Bubble Generation Research Articles

Evaluation of a hydrodynamic cavitation-type bubble generator in a prototype bench-scale flotation unit for poultry processing wastewater treatment

ABSTRACT Dissolved Air Flotation (DAF) systems are designed to remove oil and grease (O&G) and total suspended solids (TSS) in wastewater treatment. These systems require saturation tanks, water pumps, and high-pressure compressors to control the pressure, hydraulic retention time, and airflow parameters. DAF process efficiency depends on complex operational controls associated with these components, and the most critical aspect of an effectively operating DAF unit is a generated bubble size. This work presents the design and operational test of a flotation unit prototype that replaces the saturation tank and high-pressure compressors present in DAF with the CARMIN microbubble injector, the evaluation of the proposed system's TSS and O&G removal efficiency was carried out considering different initial configurations of the injector to change the generated microbubble size, four synthetic wastewater solutions, and poly aluminum chloride as a flocculant to establish the potential of this system for the poultry processing wastewater treatment. Mean microbubble size results were obtained from 47.41 µm to 116.17 µm. The average removal efficiency of TSS exceeded 65% under a high concentration of suspended particles (1,560 mg/l) and 80% under a lower TSS concentration (795 mg/l). Meanwhile, 70% and 90% of O&G were removed from high (400 mg/l) and low (100 mg/l) initial O&G concentrations, respectively. These removal levels are similar to those reported in the literature for DAF for poultry processing wastewater, albeit with a simple configuration and better controllability and scalability.

Improving gas dispersion and decreasing bubble size in saline water with oscillatory air supply

The present paper describes gas dispersion in an air-sparged laboratory scale bubble column containing aqueous solutions of single electrolytes (NaCl, CaCl2, MgCl2, MgSO4, and AlCl3) and mixed electrolytes (NaCl-CaCl2 and NaCl-CaCl2-MgCl2-AlCl3) at different ionic strengths and air flow rates, with oscillatory air supply and conventional steady air supply, respectively. The gas dispersion was investigated by a light-scattering technique (turbidity measurement) and bubble size measurement. The oscillatory air supply was generated from steady air flow using a fast-switching solenoid valve and was characterized by measuring the pressure of the output air of the solenoid valve as a function of time. It was found that for each electrolyte solution tested, oscillatory air supply at a certain frequency and amplitude could give better gas dispersion than steady air supply, and the degree of improvement of gas dispersion varied with the electrolyte type and concentration and air flow rate. At each air supply mode tested, either the intensity of scattered light or the Sauter mean diameter of bubbles would scale with the ionic strength, following a decreasing trend. A cooperative effect of electrolyte addition and oscillatory air supply on gas dispersion was generally found. The combined effect of electrolyte addition and oscillatory air supply on bubble size was significant, being up to almost a 2/3 decrease in the Sauter mean diameter. The present work highlights the importance of flow condition for controlling bubble generation and coalescence in saline water and has significant implications for seawater flotation of minerals.

Accuracy Assessment of the Eulerian Two-phase Model for the CFD Simulation of Gas Bubbles Dynamics in Alkaline Electrolyzers

To date, the most industrially developed technology to produce green hydrogen is represented by alkaline water electrolysis (AWE). To improve on design and efficiency of these devices, however, multiphysics simulations based on Computational Fluid Dynamics (CFD) are needed, able to account for electrophysical phenomena and multiphase flows. Focusing on internal flow optimization, the requirements for CFD simulations are anyhow extremely challenging, since solving the gas bubbles’ motion implies the solution of a two-phase flow characterized by very low Reynolds numbers and a high fraction of dispersed gas. Despite some interesting studies have been presented in the literature so far, validation of CFD results with detailed experimental measurements is quite rare and, therefore, the reliability of the adopted modelling approaches is not assessed yet. This study presents the results of a multivariate CFD analysis of an electrochemical cell and its validation through a literature test case. Bubbles generation is introduced as a source term, thus overlooking for the moment the electrochemistry to focus on fluid-dynamics. In particular, attention is given to the Eulerian multiphase modelling, investigating the influence of both the inter-phase interaction sub-models’ settings (e.g., lift and drag forces, virtual-mass force) and the general settings of the simulation. The mean velocity field of the PIV-measured bubbles is considered to assess the accuracy of numerical predictions, while the available high-definition flow pictures allow a qualitative assessment of the bubbles size and location. CFD results are shown to be in decent agreement with experimental data and able to reproduce the key flow features such as the spreading of the bubble curtains and the gas shifting towards the inner part of the cell. The effect of the bubbles’ diameter and of source layer thickness is also discussed.

The Generation and Migration of Bubbles in Oil-Pressboard Insulation Needle-Plate System

Bubbles in transformer oil can easily lead to partial discharge (PD), which can deteriorate the transformer oil and even breakdown the transformer insulation. To clarify the migration process and the characteristics of bubbles generated in an oil-immersed power transformer exposed to an extremely uneven electric field, we experimentally monitor these phenomena under an extremely nonuniform ac electric field and numerically simulate the migration distance and the migration speed of bubbles with different initial positions and sizes. The results show that the streamer discharge channel formed by a PD in oil is gasified into a bubble channel. After it collides with the surface of the pressboard, its morphology is transformed into approximately spherical bubbles due to the surface tension of the gas–liquid interface. After bubbles are generated in the oil, they move away from areas with a strong electric field due to the electric-field force and gradually approach the oil surface due to the buoyancy force. The experimental results are consistent with the simulation results, which verify the rationality of the simulation model.

Research on Defects of High Voltage Cable Fusion Joint Using XLPE Insulation Material

High-voltage cable fusion joints are made on site. Affected by environmental conditions, fusion joints are prone to small defects that are not easy to find, resulting in frequent accidents after the joints are put into operation. There is currently no effective method to detect small defects inside the joint. Therefore, this paper proposes to use digital radiography nondestructive test technology to detect the main insulation and conductor recovery process of high-voltage cable fusion joints. The results show that the digital radiography nondestructive test method can effectively find defects such as microporous bubbles, conductor bending, and eccentricity in the main insulation of XLPE fusion joints. The on-site anatomy demonstrates the accuracy of the test results, and the digital radiography nondestructive test can effectively detect the defects of high-voltage cable fusion joints. Experiments have verified that the decrease of cross-linked pipe temperature and nitrogen pressure can lead to the generation of microporous bubbles in the XLPE main insulation. For conductor bending and eccentricity defects, the limit value of the welding conductor bending gap is 11mm through simulation calculation.

An optimized recipe for making giant bubbles.

Large bubbles are primarily used in physics laboratories to study 2D turbulence, surface wavers, and fundamental properties of soap systems. Outside of physics, blowing large bubbles is also a part of many performances and shows. Both the scientific and artistic communities usually want to get reasonably stable foam films. The purpose of this article is to propose the main physical ingredients needed for an effective recipe to make stable films and bubbles. We propose controlled experiments to measure both the ease of generating a bubble, and its stability in different stabilizing solutions, which we choose by adding the ingredients contained in a bubble artist's recipe one by one. Our main findings are that (i) the surfactant concentration must not be too high, (ii) the solution must contain long flexible polymer chains to allow for easy bubble generation and (iii) the addition of glycerol provides improved bubble stability by preventing evaporation. Finally, we propose an efficient recipe, which takes into account these considerations.

Vibration method for the characterization of bubble behavior in sodium flows

Real-time bubble behavior characterization is critical in the chemical industry, especially for leakage in sodium–water reactions. In this study, the injection of gas into sodium flows was conducted to evaluate bubble characterization methods. Simulation research was performed to dynamically characterize bubble generation, growth and bursting in flowing sodium, and experimental research was conducted to explore the time-frequency and time domain statistical characteristics of bubble signals. After gas injection, significant low-amplitude signals were observed in the time domain, the power spectral density fluctuated in the 0–20 kHz band; its spectral components were more abundant above 8 kHz, and the short-time Fourier transform of the energy spectrum exhibited a nonlinear intermittent distribution in the 8–20 kHz band. Based on these differences, the bubble signal was effectively detected when the gas injection rate was 0.11 m3/h with a signal-to-noise ratio less than 0.5 dB. Thus, this study complemented the present techniques and knowledge in the field of chemical engineering.

Microstructure and oxidation behavior of CrCN/TiSiCN nano-multilayer coatings on Zircaloy in high-temperature steam

CrCN/TiSiCN nano-multilayer coatings with different C/N composition ratios were prepared on Zircaloy by hybrid multi-arc ion plating and magnetron sputtering technique, and the microstructures and oxidation behaviors of the coatings in 1000 °C steam were investigated. At the C/N ratio of 1.0, the coating structure changed from a quasi-bicrystalline layer periodic arrangement to a crystalline/amorphous layer arrangement. Increasing C/N ratio inhibited the generation and mobility of N2 bubbles in the coatings during oxidation, avoiding crack generation and coating flaking. The coating with C/N of 1.0 exhibited optimal high-temperature oxidation resistance. Moreover, the oxidation mechanisms of CrCN/TiSiCN nano-multilayer coatings were discussed.

Preparation of textured epoxy resin coatings for excellent hydrophobicity and corrosion resistance

Because of the severe corrosion of marine structural steel under the splash zone, it is proposed to prepare surface textures in widely used epoxy resin (ER) coatings to improve the hydrophobicity and corrosion resistance. However, the surface texture preparation technology of organic coatings is flawed and needs to be improved. Template method, one of the textured preparation technologies of organic coatings, has the trouble that the air bubbles, which diminish the coating quality. In this paper, the method of curing at room temperature and vacuum treatment is used to reduce the generation of air bubbles. Scanning electron microscope (SEM) images show the complete morphology and texture dimensions of the textured ER coatings. The hydrophobic properties of the textured ER coatings were systematically investigated, the contact angles of the textured ER coatings were significantly improved, and all the textured ER coatings samples met the hydrophobic criteria. And the wetting state model of the textured ER coatings was determined by modifying the Wenzel wetting mode. Furthermore, the electrochemical impedance spectroscopy (EIS) analysis of the textured ER coatings shows that the corrosion resistance is improved by two orders of magnitude compared to the smooth ER coatings. That shows excellent corrosion resistance, and the effect of the texture parameters on the corrosion resistance and hydrophobic properties of the textured ER coatings showed a high degree of consistency. The preparation technology of surface textured coatings has great potential in the preparation and application of marine anti-corrosion organic coatings.

Preparation and characterization of solvent-free anti-corrosion polyurethane-urea coatings

Metal corrosion can cause severe damage to metal facilities, leading to huge economic loss. Anti-corrosion coating is considered to be effective and economic for the protection of metal facilities. Herein, a series of polyurethane-urea coatings were prepared with aromatic isocyanate, amino terminated polyether, castor oil-based polyol and polyaspartic resin as reactants and conductive mica powders as filler. The effect of isocyanate index on the chemical structure, static conductivity and corrosion resistance of the polyurethane-urea coatings were studied, and the micromorphology of the coating layers were observed. With the increase of isocyanate index, the cross-linking degree of the coating increased, while the compactness and electrostatic conductivity decreased, which was caused by the generation of carbon dioxide bubbles in the coating layer. Electrochemical impedance spectroscopy and salt spray test indicated that as the isocyanate index increased, the anti-corrosion ability of the polyurethane-urea coating on the flawless area increased, but that on the defect area decreased. The polyurethane-urea coatings prepared with isocyanate index of 1.0-1.1 possessed good corrosion resistance and electrostatic conductivity. The findings conceivably provided a new strategy to prepare solvent-free anti-corrosion coatings for the protection of metal and metallic facilities and promote the rapid development and practical application of anticorrosive materials.

Effect of Aeration on Blockage Regularity and Microbial Diversity of Blockage Substance in Drip Irrigation Emitter

Aerated drip irrigation is rendered as a new water-saving irrigation method based on drip irrigation technology, which is endowed with the function of effectively alleviating the problem of rhizosphere hypoxia in crop soil, enhancing the utilization rate of water and fertilizer; as a result, it improves the harvest and quality of crops. However, clogged emitters are important indexes, among others, that pose an influence to the service effect and life duration of drip irrigation systems. At present, the working principle and mechanism of the influence of air feeding on the blockage of drip irrigation emitters remain unclear. Therefore, based on the two gas filling methods of the micro/nano bubble generator and Venturi injector, the dynamic change process for the average flow ratio of an air-filled drip irrigation emitter was studied by the method of emitter plugging test. 16S rRNA sequencing was used to analyze the microbial diversity of the emitter plugs. The results show that the air injection can pose influence on the clogging procedure of drip irrigation emitters, and more importantly, it makes the distribution of blocked emitters more uniform, thus improving the uniformity of the system. Different filling methods have different effects on the blockage of the emitter. Among them, the blockage time of drip irrigation system under the micro/nano aerated drip irrigation (MAI) mode is 5.73 times longer than that under unaerated drip irrigation (UVI), and similarly, Venturi gas drip irrigation (VAI) is close to that under UVI. The filling method changed the microbial diversity of the blockage in the emitter. Among them, the number of operational taxonomic unit (OTU) unique to MAI was 2.1 times that of UVI, and the number of OTU unique to VAI was 1.3 times that of UVI. Meanwhile, gas addition will inhibit the growth of Nitrospirae and Proteobacteria microbial communities and promote the growth of Firmicutes and Actinobacteria microbial communities. Furthermore, the increase in microbial extracellular polymer in the plugging material of the emitter was inhibited and the plugging process of the emitter was slowed down. The research results are of great significance in the disclosure of the clogging mechanism of drip irrigation emitter and constructing the green, anti-blockage technology of aerated drip irrigation.

Boiling in nanopores through localized Joule heating: Transition between nucleate and film boiling

The transition from nucleate to film boiling on micro/nanotextured surfaces is of crucial importance in a number of practical applications, where it needs to be avoided to enable safe and efficient heat transfer. Previous studies have focused on the transition process at the macroscale, where heat transfer and bubble generation are activated on an array of micro/nanostructures. In the present study, we narrow down our investigation scale to a single nanopore, where, through localized Joule heating within the pore volume, single-bubble nucleation and transition are examined at nanosecond resolution using resistive pulse sensing and acoustic sensing. Akin to macroscale boiling, where heterogeneous bubbles can nucleate and coalesce into a film, in the case of nanopores also, patches of heterogeneous bubbles nucleating on the cylindrical pore surface can form a torus-shaped vapor film blanketing the entire pore surface. In contrast to conventional pool boiling, nanopore boiling involves a reverse transition mechanism, where, with increased heat generation, film boiling reverts to nucleate boiling. With increasing bias voltage across the nanopore, the Joule heat production increases within the pore, leading to destabilization and collapse of the torus-shaped vapor film.

Realistic Rendering Algorithm for Bubble Generation and Motion in Water

A simplified bubble model and its solver optimization method are proposed to solve the problem of poor realistic simulation and complex solutions for bubble-motion behavior in water. Firstly, the internal velocity of the bubble was avoided, and the bubble model was established by only considering the net flux of the inlet and outlet bubbles, which reduced the computational complexity. The bubble constraint was then introduced into the motion equation of water, and the mixed Euler–Lagrangian method was used to solve it. FLIP particles tracked the bubble position, velocity, and deformation, and the mesh updated the vector field. At the same time, the viscosity term was simplified. Finally, it was combined with implicit incompressible SPH particles to achieve the purpose of volume correction. The experimental results show that the method in this paper can present a simulation effect of bubbles in water with rich detail and a realistic sense, whether compared with actual pictures or with existing methods.

Experimental investigation of side-discharge sparger on pressure oscillation in steam jet condensation

Experiments were conducted to investigate the pressure oscillation of steam jet condensation through a side-discharge sparger in stagnant water. Simultaneous analysis based on the synchronous measurement in the time domain was conducted between pressure oscillation and steam interfacial behaviors, and quantitative discussion about the intensity was taken according to the power spectrum of pressure oscillation. First, a regime map suitable for a sparger of 16 mm was presented and the bubbling, chugging, and jetting regimes were observed in this experiment. Compared with other regime maps, the bubbling regime is more narrow in the range of the steam mass flux when it is at low temperature, while is wider at high temperature. Second, it was found that different frequencies dominate the pressure oscillation under different conditions named the collapse frequency, the condensation frequency, and the generation frequency. The collapse frequency created by the collapse of detached bubbles prevails in the region with the temperature higher than 75 °C, while the condensation frequency induced by the condensation of bubbles controls the pressure oscillation in regions with the temperature lower than 60 °C. The generation frequency caused by the generation of bubbles mainly appears in the bubbling regime and is the secondary influence of pressure oscillation. This work provides a better understanding of steam jet condensation and could be helpful for the analysis of accident scenarios of pressure suppression systems.

Oil Cavitation Morphology Analysis of Textured Cylindrical Rotating Friction Pair on Circular-Disk End Faces

Cavitation is a destructive hydraulic phenomenon; it causes damage to small hydraulic machinery and large water-conservancy projects, and cavitation and cavitation corrosion problems are long-standing problems for hydraulic machinery experts. Micro-textures are often machined onto the end face near the indirect contact point to increase the oil film bearing capacity and reduce the transfer torque in hydraulic machinery, but the pressure disturbance caused by the texture dimple and boundary can induce cavitation. To delay or eliminate cavitation on the textured surface of equipment such as mechanical seals and dynamic bearings, the cavitation effect of a cylindrical texture on a rotating frictional pair was investigated from theoretical, imitational, and experimental perspectives. Low pressure is extremely important for inducing cavitation; once cavitation conditions are met, a full bubble can be formed in about 0.015 s. Cavitation mainly occurs at the outer radius of the rotating end face, i.e., in the high-speed region. There is a mutual mass transfer between gas and liquid phases when cavitation occurs, but the generation of cavitation bubbles is faster than its collapse, thus causing an increasing cavitation region.

Evaluation indicators of saturation on partially saturated sand induced by microbial gas

The induced partial saturation (IPS) technique introduces a certain amount of air/gas into soil voids to mitigate sand liquefaction. The microbial desaturation method utilises biological metabolic processes which are good for environmental protection and disaster prevention. Real time monitoring and quantitative analysis of the saturation after microbial bubble generation are the first step in the field application of the IPS method. In this study, shear wave velocity measurements were taken using the pre-vibration technique through an array of accelerometers at different depths on the basis of shaking table model tests. The shear wave velocity was obtained during the desaturation of microbial gas production. The arrival time of the first wave peak increased and the shear wave velocity decreased with the decreasing saturation. The shear wave velocity decreased with the saturation, and the rate of shear wave velocity decreased when the degree of saturation reduced to 90 %. This result indicates that the shear wave speed is effective as an indicator of saturation in microbial gas production. The shear wave velocity decreases with the saturation, and the relationship between the two satisfies the exponential functions. A quantitative relationship was established between the saturation and shear wave velocity during microbial gas production. The maximum shear modulus decreases with the saturation during the gas production process. Meanwhile, the relationship between shear wave velocity and resistivity was established using resistivity meters. The resistivity value decreases with the degree of saturation. The power function relationship between resistivity and saturation was satisfied in this experiment. The explicit analysis of the saturation data at each point in the desaturation method has significant research value for field trials using high-density electrical resistivity tomography. The research results obtained in this experiment can help in the quantitatively analysis of the uniformity and effectiveness of bubble distribution in the field test application of the microbial desaturation method and provide a basis for the engineering application of the microbial desaturation method in field tests.

Improving microalgae growth and carbon capture through micro-size bubbles generation in flat-panel photobioreactors: Impacts of different gas sparger designs on mixing performance

Anthropogenic emissions like carbon dioxide (CO2) necessitate the need for immediate mitigation of excessive CO2 concentrations, so as to ease the impacts of climate change. Mass carbon capture and storage (CCS) via microalgae cultivations have been proven prominent for their CO2 capturing abilities and prospects of converting CO2 into commercially valuable bioproducts. However, current microalgae cultivation systems have yet to achieve the satisfactory production rates required for mass CCS. To improve microalgae growth, CO2 gas bubbles are often fed to sustain microalgae cultures in photobioreactors (PBRs). Studies have shown significant increases in microalgae growth and CO2 capturing rates with the reduction of gas bubble sizes to micro-size. In this work, 3 distinct gas spargers were designed, and their mixing performances were investigated in a 10 L flat-panel PBR. Detailed analysis of micro-size bubbles generation via gas spargers, along with the cost analysis and carbon tax estimations for the sparger designs and operations, were discussed. The results of this work showed that spargers with micro-size pores were not able to produce micro-size bubbles. The customised sintered metal spargers with novel double sparger orientation resulted in higher mixing performances, compared to perforated spargers and commercial spargers. Findings advance the understanding of mixing and CO2 mass transfer, important milestones for photobioreactor scale-up studies and partly fulfil the circular bioeconomy concept where microalgae cultivations address both environmental and economic aspects.

Bubble growth and detachment process in surfactant solutions: a study of 90% pure sodium dodecyl sulfate

We have investigated the effect of the surfactant Sodium Dodecyl Sulfate on the bubble growth and detachment processes in a bubble column system. A modified pendant drop model was employed to determine the dynamic surface tension. The dynamic surface tension approached the static surface tension value within a few seconds. A photographic technique and a laser-based electro-optical method were used to study the bubble size variation and the generation frequency, respectively. Parameters like bubble generation frequency, size, growth rate, and contact angle were found to be a strong function of surfactant concentration. Our results show that the addition of surfactant significantly reduces the bubble lifetime at the orifice. Such a decrease in the bubble lifetime has a direct impact on the bubble growth rate and departure size. Furthermore, bubble parameters vary with surfactant concentration only up to the critical micellar concentration.

Effect of welding stability on process porosity in laser arc hybrid welding of dissimilar steel

Welding porosity is an important factor affecting the mechanical properties of welded joints. This study investigated the effect of welding stability on weld porosity to solve the problem of welding porosities in laser-arc hybrid welding of dissimilar steels. The type and formation mechanism of welding porosity were analyzed. The study results showed that the porosities generated during the welding process were process welding porosities. The generation of bubbles was closely related to the instability of the steam pressure inside laser keyhole. The droplet transfer form was an essential factor affecting the stability of the laser keyhole. When the droplet was in the projected transfer mode, the weld porosity could be effectively reduced. Penetration weld was also greatly helpful in reducing weld porosity.

Experimental and theoretical investigation on the effect of surface porosity on the nucleate flow boiling heat transfer

The surface porosity of accident tolerant fuel cladding materials is different from the fuel material inservice at nuclear power plant, which leads to differences in the heat transfer characteristics. An experiment was conducted to investigate the effect of surface porosity on the nucleate flow boiling heat transfer. The results showed that bubbles on the SiC surface appear to be more uniform and remain isolated longer after bubble generation, and the average diameter of bubbles on the SiC surface tends to be smaller than that on the FeCrAl alloy surface. The nucleation site density is power-law dependent on porosity and superheat, and negative power-law dependent on critical cavity diameter and coolant subcooling. For the same degree of temperature increase on both surfaces, the number of nucleation sites on the surface with larger porosity increases much more than that on the surface with lower porosity, while the coolant subcooling degree inhibits the bubble nucleation and shrinks the bubble detachment diameter accordingly. A new correlation was proposed based on dimensional analysis, and the predictions were compared with the present experimental data and the other research data, with an 86% confidence interval of ± 50% for the consolidated datasets. Furthermore, the side-by-side comparison of the performance on these correlations showed that the present correlation, which includes surface porosity and roughness factors, significantly reduces the prediction error compared to the previous correlations.