Theoretical Study of Noble Gas Bubble Behavior in Mercury

Spallation Neutron Source (SNS) uses heavy liquid metal (mercury) as the target material for high power proton beam bombardment to produce neutrons for scientific research. Though the liquid target is not subject to material degradation due to radiation damage, the stainless steel pressure boundary confining the liquid metal flow is damaged by radiation and cavitation erosion induced by the thermal shock waves caused by the deposition of the incoming high-power proton beam. This puts a limit on the lifetime of the target holder. To mitigate the cavitation-induced erosion damage to the target holder, it is aimed to introduce microbubbles to the target mercury with expected nominal size of 30μm diameter and volume fraction of 0.5%, which can substantially lower the pressure amplitude resulting from the proton beam deposition due to the added compressibility. The noble gas bubble behavior in mercury is studied in this thesis. The acoustics of the two-phase mixture under the perturbation due to beam deposition, specifically acoustic streaming, is simulated in a bubbly two-phase flow for the first time in the literature. The numerical simulation shows the magnitude of obtained streaming velocity is much smaller than the pumped mercury flow in the target and will not cause distortion to flow patterns and heat transfer in the target. Single bubble dynamics, which includes noble gas solubility evaluation in mercury and the bubble radius evolution under the effect of mass diffusion across the bubble wall, is also simulated. Two different profiles of bubble size distribution are studied. The solubility evaluation provides a theoretical basis for the inert gas solubility measurement experiments. The mass diffusion induced bubble behavior simulation based on the solubility results indicates that xenon bubbles creates a more viable and stable bubble population in mercury than helium bubbles, which means xenon is a possible better candidate to add compressibility to pure mercury in the SNS target

Research study for determination of liquid surface profile in a cryogenic tank during gas injection annual report, 18 jun. 1964 - 17 jun. 1965

Equation for free liquid surface profile caused by gas bubble injection into tank of liqui

The Study of Bubble Growth Hydrodynamics in the Supersaturated Liquids

Bubble formation and dissolution have a wide range of industrial applications, from the production of beverages to foam manufacturing processes. The rate at which the bubble expands, or contracts has a significant effect on these processes. In the current work, the hydrodynamics of an isolated bubble expanding due to mass transfer in a pool of supersaturated gas-liquid solution is investigated. The complete scalar transportation equation (advection-diffusion) is solved numerically and it has been observed that the present model predicted an accurate bubble growth when compared with existing approximated models and experiments. The effect of gas-liquid solution parameters such as inertia, viscosity, surface tension, diffusion coefficient, system pressure, and solubility of the gas has been investigated. It is found that the surface tension and inertia have a very minimal effect during the bubble expansion. However, it is observed that the viscosity, system pressure, diffusion, and solubility have a considerable effect on bubble growth

Stochastic modeling of flow behavior and cell structure formation during extrusion of biopolymer melts

Master of ScienceDepartment of Grain Science and IndustrySajid AlaviExtrusion is a widely used processing technology for various food products and is also commonly applied in non-food applications involving plastics, rubber and metal. Expanded products for human and animal consumption such as snacks, breakfast cereal, pet food and aquatic food typically consist of a biopolymer matrix of starch and proteins that have natural physical, chemical and polymeric variability. Additionally, variability in extrusion parameters such as water injection and screw speed is often observed depending on the process controls employed. This can potentially lead to inconsistency in product quality. Stochastic modeling helps in studying the impact of variability of various parameters on the end product, which in turn helps in better process and product quality control. The primary purpose of this research was to develop a mathematical model for flow behavior of biopolymer melts inside extruder barrel and bubble growth dynamics after exiting the extruder using mass, heat and momentum transfer equations. This model was integrated with a Monte-Carlo based stochastic interface for input of randomly generated process data (based on experimental data acquisition) and output of simulated distributions of end-product properties such as expansion ratio and cellular architecture parameters (cell size and wall thickness). The mathematical model was experimentally validated using pilot-scale twin screw extrusion for processing of cereal-based cellular products. Process and product data were measured at different in-barrel moisture contents (19-28% dry basis) and experimental screw speeds (250-330 rpm). Experimental process parameters such as specific mechanical energy (212.8-319.3 kJ/kg), die temperature (120.7-170.6oC), die pressure (3160-7683 kPa) and product characteristics such as expansion ratio (3.29-16.94) and cell size or bubble radius (435-655 microns) compared well with simulated results from the mathematical model viz., specific mechanical energy (217.6-323.9 kJ/kg), die temperature (116.8-176.1oC), die pressure (3478-6404 kPa), expansion ratio (4.56-19.4) and bubble radius (426-728 microns). Experimental variability in product characteristics was quantified using coefficient of variation which compared well with simulation results (example, 2.5-4.9% versus 0.24-3.1% respectively for expansion ratio). The stochastic model was also used to conduct sensitivity analysis for understanding which raw material and process characteristics contribute most to product variability. Sensitivity analysis showed that the water added in extruder affects the magnitude and variability of expansion ratio the most, as compared to screw speed and consistency index

Fundamental study of underfill void formation in flip chip assembly

Flip Chip in Package (FCIP) has been developed to achieve the assembly process with area array interconnects. Particularly, a high I/O count coupled with finer pitch area array interconnects structured FCIP can be achieved using no-flow underfill assembly process. Using the assembly process, a high, stable yield assembly process recently reported with eutectic lead-tin solder interconnections, 150 µm pitch, and I/O counts in excess of 3000. The assembly process reported created a large number of voids among solder interconnects in FCIP. The voids formed among solder interconnections can propagate, grow, and produce defects such as solder joint cracking and solder bridging. Moreover, these voids can severely reduce reliability performance. Indeed, many studies were conducted to examine the void formation in FCIP. Based on the studies, flip chip geometric design, process conditions, and material formulation have been considered as the potential causes of void formation. However, the present research won't be able to identify the mechanism of void formation, causing a lot of voids in assembly process without consideration of chemical reaction in the assembly process with a fine-pitch, high I/O density FCIP. Therefore, this research will present process technology necessary to achieve high yields in FCIP assemblies using no-flow underfills and investigate the underlying problem of underfill void formation in these assemblies. The plausible causes of void formation will be investigated using experimental techniques. The techniques will identify the primary source of the void formation. Besides, theoretical models will be established to predict the number of voids and to explain the growth behavior of voids in the FCIP. The established theoretical models will be verified by experiments. These models will validate with respect to the relationship between process parameters to achieve a high yield and to minimize voids in FCIP assemblies using no-flow underfill materials regarding process as well as material stand points. Eventually, this research provides design guideline achieving a high, stable yield and void-free assembly process.Ph.D.Committee Chair: Baldwin, Daniel; Committee Member: Colton, Jonathan; Committee Member: Ghiaasiaan, Mostafa; Committee Member: Moon, Jack; Committee Member: Tummala, Ra

The development of h Homogeneous nucleation rate model for thermoplastic foams based on a molecular partition function and Fickian diffusion

An improved homogeneous nucleation rate model for thermoplastic foams has been developed. This model does not rely on experimentally determined parameters and only uses pure component physical property data and a binary diffusion coefficient. This model, like those derived from classical nucleation theory, is made up of two parts, one that determines the size of the energy barrier in the nucleation process and one that estimates the forward rate of the process. A statistical-mechanical approach was used to create an energy term that is based on a molecular partition function. In this approach, the bulk phase (polymer and blowing, agent mixture) of the system is treated as a regular solution and the potential energy of this phase is estimated from regular solution theory. The rate component of the model is obtained by utilizing a diffusion-based model derived from Fick\u27s law, Additional approaches including a diffusion only based approach, a fluctuation theory based approach, and a lattice model based approach were all unsuccessfully investigated. The predictions obtained from the model have been compared to a polymethylmethacrylate/carbon system with limited success. Althoulgh the model results do not match the experimental data, there is a significant improvement over the results obtained from current models available in the literature. The data is limited to the one system described above as there was significant evidence of heterogeneous nucleation in most other systems identified in the literature. Finally, the work also provides a comprehensive review of the literature on foam nucleation in thermoplastics. The review covers both homogeneous and heterogeneous models and looks at results obtained experimentally and theoretically. This review clearly identifies the need for an improved nucleation model that is not dependent on experimental parameters like the one developed in this work

Plasticisation effects of high-pressure carbon dioxide on polymers

This thesis examines the effects derived from the ability of high pressure carbon dioxide to soften polymers. This has potential applications in the shape forming of polymers at lower temperatures, dye impregnation and the foaming of polymers. This study was conducted in two parts: (i) mechanical measurement of polymer softening under CO2 at high pressure; and (ii) foaming behaviour of polymers containing dissolved CO2 during depressurisation. In the first study the softening of polymers as a function of applied CO2 pressure and temperature was measured using a novel mechanical 3-point bend test rig. In initial experiments the temperature was slowly ramped upwards and the nominal glass transition temperature was recorded as where the central deflection suddenly begins to increase. Significant reductions in the bending onset temperatures were observed on the application of CO2 for polycarbonate, poly(methyl-methacrylate), glycol modified poly(ethylene-terepthalate) and polystyrene, of typically 50–100°C over the range of pressures applied (24 to 120 bar). [Continues.