The chemical resistance of glass is an important property for many applications. This property has been extensively studied for many types of glass under static conditions (no liquid is removed during the experiment). There has been little research conducted on the effects of additional stimuli on the dissolution of glass. For this research lithium disilicate was leached in deionized water at multiple temperatures while microwave radiation, ultrasonication or flow conditions were also applied to the system. These results were then compared to static baseline to determine if these stimuli would cause any change to the mechanisms and kinetics of the reaction. It was determined that for the experimental conditions used there was little to no change in dissolution when 2.45 GHz microwave radiation instead of conventional methods was used to heat the reaction. Results from ultrasonication found that samples that experienced erosion showed an increase in dissolution with an increase in dissolution following heavier erosion. This was thought to be due to both an increase in the surface area of the sample to volume of solution (SA/V) ratio (erosion would modify the surface area and release small particulates) and the accelerated removal of the depleted layer due to erosion. Stereoscopic reconstruction was used to semi-quantitatively measure the change in surface area. Regions that experienced minor erosion showed a 3-6% increase in surface area while those that experienced heavy erosion showed a 29-35% increase in surface area. Due to inconsistencies in the size of the eroded area it was not possible to determine the effects of power intensity with this research. Flow dissolution showed similar trends in concentration and different trends for the total normalized mass loss (TNL) to previously published research on more complex glasses. The elemental concentration initially increased before reaching a peak and decreasing to steady state. This peak was thought to be caused by the combination of flow, increasing thickness in the depleted layer, and an initial fluctuation in the forward reaction rate due to changes in pH. For the lithium disilicate glass used in this research both the elemental concentration and the TNL increased with increasing temperature and decreasing flow rate (silica dissolution was an exception as it did not show any change in TNL due to flow). All experimental conditions were shown to achieve steady state (dC/dt~0) by the seventh day of leaching. The contrast in the observed TNL trends between lithium disilicate and more complex glasses was thought to be due to differences in reaction rates and the presence of an additional surface layer in the complex glasses due to precipitation. Microscopy of the leached glass showed that surface features introduced during grinding (scratch lines and microcracks) were preferentially leached and grew in size and number visible during dissolution. A semi-quantitative model was created using stereoscopic reconstruction to describe the preferential leaching of the microcracks as there was little available discussion found in literature outside of associating the growth of these features with localized network dissolution. In this model the microcracks experience preferential dissolution leading to a change in size and shape. The SA/V ratio inside the crack would be much larger than the bulk system (calculated to initially be ~768,000cm-1 compared to the bulk's 0.1cm-1). This would cause massive acceleration in the initial ion exchange, raising the pH of the solution which would in turn cause network dissolution to occur much faster inside the crack. Based on static experiments on lithium disilicate frit (SA/V of 1,010cm-1) the pH inside the crack would jump to above 11 in minutes. As the crack grows, the SA/V ratio inside it would decrease (largest cracks were found to have a ratio ~100,000cm-1). The accelerated leaching caused by these features could have a noticeable effect on the dissolution results. In addition to the accelerated leaching inside a crack, the size of the depleted layer under the crack would be different from the bulk glass. / Doctor of Philosophy / The chemical resistance of glass is an important property for many applications. This property has been extensively studied for many types of glass under static conditions where no liquid was removed and temperature was the major variable. For this research lithium disilicate was leached in deionized water at multiple temperatures while the additional stimuli of microwave radiation, ultrasonication or flow conditions were also applied to the system. The question that this research addressed was how does the aqueous dissolution of glass change when a system is exposed to these additional stimuli? Although glasses are subjected to these stimuli in many everyday applications, their influence on dissolution has not been studied extensively. Lithium disilicate glass was selected because it contains components used in many commercial glasses, has sufficient reactivity in water to allow experiments to be completed in a reasonable time, and because its mechanisms for dissolution under static conditions were well known. Glass is frequently selected to be the container when microwaves are used to heat food or materials. Flow is an important part of many applications involving glass including the storage of nuclear waste glass, glass-lined tanks used in the chemical industries, in the use of glass in the human body (bioglass and dental crowns), and in typical window and laboratory glasses where intermittent aqueous contact and runoff may occur. Examining how cavitation via ultrasonication can be controlled to either minimize or maximize element extraction is important, with the removal of rare earth elements from fly ash being one example.

<div>Nucleate boiling is an efficient heat transfer mechanism that enables the dissipation of high heat fluxes at low temperature differences. Heat transfer phenomena during nucleate boiling are closely linked to the two-phase flow morphology that evolves in time and based on the operating conditions. In particular, the critical heat flux, which is the upper limit for the nucleate boiling regime, can be triggered by hydrodynamic mechanisms resulting from interactions between the liquid and vapor phases. The aim of this thesis is to characterize the two-phase flow morphology evolution during nucleate boiling at high heat fluxes in two configurations: pool boiling, and confined and submerged two-phase jet impingement. The characterization is performed via non-invasive, high-speed optical based diagnostic tools. </div><div>Experimental characterization of liquid-vapor interfaces during boiling is often challenging because the rapidly evolving vapor structures are sensitive to invasive probes and multiple interfaces can occlude one another along a line of sight. In this thesis, a liquid-vapor interface reconstruction technique based on high-speed stereo imaging is developed. Images are filtered for feature enhancement and template matching is used for determining the correspondence of local features of the liquid-vapor interfaces between the two camera views. A sampling grid is overlaid on the reference image and windows centered at each sampled pixel are compared with windows centered along the epipolar line in the target image to obtain a correlation signal. To enhance the signatures of true matches, the correlation signals for each sampled pixel are averaged over a short time ensemble correlation. The three-dimensional coordinates of each matched pixel are determined via triangulation, which yields a set of points in the physical world representing the liquid-vapor interface. The developed liquid-vapor interface reconstruction technique is a high-speed, flexible and non-invasive alternative to the various existing methods for phase-distribution mapping. This technique also has the potential to be combined with other optical-based diagnostic tools, such as tomographic particle image velocimetry, to further understand the phase interactions.<br></div><div>The liquid-vapor interface reconstruction technique is used to characterize liquid-vapor interfaces above the heated surface during nucleate pool boiling, where the textured interface resulting from the boiling phenomena and flow interactions near the heated surface is particularly suited for reconstruction. Application of the reconstruction technique to pool boiling at high heat fluxes produces a unique quantitative characterization of the liquid-vapor interface morphology near heated surface. Analysis of temporal signals extracted from reconstructions indicate a clear transition in the nature of the vapor flow dynamics from a plume-like vapor flow to a release mode dominated by vapor burst events. Further investigation of the vapor burst events allows identification of a characteristic morphology of the vapor structures that form above the surface that is associated to the square shape of the heat source. Vapor flow morphology characterization during pool boiling at high heat fluxes can be used to inform vapor removal strategies that delay the occurrence of the critical heat flux during pool boiling.</div><div>As compared to pool boiling, nucleate boiling can be sustained up to significantly higher heat fluxes during two-phase jet impingement. The increases in critical heat flux are explained via hydrodynamic mechanisms that have been debated in the literature. The connection between two-phase flow morphology and the extension of nucleate boiling regime is investigated for a single subcooled jet of water that impinges on a circular heat source via high-speed visualization from two synchronized top and side views of the confinement gap. When boiling occurs under subcooled exit flow conditions and at moderate heat fluxes, the regular formation and collapse of vapor structures that bridge the heated surface and the orifice plate is observed, which causes significant oscillations in the pressure drop across. Under saturated exit flow conditions, the vapor agglomerates in the confinement gap into a bowl-like vapor structure that recurrently shrinks, due to vapor break-off at the edge of the orifice plate, and replenishes due to vapor generation. The optical visualizations from the top of the confinement gap provide a unique perspective and indicate that the liquid jet flows downwards through the vapor structure, impinges on the heated surface, and then flows underneath the vapor structure, as a fluid wall jet the keeps the heated surface wetted such that discrete bubbles continue to nucleate. At high heat fluxes, intense vapor generation causes the fluid wall jet to transition from a bubbly to a churn-like regime, and some liquid droplets are sheared off into the vapor structure. The origin of critical heat flux appears to result from a significant portion of the liquid in the wall jet being deflected off the surface, and the remaining liquid film on the surface drying out before reaching the edge of the heater.</div><div>The flow morphology characterizations presented in this dissertation further the understanding of flow and heat transfer phenomena during nucleate boiling. In the pool boiling configuration, the vapor release process was quantitatively described; during two-phase jet impingement, a possible mechanism for critical heat flux was identified. Opportunities for future work include the utilization of image processing techniques to extract quantitative measurements from two-phase jet impingement visualizations. Also, the developed liquid-vapor interface reconstruction technique can be applied to a boiling situation with a simpler liquid-vapor interface geometry, such as film boiling, to generate benchmark data for validation and development of numerical models.</div><div><br></div>