Studies in bubble formation—I bubble formation under constant flow conditions

Fracture and relaxation in dense cornstarch suspensions

Dense suspensions exhibit the remarkable ability to switch dynamically and reversibly from a fluid-like to a solid-like, shear-jammed (SJ) state. Here, we show how this transition has important implications for the propensity for forming fractures. We inject air into bulk dense cornstarch suspensions and visualize the air invasion into the opaque material using time-resolved X-ray radiography. For suspensions with cornstarch mass fractions high enough to exhibit discontinuous shear thickening and shear jamming, we show that air injection leads to fractures in the material. For high mass fractions, these fractures grow quasistatically as rough cavities with fractured interfaces. For lower mass fractions, remarkably, the fractures can relax to smooth bubbles that then rise under buoyancy. We show that the onset of the relaxation occurs as the shear rate induced by the air cavity growth decreases below the critical shear rate denoting the onset of discontinuous shear thickening, which reveals a structural signature of the SJ state.

The Role of the Vocal Tract's Acousto-Mechanical Resonance on the Mechanism of Bubble Release From Tubes Used in SOVTE

AND OBJECTIVES

Semi-occluded vocal tract exercises involving a tube with a distal end immersed in water have been used as a voice rehabilitation technique for nearly 60 years. Advantages of this technique include a constant flow resistance, which is mainly dependent on the tube's immersion depth, and an oscillatory component of the back pressure due to bubble release, which is assumed to provide relief of muscle hypertension around the laryngeal and pharyngeal regions. The goal of this study is twofold. First, to shed more light on the contribution of geometrical parameters, such as tube inner diameter and inclination, on the amplitude of the back pressure oscillation and on the bubble release frequency (BRF). The second goal aims to understand the role of vocal tract compliance on the mechanism of bubble release. We hypothesize that the low-frequency acousto-mechanical resonance of a real compliant vocal tract becomes strongly coupled with the mechanism of bubble release, thereby augmenting the back pressure oscillation and dictating the BRF.

METHODS

The BRF and the back pressure are assessed experimentally as a function of flow rate for tubes with a non-compliant upstream condition, considering different diameters and inclinations. These results are compared with theoretical predictions and the experimental data obtained with six adult subjects, from which the vocal tract acousto-mechanical resonance frequencies are also assessed.

RESULTS

The results obtained with tubes involving a non-compliant upstream condition agreed well with the theoretical results and did not indicate a significant influence of the inclination nor the inner diameter on the BRF and on the oscillatory component of the back pressure. Despite the good agreement with the theory, the results obtained from the tube with a non-compliant upstream termination diverged significantly from those obtained with human subjects. In the latter case, the back pressure fluctuation was considerably higher and the BRF measured for each subject coincided with the frequency associated their respective vocal tract acousto-mechanical resonance.

CONCLUSIONS

Results indicate that the BRF is essentially driven by the acousto-mechanical resonance of the vocal tract. Furthermore, the substantial increase on the amplitude of the oscillatory component of the back pressure suggests a strong feedback loop between the source of pressure (bubble release mechanism) and the resonator (vocal tract). The high-pressure fluctuations within the oral cavity implies that subjects need a considerable extra effort to adjust their vocal tract in order to achieve an adequate impedance match between vocal folds and vocal tractsd to produce voice.

Dynamics of Drop Formation in the Presence of Interfacial Mass Transfer

The dynamics of drop formation have been investigated in the presence of interfacial mass transfer through controlled flow visualization experiments. The mixtures of n-hexane (solvent) and acetone (solute) were used as a dispersed phase, having different initial compositions varying over a broad range. Drops were formed at the submerged position in the continuous phase (water) at the same operating flow conditions. The unsteady force balance model is developed to analyze the implications of the simultaneously occurring interfacial transfer of the solute on the formation dynamics in real time, and predictions are validated with experimental results. Based on initial compositions, the analysis of the transient drop shape shows a sharp transition in the drop formation regime. At lower initial solute concentrations, i.e., ϕ0 < 0.2, axisymmetric drop formation occurs and the interfacial solute transfer has negligible effects on the formation dynamics. Over an intermediate range of solute concentrations, i.e., 0.2 < ϕ0 < 0.5, Marangoni instability is triggered along the evolving interface, and therefore, the interface deformations and contractions occur during the drop formation. At ϕ0 = 0.5, the drop takes highly nonaxisymmetric shapes and remains away from equilibrium until its detachment from an orifice. For ϕ0 > 0.5, the spontaneous ejection of plumes of the solute results in the rapid generation of multiple droplets of smaller size. This work shows that higher solute concentration gradients not only lead to faster solute transport but also induce strong interfacial instability simultaneously. Thus, the coupled effects of transient change in composition and fluid properties govern the drop size and its formation time in such systems under non-equilibrium.

Experimental Determination of the Conversion Rate of Molten White Metal by Supplying Individual Air Bubbles

The conversion of copper sulfide mattes lacks fundamental kinetic information for understanding the process. This work presents the results of the experimental measurement of the rate of oxidation of molten white metal by supplying individual air bubbles. The bubbles were characterized from information collected during the experiments and through theoretical and empirical correlations. Conversion tests were carried out at different temperature conditions and injection dynamics of the oxidizing gas. The results indicate that the conversion is controlled by the transport of oxygen into the bubble. A dependency between the characteristics of the injector and the shape, size and surface of the bubbles was identified. The oxidation rate of the white metal depended on the characteristics of the bubbles and the oxygen available for conversion. The results of this research provide relevant information to improve the operation of industrial conversion furnaces by controlling gas injection through the tuyeres. The criterion for improving conversion systems is the balance between the retention of the bubbles in the molten white metal and their size, such that the availability of oxygen for the conversion is guaranteed.

Design and Investigation of a 3D-Printed Micro-Fluidized Bed

Micro-fluidized bed has aroused much attention due to its low-cost, intensified-process and fast-screening properties. In this paper, a micro-fluidized bed (15 × 15 mm in cross-section) was designed and fabricated with the use of the stereolithography printing technique, for the investigation of bubbles’ hydrodynamics and comparison of the solids (3D-printed particles VS fungal pellets) fluidization characteristics. In a liquid–gas system, bubble flow regime started from mono-dispersed homogeneous regime, followed by poly-dispersed homogeneous regime, transition bubble regime and heterogeneous bubble regime with increasing gas flowrates from 3.7 mL/min to 32.7 mL/min. The impacts from operating parameters such as gas flowrate, superficial liquid velocity and gas sparger size on bubble size, velocity and volume fraction have been summarized. In liquid–solid fluidization, different solid fluidization regimes for both particles bed and pellets bed were identified. From the bed expansion results, much higher Umf of 7.8 mm/s from pellets fluidization was observed compared that of 2.3 mm/s in particles fluidization, because the hyphal structures of fungal pellets increased surface friction but also tended to agglomerate. The similar R–Z exponent n (5.7 and 5.5 for pellets and particles, respectively) between pellets and particles was explained by the same solid diameter, but much higher Ut of 436 µm/s in particles bed than that of 196 µm/s in pellets bed is a consequence of the higher density of solid particles. This paper gives insights on the development of MFB and its potential in solid processing.

Collaborative Control Applied to BSM1 for Wastewater Treatment Plants

This paper describes a design procedure for a collaborative control structure in Plant Wide Control (PWC), taking into account the existing controllable parameters as a novelty in the procedure. The collaborative control structure includes two layers, supervisory and regulatory, which are determined according to the dynamics hierarchy obtained by means of the Hankel matrix. The supervisory layer is determined by the main dynamics of the process and the regulatory layer comprises the secondary dynamics and controllable parameters. The methodology proposed is applied to a wastewater treatment plant, particularly to the Benchmark Simulation Model No 1 (BSM1) for the activated sludge process, comparing the results with the use of a Model Predictive Controller in the supervisory layer. For determining controllable parameters in the BSM1 control, a new specific oxygen mass transfer model in the biological reactor has been developed, separating the kLa volumetric mass transfer coefficient into two controllable parameters, kL and a.

Modification of the Young-Laplace equation and prediction of bubble interface in the presence of nanoparticles

Bubbles are fundamental to our daily life and have wide applications such as in the chemical and petrochemical industry, pharmaceutical engineering, mineral processing and colloids engineering. This paper reviews the existing theoretical and experimental bubble studies, with a special focus on the dynamics of triple line and the influence of nanoparticles on the bubble growth and departure process. Nanoparticles are found to influence significantly the effective interfacial properties and the dynamics of triple line, whose effects are dependent on the particle morphology and their interaction with the substrate. While the Young-Laplace equation is widely applied to predict the bubble shape, its application is limited under highly non-equilibrium conditions. Using gold nanoparticle as an example, new experimental study is conducted to reveal the particle concentration influence on the behaviour of triple line and bubble dynamics. A new method is developed to predict the bubble shape when the interfacial equilibrium conditions cannot be met, such as during the oscillation period. The method is used to calculate the pressure difference between the gas and liquid phases, which is shown to oscillate across the liquid-gas interface and is responsible for the interface fluctuation. The comparison of the theoretical study with the experimental data shows a very good agreement, which suggests its potential application to predict bubble shape during non-equilibrium conditions.

Fischer–Tropsch Synthesis in Slurry Bubble Column Reactors: Experimental Investigations and Modeling – A Review

This paper presents an extensive review of the kinetics, hydrodynamics, mass transfer, heat transfer and mathematical as well as computational fluid dynamics (CFD) modeling of Low-Temperature Tropsch Synthesis (LTFT) synthesis in Slurry Bubble Column Reactors (SBCRs), with the aim of identifying potential research and development areas in this particular field. The kinetic expressions developed for F-T synthesis over iron and cobalt catalysts along with the water gas shift (WGS) reactions are summarized and compared. The experimental data and empirical correlations to predict the hydrodynamics (gas holdup, Sauter mean bubble diameter, and bubble rise velocity), mass transfer coefficients and heat transfer coefficients are presented. The effects of various operating variables, including pressure, temperature, gas velocity, catalyst concentration, reactor geometry, and reactor internals on the hydrodynamic and transport parameters as well as the performance of SBCRs are discussed. Additionally, modeling efforts of SBCRs, using axial dispersion models (ADM), multiple cell recirculation models (MCCM) and computational fluid dynamics (CFD), are addressed. This review revealed the following: (1)

Bubble formation in a quiescent pool of gold nanoparticle suspension

This paper begins with an extensive review of the formation of gas bubbles, with a particular focus on the dynamics of triple lines, in a pure liquid and progresses into an experimental study of bubble formation on a micrometer-sized nozzle immersed in a quiescent pool of aqueous gold nanofluid. Unlike previous studies of triple line dynamics in a nanofluid under evaporation or boiling conditions, which are mainly caused by the solid surface modification due to particle sedimentation, this work focuses on the roles of nanoparticles suspended in the liquid phase. The experiments are conducted under a wide range of flow rates and nanoparticle concentrations, and many interesting phenomena are revealed. It is observed that nanofluids prevent the spreading of the triple line during bubble formation, i.e. the triple line is pinned somewhere around the middle of the tube wall during the rapid bubble formation stage whereas it spreads to the outer edge of the tube for pure water. A unique 'stick-slip' movement of the triple line is also observed for bubbles forming in nanofluids. At a given bubble volume, the radius of the contact line is found to be smaller for higher particle concentrations, but a reverse trend is found for the dynamic bubble contact angle. With the increase of particle concentration, the bubble frequency is raised and the bubble departure volume is decreased. The bubble shape is found to be in a good agreement with the prediction from Young-Laplace equation for given flow rates. The influence of nanoparticles on other detailed characteristics related to bubble growth inside, including the variation of bubble volume expansion rate, the radius of the curvature at the apex, the bubble height and bubble volume, is revealed. It is suggested that the variation of surface tensions and the resultant force balance at the triple line might be responsible for the modified dynamics of the triple line.