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141 
Experimental Investigation of Bubble Lateral Motion in Shear FlowKe Tang (5930894) 03 January 2019 (has links)
In twophase flow, the void fraction and its distribution are two major factors describing the characteristic of flow patterns. Better understanding of void fraction distribution in twophase flow would help improve safety and efficiency in the nuclear industry as the heat transfer process is significantly affected by the void distribution in nuclear reactor fuel bundles. Lift force is proposed to explain the lateral migration of bubbles in the shear flow (Feng & Bolotnov, 2017, Lucas & Tomiyama, 2011, Akio Tomiyama, Tamai, Zun, & Hosokawa, 2002). However, the mechanism of lift force is unclear and the research on lift force is limited.<div><br></div><div>An experimental investigation is performed on the lift force of single bubble in weak linear shear flow field in water. In addition, characteristics of bubble motion including bubble terminal velocity, aspect ratio and oscillation amplitude are studied and comparisons are made with existing models.<br></div><div><br></div><div>It was found that the model proposed by Tomiyama et al. (A. Tomiyama, Celata, Hosokawa, & Yoshida, 2002) has the best prediction of bubble terminal velocity with introduction of a tuning factor in consideration of the asymmetric deformation of bubble. Bubble aspect ratio is found to significantly affect its terminal velocity, and a new model is proposed to best fit the experiment data. It is also observed that the shear rate magnitude has no influence on bubble aspect ratio in this study. Oscillation was observed for all the bubbles in this experiment. Oscillation amplitude scattered widely and it was difficult to correlate it only with the bubble equivalent diameter. In terms of lift force, positive lift coefficient was observed for small size bubbles and transits to negative value with growing size. Due to the high Reynolds number of flow and low viscosity of water, widely scattered data is found in the results. Although the accurate prediction of lift coefficient is difficult to obtain in the experiment, the lift coefficient transition trend is given and agrees with many other research. In addition, this research provides a database for further lift coefficient investigation.<br></div>

142 
Semiinfinite and finite bubble propagation in the presence of a channeldepth perturbationFranco Gomez, Andres January 2018 (has links)
The twophase flow displacement of a viscous fluid by a less viscous one in a confined environment leads to a viscous fingering instability commonly encountered in natural systems, for example, in flows through porous media or pulmonary airways. The classical study of viscous fingering has been conducted in rectangular channels of high aspect ratio (large channel width/height), known as HeleShaw channels where a unique, steady symmetric, semiinfinite bubble (finger) emerges. In this Journal Format thesis, the propagation of semiinfinite (open) and finite (closed) air bubbles is considered in HeleShaw channels where thin, axiallyuniform occlusions are introduced. This configuration is known to generate symmetric, asymmetric and oscillatory modes with complex interactions and rich behaviour. Numerical results of finger propagation using a depthaveraged model in these constricted channels are found to be in quantitative agreement with experimental results once the aspect ratio reaches a value of $\alpha\geq40$ and capillary numbers below $Ca\leq 0.012$. The same evolution of the bifurcation scenario between multiple modes is found, however, it occurs for decreasing values of occlusion height as the value of aspect ratio is increased that the system exhibits sensitivity to small but finite depthvariations. The numerical simulations reveal multipletipped unstable symmetric solutions which interact with the single symmetric mode at vanishing occlusion heights resulting in stabilisation of the asymmetric and oscillatory modes. Moreover, deviations from the single symmetric mode are predicted when depthvariations of order of the roughness of the channel walls ($\sim 1$ $\mu$m) are introduced for larger aspect ratios of $\alpha\geq 155$. The propagation of finite bubbles is studied in a channel with constant aspect ratio of $\alpha=30$ and where the height of the occlusion, termed rail, is $1/40$ of the channel height. For bubble diameters of the order of the rail width, a tongueshaped stability boundary for symmetric (onrail) propagation is encountered so that for flow rates marginally larger than a critical value, a narrow band of bubble sizes can propagate (stably) over the rail while bubbles of other sizes segregate to the side of the rail. The numerical depthaveraged model is adapted for bubble propagation and captures in qualitative agreement the experimental observations. Timedependent calculations are additionally performed, showing that onrail bubble propagation is the result of a nontrivial dynamical interaction between capillary and viscous forces.

143 
Twophase flow properties upscaling in heterogeneous porous mediaFranc, Jacques 18 January 2018 (has links) (PDF)
The groundwater specialists and the reservoir engineers share the same interest in simulating multiphase flow in soil with heterogeneous intrinsic properties. They also both face the challenge of going from a wellmodeled micrometer scale to the reservoir scale with a controlled loss of information. This upscaling process is indeed worthy to make simulation over an entire reservoir manageable and stochastically repeatable. Two upscaling steps can be defined: one from the micrometer scale to the Darcy scale, and another from the Darcy scale to the reservoir scale. In this thesis, a new second upscaling multiscale algorithm Finite Volume Mixed Hybrid Multiscale Methods (FvMHMM) is investigated. Extension to a twophase flow system is done by weakly and sequentially coupling saturation and pressure via IMPESlike method.

144 
Experimental and CFD simulation investigations into fouling reduction by gasliquid twophase flow for submerged flat sheet membranesNdinisa, Nkosinathi Vincent, Chemical Sciences & Engineering, Faculty of Engineering, UNSW January 2006 (has links)
Submerged flat sheet membranes are mostly used in membrane bioreactors for wastewater treatment. The major problems for these modules are concentration polarization and subsequent fouling. By using gasliquid twophase flow, these problems can be ameliorated. This thesis aimed to optimize the use of gasliquid twophase flow as a cleaning mechanism for submerged flat sheet membrane. The effect of various hydrodynamic factors such as airflow rate, nozzle size, nozzle geometry, intermittent bubbling, intermittent filtration, channel gap width, feed concentration and membrane baffles were investigated for model feed materials (yeast suspensions and mixed liquor from activated sludge plants). Insights into mechanisms by which twophase flow reduces fouling for submerged flat sheet membranes were obtained by using Computational Fluid Dynamics. Experiments conducted showed that an optimal airflow rate exists beyond which no further flux enhancement was achieved. Fouling reduction increased with nozzle size at constant airflow. Nozzles of equal surface area but different geometries performed differently in terms of fouling reduction. Bubble size distribution analyses revealed that the percentage of larger bubbles and bubble rise velocities increased with the airflow rate and nozzle size. Thus the results of this study suggest that the effectiveness of twophase flow depends on the bubble size. CFD simulations revealed that average shear stress on the membrane increased with airflow rate and bubble size and further indicated that an optimal bubble size possible exists. Using intermittent filtration as an operating strategy was found to be more beneficial than continuous filtration. This study also showed the importance of the size of the gap between the submerged flat sheet membranes. Increasing the gap from 7 mm to 14 mm resulted in an increase in fouling by about 40% based on the rate of increase in suction pressure (dTMP/dt). Finally, this is the first study which investigated the effect of baffles in improving air distribution across a submerged flat sheet membrane. It was found that baffles decreased the rate of fouling at least by a factor of 3.0 based on the dTMP/dt data.

145 
The twophase plane turbulent mixing layer / by Duncan Estcourt WardWard, Duncan Estcourt January 1986 (has links)
One microfilm reel (16 mm.) in pocket / Bibliography: leaves 194201 / xiii, 212, 6 leaves, [9] leaves of plates : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)University of Adelaide, Dept. of Mechanical Engineering, 1987

146 
Microgravity flow pattern identification using void fraction signalsValota, Luca 29 August 2005 (has links)
Knowledge of the twophase flow state is fundamental for twophase flow system design and operation. In traditional twophase flow studies, the flow regime refers to the physical location of the gas and liquid in a conduit. Flow configuration is important for engineering correlations of heat and mass transfer, pressure drop, and wall shear. However, it is somewhat subjective since it is mostly defined by experimental observation, resulting in an approximate and equivocal definition. Thus, there is need for a better, objective flow regime identification. The void fraction is a key parameter in monitoring the operating state of a twophase system and several tools have been developed in order to measure it. The purpose of this study is to use the void fraction and other parameters of the system to achieve a model for flow pattern identification. Recently, an experimental program using the FosterMiller twophase flow test bed and Creare Inc. capacitance void fraction sensors was conducted in the microgravity environment of the NASA KC135 aircraft. Several data types were taken for each phase, such as flow rate, superficial velocity, density and transient void fraction at 100Hz. Several analytical approaches were pursued, including a statistical approach of the fluctuation of the void fraction, Martinelli analysis, and Drift Flux analysis, in order to reach a model for flow pattern identification in microgravity conditions. Several parameters were found to be good flow pattern identifiers such as the statistical moments variance and skewness, Signal to noise ratio (SNR), Half Height Value (HHV) and Linear Area Difference (LAD). Moreover, relevant conclusions were achieved using the Martinelli parameter and the Drift Flux model in microgravity conditions. These results were compared with the basic literature.

147 
Investigation of Twophase Microchannel Flow and Phase Equilibria in Micro Cells for Applications to Enhanced Oil RecoveryForoughi, Hooman 21 August 2012 (has links)
The viscous oilwater hydrodynamics in a microchannel and phase equilibria of heavy oil and carbon dioxide gas have been investigated in connection with the enhanced recovery of heavy oil from petroleum reservoirs.
The oilwater flow was studied in a circular microchannel made of fused silica with an I.D. of 250 µm. The viscosity of the silicone oil (863 mPa.sec) was close to that of the gassaturated heavy oil in reservoirs. The channel was always initially filled with the oil. Two different sets of experiments were conducted: continuous oilwater flow and immiscible displacement of oil by water. For the case of continuous water and oil injection, different types of liquidliquid flow patterns were identified and a flow pattern map was developed based on Reynolds, Capillary and Weber numbers. Also, a simple correlation for pressure drop of the two phase system was developed.
In the immiscible displacement experiments, the water initially formed a coreannular flow pattern, i.e. a water core surrounded by a viscous oil film. The initially symmetric flow became asymmetric with time as the water core shifted off centre and also the waves at the oilwater interface became asymmetric. A linear stability analysis for coreannular flow was also performed. A characteristic equation which predicts the growth rate of perturbations as a function of the core radius, Reynolds number, and viscosity and density ratios of the two phases was developed.
Also, two micro cells for gas solubility measurements in oils were designed and constructed. The blind cell had an internal volume of less than 2 ml and the micro glass cell had a volume less than 100 µl. By minimizing the cell volume, measurements could be made more quickly. The CO2 solubility was determined in bitumen and ashphaltenefree bitumen samples to show that ashphaltene has a negligible effect on CO2 solubility.

148 
A general theory of flooding implementing the cuspoid catastropheLafi, Abd Y. 06 June 1990 (has links)
The flooding phenomenon can be defined as the maximum attainable flow
condition beyond which the well defined countercurrent flow pattern can no longer
exist. Thus the countercurrent flow limit (CCFL) or the flooding limit may be thought
of as the flow condition at which the strong interaction between the two phases
occurs.
Considerable effort has been devoted to understanding and analyzing the
flooding transition in many fields. For example; the flooding phenomenon is one of the
important phenomena encountered in the safety analysis of light water reactors
(pressurized water reactors and boiling water reactors). Accurate predictions of
flooding behavior are particularly important in the assessment of emergency core
cooling system (ECCS) performance. Currently, the postulated lossofcoolant
accident (LOCA) is considered the design basis accident. A physical understanding of
the flooding phenomenon will help assess core refill during the course of a LOCA.
Understanding the physical mechanisms of the flooding phenomenon might help
establish more reliable equations and correlations which accurately describe the
thermal hydraulic behavior of the system. The models can provide bestestimate
capability to the design codes used in the evaluation of ECCS performance.
The primary concern of this study was to:
1. Understand the physical mechanisms involved in the flooding phenomenon in
order to derive a suitable analytical model.
2. Show that the combination of:
a. Linear Instability Theory
b. Kinematic Wave Theory
c. Catastrophe Theory
can provide a general model for flooding phenomenon.
The theoretical model derived using the aforementioned combination of theories
indicates good agreement between the experimental and the predicted values.
Comparisons have been made using a large volume of airwater flooding data. / Graduation date: 1991

149 
Investigation of Twophase Microchannel Flow and Phase Equilibria in Micro Cells for Applications to Enhanced Oil RecoveryForoughi, Hooman 21 August 2012 (has links)
The viscous oilwater hydrodynamics in a microchannel and phase equilibria of heavy oil and carbon dioxide gas have been investigated in connection with the enhanced recovery of heavy oil from petroleum reservoirs.
The oilwater flow was studied in a circular microchannel made of fused silica with an I.D. of 250 µm. The viscosity of the silicone oil (863 mPa.sec) was close to that of the gassaturated heavy oil in reservoirs. The channel was always initially filled with the oil. Two different sets of experiments were conducted: continuous oilwater flow and immiscible displacement of oil by water. For the case of continuous water and oil injection, different types of liquidliquid flow patterns were identified and a flow pattern map was developed based on Reynolds, Capillary and Weber numbers. Also, a simple correlation for pressure drop of the two phase system was developed.
In the immiscible displacement experiments, the water initially formed a coreannular flow pattern, i.e. a water core surrounded by a viscous oil film. The initially symmetric flow became asymmetric with time as the water core shifted off centre and also the waves at the oilwater interface became asymmetric. A linear stability analysis for coreannular flow was also performed. A characteristic equation which predicts the growth rate of perturbations as a function of the core radius, Reynolds number, and viscosity and density ratios of the two phases was developed.
Also, two micro cells for gas solubility measurements in oils were designed and constructed. The blind cell had an internal volume of less than 2 ml and the micro glass cell had a volume less than 100 µl. By minimizing the cell volume, measurements could be made more quickly. The CO2 solubility was determined in bitumen and ashphaltenefree bitumen samples to show that ashphaltene has a negligible effect on CO2 solubility.

150 
The thermocapillary flow effects on a free surface deformation during solidificationChan, ChengYu 28 July 2010 (has links)
This study uses the Phasefield method to simulate the transient thermal current of the metal surface heated and molten by a massing energy. The flow field uses a twodimension module, considered with the mass conservation equation, momentum equation, energy equation and levelset equation, to solve for the distribution in whole domain, including
the interface, of temperature, velocity, pressure and levelset number. We ignore the effect of concentration diffusion, but consider about the effect
of heat translation on the flow field. Finally the results will display the flows of air around molten area forced by buoyancy which is caused by high temperature, and the flows in molten area forced by thermocapillary
which is caused by temperature gradient.

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