Bubble columns are multiphase contactors, which are often used for the operation of multiphase reactions. These reactors are characterized by large interfacial areas, intense mixing in the column and dynamic flow structures. For the design of bubble columns numerous empirical correlations for hydrodynamic and mass transfer parameters exist, which however, only cover a specific design range, e.g. column diameter and reaction system. Furthermore, they are often based on integral measurements. High deviations occur especially at moderate and high gas holdup, since fluid dynamics and mass transfer are essentially determined by local effects, which has received little attention so far due to lack of appropriate measurement techniques. For this reason the design of bubble columns is carried out with considerable safety margins. Therefore, bubble columns have a high potential for savings in terms of efficiency and operating costs if the unterstanding of the hydrodynamics and mass transfer in bubble columns is improved. The fluid dynamics in bubble columns are essentially characterized by the bubble size distribution and with increasing gas holdup by enhanced bubble interaction, which leads to intense motion in the liquid phase. A cross-scale study of the fluid dynamics is necessary to better understand the factors influencing the global fluid dynamics and to optimize the design of bubble columns. The major limitations in the experimental investigation of dense bubbly flow are the available measurement techniques. As the fraction of the dispersed phase increases, the optical accessibility decreases and limits the selection of appropriate measurement techniques. This work deals with the experimental study of fluid dynamics and mass transfer with chemical reaction using tomographic measurement techniques. The experiments were performed in a cylindrical bubble column with 0.10 m inner diameter. The gas phase fluid dynamics were studied by means of ultrafast X-ray computed tomography (UFXCT). Additionally, the minimally intrusive wire-mesh sensor was qualified to assess chemical species concentrations during chemical absorption of CO2 and to derive local mass transfer related parameters. Among the global description of the fluid dynamics also local parameters of the dispersed phase were extracted. Advanced post-processing algorithms for ultrafast X-ray CT data allow a more accurate determination of the Sauter diameter and interfacial area due to the extraction of the bubbles surface area. The interaction of bubbles within dense bubbly flow was studied by determining and evaluating distance parameters of the bubbles. Based on that, the near order of bubbles was studied and conclusions on their preferential arrangement in dense bubbly flows were derived. Furthermore, the fluid dynamics and mass transfer were studied in the presence of a chemical reaction. For this purpose experiments on the chemical absorption of CO2 in sodium hydroxide solution were performed at various gas flow rates and initial pH values of the solution. The change of the fluid dynamics and the conversion of the chemical species is analyzed for different experimental conditions. A core topic of the present work is the determination of concentration of a chemical species during chemical absorption of CO2. Within this work, the wire-mesh sensor was qualified for this new field of application and was used to determine cross-sectional data of the species conversion. Through combined use of the wire-mesh sensor and ultrafast X-ray CT an extensive data base on the chemical absorption of CO$_2$ was obtained, which can be used for numerical validation of bubbly flows with gas holdups < 0.17 / Blasensäulen sind in der chemischen Industrie sehr häufig genutzte Kontaktapparate für die Durchführung von Mehrphasenreaktionen. Sie zeichnen sich durch eine hohe Gas-Flüssig-Aus-tauschfläche, starke Vermischung und dynamische Strömungsstrukturen aus. Bisher basiert die Auslegung von Blasensäulen vorrangig auf empirischen Korrelationen. Hohe Abweichungen dieser Korrelationen treten vor allem bei moderaten und hohen Gasgehalten auf, da die Fluiddynamik und der Stofftransport wesentlich durch lokale Effekte bestimmt sind, die bisher aufgrund fehlender Messtechnik nur wenig Beachtung finden. Aus diesem Grund erfolgt die bisherige Auslegung von Blasensäulenreaktoren mit erheblichen Sicherheitszuschlägen. Daraus lässt sich ableiten, dass Blasensäulen in Bezug auf deren Effizienz und die Höhe der Betriebskosten ein enormes Einsparungspotential aufweisen, wenn das Verständnis von Hydrodynamik und Stofftransport in Blasensäulen verbessert wird. Die Fluiddynamik in Blasensäulen wird wesentlich durch die Blasengrößenverteilung und bei steigendem Gasgehalt durch die durch zunehmende Blaseninteraktion induzierte Flüssigkeitsbewegung charakterisiert. Eine skalenübergreifende Untersuchungen der Fluiddynamik ist erforderlich, um die Einflussfaktoren der globalen Fluiddynamik besser verstehen zu können und die Auslegungsgleichungen optimieren zu können. Die größte Limitierung der experimentellen Untersuchung von dichten Blasenströmung stellt die verfügbare Messtechnik dar. Mit zunehmenden Anteil der Dispersphase nimmt die optische Zugänglichkeit ab und schränkt die Möglichkeiten bei der Auswahl geeigneter Messtechniken ein. Diese Arbeit beschäftigt sich mit der experimentellen Untersuchung der Fluiddynamik und des Stofftransports bei chemischer Reaktion mit Hilfe tomographischer Messverfahren. Die Experimente wurden in einer zylindrischen Blasensäule mit 0,10 m Durchmesser durchgeführt. Die Fluiddynamik der Gasphase wurde mit Hilfe der ultraschnellen Röntgen-Computertomographie (UFXCT) untersucht. Zusätzlich wurde der minimal-intrusive Gittersensor qualifiziert, um die Konzentrationen einer relvanten chemischen Spezies während der chemischen Absorption von CO2 zu bestimmen und stofftransportbezogene Parameter auf lokaler Ebene berechnen. Neben der räumlich und zeitlich gemittelten Beschreibung der Fluiddynamik wurden auch lokale Parameter der dispersen Phasen bestimmt. Fortgeschrittene Auswertungsalgorithmen ermöglichen eine genauere Berechnung des Sauterdurchmessers und der Phasengrenzfläche durch direkte Bestimmung der Blasenoberfläche. Die Interaktion von Blasen in der dichten Blasenströmung wurde durch die Berechnung von Abstandsparametern untersucht. Dies ermöglicht die Analyse der Nahordnung von Blasen und lässt damit Rückschlüsse auf deren bevorzugte Anordnung in dichten Blasenströmungen zu. Darüber hinaus wurden die Fluiddynamik und zum Stofftransport in Gegenwart einer chemischen Reaktion untersucht. Dazu wurden Experimente zur chemischen Absorption von CO2 n Natronlauge in der Blasensäule bei verschiedenen Gasdurchsätzen und unterschiedlichen initialen pH-Werten der Lösung durchgeführt. Die Veränderung der Fluiddynamik der Gasphase und die Umwandlung der chemischen Spezies wurden unter verschiedenen Betriebsbedingungen analysiert. Ein Kernthema der Arbeit stellt die Erfassung der Konzentration einer chemischen Spezies während der chemischen Absorption dar. Dazu wurde im Rahmen dieser Arbeit der Gittersensor für dieses neue Anwendungsgebiet qualifiziert und eingesetzt, um querschnittsaufgelöste Daten über den Verbrauch der charakteristischen Spezies zu ermitteln. Durch den kombinierten Einsatz der Gittersensormesstechnik und der ultraschnellen Röntgentomographie wurden umfangreiche Daten zur Hydrodnamik und zum Stofftransport während der chemischen Absorption von CO2 erhalten. Diese stellen eine umfassende Datenbasis zur numerischen Validierung von Blasenströmungen mit Gasgehalten bis zu 17% dar.

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The viability of a drug delivery system which encapsulates chemotherapeutic drugs (Doxorubicin) in the hydrophobic core of polymeric micelles and triggers release by ultrasound application was investigated at an applied frequency of 500 kHz. The investigation also included elucidating the mechanism of drug release at 70 kHz, a frequency which had previously been shown to induce drug release. A fluorescence detection chamber was used to measure in vitro drug release from both Pluronic and stabilized micelles and a hydrophone was used to monitor bubble activity during the experiments. A threshold for release between 0.35 and 0.40 in mechanical index was found at 70 kHz and shown to correspond with the appearance of the subharmonic signal in the acoustic spectrum. Additionally, drug release was found to correlate with increase in subharmonic emission. No evidence of drug release or of the subharmonic signal was detected at 500 kHz. These findings confirmed the role of cavitation in ultrasonic drug release from micelles. A mathematical model of a bubble oscillator was solved to explore the differences in the behavior of a single 10 um bubble under 70 and 500 kHz ultrasound. The dynamics were found to be fundamentally different; the bubble follows a period-doubling route to chaos at 500 kHz and an intermittent route to chaos at 70 kHz. It was concluded that this type of "intermittent subharmonic" oscillation is associated with the apparent drug release. This research confirmed the central role of cavitation in ultrasonically-triggered drug delivery from micelles, established the importance of subharmonic bubble oscillations as an indicator, and expounded the key dynamic differences between 70 and 500 kHz ultrasonic cavitation.

A pool boiling experimental facility has been designed and built to investigate nucleate pool boiling in water under sub atmospheric pressure. Liquid crystal thermography, a non intrusive technique, is used for the determination of surface temperature distributions. This technique uses encapsulated liquid crystals that reflect definite colors at specific temperatures and viewing angle. Design of the test section is important in this experimental study. Since a new TLC is required for every new set of test conditions, a permanently sealed test section is not an option. The real challenge is to design a leak proof test section which is flexible so that it can be taken apart easily. A plexiglass test section, including a top chamber with an internal volume of 60.9 x 60.9 x 66.4 mm and a bottom plate of 5.5mm thickness is designed and assembled together using quick grips. In the test section, water is boiled using 85.0mm x 16.0mm and 0.050mm thick Fecralloy® as the heating element. The TLC sheet is attached to the bottom plate and the heating element is placed on top of TLC so that the temperature distribution of the heating element during boiling can be interpreted from TLC. A camera system fast enough to capture the thermal response of the TLC and an arrangement to capture both hue of the TLC and growth of the bubble on the same frame has been designed and successfully used. This system allowed recording of position, size and shape of the bubble with synchronized surface temperature. In order to get hue vs. temperature relation, in-situ calibration of the TLC is performed for each test condition with the present experimental setup and lighting conditions. It is found that the calibration curve of the TLC at atmospheric pressure is different from the calibration curve of the same TLC at subatmospheric pressures. The maximum temperature difference between the two curves for the same hue is found to be only 0.6°C. The experiment is run at four different test conditions of subatmospheric pressure and low heat flux. It is run at system pressures of 6.2kPa (0.89Psi) and 8.0kPa (1.16Psi) with a constant heat flux of 1.88kW/m2 and 2.70kW/m2, and a constant heat flux of 2.70kW/m2, 3.662kW/m2 and 4.50 kW/m2 respectively. Analysis of nucleating surface temperatures using thermochromic liquid crystal technique is performed for these test conditions and the bubble dynamics is studied. The temperature distribution is quite varied in each case and the temperature is at its maximum value at the center of the bubble and it decreases radially from the center. The dry spot observed during the experiments indicates that the process of evaporation of the microlayer is dominant at subatmospheric pressures. It is observed that at very low pressure and heat flux the bubble growth is accompanied by the neck formation. Boiling parameters such as bubble frequency, bubble size and contact are also analyzed and a summary of these results for four different test conditions is presented and the relevant differences between the cases are discussed and the effect of increase in pressure and heat flux is noted.

A heater designed to monitor surface temperature fluctuations during pool boiling experiments while the bubbles were simultaneously being observed has been fabricated and tested. The heat source was a transparent indium tin oxide (ITO) layer commercially deposited on a fused quartz substrate. Four copper-nickel thin film thermocouples (TFTCs) on the heater surface measured the surface temperature, while a thin layer of sapphire or fused silica provided electrical insulation between the TFTCs and the ITO. The TFTCs were micro-fabricated using the liftoff process to deposit the nickel and copper metal films. The TFTC elements were 50 microns wide and overlapped to form a 25 micron by 25 micron junction. TFTC voltages were recorded by a DAQ at a sampling rate of 50 kHz. A high-speed CCD camera recorded bubble images from below the heater at 2000 frames/second. A trigger sent to the camera by the DAQ synchronized the bubble images and the surface temperature data. As the bubbles and their contact rings grew over the TFTC junction, correlations between bubble behavior and surface temperature changes were demonstrated. On the heaters with fused silica insulation layers, 1-2 C temperature drops on the order of 1 ms occurred as the contact ring moved over the TFTC junction during bubble growth and as the contact ring moved back over the TFTC junction during bubble departure. These temperature drops during bubble growth and departure were due to microlayer evaporation and liquid rewetting the heated surface, respectively. Microlayer evaporation was not distinguished as the primary method of heat removal from the surface. Heaters with sapphire insulation layers did not display the measurable temperature drops observed with the fused silica heaters. The large thermal diffusivity of the sapphire compared to the fused silica was determined as the reason for the absence of these temperature drops. These findings were confirmed by a comparison of temperature drops in a 2-D simulation of a bubble growing over the TFTC junction on both the sapphire and fused silica heater surfaces. When the fused silica heater produced a temperature drop of 1.4 C, the sapphire heater produced a drop of only 0.04 C under the same conditions. These results verified that the lack of temperature drops present in the sapphire data was due to the thermal properties of the sapphire layer. By observing the bubble departure frequency and site density on the heater, as well as the bubble departure diameter, the contribution of nucleate boiling to the overall heat removal from the surface could be calculated. These results showed that bubble vapor generation contributed to approximately 10% at 1 W/cm^2, 23% at 1.75 W/cm^2, and 35% at 2.9 W/cm^2 of the heat removed from a fused silica heater. Bubble growth and contact ring growth were observed and measured from images obtained with the high-speed camera. Bubble data recorded on a fused silica heater at 3 W/cm^2, 4 W/cm^2, and 5 W/cm^2 showed that bubble departure diameter and lifetime were negligibly affected by the increase in heat flux. Bubble and contact ring growth rates demonstrated significant differences when compared on the fused silica and sapphire heaters at 3 W/cm^2. The bubble departure diameters were smaller, the bubble lifetimes were longer, and the bubble departure frequency was larger on the sapphire heater, while microlayer evaporation was faster on the fused silica heater. Additional considerations revealed that these differences may be due to surface conditions as well as differing thermal properties. Nucleate boiling curves were recorded on the fused silica and sapphire heaters by adjusting the heat flux input and monitoring the local surface temperature with the TFTCs. The resulting curves showed a temperature drop at the onset of nucleate boiling due to the increase in heat transfer coefficient associated with bubble nucleation. One of the TFTC locations on the sapphire heater frequently experienced a second temperature drop at a higher heat flux. When the heat flux was started from 1 W/cm^2 instead of zero or returned to zero only momentarily, the temperature overshoot did not occur. In these cases sufficient vapor remained in the cavities to initiate boiling at a lower superheat.

Detailed understanding of bubbles growing on a solid surface is a fundamental requirement in many technological domains, with particular application to water electrolysis in relation to the present-day socio-economic significance of clean energy transition. Evolution of bubbles at the electrode surface greatly determines the overall efficiency and throughput of an electrolysis cell. Bubbles residing on the electrode surface creates resistance to the flow of electric current and reduces the active electro-catalytic area. Therefore, fast removal of the bubbles is desirable for efficient operation. With this motivation, this dissertation aims to build deeper understanding of the bubble dynamics during the pre-detachment and detachment stage. To this end, single hydrogen bubbles grown on microelectrodes are chosen as the object of study. Thermocapillary and electric forces acting on an electrolytic bubble are introduced and a thorough account of the forces acting on the bubble is taken. A dynamical model of the bubble motion is developed. By means mathematical and physical modeling of the forces, working mechanism is provided for a novel mode of bubble detachment, namely oscillatory bubble detachment. The model predictions of oscillation parameters are in good correlation with experimental observations. Furthermore, the equation of motion of the bubble is shown to undergo bifurcation thus providing mathematical reasoning behind the existence of different detachment modes. A deeper look is taken specifically at the oscillatory mode. The electrolyte flow velocity is computed and compared with PTV based measurements. Force variation during one oscillation cycle is characterized and correlated with relevant geometric and operational parameters. Based on dynamical conditions of the bubble motion, the surface charge at the bubble interface is quantified. The calculated values match with literature values from bubble electrophoresis experiments. A detailed look is also taken at the effect of electrode size on the thermocapillary effect. The temperature and flow velocity field in the electrolyte is computed for various electrode size. Additional details regarding the flow structure were found. The location of the interfacial temperature hotspot was quantified. The current density distribution along the electrode surface was found to be strongly non-uniform. The Marangoni and the hydrodynamic force acting on the bubble was quantified at various electrode sizes. Further a model was developed to approximate the thermocapillary effect of a bubble on a large electrode. The location of temperature hotspot was found to be different when compared to bubbles on a microelectrode. This influences the Marangoni flow structure and also the Marangoni force on the bubble. Overall, this dissertation provides a systematic framework for characterizing forces acting on the bubble and investigating the dynamics of the bubble motion, which adds to our current understanding of bubble evolution and, takes one step towards predictive detachment models.

Stirring tanks are very important equipments used for mixing, separating, chemical reaction, etc. A typical stirring tank is a cylindrical vessel with an agitator driving the fluid and generating turbulence to promote mixing. Flotation cells are widely used stirring tanks in phase separation where multiphase flow is involved. Flotation refers to the process in which air bubbles selectively pick up hydrophobic particles and separate them from hydrophilic solids. This technology is used throughout the mining industry as well as the chemical and petroleum industries. In this research, efforts were made to investigate the multi-phase flow hydrodynamic problems of some flotation cells at different geometrical scales. Pitot-static and five-hope probes were employed to lab- pilot- and commercial-scale tanks for velocity measurements. It was found that the tanks with different scales have similar flow patterns over a range of Reynolds numbers. Based on the velocity measurement results, flotation tanks' performance was evaluated by checking the active volume in the bulk. A fast-response five-hole probe was designed and fabricated to study the turbulence characteristics in flotation cells under single- and multi-phase flow conditions. The jet stream in the rotor-stator domain has much higher turbulence intensity compared with other locations. The turbulent dissipation rate (TDR) in the rotor-stator domain is around 20 times higher than that near tank's wall. The TDR could be used to calculate the bubble and particle slip velocities. An isokinetic sampling probe system was developed to obtain true samples inthe multi-phase flow and then measure the local void fraction. It was found that the air bubbles are carried out by the stream and dispersed to the whole bulk. However, some of the bubbles accumulate in the inactive regions, where higher void fractions were detected. The isokinetic sampling probe was then extended to be an isokinetic borescope system, which was used to detect the bubble-particle aggregates in the tank. Aggregates were found in the high-turbulence level zones. The isokinetic sampling probe and the isokinetic borescope provide new methods for flotation tank tests. An experiment was also set up to study the dynamics of bubble particle impact. Four different modes were found for the collision. The criterion is that if the fluid drainage time is less than the residence time, the attachment will occur, otherwise, the particle will bounce back. / Ph. D.

We present two distinct projects in this article. In the first project, an experiment aiming to quantify the impacts of material acoustic impedance and thickness on single laser-induced cavitation bubble dynamics with measurements of exerted pressure on a specific material in order to identify the primary sources most responsible for material damages is presented in this article. Two types of major pressure sources have been identified. For bubble collapsing near a rigid wall, when standoff ratio γ < 0.6, the ring collapse is the most prominent pressure source. The jet takes the strongest effects at γ = 1.12. The pressure is minimal at γ = 0.913. After the first jet impingement, a second ring collapse will follow and input the maximum pressure to the wall. By further increasing γ, a similar pressure profile of the second collapse to the first collapse is achieved, during which the pressure for the second collapse is minimal at γ = 1.41 and for the jet is maximum at γ = 1.79. Compared with the maximum pressure dealt by the first jet, the second ring collapse and jet are increasing much faster with the bubble size and eventually overwhelm the first jet. However, the first ring collapse is still the most dominant pressure source responsible for material damages. For wall featuring smaller acoustic impedance or thickness that cannot be approximated to a rigid body, the ring collapse and jet occur at smaller standoff ratios. The cavity shrinking rate suggests the maximum pressure exerted on the wall at applicable standoff ratios should be smaller than that on a rigid wall. In the second project, a comprehensive collection of dynamics of one and two laser-induced cavitation bubbles collapsing near different boundaries is presented in this article by measuring the velocity fields using particle image velocimetry (PIV) techniques. Cases include a single bubble collapsing near the hard, medium, and soft walls characterized by acoustic impedance, free collapse of two bubbles, and two bubbles collapsing near the hard and soft walls. We implemented the most significant velocity and top velocity regions derived from each velocity field to analyze the features of these cases. Before converging to free collapse, the bubble near the hard wall experienced a significant velocity decrease before collapse, the bubble near the medium wall was severely damped at a specific standoff distance, and the bubble near the soft wall collapsed much earlier and preserved a linear velocity region at low speed. Free collapse of two same bubbles underwent a decrease of acceleration before collapse. Decreasing the size of one bubble caused a jet in the other. With the presence of a hard wall near two bubbles, the bubble closer to it may be stretched to a cavity with a high aspect ratio, leading to very mild collapse. With a bigger bubble between a smaller one and the soft wall, the merging cavity may suppress the tendency of jet formation, making the velocity stay at low levels throughout the lifetime. For configurations regarding single bubbles collapsing near a wall and free collapse of two same bubbles, we performed data scaling to study the velocity variations for different bubble sizes by controlling the standoff ratios and assessed the data quality aided by curving fitting and statistics. Results indicated measured velocity regarding a single bubble collapsing near the wall over its diameter remained the same given a standoff ratio, while measured velocity did not change given a standoff ratio for free collapse of two same bubbles within the scope of the experiment. In addition, we detailed the experimental setup and water treatment for better signal-to-noise ratios as well as validated the system from both the PIV and high speed imaging approaches using free collapse of a single bubble to ensure the reliability of this experiment. / Doctor of Philosophy / The phenomenon of cavitation extensively exists. These small and transient bubbles are observed typically in fast moving fluids, e.g., shaking a bottle of water. Each bubble experi- ences a process of growth, collapse, rebound, and collapse again before it is gone. Although the bubble is tiny, the collapse of a bubble releases considerable pressure, which is intense enough to damage nearby objects over time. This interaction between bubbles and objects depends highly on the types of objects such as the materials and thickness. To study how the bubble behaves near a wall (object) and explain how the wall is damaged, we present two projects in this article. In the first project, we created a bubble near a wall at differ- ent bubble-to-wall distances and tracked how the bubble changed its shape until collapse with a fast speed camera. This work was repeated for multiple different wall materials and thickness. We then measured the pressure exerted by a bubble at a series of different bubble-to-wall distances on a specific wall equipped with a sensor. By comparing and sum- marizing results from both the bubble shape changes near different walls and the pressure measurement, we found the relationship between the magnitude of pressure and the distance between the bubble and the wall. In the second project, we implemented the particle image velocimetry (PIV) techniques to measure the velocity fields. By feeding particles into the fluid, PIV tracks the location differences of particles in two subsequent frames to determine the velocity of every point. Based on that, we obtained a collection of velocity fields of interaction between single bubbles and walls, two bubbles, and two bubbles and walls.

Faraday rotation is a process by which the position angle (PA) of background linearly polarized light is rotated when passing through an ionized and magnetized medium. The effect is sensitive to the line-of-sight magnetic field in conjunction with the electron density. This contribution highlights diagnostic possibilities of inferring the magnetic field (or absence thereof) in and around wind-blown bubbles from the Faraday effect. Three cases are described as illustrations: a stellar toroidal magnetic field, a shocked interstellar magnetic field, and an interstellar magnetic field within an ionized bubble.