The primary objective of this thesis was to develop a nucleation regime map to determine the controlling nucleation mechanism as a function of material properties and operating parameters. Two distinct regimes of nucleation were identified. The drop controlled nucleation regime occurs when nucleation conditions are ideal and one drop produces one nucleus granule and the controlling property is the droplet size. The nuclei formation kinetics are fast and the binder droplets penetrate into the powder bed pores almost immediately. In this region, the nuclei distribution reflects the drop size distribution as one drop tends to form one granule provided that (a) drops hitting the powder surface do not overlap - low spray flux Ya and (b) the drop must wet into the bed completely before bed mixing brings it into contact with another partially absorbed drop on the bed surface - low penetration time tp. If either criterion is not met, powder mixing characteristics will dominate. In the mechanical dispersion regime, the viscous or poorly wetting binder is slow to flow through the powder pores and form nuclei and good mixing is required for binder dispersion. The kinetics of nuclei formation were characterised using a simple drop penetration time test. A single drop of binder fluid was placed on a loosely packed powder bed and the time taken for the fluid to penetrate completely was measured for a range of powder and binder combinations. Loosely packed powder beds contain large macrovoids which are included in the existing Kozeny approach to estimating pore size. However it was found that these pores do not participate in liquid flow. A new two phase model was proposed where the total volume of the macrovoids was assumed to be the difference between the bed porosity and the tap porosity. A new parameter, the effective porosity eeff, was defined as the tap porosity multiplied by the estimated fraction of pores that terminate at a macrovoid and are effectively blocked pores. The pore sizes and drop penetration times were recalculated using the effective porosity and the predicted tp values were generally within an order of magnitude of the experimental results for all powders. The drop penetration time is reduced by small drops, low viscosity fluids, porous powders (but without macrovoids), large powder pores, high surface tension, low contact angle and pre-wetting of the powder bed. A new dimensionless group, the dimensionless spray flux Ya was defined to characterise the three most important operating variables in binder dispersion: liquid binder flowrate, drop size and powder flux through the spray zone. At low Ya, the majority of drops land on the powder sufficiently well separated to allow ideal “drop controlled” nucleation where one drop forms one granule. As Ya increases, the probability of drop footprints overlapping to give larger agglomerate nuclei increases. Monte-Carlo simulations were performed to validate the spray flux theory. The proportion of nuclei formed from single drops falls exponentially as Ya increases and to remain in the drop controlled regime Ya must be kept below 0.1. Analytical solutions based on the Poisson distribution for the fraction of single drop nuclei as a function of Ya were an excellent match with the Monte-Carlo data. Further validation experiments in carefully designed ex-granulator experiments and in an industrial granulator were performed. The results matched the theoretical solutions and demonstrated the ability of Ya to describe the nucleation zone in a real granulator. The proposed nucleation regime map demonstrated the interaction between drop penetration time and spray flux in nucleation. At short penetration times, such as the water and lactose system, decreasing Ya causes a shift towards the drop controlled regime and a narrower nuclei distribution. When penetration time is long, the nuclei size distribution is always larger and broader. Granulation may still be successful if the mechanical dispersion forces are able to break up the binder clumps and distribute the binder through the powder. The nucleation regime map should prove to be a useful tool for maintaining effective liquid distribution during scale-up as well as a useful trouble-shooting tool. It allows the dominant mechanism controlling the nucleation process to be easily identified using relatively simple parameters and a rational approach can then be used to control the properties of the nuclei.

Thesis (M.S.)--University of Nevada, Reno, 2007. / "December 2007." Includes bibliographical references (leaves a-b). Online version available on the World Wide Web.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1967. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.) -- Virginia Commonwealth University, 2004. / Title from title-page of electronic thesis. Prepared for: Dept. of Chemistry. Bibliography: p. 218-236.

An optical microscope coupled to a flow cell was used to investigate ice nucleation on five atmospherically relevant mineral dusts at temperatures ranging from 233 to 247 K. Kaolinite and muscovite particles were found to be efficient ice nuclei in the deposition mode, requiring relative humidities with respect to ice (RHi) below 112% in order to initiate ice crystal formation. Quartz and calcite particles, by contrast, were poor ice nuclei, requiring relative humidities close to water saturation before ice crystals would form. Montmorillonite particles were efficient ice nuclei at temperatures below 241 K, but poor ice nuclei at higher temperatures. In several cases, there was a lack of quantitative agreement between these data and previously published work. This can be explained by several factors including mineral source, particle size, observation time and surface area available for nucleation. Heterogeneous nucleation rates (Jhet) were calculated from the onset data. Jhet values ranged from 60 to 1100 cm-²s-¹ for the five minerals studied. These values were then used to calculate contact angles (θ) for each mineral according to classical nucleation theory. The contact angles measured for kaolinite and muscovite ranged from 6 to 12º; for quartz and calcite the contact angles were much higher, ranging from 25 to 27º. The contact angles measured for montmorillonite were less than 15º at temperatures below 241 K, and above 20º at higher temperatures. The reported Jhet and θ values may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere. The roles of H₂SO₄ and (NH4)₂SO₄ coatings on the ice nucleating properties of kaolinite were also investigated. Onset data was collected for H₂SO₄ coated and (NH4)₂SO₄ coated kaolinite particles at temperatures ranging from 233 to 247 K. In contrast to uncoated kaolinite particles, which were effective ice nuclei, H₂SO₄ coated particles were found to be poor ice nuclei, requiring relative humidities close to water saturation before nucleating ice at all temperatures studied. (NH4)₂SO₄ coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K. / Science, Faculty of / Chemistry, Department of / Graduate

We report the results of a series of experiments in three interrelated areas: light induced nucleation of vapor bubbles in superheated liquid helium 4, transient heat transfer in liquid helium isotopes 3 and 4, and homogeneous nucleation of vapor bubbles in superheated liquid helium 3. This work has resulted in significant contributions in each of these particular areas. Our transient heat transfer work has resulted in extremely high temperature measurements of the Kapitza thermal boundary conductance limits in helium 3 and 4, in measurements of the delay time to the onset of film boiling over a wide range of bath temperatures in helium 4, and in a determination of bubble growth rates in helium 4. These measurements have been compared with theory and have in some cases allowed the extension or elucidation of that theory. We have characterized the so called "light effect" and established photographically that small amounts of visible light will cause the formation of vapor bubbles at the interface of a solid and superheated liquid helium 4 and that this vapor can influence the quasi-steady-state heat flux vs temperature hysteresis curve. Finally, we have measured the homogeneous nucleation temperature of liquid helium 3 and found good agreement with the predictions of the Becker-Doring-Volmer-Zel'dovich Frenkel nucleation theory. This work is shown to have applications to practical cryogenic engineering, to further understanding of basic heat transfer and nucleation theory, and to practical and theoretical environmental and resource considerations.

No description available.

published_or_final_version / abstract / Physics / Master / Master of Philosophy

Latium Vetus is accepted as having possessed an urban status by the archaic period. The evolution towards this status depended upon various factors operating through centuries. From an initial stage in which the region was composed of insular settlements, the first step towards urbanisation was nucleation of settlement. This was a federal grouping of small, self-governing kin units. Such were the curiae of history; their individualism is emphasised in the topography of the cemeteries. They are revealed in the remnants of early law operating along the lines of reciprocity and collusion. This nucleation was probably a result of demographic pressures; trade and technological innovation may also be considered contributory factors. Certainly these latter emerged as conditioning elements within the development of such communities. The separate units within the settlements practised an individual prestige-goods economy. Their powers were separate from those of the community as a whole. Such powers had to be curbed as the role of manufacture and trade increased. Thus the central, 'state' power grew, as may be seen in legal and historical developments. The aristocracies which had emerged had proved a destabilising factor in the state, for they maintained economic and sociopolitical practices which artificially supported secondary activities and separatist influences. As society became more complex, so the kin basis upon which it was founded proved inadequate. Changes in the demographic constitution of the community, overly competitive economic practice and increasing functional differentiation caused the creation of a public domain, one witnessed in various ways in the source material. Urbanisation was the end-result of the functioning of a prestige-goods economy in a society formed of distinctive groups prior to the initiation of large scale trade and manufacture. The competition inherent within such a society led ultimately to the unity of the urban system.

No description available.