with an accurate measurement of a stochastic gravitational wave background.