This thesis presents the continued development of micro optical phase detection instrumentation capable of measuring void fraction, interfacial area density, interfacial velocity and bubble sizes and their application to measurements in a high speed boundary layer. The instrumentation consists of micro sized sapphire tipped probes tailored to measure in the two-phase flow of air bubbles in water. Probe tips with geometries intended to maximize field life while minimizing intrusiveness were designed, fabricated and characterized. The characterization revealed that the active region of a probe tip can go beyond the highly sensitive 45 degree tip. Controlling the active length of the tips can be achieved through a combination of taper angles and 45 degree tip size, with larger tips having shorter active lengths. The full scale bubbly flow measurements were performed on a 6 m flat bottom survey boat. The aforementioned quantities were measured on bubbles naturally entrained at the bow of the boat. Probes were positioned at the bow of the boat, near the entrainment region and at the stern where the bubbles exit after having interacted with the high shear turbulent boundary layer. Experiments were conducted in fresh water, at the Coralville Lake, IA, and salt water, at the St. Andrews Bay and Gulf Coast near Panama City, FL. The results indicate that the bubbles interact significantly with the boundary layer. At low speeds, in fresh water, bubble accumulation and coalescence is evident by the presence of large bubbles at the stern. At high speeds, in both fresh and salt water, bubble breakup dominates and very small bubbles are produced near the hull of the boat. It was observed that salt water inhibits coalescence, even at low boat speeds. Void fraction was seen to increase with boat speeds above 10 knots and peaks near the wall. Bubble velocities show slip with the wall at all speeds and exhibit large RMS fluctuations, increasing near the wall.

Bubbly turbulent flows occur in a variety of industrial, naval and geophysical problems. In these flows, the bubbles in the flow interact with turbulence and/or vortical structures present in the continuous phase, resulting in bubble motion and deformation, and at the same time modifying the turbulence and/or vortical structures. Despite the fact that this has been a subject of interest for some time, mechanisms of bubble break-up due to turbulence and turbulence modulation due to bubbles are not well understood. To help understand this two-way coupled problem, we study in this thesis, the interaction of single and multiple bubbles with vortical structures; the thesis being broadly divided in to three parts. In the first part, we study the interaction of a single bubble with a single vortical structure, namely a vortex ring, formed in the continuous phase (water). This may be thought of as a simplified case of the interaction of bubbles with vortical structures in any turbulent flow. We then increase the complexity and study the interaction of a single bubble with naturally occurring vortical structures present in a fully developed turbulent channel flow, and then finally to the case of a large number of bubbles injected in to a fully developed turbulent channel. The bubble motions and deformations in all three cases are directly imaged using high speed visualizations, while the flow field information is obtained using time-resolved Particle-Image Velocimetry (PIV) in the first two cases, and from pressure drop measurements within the channel in the latter case. The interaction of a single vortex ring with a bubble has been studied for a large range of vortex ring strengths, represented in terms of a Weber number (We). We find that in all cases, the bubble is first captured by the low pressure within the core of the ring, then stretched azimuthally within the core, and gradually broken up in to a number of smaller bubbles. Along with these bubble deformations, the vorticity within the core of the ring is also modified significantly due to bubble capture. In particular, at low We, we find that the core of the ring fragments as a result of the interaction resulting in a large reduction in the enstrophy of the ring and its convection speed. In the second part of the thesis, interaction of a single bubble with naturally occurring vortical structures present in a fully developed turbulent channel is studied. In this case, single bubbles of different sizes are injected either from bottom or top wall into a channel at Reynolds number of about 60,000. We study the trajectories of the single bubble, and also investigate the effect that such bubbles have on the naturally occurring vortical structures present in these flows. The injected bubble is found to have three broadly different types of bubble paths when injected from the bottom wall, which are sliding along the wall, bouncing motions and vertical escape from the vicinity of the wall. Even at the same bubble diameter Db and channel flow Re, we find that different realizations show considerable variations, with all three bubble paths being possible. PIV measurements of a bubble captured by a naturally occurring vortical structure in the flow, shows a more rapid decrease in enstrophy compared to naturally occurring structures in the absence of bubbles, as seen in the interaction of a bubble with a vortex ring. We also find that the bubble can interact with multiple vortical structures, depending on their strength and spatial distribution in the flow, resulting in a complex bouncing bubble motion. In the third part of the study, a large number of bubbles are injected in to the channel through porous plates fixed at the top and bottom channel walls. The main parameters here are the channel Re, bubble void fraction (α) and the orientation of injection. In this case, in addition to bubble visualizations, the pressure drop through the channel is measured at different vertical locations. These measurements show large vertical variations in the measured pressure drop due to the presence of bubbles. The overall drag reduction in these cases is obtained from an integral of the pressure drop variation along the vertical direction. The visualizations show a number of bubble dynamics regimes depending on the parameters, with possibilities of both increased and decreased drag compared to the reference no bubble case. From simultaneous measurements, we relate the variations in drag reduction to the different bubble dynamics regimes. We find that at the same void fraction (α), the drag reduction obtained can be very different due to changes in bubble dynamics regimes caused by changes in other parameters. Top wall injection is observed to give good drag reductions over a wide range of flow Re and α, but is seen to saturate beyond a threshold α. In contrast, the bottom wall injection case shows that drag reduction continuously increases with αat high Re. The present study shows a maximum of about 60% increase and a similar 60% reduction in wall drag over the entire range of conditions investigated.

Thesis (M.A. (Communication Studies)) -- University of Limpopo, 2020 / Hubbly Bubbly smoking is a visibly growing trend among young people globally. However, health promotion about the risks that result from hubbly bubbly smoking has not been vigilant so far. The study attempts to determine youth’s knowledge and perceptions towards the health risks incurred from smoking the HB. The study triangulates three theories namely: The Extended Parallel Process Model, Peer Cluster Theory and Hofstede’s Cultural Theory using a mixed method approach among University of Limpopo students. A sample of 350 students was used, made up of n=175 HB users and n=175 non-users. In the quantitive part of the study, a 2 x 2 between and within respondents design was followed. Most HB users 74% (129) and non-users 80% (140) identified lung cancer as the health risk of HB smoking. Participants perceived HB smoking health risks as exaggerated, and that HB is less addictive and less harmful. The majority of participants (75%) had not heard or seen any health communication awareness campaigns about HB risks. Focus group interviews revealed that participants perceived HB as less harmful, less expensive with no visible health warning and not addictive to every user. Findings suggest that there is inadequacy in terms of knowledge as well as negative perceptions towards HB and its health risks. There should be more health communication campaigns that will engage the youth and the public, and the media should speak more about HB and its health risks. The study contributes to the body of knowledge about health communication campaigns about existing problems. / National Research Foundation (NRF)

This thesis describes the design and evaluation process of experimental measuring of the amount of bubbly air in water. The measuring principle is based on the assessment of speed of sound, which is dependent on the presence of air. Results were compared with the concentration calculated from the calibrated air flow through nozzles.

Bubbly flows can be found in many applications in chemical, biological and power engineering. Reliable simulation tools of such flows that allow the design of new processes and optimization of existing one are therefore highly desirable. CFD-simulations applying the multi-fluid approach are very promising to provide such a design tool for complete facilities. In the multi-fluid approach, however, closure models have to be formulated to model the interaction between the continuous and dispersed phase. Due to the complex nature of bubbly flows, different phenomena have to be taken into account and for every phenomenon different closure models exist. Therefore, reliable predictions of unknown bubbly flows are not yet possible with the multi-fluid approach. A strategy to overcome this problem is to define a baseline model in which the closure models including the model constants are fixed so that the limitations of the modeling can be evaluated by validating it on different experiments. Afterwards, the shortcomings are identified so that the baseline model can be stepwise improved without losing the validity for the already validated cases. This development of a baseline model is done in the present work by validating the baseline model developed at the Helmholtz-Zentrum Dresden-Rossendorf mainly basing on experimental data for bubbly pipe flows to bubble columns, bubble plumes and airlift reactors that are relevant in chemical and biological engineering applications. In the present work, a large variety of such setups is used for validation. The buoyancy driven bubbly flows showed thereby a transient behavior on the scale of the facility. Since such large scales are characterized by the geometry of the facility, turbulence models cannot describe them. Therefore, the transient simulation of bubbly flows with two equation models based on the unsteady Reynolds-averaged Navier–Stokes equations is investigated. In combination with the before mentioned baseline model these transient simulations can reproduce many experimental setups without fitting any model. Nevertheless, shortcomings are identified that need to be further investigated to improve the baseline model. For a validation of models, experiments that describe as far as possible all relevant phenomena of bubbly flows are needed. Since such data are rare in the literature, CFD-grade experiments in an airlift reactor were conducted in the present work. Concepts to measure the bubble size distribution and liquid velocities are developed for this purpose. In particular, the liquid velocity measurements are difficult; a sampling bias that was not yet described in the literature is identified. To overcome this error, a hold processor is developed. The closure models are usually formulated based on single bubble experiments in simplified conditions. In particular, the lift force was not yet measured in low Morton number systems under turbulent conditions. A new experimental method is developed in the present work to determine the lift force coefficient in such flow conditions without the aid of moving parts so that the lift force can be measured in any chemical system easily.

Cette thèse vise à contribuer à la caractérisation, à l’aide de modélisation et d’expérience, de la dynamique de bulle en microfluidique. Deux régimes d’écoulements rencontrés en microfluidique sont étudiés, le régime bubbly flow et le régime Taylor flow. En particulier, la première partie de cette thèse traite de la dynamique d’un écoulement de type bubbly flow dans un microcanal rectiligne de section circulaire en présence de surfactants. Le code de calcul numérique JADIM est utilisé. Une méthode numérique permettant, d’une part, de simuler le transport de surfactants le long d’une interface qui bouge et qui se déforme, et d’autre part, de simuler l’effet Marangoni crée par une distribution inhomogène de ces surfactants sur cette interface, est implémentée et validée. Les simulations effectuées avec ce code concernant la dynamique d’un écoulement de type bubbly flow montrent par exemple que, le confinement créé par les parois du microcanal résulte en une distribution des surfactants sur la surface des bulles qui est fondamentalement différente d’une distribution rencontrée dans le cas d’une bulle qui se déplace dans un liquide de dimension infinie. En effet, les surfactants s’accumulent en des locations spécifiques sur la surface des bulles et créent des forces de Marangoni locale, qui influencent drastiquement la dynamique des bulles. Dans certains cas, les surfactants peuvent même engendrer une désintégration de la bulle, un mécanisme qui est rationalisé par un bilan de force à l’arrière de la bulle. La méthode numérique implémentée dans cette thèse est également utilisée pour un problème pratique concernant la production artisanale de Mezcal, une boisson alcoolisée produite au Méxique. La seconde partie de cette thèse traite de la dynamique d’un écoulement de type Taylor flow, à l’aide d’expérience et de modélisation. Une méthode expérimentale permettant de mesurer l’épaisseur du film de lubrification qui se forme entre une bulle de Taylor et les parois du microcanal est développée. Cette méthode requiert uniquement une image « brightfield » de la bulle. En plus de la mesure de l'epaisseur du film de lubrification, la méthode permet aussi de mesurer la profondeur du microcannal. Enfin, l'utilisation de la méthode proposée couplée à la mesure de la vitesse de translation de la bulle permet de déduire la tension de surface de celle-ci. Dans le dernier chapitre de cette thèse, l'influence des effets gravitaires sur la dynamique des écoulements de Taylor est quantifiée. Quoique souvent négligée en microfluidique, il est montré que les effets gravitaires peuvent avoir un impact significatif sur la dynamique des écoulements de Taylor. Ces impacts sont quantifiés à l'aide d'expériences et de modélisations. Ce travail a été réalisé à la Princeton University avec Professeur Howard A. Stone pendant un séjour de 7 mois. / This thesis aims at contributing to the characterization of the dynamics of bubbles in microfluidics through modeling and experiments. Two flow regimes encountered in microfluidics are studied, namely, the bubbly flow regime and the Taylor flow regime (or slug flow). In particular, the first part of this thesis focuses on the dynamics of a bubbly flow inside a horizontal, cylindrical microchannel in the presence of surfactants using numerical simulations. A numerical method allowing to simulate the transport of surfactants along a moving and deforming interface and the Marangoni stresses created by an inhomogeneous distribution of these surfactants on this interface is implemented in the Level set module of the research code. The simulations performed with this code regarding the dynamics of a bubbly flow give insights into the complexity of the coupling of the different phenomena controlling the dynamics of the studied system. Fo example it shows that the confinement imposed by the microchannel walls results in a significantly different distribution of surfactants on the bubble surface, when compared to a bubble rising in a liquid of infinite extent. Indeed, surfactants accumulate on specific locations on the bubble surface, and create local Marangoni stresses, that drastically influence the dynamics of the bubble. In some cases, the presence of surfactants can even cause the bubble to burst, a mechanism that is rationalized through a normal stress balance at the back of the bubble. The numerical method implemented in this thesis is also used for a practical problem, regarding the artisanal production of Mezcal, an alcoholic beverage from Mexico. The second part of the thesis deals with the dynamics of a Taylor flow regime, through experiments and analytical modeling. An experimental technique that allows to measure the thickness of the lubrication film forming between a pancake-like bubble and the microchannel wall is developed. The method requires only a single instantaneous bright-field image of a pancake-like bubble translating inside a microchannel. In addition to measuring the thickness of the lubrication film, the method also allows to measure the depth of a microchannel. Using the proposed method together with the measurment of the bubble velocity allows to infer the surface tension of the interface between the liquid and the gas. In the last chapter of this thesis, the effect of buoyancy on the dynamics of a Taylor flow is quantified. Though often neglected in microfluidics, it is shown that buoyancy effects can have a significant impact on the thickness of the lubrication film and consequently on the dynamics of the Taylor flow. These effects are quantified using experiments and analytical modeling. This work was performed at Princeton University with Professor Howard A. Stone during a seven month stay.

No description available.

In this paper we present detailed Euler-Euler Large Eddy Simulations (LES) of dispersed bubbly flow in a rectangular bubble column. The motivation of this study is to investigate potential of this approach for the prediction of bubbly flows, in terms of mean quantities. The set of physical models describing the momentum exchange between the phases was chosen according to previous experiences of the authors. Experimental data, Euler-Lagrange LES and unsteady Euler-Euler Reynolds-Averaged Navier-Stokes model are used for comparison. It was found that the presented modelling combination provides good agreement with experimental data for the mean flow and liquid velocity fluctuations. The energy spectrum made from the resolved velocity from Euler-Euler LES is presented and discussed.

Ce travail est dédié au développement d'une méthode numérique pour la simulation des écoulements liquide-bulles. La présence des bulles dans l'écoulement visqueux et incompressible est prise en compte via une méthode de pénalisation. Dans cette représentation Euler-Lagrange, les bulles supposées indéformables et parfaitement sphériques sont assimilées à des objets pénalisés interagissant avec le fluide. Une méthode VOF (Volume Of Fluid) est employée pour le suivi de la fonction de phase. Une adaptation de la discrétisation des équations de Navier-Stokes est proposée afin d'imposer la condition de glissement à l'interface entre le liquide et les bulles. Une méthode de couplage entre le mouvement des bulles et l'action du liquide est proposée. La stratégie de validation est la suivante. Dans un premier temps, une série de cas-tests est proposée; les objets pénalisés sont supposés en non-interaction avec le fluide. L'étude permet d'exhiber la convergence et la précision de la méthode numérique. Dans un second temps le couplage est testé via deux types de configurations de validation. Le couplage est d'abord testé en configuration de bulle isolée, pour une bulle en ascension dans un liquide au repos pour les Reynolds Re=17 and Re=71. Les résultats sont comparés avec la théorie établie par la corrélation de Mei pour les bulles sphériques propres décrivant intégralement la dynamique de la bulle. Enfin, des simulations en configurations de nuage de bulles sont présentées, pour des populations mono- et bidisperses dans un domaine entièrement périodique pour des taux de vide s'établissant entre 1% et 15%. Les statistiques fournies par les simulations caractérisant l'agitation induite par les bulles sont comparées à des résultats expérimentaux. Pour les simulations de nuages de bulles bidisperses, de nouveaux résultats sont présentés. / This work is devoted to the development of a numerical method for the simulation of two-phase liquid-bubble flows. We use a volume penalization method to take into account bubbles in viscous incompressible flows. The chosen Euler-Lagrange framework involves spherical and nondeformable bubbles represented as moving penalized obstacles interacting with the fluid. A VOF (Volume Of Fluid) method is used to track the phase function while a discretization of the penalized conservation equations is realized to impose slip conditions at the liquid-bubble interface. A coupling method devised from the penalized momentum equations is proposed. The validation process is set as following. First, the fluid is supposed non-acting on the bubbles; several test-cases are presented; we consider configurations with different penalized obstacles shapes (curved channel, inclined channel), the obstacles are either static or dynamic; in each configuration an analytical solution is known. The results show the compliance and the quality of our numerical closures by exposing the convergence order of the method. In order to verify the accuracy of the coupling method, numerical simulations of a 1mm diameter single bubble rising in a quiescent liquid are performed for Re=17 and Re=71. Results are compared with theory established by using Mei correlation for clean spherical bubbles describing the whole dynamics of the rising bubble. Finally, simulations of bubble swarms, in mono- and bidisperse configurations have been carried out in a fully periodic box with moderate void fractions ranging from 1% to 15%. The statistics provided by the simulations characterizing the bubble-induced agitation are compared to experimental results. For the bidisperse bubble swarm configuration, new results are presented.

<p>To fully realize the advantages of the two-fluid model, the interfacial area concentration (IAC) should be properly given by a constitutive model. The conventional flow-regime-based IAC correlations intrinsically cannot predict the dynamic flow structure change and would introduce a discontinuity and numerical instability to system codes. As a promising alternative, the interfacial area transport equation (IATE) is developed to model the interface structure mechanistically. Progress has been achieved for IATE modeling in bubbly, slug, and churn-turbulent flow during the past two decades. Aiming at a comprehensive flow structure predictor for all flow regimes, further development in two directions is highly desirable. First is extending the current experiment and modeling capability from churn-turbulent to annular flow. In this study, an advanced four-sensor droplet capable conductivity probe (DCCP-4) is developed to capture all interfaces in churn-turbulent and annular flow, including liquid film, liquid droplet, gas core, and gas bubble. A first of a kind experimental database in churn-turbulent, annular, and wispy annular flow with two-dimensional spatial distributions is established, which provides the experimental basis for the multi-field two-phase flow model development. The measured parameters include local time-averaged volume faction, IAC, and velocity for various fields of annular flow. In addition, a new constitutive model to quantify the interfacial area between the gas core and liquid film of annular flow is developed, which fills the last theoretical gap of interfacial area modeling. The other important direction is improving the current IATE model to fulfill the dynamic prediction of developing flow, especially the bubbly to slug transition flow. Vertical-upward air-water two-phase flow experiments are performed. The state-of-the-art IATE model is evaluated against the newly collected data at bubbly and slug flow, and the result shows unsatisfactory performance in predicting the developing flow with intensive bubble coalescence. A new bubble coalescence model is derived by using the log-normal bubble size distribution, which significantly improves the model prediction capability.</p>