Thesis (Ph. D.)--University of Wisconsin--Madison, 1975. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (p. 107-109).

Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Cavitation is the formation of vapor bubbles in a liquid that is a consequence of tensions acting on the liquid. It is of great interest to lots of different scientific fields such as fluid mechanics, acoustics, hydraulic engineering and biology. Although widely studied in macroscale and microscale confined liquids, heterogeneous cavitation at the nanoscale has only been experimentally observed recently in 2-D nanofluidic channels during an evaporation process, where vapor bubbles form and expand inside the nanochannels instead of menisci receding along the channels. Such evaporation-induced cavitation shows a strong correlation with the nanochannel cross-section non-uniformity and exhibited lots of interesting phenomena, including fast evaporation rate and self-controlled bubble dynamics. In this work, we further investigated this new cavitation phenomenon using a series of specially designed 2-D multi-section nanochannels. Each of these channels includes two or three sections of nanochannel with heights of 25 and/or 35 nm and the same width of 3 μm. A modified sacrificial layer etching method was developed to fabricate these nanochannel devices. Water evaporation processes in these channels were recorded using a high-speed camera mounted on an inverted microscope. We observed that cavitation only occurred in multi-section nanochannels with a “Low to High” channel design. In such nanochannels, when menisci receded to the “Low to High” step, bubbles occurred in the higher channel section and started expansion until they occupied the whole section. We explored the origin of these cavitation phenomena and discovered that that initial bubbles were formed during a snap-off process, where meniscus curvature difference induced reverse liquid flows cause air trapping right at the step. The following bubble expansion is a result of evaporation-induced negative pressure (up to -58 bars) as water inside the nanochannels is in a metastable state. We also analyzed water evaporation rates (bubble growth rates) in these nanochannels in the presence of cavitation. While most evaporation rates can be explained by classic vapor diffusion theories or the kinetic limit of evaporation, water evaporation rates in nanochannels with a Low-High-Low design in the presence of cavitation were as high as 630 μm/s, which is even much higher than the kinetic limit of evaporation and cannot be explained by any current theories. This study further expands our understanding of cavitation and provides new insights and explanations for phase-change phenomena at the nanoscale, including cavitation in plants and quick drying process in nanoporous media. The discovered ultra-high evaporation rates in the Low-High-Low nanochannels also offer a new solution to address thermal management needs for next generation electronic devices. / 2031-01-01

This research focusses on observing and modelling the suburban surface energy balance. The initial objective is to use measurements to elucidate the controls on the size and temporal variability of the latent heat flux. This is achieved by synchronous observations of suburban and rural energy balances. On the basis of this comparison it is proposed that the day-to-day variability of the partitioning of the suburban turbulent fluxes is linked both to larger-scale atmospheric influences and variations in the energy and moisture availability within the suburban 'canopy'. This hypothesis is examined through measurement and modelling. Further observations of the suburban energy balance components reveal that the size of the Bowen ratio is linked to the surface moisture availability. This is comprised of soil moisture variations in unirrigated greenspace areas and also the anthropogenic influence of lawn irrigation. However, in addition to this, the day-to-day variability of the Bowen ratio is a function of an advective influence upon the saturation deficit in the surface and mixed-layers. The mechanisms which determine this relationship are identified as meso-scale advective effects resulting from differing land-uses. This influences the nature of the mixed-layer and hence surface fluxes. In light of this interaction of scales and atmospheric processes, a model is developed that couples advectively-dominated mixed-layer dynamics with surface-layer exchanges of heat and mass. The acronym for the model is SCABLE, Suburban Canopy and Boundary Layer Evaporation model). It predicts the diurnal evolution of the mixed-layer depth, temperature and humidity. The saturation deficit of the mixed-layer is an input to the surface evaporation model. In turn this enables the surface sensible heat flux to be calculated from the surface energy balance (using measurements of the available energy). This modelled surface sensible heat flux drives the growth of this mixed-layer and thus the rate of entrainment from the capping inversion. The temperature and moisture structure of the mixed-layer is determined by both inputs from the surface-layer, and from the "free" atmosphere. The suburban canopy evaporation sub-model is based on the 'big leaf' Combination model, with a parameterisation scheme for the surface and aerodynamic resistances based upon the approaches taken by Shuttleworth (1976, 1978). The model performs adequately for simulating the day-to-day variability of the saturation deficit and surface evaporation. Its performance on an hourly basis indicates that the model weaknesses lie in the simulation of the diurnal behaviour of the surface resistance and potential temperature of the mixed-layer. It is concluded in the thesis that such an approach is necessary and valid for predicting and understanding the evaporation regime in areas the size of suburbia. This is especially true where there is likely to be a combination of factors determining the surface evaporation rate. / Arts, Faculty of / Geography, Department of / Graduate

The thesis presents work across three different subjects of investigations into the modelling of spray development and its interaction with non-reactive and reactive flow. The first part of this research is aimed to create a new and robust family of convective scheme to capture the interface between the dispersed and the carrier phases without the need to build up the interface boundary. The selection of Weighted Average Flux (WAF) scheme is due to this scheme being designed to deal with random flux scheme which is second-order accurate in space and time. The convective flux in each cell face utilizes the WAF scheme blended with Switching Technique for Advection and Capturing of Surfaces (STACS) scheme for high resolution flux limiters. However in the next step, the high resolution scheme is blended with the scheme to provide the sharpness and boundedness of the interface by using switching strategy. The proposed scheme is tested on capturing the spray edges in modelling hollow cone type sprays without need to reconstruct two-phase interface. A test comparison between TVD scheme and WAF scheme using the same flux limiter on convective flow on hollow cone spray is presented. Results show that the WAF scheme gives better prediction than the TVD scheme. The only way to check the accuracy of the presented models are evaluations according to physical droplets behaviour and its interaction with air. In the second part, due to the effect of evaporation the temperature profile in the released fuel vapour has been proposed. The underlying equation utilizes transported vapour mass fraction. It can be used along with the solution of heat transfer inside a sphere. After applying boundary conditions, the equation can provide a solution of existing conditions at liquid-gas interface undergoing evaporation and it is put in a form similar to well-known one-third rule equation. The resulting equation is quadratic type that gives an accurate prediction for the thermo-physical properties due to the non-linear relation between measured properties and temperature. Comparisons are made with one-third rule where both equations are implemented in simulating hollow cone spray under evaporation conditions. The results show the presumed equation performs better than one-third rule in all comparisons. The third part of this research is about a conceptual model for turbulent spray combustion for two combustion regimes that has been proposed and tested for n-heptane solid cone spray type injected into a high-pressure, high-temperature open reactor by comparing to the available experimental data and to results obtained using two well known combustion models named the Combined Combustion Model (CCM) and the unsteady two-dimensional conditional moment closure (CMC) model. A single-zone intermittent beta-two equation turbulent model is suggested to characterise the Lumped zone. This model can handle both unburned and burned zones. Intermittency theory is used to account for the spatially non-uniform distribution of viscous dissipation. The model suggests that the Lumped zone can be identified by using the concept of Tennekes and Kuo-Corrsion of isotropic turbulence that suggests that dissipative eddies are most probably formed as vortex tubes with a diameter of the order of Kolmogorov length scale and a space of the order of Taylor length scale. Due to the complexity of mixture motion in the combustion chamber, there exist coherent turbulent small scale structures containing highly dissipative vortices. The small size eddies play an important role in extinguishing a diffusion spray flame and have an effect on the combustion reaction at molecular scale because small scales turbulence increase heat transfer due to the dissipation. A common hypothesis in constructing part of the model is if the Kolmogorov length scale is larger than the turbulent flame thickness. The Lumped strategy benefits from capturing small reactive scales information provided by numerics to improve the modelling and understand the exact implementation of the underlying chemical hypothesis. The Lumped rate is estimated from the ratio of the turbulent diffusion to reaction flame thickness. Three different initial gas temperature test cases are implemented in simulations. Lumped spray combustion model shows a very good agreement with available experimental data concerning auto-ignition delay points.

Abstract to“Interfacial colloidal particle films andtheir structure formation”; a licentiate thesis, whichwill be presented by Sandra Rödner in Q2, 29 November 2002at 13.00. Colloidal particles can be made to organise themselves intoordered arrays. These colloidal structures acquire interestingand useful properties, not only from their constituentmaterials but also from the spontaneous emergence of mesoscopicorder that characterises their internal structure. Orderedarrays of colloidal particles, with lattice constants rangingfrom a few nanometers to a few microns, have potentialapplications as optical computing elements and chemicalsensors, and also has an important influence on the mechanicalproperties and optical appearance of paint films and papercoatings. The control of colloidal structure formation starts with theparticle interactions (attractive or repulsive) and colloidaldynamics, which is the topic of this thesis. To enable adetailed understanding of the different factors that controlthe formation of dense 2D colloidal films, a method forstructural characterisation was developed. The degree of orderin the hexagonal close-packed structure, displayed by thecolloidal films, was characterised by the size of ordereddomains and by the distribution of pore sizes. The size ofordered domains was obtained from the pair distributionfunction, and the distribution of pores from a Delaunaytriangulation procedure. These methods are based on theparticle positions in the film, which were determined by lightmicroscopy and processed digital images. The two methods were used to study the effect of particleinteractions on the structure of colloidal monoparticulatefilms, formed at the air-liquid interface. The size of theordered domains decreased exponentially with increasing bondstrength, while the pore density increased. The transfer andsubsequent drying of the formed film on a solid substrateinduced structural changes; the capillary forces transformedsmall pores into triangular order while some of the largervoids and cracks increased in size. The structural features of colloidal monolayers, formed bydrying a dilute silica suspension on a substrate, wereinvestigated. Addition of small amounts of salt resulted indrastic changes of the particle film structure. The size of theordered domains decreased exponentially with increasing amountsof added salt (0-2.9% NaCl/Silica ratio), with a simultaneousincrease of the concentration of large defects. This suggeststhat loss of colloidal stability and onset of particle adhesionto the substrate inhibit rearrangement and ordering. Theevaporation rate was controlled by varying the relativehumidity during drying. Colloidal monolayers with the largestordered domains and the lowest concentration of stacking faultswere formed at an intermediate humidity (55% RH). The rearrangement process during drying of dilute silicasuspensions was followed in detail by studying the changes inthe structural features during growth of colloidal monolayers.Low crystal growth rate promoted the transition of squarelattice domains to a hexagonal close-packed structure. Additionof salt to the electrostatically stabilised dispersionincreased the formation of square structured regions at thecrystal-suspension interface, due to increasing adhesion to thesubstrate. The loss of colloidal stability inhibited therearrangement process, resulting in higher concentrations ofsquare lattice domains at large distances from the crystal edgecompared to systems without added salt. / NR 20140805

No description available.

Over the coming decades, both food consumption and agricultural water use are expected to increase in response to growing populations. In light of these concerns, there has been a growing awareness and appreciation of the objectives of agricultural sustainability, which has the broad aim of securing food and water resources, without adversely affecting the environment or disenfranchising future generations. To ensure that irrigated fields optimize their water use towards a more sustainable application while remaining compliant with any imposed restrictions on access to water supplies (i.e. through water licensing), it is necessary to understand and quantify the water consumption of crops at appropriate spatial and temporal scales. Evaporation (E), also commonly referred to as evapotranspiration (ET), is the physical process of water vapor transport from the surface into the atmosphere. Evaporation can be estimated via interpretive modeling approaches that combine meteorological, radiative, vegetation, and other related properties to estimate land surface fluxes at any given time. The research presented herein aims to investigate the evaporative response of agricultural croplands across a range of spatial and temporal scales, with a focus on high-resolution and field-scale estimation. In particular, we explore the utility of novel CubeSat imagery to produce the highest spatial resolution (3 m) crop water use estimates ever retrieved from space. These high-resolution results are expanded through time by retrieving a daily evaporation product, offering an enhanced capacity to provide new insights into precision agriculture. The effects and implications of higher spatiotemporal resolutions are explored and contrasted against governmental satellite missions that operate at lower resolutions. An exploratory study on the use of unmanned aerial vehicles (UAVs) is also performed, specifically in the context of their capacity to mount miniaturized thermal sensors: with the accuracy and limitations of these sensors for deriving evaporation-type products examined. The overarching goal of this research is to advance the utility of space-based estimates of evaporation for precision agricultural applications, and to provide new high-spatial and temporal agricultural insights that can be directed towards improving water management and address food security concerns in a more sustainable manner.

The hydrophobicity of a surface is defined as the degree to which it repels water molecules, and the internal contact angle that the droplet makes with the surface is a measure of the hydrophobicity. Contact angles less than 90° occur on hydrophilic surfaces, while contact angles greater than 90° occur on hydrophobic surfaces. If a surface's contact angle is greater than 120° the surface is commonly defined as superhydrophobic (SH). Superhydrophobicity is accomplished through a combination of microscale surface roughness and water repellant surface chemistry. The roughness creates cavities, or pockets, of vapor underneath the droplet which act to increase the effects of surface tension and lead to increased contact angles. The cavity fraction, F_c, of a surface is a measure of the surface roughness and is defined as the ratio of the projected cavity area to the projected total area of the surface. This thesis investigates the effects of varying cavity fraction, F_c, and substrate temperature, T_s, on heat transfer to evaporating water droplets. Distilled water droplets of nominally 3 mm in diameter were placed on heated SH substrates of varying F_c (0.5, 0.8, and 0.95). A smooth hydrophobic surface was included in the experiments for comparative purposes. The temperature of the surface was held constant at temperatures ranging from 60 to 230°C while the droplet evaporated. Measurements of droplet temperature and size were taken throughout the evaporation process using CCD and infrared camera images. These images were analyzed to yield heat transfer rates for the various surface types and surface temperatures studied. At temperatures below the saturation point of water, average droplet temperatures and heat transfer rates decrease with increasing cavity fraction. Differences in heat transfer rate between substrates increase with substrate temperature. Nusselt number decreases as cavity fraction is increased. Cavity fractions less than about 0.5 show only modest differences in Nusselt number between surfaces. As cavity fraction approaches unity, differences in Nusselt number become amplified between surfaces. At temperatures above the saturation point of water, boiling behavior on SH surfaces deviates dramatically from that of smooth untextured surfaces. Average heat transfer rates decrease with increasing cavity fraction. Nucleate boiling is delayed to highter superheats than normal or is not observed. The Liedenfrost point is advanced to lower superheats as cavity fraction is increased. Similar heat transfer rates are observed beyond the Leidenfrost point.

Inspired by the dilute spray regime in spray combustion, this dissertation explores the evaporation and combustion of an isolated droplet. Under a highly convective environment inside a gas combustor, due to imbalance of inertial and surface tension forces, the droplets of larger size in sprays exhibit notable deformations from spherical to non-spherical shapes. Such shape changes are generally observed but not quantified in experimental studies. Therefore, the effect of this deformation on droplet combustion dynamics is unknown yet. To bridge this gap, a comprehensive investigation of an isolated freely deforming droplet can be insightful as it can reveal more about the interaction of droplet shape with its evaporation and combustion. This work attempts to analyze and quantify the impact of such deformations on evaporation and combustion using interface-capturing Direct Numerical Simulation approach. With the focus on small-scale processes involved in evaporation as it is a pre-step for combustion, this dissertation first covers a thorough examination on evaporation of a deformable droplet under both natural and forced convection. A single component jet-fuel surrogate n-decane is chosen. To ensure that the droplet remains stationary throughout its lifetime, a novel numerical method called "gravity update method" is developed and implemented. The results obtained from these two separate studies are validated against experimental results and analytical correlations respectively. The findings from the investigation of droplet evaporation under forced convective flow at moderate Reynolds numbers are noteworthy. The droplet shape under such flow conditions is governed by Weber number (We) which is a ratio of inertial force to surface tension force. The results demonstrated upto 20% en- hancement in total evaporation rate for highly deformed droplets. This improvement is a net results of increased droplet surface area and alteration in the distribution of local evaporation flux ( m'' ). It is found that m'' is proportional to its curvature up to the point of flow separation which agrees with low Re theories on droplet evaporation by Tonini and Cossalli (International Journal of Heat and Mass Transfer 2013), Palmore (Journal of Heat Transfer 2022). Beyond the flow separation point, evaporation flux distribution depends on the boundary layer development and flow evolution downstream of the droplet. For highly deformed droplets, a larger wake region creates favorable fuel vapor gradients and promotes mixing in droplet wake, hence higher evaporation flux. Such positive impact of droplet deformation on total evaporation rate motivated further investigation on droplet combustion under a low Reynolds number convective flow. High pressure and temperature gas flow leads to Damköhler number is higher than 1. This fa- vors the generation of envelope type flame. The results show overall little sensitivity to combustion related parameters despite the droplet shape change and significant (upto 9%) enhancement in total evaporation rate. It is also noted that while burning, droplets do not reach critical deformation conditions and break-up even beyond the critical Weber number, suggesting the suppression of deformation due to faster evaporation rate. The findings presented in these studies provide substantial evidence for the interaction between droplet shape and flow dynamics. Therefore, it demonstrates the potential for enhancing the existing numerical models and analytical correlations by accounting the influence of droplet shape. / Doctor of Philosophy / This work is inspired by the spray combustion in gas turbines where the pressurized liquid fuel jet is injected in the combustion chamber and converted into dilute sprays after undergoing a series of processes. Due to the presence of higher air to fuel ratio for these spray droplets, they become the localized combustion sites with rapid evaporation rates. Understanding the evaporation of these droplets becomes crucial, as it sets the stage for their subsequent combustion. In an attempt to understand this chemically and fluid-dynamically complex phenomenon, abundant experimental studies are available with focus on overall atomization process and velocity field evolution. However, they lack in resolving the small-scale processes which govern the evaporation, therefore combustion. With the intent to investigate in detail about the combustion aspect, this problem is reduced to analyzing behavior of isolated droplets. Despite the sophisticated measurement technologies particle-scale processes such as temperature and species mass fraction evolution are yet unknown. Moreover, the majority of these studies are performed with simplifying assumptions. assumption has been that the droplet remains spherical throughout its lifetime. However, in practical applications, particularly when exposed to convective and turbulent environments, droplets can undergo significant deformation due to the presence of inherent surface tension of liquid. This deformation can influence their evaporation and burning rates. Additionally, the droplet's shape governs the flow field around it, potentially altering droplet-droplet interactions. Direct Numerical Simulation (DNS) approach is one of the numerical methods which can resolve both the phases. It offers a promising approach to reveal these small-scale details, such as droplet shape, vapor and temperature field around a droplet, droplet-droplet interaction, droplet motion etc. With the aim to bridge this gap, this dissertation focuses on the study of evaporation and combustion of an isolated deformable droplet under various conditions.

<p> As part of a CdS-cu2s thin film solar cell research project, a CdS evaporation system was designed and built using an Edwards 19E6 coating unit. With the overall aims of the project in mind, the apparatus was designed as part of a CdS-Cu2s dual, in situ, evaporation system. CdS films, ranging in thickness from lμ to 25μ, produced by this system, were characterized optically, electrically and crystallographically as functions of the various evaporation parameters. </p> / Thesis / Master of Engineering (MEngr)