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<p>In an effort to better understand the physical mechanisms responsible for pool boiling heat transfer, an analytical model is developed that better describes the changing shape and size of a growing bubble. Indeed, any analysis of thermal transport due to nucleate pool boiling requires bubble frequency predictions which are intimately linked to bubble volume. The model is developed and validated for quasi-static bubble growth due to gas injection and for bubble growth due to vaporization within the heat-transfer controlled growth regime; it highlights the need to include the asymmetric nature of growing bubbles when modeling bubble growth.</p> <p>In addition, a numerical study of quasi-static bubble shape for both adiabatic bubble growth and vapour bubble growth provides insight into the dependence the bubble shape evolution has on the Bond number. In so doing, bubble profiles generated from a numerical treatment of the Capillary equation are benchmarked to quasi-static gas injected bubble formations and to heat-transfer controlled vapour bubble formations.</p> <p>The numerical treatment of bubble shape evolution leads to a simplifying bubble geometry for low Bond number applications. The geometric model accounts for bubble shape transformation throughout the bubble growth cycle including the necking phenomenon. An analytical model of quasi-static adiabatic bubble growth is accordingly developed based on the proposed low Bond number geometric model; it is coupled with a geometric detachment relation and a force balance detachment criterion that are dependent on the Bond number. The resulting predicted bubble growth characteristics, such as profile, volume, centre of gravity and aspect ratio, are validated with the benchmarked numerical treatment of the problem.</p> <p>Furthermore, the low Bond number geometric model is applied to bubble growth due to vaporization. In order to solve the mass-energy balance at the vapour bubble interface, a spherical surface area is commonly assumed. This leads to the need for correction factors and provides little insight into the physical mechanism responsible for bubble shape. In this study, the transitioning shape of a vapour bubble is considered in the integral analysis of the interfacial mass-energy balance. The model predicts the following bubble growth characteristics: profile, volume, centre of gravity, and aspect ratio.</p> / Doctor of Philosophy (PhD)

Computational fluid dynamics (CFD) modeling was used to predict the hydrodynamics of a column reactor. Bubble columns have applications across many engineering disciplines and improved modeling techniques help to increase the accuracy of numerical predictions. An Eulerian-Eulerian multi-fluid model was used to simulate fluidization and to capture the complex physics associated therewith. The commercial code ANSYS Fluent was used to study two-dimensional gas-liquid bubble columns. A comprehensive parameter study, including a detailed investigation of grid resolution was performed. Specific attention was paid to the bubble diameter, as it was shown to be related to cell size have significant effects on the characteristics of the flow. The parameters used to compare the two-dimensional (2D) cases to experimental results of Rampure, et. al. were then applied to a three-dimensional (3D) geometry. It was demonstrated that the increase in accuracy from 2D to 3D is negligible compared to the increase in CPU required to simulate the entire 3D domain. Additionally, the reaction chamber of a microbial fuel cell (MFC) was modeled and a preliminary parameter study investigating inlet velocity, temperature, and acetate concentration was conducted. MFCs are used in wastewater treatment and have the potential to treat water while simultaneously harvesting electricity. The spiral spacer and chemical reactions were modeled in a 3D geometry, and it was determined that inlet velocity was the most influential parameter that was simulated. There were also significant differences between the 2D and 3D geometries used to predict the MFC hydrodynamics. / Master of Science

An accurate measurement of bubbly flow has a significant value for understanding the bubble behavior, heat and energy transfer pattern in different engineering systems. It also helps to advance the theoretical model development in two-phase flow study. Due to the interaction between the gas and liquid phase, the flow patterns are complicated in recorded image data. The segmentation and reconstruction of overlapping bubbles in these images is a challenging task. This dissertation provides a complete set of image processing algorithms for bubbly flow measurement. The developed algorithm can deal with bubble overlapping issues and reconstruct bubble outline in 2D high speed images under a wide void fraction range. Key bubbly flow parameters such as void fraction, interfacial area concentration, bubble number density and velocity can be computed automatically after bubble segmentation. The time-averaged bubbly flow distributions are generated based on the extracted parameters for flow characteristic study. A 3D imaging system is developed for 3D bubble reconstruction. The proposed 3D reconstruction algorithm can restore the bubble shape in a time sequence for accurate flow visualization with minimum assumptions. The 3D reconstruction algorithm shows an error of less than 2% in volume measurement compared to the syringe reading. Finally, a new image synthesis framework called Bubble Generative Adversarial Networks (BubGAN) is proposed by combining the conventional image processing algorithm and deep learning technique. This framework aims to provide a generic benchmark tool for assessing the performance of the existed image processing algorithms with significant quality improvement in synthetic bubbly flow image generation. / Ph. D. / Bubbly flow phenomenon exists in a wide variety of systems, for example, nuclear reactor, heat exchanger, chemical bubble column and biological system. The accurate measurement of the bubble distribution can be helpful to understand the behaviors of these systems. Due to the complexity of the bubbly flow images, it is not practical to manually process and label these data for analysis. This dissertation developed a complete suite of image processing algorithms to process bubbly flow images. The proposed algorithms have the capability of segmenting 2D dense bubble images and reconstructing 3D bubble shape in coordinate with multiple camera systems. The bubbly flow patterns and characteristics are analyzed in this dissertation. Finally, a generic image processing benchmark tool called Bubble Generative Adversarial Networks (BubGAN) is proposed by combining the conventional image processing and deep learning techniques together. The BubGAN framework aims to bridge the gap between real bubbly images and synthetic images used for algorithm benchmark and algorithm.

In this thesis, research by Crum and Mao [J. Acoust. Soc. Am. 99, 2898--2907 (1996)] and Houser, Howard, and Ridgway [J. Theor. Biol. 213, 183--195 (2001)] is extended by numerically investigating bubble growth (initial radius of 10 microns) during rectified diffusion for gas supersaturations up to 300% by using the Fyrillas-Szeri equation [J. Fluid Mech. 277, 381--407 (1994)]. Bubble growth is simulated for a range of frequencies (100 Hz to 10 kHz), sound pressure levels (205 dB to 215 dB re 1 micropascal), and gas supersaturations (150% to 300%). Simulations are presented for continuous and monofrequency excitation, repeated tone bursts, and pulsed frequency-modulated waveforms. The potential for bubble growth to occur in marine mammals is also considered. For the parameters considered, static diffusion becomes the dominant growth mechanism as supersaturation is increased, bubble growth is frequency independent away from bubble resonance, and bubble growth due to duty-cycle excitation can be modeled as an effective continuous source with a reduced sound pressure level. A more accurate model of in vivo marine mammal tissue is required to determine if rectified diffusion can trigger bubble growth at the levels predicted in this thesis. / text

<p>The growth and removal of inclusions in stirred ladles hasbeen studied. First, the importance of different growthmechanisms suggested in the literature were studied. Simulationresults from a fundamental model of an induction-stirred ladlehave been used as input in the calculations. Based on thegrowth calculations it was concluded that four of the growthmechanisms need not to be considered since they contribute solittle: i) diffusion of oxygen and aluminum to the inclusionsurface, ii) diffusion coalescence, iii) Brown motioncollision, and iv) laminar shear collision. The majorcontributor to inclusion growth is turbulent collision. Growthdue to Stoke's collisions is also somewhat important if largedifferences among inclusion sizes exist.</p><p>Growth of inclusions in gas stirred ladles was studied usinga similar approach as the one for induction stirred ladles, butwith use of simulation results from a fundamental mathematicalmodel of a gas-stirred ladle. Similarly to what was found inthe case of induction stirring, it was found that turbulentcollisions and Stokes collisions appeared to be the majormechanisms for inclusion growth. The contribution of laminarshear collisions to growth was deemed negligible compared tothat of turbulent collisions.</p><p>For the gas stirred ladle different removal mechanisms werealso studied, based on input data from a mathematical model ofa gas-stirred ladle. It was found that different modelssuggested to predict the inclusion removal due to bubbleflotation gave very different results. Also, all models assumeda spherical shape of the gas bubbles, which was found to beless realistic. Therefore, a new model for inclusion removal byspherical cap bubble flotation was developed. In the newcalculations, the most important mechanisms of inclusionremoval were found to be removal to the top slag and removal bybubble flotation, assuming spherical-cap bubbles and planecontact. When the bubbles were assumed to be spherical,resulting removal rates were lower than when they were assumedto be spherical caps. Based on these results it is concludedthat more research is needed to obtain a better understandingof the importance of bubble flotation on inclusion removal.Experiments are clearly needed to determine which modelconcepts produce predictions in best agreement withcorresponding data from actual steelmaking processes.</p>

This research addresses a number of fundamental issues concerning convective mass transfer across fluid-fluid interfaces in porous media. Mass transfer to/from distributed sinks/sources is considered for i) the slow dissolution of liquid filaments of a wetting non-aqueous phase liquid (NAPL) held in the corners of angular pores or throats and ii) the fate of gas bubbles generated during the flow of a supersaturated aqueous phase in porous media. 1. Effects of the stability of NAPL films on wetting NAPL dissolution Wettability profoundly affects the distribution of residual NAPL contaminants in natural soils. Under conditions of preferential NAPL wettability, NAPL is retained within small pores and in the form of thick films (liquid filaments) along the corners and crevices of the pore walls. NAPL films in pore corners provide capillary continuity between NAPL-filled pores, dramatically influencing the behaviour of NAPL dissolution to the flowing aqueous phase by convection and diffusion. A pore network model is developed to explore the dissolution behaviour of wetting NAPL in porous media. The effects of initial NAPL distribution and NAPL film stability on dissolution behaviour are studied using the simulator. NAPL phase loses continuity and splits into disconnected clusters of NAPL-filled pores due to rupture of NAPL films. Quasi-state drainage and fingering of the aqueous phase into NAPL-filled pores is treated as an invasion percolation process and a stepwise procedure is adopted for the solution of flow and solute concentration fields. NAPL film stability is shown to critically affect the rate of mass transfer as such that stable NAPL films provide for more rapid dissolution. The network simulator reproduces the essential physics of wetting NAPL dissolution in porous media and explains the concentration-tailing behaviour observed in experiments, suggesting also new possibilities for experimental investigation. 2. Convective Mass Transfer across Fluid Interfaces in Straight Angular Pores Steady convective mass transfer to or from fluid interfaces in pores of angular cross-section is theoretically investigated. The model incorporates the essential physics of capillarity and solute mass transfer by convection and diffusion in corner fluid filaments. The geometry of the corner filaments, characterized by the fluid-fluid contact angle, the corner half-angle and the interface meniscus curvature, is accounted for. Boundary conditions of zero surface shear (???perfect-slip???) and infinite surface shear (???no-slip???) at the fluid-fluid interface are considered. The governing equations for laminar flow within the corner filament and convective diffusion to or from the fluid-fluid interface are solved using finite-element methods. Flow computations are verified by comparing the dimensionless resistance factor and hydraulic conductance of corner filaments against recent numerical solutions by Patzek and Kristensen [2001]. Novel results are obtained for the average effluent concentration as a function of flow geometry and pore-scale Peclet number. These results are correlated to a characteristic corner length and local pore-scale Peclet number using empirical equations appropriate for implementation in pore network models. Finally, a previously published ???2D-slit??? approximation to the problem at hand is checked and found to be in considerable error. 3. Bubble evolution driven by solute diffusion during the process of supersaturated carbonated water flooding In situ bubble growth in porous media is simulated using a pore network model that idealizes the pore space as a lattice of cubic chambers connected by square tubes. Evolution of the gas phase from nucleation sites is driven by the solute mass transfer from the flowing supersaturated water solution to the bubble clusters. Effects of viscous aqueous phase flow and convective diffusion in pore corners are explicitly accounted for. Growth of bubble clusters is characterised by a pattern of quasi-static drainage and fingering in the gas phase, an invasion percolation process controlled by capillary and gravitational forces. A stepwise solution procedure is followed to determine the aqueous flow field and the solute concentration field in the model by solving the conservation equations. Mobilization of bubbles driven by buoyancy forces is also studied. Results of bubble growth pattern, relative permeability and macroscopic mass transfer coefficient are obtained under different gas saturations and aqueous flow conditions.

Bubble and cavitation effects phenomena can be encountered in two-phase gas-liquid systems in industry. In certain industries, particularly high-risk systems such as a nuclear reactor/plant, the detection of bubble dynamics, and the monitoring and measurement of their characteristics are necessary in controlling temperature. While in the petro-chemical engineering industry, such as oil transportation pipelines, the detection and monitoring of bubbles/cavitation phenomena are necessary to minimise surface erosion in fluid carrying components or downstream facilities. The high sensitivity of Acoustic Emission (AE) technology is feasible for the detection and monitoring of bubble phenomena in a two phase gas-liquid system and is practical for application within the industry. Underwater measurement of bubble oscillations has been widely studied using hydrophones and employing acoustic techniques in the audible range. However, the application of Acoustic Emission (AE) technology to monitor bubble size has hitherto not been attempted. This thesis presents an experimental investigation aimed at exploring AEs from gas bubble formation, motion and destruction. AE in this particular investigation covers the frequency range of between 100 kHz to 1000 kHz. The AE waveform analysis showed that the AE parameter from single bubble inception and burst events, i.e. AE amplitude, AE duration and AE energy, increased with the increase of bubble size and liquid viscosity. This finding significantly extends the potential use of AE technology for detecting the presence of bubbles in two-phase flow. It is concluded that bubble activity can be detected and monitored by AE technology both intrusively and non-intrusively. Furthermore, the bubble size can be determined by measurement of the AE and this forms the significant contribution of this thesis.

Today nucleate boiling is widely used in numerous industrial applications such as cooling processes because of the high achieved heat transfer rates for low temperature differences. It remains a possible cooling solution for the next generation of central processing units (CPU), which dissipate heat fluxes exceeding the capabilities of today’s conventional forced air cooling. However, nucleate boiling is a very complex and elusive process involving many mechanisms which are not fully understood yet and a comprehensive model is still missing. For this study a new experimental setup was designed, constructed and commissioned to investigate bubble nucleation, growth, departure and interaction during nucleate pool boiling from a silicon device fully immersed in fluorinert FC-72. The location of bubble nucleation is controlled by artificial cavities etched into the silicon substrate. Boiling is initiated with a heater integrated on the back and micro-sensors indicate the wall temperature at the bubble nucleation site. During this work three different silicon test section designs were fabricated and boiling experiments on these substrates successfully conducted. Bubble growth, bubble departure frequencies and bubble departure diameters for different dimensioned artificial cavities, varied pressure and increasing wall temperature were measured from high-speed imaging sequences. Bubble interactions like vertical and horizontal coalescence were visualised and their impact on the boiling heat transfer investigated. The influence of spacing between two neighbouring artificial cavities on bubble nucleation and departure frequencies, vertical coalescence frequencies and departure diameters was analysed. The acquired data are used as input for a numerical code developed by our collaborators (Brunel University, UK and Los Alamos National Laboratories, USA) and are a first step to validate the code. The code studies the interactions between bubble nucleation sites on solid surfaces as a network. The simulations will help design boiling substrates utilised for chip cooling applications with optimal artificial cavity distribution to maximise the cooling heat transfer.

It is estimated in the last five years Chinese investors have poured over $93 billion into the United States residential real estate market targeting high-end housing sectors. I analyze the implications of the investment and how it has affected the Bay Area housing prices. In order to find out why large outflows are targeting the United States I compare China’s economy with Japan’s economy in the late 1980’s when Japanese investors invested over $300 billion into high profile real estate properties. I find many similarities, suggesting China has a bubble economy such as Japan before the lost decade. To combat their bubble, China has implemented new restrictions on capital outflows in order to stabilize their volatile markets. In terms of the Bay Area real estate market I gathered evidence a recession is imminent due to the demand falling for high-end housing. The housing market mirrors economic health and indicates whether an economy is in a boom or bust.