Spray models are increasingly becoming the principal tools in the design and development of gas turbine combustors. Spray modeling requires a knowledge of the liquid atomization process, and the sizes and velocities of subsequently formed droplets as initial conditions. In order to have a better understanding of the liquid atomization process,the breakup characteristics of plane liquid sheets in co-flowing gas streams are investigated by means of linear and nonlinear hydrodynamic instability analyses. The liquid sheet breakup process is studied for initial sinuous and varicose modes of disturbance. It is observed that the sheet breakup occurs at half-wavelength intervals for an initial sinuous disturbance and at full-wavelength intervals for an initial varicose disturbance. It is also found that under certain operating conditions, the breakup process is dictated by the initial varicose disturbance compare to its sinuous counterpart. Further, the breakup process is studied for the combined mode and it is found that the sheet breakup occurs at half- or full-wavelength intervals depending on the proportion of the individual sinuous and varicose disturbances. In general, the breakup length decreases with the increase in the Weber number, gas-to-liquid velocity and density ratios. A predictive model of the initial droplet size and velocity distributions for the subsequently formed spray is also formulated here. The present model incorporates the deterministic aspect of spray formation by calculating the breakup length and the mass-mean diameter and the stochastic aspect by statistical means through the maximum entropy principle based on Bayesian entropy. The two sub-models are coupled together by the various source terms signifying the liquid-gas interaction and a prior distribution based on instability analysis, which provides information regarding the unstable wave elements on the two liquid-gas interfaces. Experimental investigation of the breakup characteristics of the liquid sheet is performed by a high speed CCD camera and the measurement of the initial droplet size and distributions is conducted by phase-Doppler interferometry. Good agreement of the theoretical breakup length with the experiment is obtained for a planar, an annular and a gas turbine nozzle. The predicted initial droplet size and velocity distributions show reasonably satisfactory agreement with experimental data for all the three types of nozzles. Hence this spray model can be utilized to predict the initial droplet size and velocity distributions in sprays, which can then be implemented as a front-end subroutine to the existing computer codes.

Spray models are increasingly becoming the principal tools in the design and development of gas turbine combustors. Spray modeling requires a knowledge of the liquid atomization process, and the sizes and velocities of subsequently formed droplets as initial conditions. In order to have a better understanding of the liquid atomization process,the breakup characteristics of plane liquid sheets in co-flowing gas streams are investigated by means of linear and nonlinear hydrodynamic instability analyses. The liquid sheet breakup process is studied for initial sinuous and varicose modes of disturbance. It is observed that the sheet breakup occurs at half-wavelength intervals for an initial sinuous disturbance and at full-wavelength intervals for an initial varicose disturbance. It is also found that under certain operating conditions, the breakup process is dictated by the initial varicose disturbance compare to its sinuous counterpart. Further, the breakup process is studied for the combined mode and it is found that the sheet breakup occurs at half- or full-wavelength intervals depending on the proportion of the individual sinuous and varicose disturbances. In general, the breakup length decreases with the increase in the Weber number, gas-to-liquid velocity and density ratios. A predictive model of the initial droplet size and velocity distributions for the subsequently formed spray is also formulated here. The present model incorporates the deterministic aspect of spray formation by calculating the breakup length and the mass-mean diameter and the stochastic aspect by statistical means through the maximum entropy principle based on Bayesian entropy. The two sub-models are coupled together by the various source terms signifying the liquid-gas interaction and a prior distribution based on instability analysis, which provides information regarding the unstable wave elements on the two liquid-gas interfaces. Experimental investigation of the breakup characteristics of the liquid sheet is performed by a high speed CCD camera and the measurement of the initial droplet size and distributions is conducted by phase-Doppler interferometry. Good agreement of the theoretical breakup length with the experiment is obtained for a planar, an annular and a gas turbine nozzle. The predicted initial droplet size and velocity distributions show reasonably satisfactory agreement with experimental data for all the three types of nozzles. Hence this spray model can be utilized to predict the initial droplet size and velocity distributions in sprays, which can then be implemented as a front-end subroutine to the existing computer codes.

This study focuses on the importance of solute dissolution kinetics for cloud droplet formation. To comprehensively account for the kinetics, a numerical model of the process was developed. Simulations of cloud droplet growth were performed for solute diffusivity, droplet growth rates, dry particle and droplet diameters relevant for ambient conditions. Simulations suggest that high ambient supersaturations and a decrease in solute diffusivity are major contributors to significant decreases in effective solute surface concentrations. The numerical simulations were incorporated into Khler theory to assess the impact of dissolution kinetics on the droplet equilibrium vapor pressure. For CCN composed of partially soluble material, a significant increase was found in the equilibrium supersaturation of CCN.

A droplet may be prevented from wetting a solid surface by the existence of a lubricating film of air, driven by theromcapillary convection, between liquid and solid surfaces. The noncontact nature and the load-carrying capability of a nonwetting droplet lead to potential engineering applications, e.g., low-friction bearings. The present research consists of two thrusts. The first is aimed at quantifying nonwetting-system failures (film and pinning) triggered by application of a mechanical load, gaining insights to failure mechanisms. Experimental results show that film failure occurs over a wide range of droplet volumes when the temperature difference between the droplet and the plate, the driving potential of the free-surface motion, is small. Interferometric observations reveal flow instability just prior to film failure, with the growth of a nonaxisymmetric disturbance on a free surface (m = 1). Pinning failure becomes more prevalent as the temperature difference is increased, stabilizing the film flow. As part of the present investigation, a system was devised, allowing an oscillating free-surface to be reconstructed from a series of interferograms. The dynamic responses of the free surface reveal mode coupling, with harmonics of the input frequency excited through nonlinearity. The second thrust of the research succeeded in levitating and translating a droplet using the mechanism of permanent nonwetting. In this scheme, the droplet is heated by a CO2 laser and is placed above a cooled glass surface in order to drive the lubricating film that supports the weight of the drop. Furthermore, the position of the droplet can be controlled by moving the heating location, which leads to an asymmetry of the flow fields, driving air from the cooler-end of the droplet and propelling it towards the heat source. These demonstrations suggest the techniques potential use as a liquid-delivery scheme in a Lab-On-a-Chip system. Modeling is carried out to estimate propulsive forces on the droplet and to explain oscillatory behavior observed when excessive heating is applied on the drop. The concept to sandwich a droplet between two plates, a necessary configuration for levitating smaller droplets (less than mm-scale), is also discussed.

The current dissertation is motivated by the need for an improved understanding of aerosol water interactions both in subsaturated and supersaturated atmospheric conditions with a strong emphasis on air pollution and climate change modeling. A cloud droplet formation parameterization was developed to i) predict droplet formation from a lognormal representation of aerosol size distribution and composition, and, ii) include a size-dependant mass transfer coefficient for the growth of water droplets which explicitly accounts for the impact of organics on droplet growth kinetics. The parameterization unravels most of the physics of droplet formation and is in remarkable agreement with detailed numerical parcel model simulations, even for low values of the accommodation coefficient. The parameterization offers a much needed rigorous and computationally inexpensive framework for directly linking complex chemical effects on aerosol activation in global climate models. The new aerosol activation parameterization was also tested against observations from highly polluted clouds (within the vicinity of power plant plumes). Remarkable closure was achieved (much less than the 20% measurement uncertainty). The error in predicted cloud droplet concentration was mostly sensitive to updraft velocity. Optimal closure is obtained if the water vapor uptake coefficient is equal to 0.06. These findings can serve as much needed constraints in modeling of aerosol-cloud interactions in the North America. Aerosol water interactions in ambient relative humidities less than 100% were studied using a thermodynamic equilibrium model for inorganic aerosol and a three dimensional air quality model. We developed a new thermodynamic equilibrium model, ISORROPIA-II, which predicts the partitioning of semi-volatiles and the phase state of K+/Ca2+/Mg2+/NH4+/Na+/SO42-/NO3-/Cl-/H2O aerosols. A comprehensive evaluation of its performance was conducted against the thermodynamic module SCAPE2 over a wide range of atmospherically relevant conditions. Based on its computational rigor and performance, ISORROPIA-II appears to be a highly attractive alternative for use in large scale air quality and atmospheric transport models. The new equilibrium model was also used to thermodynamically characterize aerosols measured at a highly polluted area. In the ammonia-rich environment of Mexico City, nitrate and chloride primarily partition in the aerosol phase with a 20-min equilibrium timescale; PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. When RH is below 50%, predictions improve substantially if the aerosol follows a deliquescent behavior. The impact of including crustal species (Ca2+, K+, M2+) in equilibrium calculations within a three dimensional air quality model was also studied. A significant change in aerosol water (-19.8%) and ammonium (-27.5%) concentrations was predicted when crustals are explicitly included in the calculations even though they contributed, on average, only a few percent of the total PM2.5 mass, highlighting the need for comprehensive thermodynamic calculations in the presence of dust.

Spray cooling has proven to be efficient in managing thermal load in high power applications. Reliability of electronic products relies on the thermal management and understanding of heat transfer mechanisms including those related to spray cooling. However, to date, several of the key heat transfer mechanisms are still not well understood. An alternative approach for improving the heat transfer performance is to change the film dynamics through surface modification. The main goal of this study is to understand the effects of nano-scale features on flat heater surfaces subjected to spray cooling and to determine the major factors in droplet impingement cooling to estimate their effects in the spray cooling system. Single droplet stream and simultaneous triple droplet stream with two different stream spacings (500 μm and 2000 μm), experiments have been performed to understand the droplet-surface interactions relevant to spray cooling systems. Experiments have been conducted on nano-structured surfaces as well as on flat (smooth) surfaces. It is observed that nano-structured surfaces result in lower minimum wall temperatures, better heat transfer performance, and more uniform temperature distribution. A new variable, effective thermal diameter (de), was defined based on the radial temperature profiles inside the impact zone to quantify the effects of the nano-structured surface in droplet cooling. Results indicate that larger effective cooling area can be achieved using nano-structured surface in the single droplet stream experiments. In triple stream experiments, nano-structured surface also showed an enhanced heat transfer. In single stream experiments, larger outer ring structures (i.e. larger outer diameters) in the impact crater were observed on the nano-structured surfaces which can be used to explain enhanced heat transfer performance. Smaller stream spacing in triple stream experiments reveal that the outer ring structure is disrupted resulting in lower heat transfer. Lower static contact angle on the nano-structured surface has been observed, which implies that changes in surface properties result in enhanced film dynamics and better heat transfer behavior. The results and conclusions of this study should be useful for understanding the physics of spray cooling and in the design of better spray cooling systems.

This dissertation, studies two specific topics related to the research of surface science by employing the molecular dynamics (MD) approach, that of a water droplet deposited on a poly (methyl methacrylate) (PMMA) substrate and that of a single tricarboxylic acid derivative, 1, 3, 5-tri(carboxymethoxy) benzene (TCMB, C6H3(OCH2COOH)3 ) adsorbed on gold (100) and (110) surfaces. These can help engineers clarify the characteristics and phenomena of physical adsorption of the molecule, as well as contributing to the application of surface science. This work is divided into two parts. Effect of droplet size on the structural and dynamical behavior of a water droplet spreading on a PMMA amorphous surface: note that most experts prefer to consider a rigid model as the substrate in research of surface wetting because it is more efficient to run the MD simulation such that a long simulation can be accomplished in a short time. The results verify that the rigid model is not suitable to act as the PMMA substrate in simulation because it prevents the diffusion of PMMA molecules, which then affects the penetration behavior of water molecules in the droplet upon impact with the PMMA surface. Several sizes of water droplets are considered in order to understand the size influence of the droplet on the properties of water molecules and on the PMMA surface. The penetrated water molecules and the local roughness increase with a decrease in the size of the droplet, which also leads to a smaller contact angle of the water droplet on the PMMA substrate. When the droplet is composed of more than 1000 water molecules, the contact angle shows agreement with experimental results. As regarding the structure of the water molecule in the droplet on PMMA substrate, the average number of hydrogen-bonded penetrating water molecules is in inverse proportion to the size of the droplet By examining the velocity field, the regular motion of the water droplet is found during the equilibrium process and after the droplet reaches the equilibrium state. The diffusion of the water molecules shows a significant decrease for the penetrated water molecules and an increase as it gradually approaches the vapor/liquid interface. Finally, calculations at different regions are made for the vibration spectrum of the oxygen atom, life time, and the relaxation time of the hydrogen bond. The changes of the hydrogen-bond dynamics of the hydrogen bond are consistent with the change of the distribution of the hydrogen bond angle. Effect of surface structure on the structural and dynamical behavior of a tricarboxylic acid derivative molecule on Au surfaces: the dynamical behavior of the single tricarboxylic acid derivative, 1, 3, 5-tri(carboxymethoxy) benzene (TCMB, C6H3(OCH2COOH)3 ) on Au (100) and (110) surfaces by molecular dynamics simulation approach is studied to provide better understanding of surface diffusion. Four possible conformations of the adsorbed TCMB molecule on the Au surface are found, with differences arising from different numbers of CH2 groups adsorbed on the Au substrate. Both the number of CH2 groups in the TCMB molecule that interact with Au surface and the different geometric relationship between the TCMB molecule and the Au surface strongly affect the translational motion, rotational motion, interaction energy and the Lock-and-Key behaviors of the TCMB molecule. A poor complementarity between the TCMB molecule shape and atomic structure of the surface results in significant migration of the molecule and is therefore an unstable adsorption. These results will be useful for the design of a molecular monolayer.

The focus of this Ph.D. thesis is the theoretical, computational, and experimental analysis of electrohydrodynamics and ionization in the Array of Micromachined UltraSonic Electrospray (AMUSE) ion source. The AMUSE ion source, for mass spectrometry (MS), is a mechanically-driven, droplet-based ion source that can independently control charge separation and droplet formation, thereby conceptually differing from electrospray ionization (ESI). This aspect allows for low voltage soft ionization of a variety of analytes and flexibility in the choice of solvents, providing a multifunctional interface between liquid chromatography and mass spectrometry for bioanalysis. AMUSE is a versatile device that operates in an array format, enabling a wide range of configurations, including high-throughput and multiplexed modes of operation. This thesis establishes an in-depth understanding of the fundamental physics of analyte charging and electrokinetic charge separation in order to enhance droplet charging and ionization efficiency. A detailed electrohydrodynamic (EHD) computational model of charge transport during the droplet formation cycle in the AMUSE ion source is developed, coupling fluid dynamics, pressure and electric fields, and charge transport in multiphase flow. The developed EHD model presents a powerful tool for optimal design and operation of the AMUSE ion source, providing insight into the microscopic details of physicochemical phenomena, on the microsecond time scale. Analyte charging and electrohydrodynamics in AMUSE are characterized using dynamic charge generation measurements and high-spatial-resolution stroboscopic visualization of ejection phenomena. Specific regimes of charge transport, which control the final droplet charging, have been identified through experimental characterization and simulations. A scale analysis of the ejection phenomena provides a parametric regime map for AMUSE ejection modes in the presence of an external electric field. This analysis identifies the transition between inertia-dominated (mechanical) and electrically-dominated (electrospraying) ejection, where inertial and electric forces are comparable, producing coupled electromechanical atomization. The understanding of analyte charging and charge separation developed through complimentary theoretical and experimental investigations is utilized to improve signal abundance, sensitivity, and stability of the AMUSE-MS response. Finally, these tools and fundamental understanding provide a sound groundwork for the optimization of the AMUSE ion source and future MS investigations.

Includes vita. / Title from PDF of title page. / Document formatted into pages; contains 224 pages. / Thesis (Ph.D.)--University of South Florida, 2004. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: Particle Nucleation is the forcing function in the Emulsion Polymerization processes and it plays an important role in dictating the final properties of the latex produced. Identification of the main nucleation sites and characterizing them in terms of their size and composition is important for elucidating the mechanism of particle nucleation. This research focuses on identifying the most likely nucleation locus in emulsion polymerization processes by characterizing the initial conditions of the reaction mixture. In order to achieve this objective, a methodology was devised, which used a non-reacting model emulsion system instead of the original emulsion. The model emulsion system selected has the same dispersion properties as that of the monomer emulsion system, but different optical properties. The model emulsion system enabled the study of the distribution of the emulsifier using Uv vis spectroscopy. / ABSTRACT: This approach also eliminated the time constraint associated with sampling during a polymerization reaction. A quantitative deconvolution using the turbidity equation, was done on the transmission Uv vis spectra of the emulsions. This enabled the characterization of the emulsions in terms of their particle size distribution, particle number and the composition of the droplet populations comprising them. The studies conducted provide the experimental evidence for a previously unidentified nano-droplet population of size range 30 to 100nm in diameter. To further support this experimental evidence, calculations were performed to obtain the emulsifier distribution over the nano-droplet population. The calculations suggest the probability of existence of the nano-droplet population to be much higher than the probability of the existence of the swollen micelles. / ABSTRACT: The results, depending upon the emulsification conditions, indicate the presence of about 15 % to 80% of the dispersed phase in the nano-droplet population. The large interfacial area offered by the nano-droplet population due to their high particle numbers and high percentage of the dispersed oil phase in them, make them the most probable particle nucleation loci in emulsion polymerization processes. Designed experiments were performed to experimentally observe the changes in the nano-droplet populations. The effects of the process variables, namely pH, surfactant concentration and temperature, on the size and compositional characteristics of the nano-droplet population were investigated. The results suggested that the surfactant to oil ratio was the dominating factor governing the size and the weight percent of the dispersed phase in the nano-droplet population. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

Effective design of liquid fuel injection systems is a function of good understanding of liquid breakup mechanisms. A transient liquid breakup model is developed on the classical interfacial breakup theory by modifying the classical linear perturbation process to include time-dependent base and perturbed flow parameters. The non-isothermal condition on liquid jet instability and breakup is theoretically modelled; with the particular consideration of a spatially variation of surface tension along the liquid-gas interface. The model combines the classical interface hydrodynamic instability and breakup theory and heat-transfer through semi-infinite medium. Analytical liquid breakup model, which combines transient and non-isothermal effects on liquid jet breakup, is suggested. The suggested model could be simplified to the transient breakup model and the non-isothermal breakup model equivalents. A novel mechanistic model, which is based on a simple momentum balance between the injected jet and the aerodynamic drag force, is suggested for breakup length. A new model, which combines energy criterion and dual-timescale for turbulent shear in droplet dispersion, is suggested for droplet breakup criteria on the basis of critical Webber number. All developed models showed good predictions of available experimental data, and established empirical correlation, within the operational conditions of contemporary ICEs, specifically diesel engines. Continued research in these areas could benefit the development of the next generation of liquid fuel injectors and combustors – by accounting for transient effects and non-isothermal conditions in liquid jet breakup, and turbulent shear in droplet breakup.