No

The hydrodynamic phenomena occurring inside the enclosed downcomer section of a plunging jet bubble column are described in the study. The gas entrainment rate for a plunging liquid jet was found to consist of two components, namely the gas trapped within the effective jet diameter at the point of impact, and the gas contained within the film between the jet and induction trumpet surface at the point of rupture. Entrainment within the effective jet diameter has been examined by McCarthy (1972). In this study, a model was supported by the experimental results, provided the film attained a region of constant thickness. When the induction trumpet was ruptured prior to a constant film thickness being reached, the measured rate of filmwise entrainment was higher than the prediction. Filmwise entrainment was found to be initiated once a critical velocity along the surface of the induction trumpet was reached. The critical velocity was a function only of the liquid physical properties and was independent of the jet conditions and downcomer diameter. The velocity of the free surface of the induction trumpet was obtained from the velocity profile for the recirculating eddy generated by the confined plunging liquid jet. The jet angle used to describe the expansion of the submerged jet inside the downcomer was predicted from the radial diffusion of jet momentum into the recirculation eddy. The model was able to predict the jet angle when it was assumed that the radial diffusion of jet momentum was a function of the Euler number based on the jet velocity and absolute pressure in the headspace at the top of the downcomer. The model was also developed to predict the maximum stable bubble diameter generated within the submerged jet volume, where the energy dissipation attributed to bubble breakup was given by the energy mixing loss derived for the throat section of a liquid-jet-gas-pump. Good agreement was found between the measured and predicted maximum bubble diameter values. The average experimental Sauter mean/maximum diameter ratio was found to be 0.61, which was similar to that for other bubble generation devices. It was found that for turbulent liquid conditions in the uniform two-phase flow region, a transition from bubble to churn-turbulent flow occurred at a gas void fraction of approximately 0.2 when the gas drift-flux was zero. Under laminar liquid flow, this transition took place at a gas void fraction above 0.3. For the bubbly flow regime the Distribution parameter Co used by Zuber and Findlay (1965) to describe the velocity and gas void fraction profile, was found to be a function of the liquid Reynolds number. For laminar liquid flow, values of Co greater than unity were obtained. As the liquid Reynolds number was increased it was found that Co decreased, until a constant value of unity was obtained for fully turbulent flow. For the churn-turbulent regime it was found that the gas void fraction measurements for all of the experimental runs could be collapsed onto a single curve when a modified gas void fraction was plotted against the gas-to-liquid volumetric flow ratio. The modified gas void fraction included a correction factor to account for the difference in the bubble slip velocity between the experimental runs. The experimental results also indicated that the value of the constant in the gas void fraction correction factor was different for laminar and turbulent flow. Prior to bubble coalescence, it was found that the experimental drift-flux curves could be predicted from the measured bubble diameter, using the separated flow model development by Ishii and Zuber (1979). After the onset of coalescence the drift flux measurements departed from the original drift-flux curves at a rate which increased linearly with increasing gas void fraction. It was found that the slope of the line fitted to the coalesced region of the drift-flux curves increased with increasing liquid Reynolds number and reached a constant value under fully turbulent flow conditions. The model developed, together with the implications of the experimental results, are discussed with regard to optimising the design of an industrial plunging jet bubble column. / PhD Doctorate

Understanding the mechanism of bubble-particle interactions plays a critical role in advancing flotation technology. In this study, submicron size bubbles with an average diameter less than 1 μm and a life time of at least several hours were generated using a novel hydrodynamic cavitation method. Effect of mechanical force and water chemistry on generation and stability of submicron size bubbles is investigated. With recent development in measuring zeta potential distributions of colloidal systems, interactions of bubbles and fine solid particles in various electrolyte, surfactant and frother solutions as well as in industrial process water were studied using the stable submicron size bubbles generated by hydrodynamic cavitation. The outcome of this study provides not only a better understanding of bubble-particle attachment mechanism and its role in flotation, but also a direct evidence of armour-coating of bubbles and enhanced bubble-particle interactions by in situ gas nucleation. / Chemical Engineering

The effects of a carbon nanotube (CNT) coating on bubble departure diameter and frequency in pool boiling experiments was investigated and compared to those on a bare silicon wafer. The pool boiling experiments were performed at liquid subcooling of 10 degrees Celsius and 20 degrees Celsius using PF-5060 as the test fluid and at atmospheric pressure. High-speed digital image acquisition techniques were used to perform hydrodynamic measurements. Boiling curves obtained from the experiments showed that the CNT coating enhanced critical heat flux (CHF) by 63% at 10 degrees Celsius subcooling. The CHF condition was not measured for the CNT sample at 20 degrees Celsius subcooling. Boiling incipience superheat for the CNT-coated surface is shown to be much lower than predicted by Hsu's hypothesis. It is proposed that bubble nucleation occurs within irregularities at the surface of the CNT coating. The irregularities could provide larger cavities than are available between individual nanotubes of the CNT coating. Measurements from high-speed imaging showed that the average bubble departing from the CNT coating in the nucleate boiling regime (excluding the much larger bubbles observed near CHF) was about 75% smaller (0.26 mm versus 1.01 mm)and had a departure frequency that was about 70% higher (50.46 Hz versus 30.10 Hz). The reduction in departure diameter is explained as a change in the configuration of the contact line, although further study is required. The increase in frequency is a consequence of the smaller bubbles, which require less time to grow. It is suggested that nucleation site density for the CNT coating must drastically increase to compensate for the smaller departure diameters if the rate of vapor creation is similar to or greater than that of a bare silicon surface.

To simulate dispersed two-phase flows CFD tools for predicting the local particle number density and the size distribution are required. These quantities do not only have a significant effect on rates of mixing, heterogeneous chemical reaction rates or interfacial heat and mass transfers, but also a direct relevance to the hydrodynamics of the total system, such as the flow pattern and flow regime. The Multiple Size Group (MUSIG) model available in the commercial codes CFX-4 and CFX-5 was developed for this purpose. Mathematically, this model is based on the population balance method and the two-fluid modeling approach. The dispersed phase is divided into N size classes. In order to reduce the computational cost, all size groups are assumed to share the same velocity field. This model allows to use a sufficient number of particle size groups required for the coalescence and breakup calculation. Nevertheless, the assumption also restricts its applicability to homogeneous dispersed flows. We refer to the CFX MUSIG model mentioned above as the homogeneous model, which fails to predict the correct phase distribution when heterogeneous particle motion becomes important. In many flows the non-drag forces play an essential role with respect to the bubble motion. Especially, the lift force acting on large deformed bubbles, which is dominated by the asymmetrical wake, has a direction opposite to the shear induced lift force on a small bubble. This bubble separation cannot be predicted by the homogeneous MUSIG model. In order to overcome this shortcoming we developed an efficient inhomogeneous MUSIG model in cooperation with ANSYS CFX. A novel multiple velocity multiple size group model, which incorporates the population balance equation into the multi-fluid modeling framework, was proposed. The validation of this new model is discussed in this report.

No description available.

Uranium-zirconium alloy nuclear fuels have many advantages as compared with ceramic fuels, especially for fast reactor systems. However, metallic fuels aren’t currently used in commercial power production due in part to issues with fuel swelling during irradiation. A major contributor to this expansion issue, the formation of fission gasses into bubbles, is examined here. Methods to evaluate evolution of fission gas bubbles within a U-Zr alloy are discussed and refined. Specifically, transmission electron microscopy (TEM) for viewing bubbles within the interior of the alloy is investigated. One constraint on the use of the TEM is the lack of literature pertaining to what thinning techniques are successful for U-10Zr alloy. Both initial and final thinning techniques were investigated. After thinning was complete, the specimens were viewed via TEM to determine their suitability. In addition, samples of U-Zr alloy were irradiated with gas atoms in an accelerator to simulate bubble initiation and formation and viewed via TEM. Only preliminary investigations were completed. Evaluated electrolyte solutions included one part phosphoric acid to two parts sulfuric acid and two parts water (A), one part phosphoric acid to one part ethanol and one part glycerol (B), one part hydrochloric acid to one part water (C), two parts methanol to 13 parts phosphoric acid (D), and one part perchloric acid to nine parts acetic acid (G). Positive responses were received from the solutions A, C, and G which generated electron transparent areas with few to no process induced artifacts. TEM trials with each electrolyte were performed on un-irradiated U-10Zr alloy. These trials indicated that the polishing methods would work but are not yet optimal. In addition, U-10Zr alloy was irradiated and viewed in the TEM where those polished with electrolyte A consistently included the presence of large circular features that could indicate bubbles or voids. The foundation was laid for further study to be done on this topic through the use of electropolishing solutions as final thinning techniques. Additional work that is recommended includes: electropolishing parameter refinement; implantation of heavier gas atoms or at elevated temperatures; annealing of the implanted alloy; and computer modeling of resulting first principles phenomena.

To simulate dispersed two-phase flows CFD tools for predicting the local particle number density and the size distribution are required. These quantities do not only have a significant effect on rates of mixing, heterogeneous chemical reaction rates or interfacial heat and mass transfers, but also a direct relevance to the hydrodynamics of the total system, such as the flow pattern and flow regime. The Multiple Size Group (MUSIG) model available in the commercial codes CFX-4 and CFX-5 was developed for this purpose. Mathematically, this model is based on the population balance method and the two-fluid modeling approach. The dispersed phase is divided into N size classes. In order to reduce the computational cost, all size groups are assumed to share the same velocity field. This model allows to use a sufficient number of particle size groups required for the coalescence and breakup calculation. Nevertheless, the assumption also restricts its applicability to homogeneous dispersed flows. We refer to the CFX MUSIG model mentioned above as the homogeneous model, which fails to predict the correct phase distribution when heterogeneous particle motion becomes important. In many flows the non-drag forces play an essential role with respect to the bubble motion. Especially, the lift force acting on large deformed bubbles, which is dominated by the asymmetrical wake, has a direction opposite to the shear induced lift force on a small bubble. This bubble separation cannot be predicted by the homogeneous MUSIG model. In order to overcome this shortcoming we developed an efficient inhomogeneous MUSIG model in cooperation with ANSYS CFX. A novel multiple velocity multiple size group model, which incorporates the population balance equation into the multi-fluid modeling framework, was proposed. The validation of this new model is discussed in this report.

Ordet bostadsbubbla spreds som en löpeld över världen efter den stora sub-primekrisen i USA 2008 som påverkade hela världens finansiella system. Sedan dess är oron för nya bubblor konstant närvarande. Denna uppsats kommer undersöka de argument som framförs för att en bostadsbubbla existerar i Sverige. Jag studerar den aktuella debatten och granskar argumenten som framförs. Existerar bostadsbubblan eller inte? Uppsatsen tar också reda på vilka riskfaktorer som skulle kunna orsaka ett prisfall på bostäder, oavsett om det råder en bostadsbubbla eller inte. En jämförelse mellan boendekostnaderna för hyresrätt kontra bostadsrätt görs med spännande resultat. Dessutom tillfrågar jag drygt 100 fastighetsmäklare om deras tro på den svenska bostadsmarknaden. Mäklarna tillfrågas om vilka faktorer de tror kan orsaka ett prisfall på den svenska bostadsmarknaden och hur stor sannolikhet det är att dessa faktorer faktiskt inträffar. Resultatet visar att ökningen av bostadspriser i Sverige har stöd av fundamentala faktorer och prisökningen kan i stor utsträckning förklaras av låga räntor och ökade disponibla inkomster. Det största hotet mot prisutvecklingen på bostadsmarknaden är en ihållande lågkonjunktur med minskade disponibla inkomster vid redan låga räntenivåer. / The word housing bubble spread like wildfire across the world after the great sub-prime crisis in the United States in 2008 that affected the financial systems of the entire world. Since then, the fear of new bubbles is constantly present. This paper will examine the arguments put forth wheter or not a housing bubble exists in Sweden. I study the current debate to see if the arguments are valid. Does the bubble exist or doesn’t it? The essay also aims to find the risk factors that could cause a fall in house prices, regardless of the fact that there is a housing bubble or not. In addition, I ask more then 100 real estate agents about their faith in the Swedish housing market. The brokers are asked which factors they think may cause a drop in the Swedish housing market and how likely it is that these factors actually occurs. The result shows that the increase in housing prices in Sweden are supported by fundamental factors and the price increase can largely be explained by low interest rates and increased disposable income. The biggest threat to prices in the housing is a persistent recession with reduced disposable income at already low interest rates.

This study investigates the factors that affect the lifetime and popping location of surface bubbles. The experiment was conducted using three different liquids (water, Sodium Dodecyl Sulfate, and Decane) with varying bubble sizes, using three different needle sizes. Each setup was tested 50 times. For pure water bubbles, the foot of the bubble is the most critical location because it typically has the highest temperature gradient, which creates a localized Marangoni flow that thins the film and eventually leads to the bubble bursting at the foot. When SDS was added to water, the bubble lifetime increased significantly. This is because the Marangoni stresses were reduced, and the bubble film thinned mainly due to curvature-driven drainage flow. The lifetime of the SDS bubble had a positive correlation with increasing bubble size. For Decane bubbles, the volatility of the liquid plays a significant role in the lifetime and popping location of the bubble. When the Decane was heated to 40°C, the lifetime of the bubbles increased significantly from 0-20 seconds to 8-12 minutes. This is because the high volatility of the Decane caused rapid evaporation of the bubble cap at the interface, which cooled the surface of the liquid. This temperature difference creates a difference in surface tension, which causes the liquid to flow from the bulk liquid into the apex of the bubble, thickening the cap film until it cools down. Then, it pops from the top due to the curvature-driven drainage.