Dissertations / Theses on the topic 'Droplet Collision'

本文データは平成22年度国立国会図書館の学位論文(博士)のデジタル化実施により作成された画像ファイルを基にpdf変換したものである
Kyoto University (京都大学)
0048
新制・論文博士
博士(工学)
乙第8868号
論工博第2977号
新制||工||996(附属図書館)
UT51-95-D461
(主査)教授 八田 夏夫, 教授 鈴木 健二郎, 教授 赤松 映明
学位規則第4条第2項該当

Spray drying is a process, which produces powders from the fluid state. This type of process is mostly used in the industrial sector. In this process, a liquid slurry is atomized, forming droplets, which are dried with hot air. During spray drying these droplets will interact and upon impact can show different types of interactions; droplet-droplet collisions as well as interactions with partially or completely dried particles, leading to agglomeration. The result of collision gives properties of the dried powder. The focus of the thesis is to investigate the droplet-droplet collision outcomes of WPC 80 (Whey Protein Concentrate 80) and Lactose. Then the effects of the absolute droplet diameter and the droplet diameter ratios are to be determined. Existing experimental setup and Image Processing Tool of MATLAB is used to study the collision outcome. The outcomes are shown in a regime map. The present results are compared with different products result and literature study. It is observed that there is an effect on collision outcome for different droplet size ratios and no effect for absolute droplet diameter.

Thesis (Ph. D.)--Ohio State University, 2005.
Title from first page of PDF file. Document formatted into pages; contains xxi, 225 p.; also includes graphics (some col.). Includes bibliographical references (p. 218-225). Available online via OhioLINK's ETD Center

Ce travail consiste en une étude des phénomènes de coalescence dans un nuage de gouttes, par la simulation numérique directe d'un écoulement turbulent gazeux, couplée avec une approche de suivi Lagrangien pour la phase dispersée. La première étape consiste à développer et valider une méthode de détection des collisions pour une phase polydispersée. Elle est ensuite implémentée dans un code couplé de simulation directe et de suivi Lagrangien existant. Des simulations sont menées pour une turbulence homogène isotrope de la phase continue et pour des phases dispersées en équilibre avec le fluide. L'influence de l'inertie des gouttes et de la turbulence sur le taux de coalescence des gouttes est discutée dans un régime de coalescence permanente. Un aperçu est donné de la prise en compte d'autres régimes de collision et de coalescence entre gouttes. Ces simulations sont la base de développement et de validation des approches utilisées dans les calculs à l'échelle industrielle. En particulier, les résultats des simulations sont comparés avec les prédictions d'une approche Lagrangienne de type Monte-Carlo et de l'approche Eulerienne 'Direct Quadrature Method of Moments' (DQMOM). Différents types de fermeture des termes de coalescence sont validés. Les uns sont basés sur l'hypothèse de chaos-moléculaire, les autres sont capables de prendre en compte des corrélations de vitesses des gouttes avant la collision. Il est montré que cette derniere approche prédit beaucoup mieux le taux de coalescence par comparaison avec les résultats des simulations déterministes
Coalescence in a droplet cloud is studied in this work by means of direct numerical simulation of the turbulent gas flow, which is coupled with a Lagrangian tracking of the disperse phase. In a first step, a collision detection algorithm is developed and validated, which can account for a polydisperse phase. This algorithm is then implemented into an existing code for direct numerical simulations coupled with a Lagrangian tracking scheme. Second, simulations are performed for the configuration of homogeneous isotropic turbulence of the fluid phase and a disperse phase in local equilibrium with the fluid. The influence of both droplet inertia and turbulence intensity on the coalescence rate of droplets is discussed in a pure permanent coalescence regime. First results are given, if other droplet collision outcomes than permanent coalescence (i.e. stretching and reflexive separation) are considered. These results show a strong dependence on the droplet inertia via the relative velocity of the colliding droplets at the moment of collision. The performed simulations serve also as reference data base for the development and validation of statistical modeling approaches, which can be used for simulations of industrial problems. In particular, the simulation results are compared to predictions from a Lagrangian Monte-Carlo type approach and the Eulerian 'Direct Quadrature Method of Moments' (DQMOM) approach. Different closures are validated for the coalescence terms in these approaches, which are based either on the assumption of molecular-chaos, or based on a formulation, which allows to account for the correlation of droplet velocities before collision by the fluid turbulence. It is shown that the latter predicts much better the coalescence rates in comparison with results obtained by the performed deterministic simulations

Dans cette thèse expérimentale, on s'intéresse aux collisions de gouttes mettant en jeu des interfaces asymétriques, soit deux gouttes constituées de liquide différent ou des gouttes de taille différente et recouvertes (ou non) d'une couche de particules hydrophobes. Dans une première partie, on étudie les collisions de gouttes de liquide immiscible. L'asymétrie de tels systèmes repose alors sur le contraste des propriétés des deux liquides : la tension de surface, la viscosité et la densité. Le résultat de ces collisions est une encapsulation totale d'un liquide par un autre ou une encapsulation suivie d'une fragmentation. On s'attache à décrire les régimes observés et à établir des lois permettant de prédire les limites de fragmentation de l'objet obtenu. La seconde partie est consacrée aux interfaces couvertes de particules hydrophobes. Pour ces systèmes, l'asymétrie réside à la fois dans la présence des particules sur une interface et pas sur l'autre et dans le contraste de taille entre les objets étudiés. Ainsi, on considère l'impact entre une petite goutte (recouverte ou non de particules) et une très grosse goutte (recouverte ou non de particules). On caractérise tout d'abord les propriétés mécaniques de ces interfaces via la propagation d'ondes de surface, notamment en terme de tension de surface effective et de module de courbure. Puis, on sonde, dans différentes situations d'impact, la robustesse de ces objets afin d'évaluer la capacité de ces couches particulaires à prévenir la coalescence

The complex nature of multiphase flows, particularly in the presence of non-Newtonian rheologies in the phases, limits the applicability of theoretical analysis of physical equations as well as setting up laboratory experiments. As a result, Computational Fluid Dynamics (CFD) techniques are essential tools to study these problems. Despite the advances in numerical simulation techniques in this field in the past decade, the applicability of these approaches are limited by challenges appearing in specific applications, and particular consideration must be taken into account for each of these problems. The present thesis aims at three-dimensional numerical solution of Newtonian/non-Newtonian multiphase flow problems in the context of finite-volume discretization approach with applications in different natural and industrial processes. This thesis is organized in five chapters. The first chapter aims at providing an introduction to the motivation behind this work. We also present some application of the context of this thesis in industrial processes, followed by a small introductory on the CTTC research group, objectives and the outline of the thesis. The core of this thesis lays within chapters two, three and four. In chapter 2, using a conservative level-set method, three-dimensional direct numerical simulation of binary droplets collision is performed. A novel lamella stabilization approach is introduced to numerically resolve the thin lamella film appeared during a broad range of collision regimes. This approach demonstrates to be numerically efficient and accurate compared with experimental data, with a significant save-up on computational costs in three-dimensional cases. The numerical tools introduced are validated and verified against different experimental results for a wide range of collision regimes where very good agreement is seen. Besides, for all the cases studied in this chapter, a detailed study of the energy budgets are provided. In chapter 3, the physics of a single droplet subjected to shear flow is studied in details, with a primary focus on the effect of viscosity on walls critical confinement ratio. First, we highly validate the ability of the numerical tools on capturing the correct physics of droplet deformation. This chapter continues by three-dimensional DNS study of subcritical (steady-state) and supercritical (breakup) deformations of the droplet for a wide range of walls confinement in different viscosity ratios. The results indicate the existence of two steady-state regions in a viscosity ratio-walls confinement ratio graph, which are separated by a breakup region. Overall, these achievements indicate a promising potential of the current approach for simulating droplet deformation and breakup, in applications of dispersion science and mixing processes. In chapter 4, with the help of experience gained in the previous chapters, a finite-volume based conservative level-set method is used to numerically solve the non-Newtonian multiphase flow problems. One set of governing equations is written for the whole domain where different rheological properties may appear. Main challenging areas of numerical simulation of multiphase non-Newtonian fluids, including tracking of the interface, mass conservation of the phases, small timestep problems encountered by non-Newtonian fluids, numerical instabilities regarding the high Weissenberg Number Problem (HWNP), instabilities encouraged by low solvent to polymer viscosity ratio in viscoelastic fluids and instabilities encountered by surface tensions are discussed and proper numerical treatments are provided in the proposed method. The numerical method is validated for different types of non-Newtonian fluids, e.g. shear-thinning, shear-thickening and viscoelastic fluids using structured and unstructured meshes, where the extracted results are compared against analytical, numerical and experimental data available in the literature.
La naturaleza compleja de los flujos multifásicos, particularmente en presencia de reologías no newtonianas, limita la aplicabilidad del análisis teórico de ecuaciones físicas y también de los experimentos de laboratorio. Por lo tanto, las técnicas de dinámica de fluidos computacional (CFD) son esenciales para estudiar estos problemas. A pesar de los avances en las técnicas de simulación numérica en esta área durante la última década, la aplicabilidad de estos enfoques está limitada por los desafíos que aparecen en las aplicaciones específicas, y se debe considerar de forma particular cada uno de estos problemas. La presente tesis tiene como objetivo la solución numérica tridimensional de los problemas de flujo multifase newtoniano / no newtoniano en el contexto del enfoque de discretización de volúmenes finitos con aplicaciones en diferentes procesos naturales e industriales. Esta tesis está organizada en cinco capítulos. El primer capítulo proporciona una introducción y la motivación de este trabajo. También presentamos alguna aplicación de esta tesis en procesos industriales, seguida de una corta introducción al grupo de investigación del CTTC, los objetivos y el resumen de la tesis. En el capítulo 2, utilizando un método CLS, se realiza una simulación numérica directa (DNS) tridimensional de colisión de gotitas binarias. Se introduce un nuevo enfoque de estabilización de lamella para resolver numéricamente la capa delgada de fluido ("lamella") que aparece durante muchos regímenes de colisión. Este enfoque demuestra ser numéricamente eficiente y preciso en comparación con los datos experimentales, con una importante reducción de costos computacionales en casos tridimensionales. Las herramientas numéricas introducidas se validan y verifican con diferentes resultados experimentales para diferentes casos de colisión en los que se observa un muy buen acuerdo. Además, para todos los casos estudiados en este capítulo, se proporciona un estudio detallado de los balances de energía. En el capítulo 3, se estudia en detalle la física de una sola gota sometida a flujo de cizallamiento, con un enfoque principal en el efecto de la viscosidad en el confinamiento crítico de las paredes. Primero, validamos la capacidad de las herramientas numéricas para capturar la física correcta de la deformación de las gotitas. Este capítulo continúa con el estudio tridimensional DNS de las deformaciones subcríticas (estado estable) y supercríticas (ruptura) de la gota para un amplio rango de confinamiento de paredes en diferentes relaciones de viscosidad. Los resultados indican la existencia de dos regiones de estado estable en un gráfico de una relación de confinamiento de las paredes y la viscosidad, que están separados por una región de ruptura. En general, estos logros indican un potencial importante del enfoque actual para simular la deformación y ruptura de las gotitas, en aplicaciones de la ciencia de la dispersión y los procesos de mezcla. En el capítulo 4, con la ayuda de la experiencia adquirida en los capítulos anteriores, se utiliza un método CLS de volumen finito para resolver numéricamente los problemas de flujo multifase no newtonianos. Las principales áreas desafiantes de la simulación numérica de fluidos multifásicos no newtonianos incluso el seguimiento de la interfaz, la conservación de masa de las fases, los problemas de pequeños paso de tiempo encontrados por los fluidos no newtonianos, las inestabilidades numéricas relacionadas con el problema del alto número de Weissenberg (HWNP), inestabilidades fomentadas por una baja relación de viscosidad de disolvente a polímero en fluidos viscoelásticos y las inestabilidades encontradas por las tensiones superficiales son discutidos y se proporcionan tratamientos numéricos adecuados para el método propuesto. El método numérico se valida para diferentes tipos de fluidos no newtonianos, utilizando diluyentes de cizallamiento, espesamiento de cizallamiento y fluidos viscoelásticos utilizando mallas estructuradas y no estructuradas, donde los resultados extraídos se comparan con los datos analíticos, numéricos y experimentales disponibles en la literatura.

Dans cette thèse, nous étudions deux aspects de la dynamique d'impuretés atomiques interagissant avec des nanogouttes d'hélium superfluide (He) : la photo-excitation d'alcalins sur une nanogoutte et le dopage de nanogouttes contenant des tourbillons (vortex) quantiques avec des atomes de gaz rares. Nous utilisons la théorie de la fonctionnelle de la densité d'hélium ainsi que sa version dépendante du temps pour en faire la description théorique. Le premier aspect a été effectué dans le cadre d'une collaboration avec des expérimentateurs sur la photo-excitation du rubidium (Rb). Les alcalins sont une sonde très intéressante des gouttelettes d'hélium car ils résident dans leur zone de surface, où il a été prédit qu'un taux de condensation de Bose-Einstein de 100% était possible en raison d'une densité inférieure à celle de l'hélium superfluide. Nos simulations montrent que les états excités 5p et 6p désorbent à des échelles de temps très différentes, séparées par 2 ordres de grandeur (~100 ps et ~1 ps pour 5p et 6p respectivement). Ces résultats sont en accord avec ceux de l'expérience pompe-sonde à l'échelle femtoseconde qui a étudié la photodesorption d'atomes de Rb. Cependant, dans nos simulations, l'excitation vers 5pPi_{3/2} aboutit à un exciplexe RbHe lié à la surface, contrairement à l'expérience où RbHe est éjecté. L'introduction de la relaxation de spin de Pi_{3/2} à Pi_{1/2} nous a permis de résoudre ce désaccord, l'exciplexe RbHe ayant alors assez d'énergie pour désorber. Le deuxième aspect concerne une investigation purement théorique inspirée par les travaux récents de Gomez et Vilesov et al., où les tourbillons quantiques étaient visualisés en dopant les nanogouttes d'hélium avec des atomes d'argent, puis en les faisant atterrir en douceur (soft landing) sur un écran de carbone. Les images au microscope électronique montrent de longs filaments d'agrégats d'atomes d'argent qui s'étaient accumulés le long des coeurs des vortex. La formation de réseaux de tourbillons quantiques à l'intérieur de nanogoutelettes dopées par du xénon est également mise en évidence par diffraction de rayons X qui montrent des pics de Bragg caractéristiques d'agrégats de xénon piégés dans les coeurs des vortex. Nous avons d'abord étudié des collisions frontales entre un atome de xénon, héliophile, et une nanogoutte de 1000 héliums, et comparé les résultats à ceux d'une étude précédente sur le même processus avec le césium (Cs), qui est héliophobe. Dans le cas de Xe une «boule de neige» se forme autour de lui quand il entre dans la nanogoutte, et il lui faut beaucoup plus d'énergie qu'au Cs pour qu'il puisse en ressortir. Quand il le fait, il emporte des héliums avec lui, contrairement au Cs. Nous avons ensuite simulé des collisions entre Ar/Xe et des nanogouttes d'hélium superfluides pour différentes vitesses initiales et paramètres d'impact afin de déterminer leur section efficace de capture. Ces simulations ont ensuite été répétées pour des gouttelettes hébergeant un vortex quantique. On observe que l'impact des impuretés induit de grandes déformations de flexion et de torsion de la ligne de vortex, allant jusqu'à la génération d'ondes de Kelvin hélicoïdales qui se propagent le long du coeur du vortex. Ar/Xe est bien finalement capturé par le vortex, mais pas dans son coeur. Nous avons également découvert que l'existence d'un réseau de 6 lignes de vortex dont les noyaux sont remplis d'atomes d'Ar donne une rigidité accrue à la nanogoutte qui permet de stabiliser le système nano-goutte + vortex même à de faibles vitesses angulaires. Nos simulations impliquant des nanogouttes d'hélium comportant des tourbillons quantiques ouvrent la voie à d'autres investigations sur des nanogouttes hébergeant un ensemble de vortex, en collision avec de multiples impuretés
In this thesis we investigate two aspects of the dynamics of atomic impurities interacting with superfluid helium (He) nanodroplets, namely the photo-excitation of alkalis on a nanodroplet and the doping process of nanodroplets hosting quantised vortices with noble gas atoms. For the theoretical investigations we use He density functional theory and its time-dependent version. The first aspect involves a joint experimental and theoretical collaboration that focusses on the photo-excitation of the alkali rubidium (Rb). Alkalis are a very interesting probe of He droplets since they reside in their surface region, where it has been argued that almost 100% Bose-Einstein condensation could be achieved due to a density that is lower than in bulk superfluid He. In our simulations we find that states excited to the 5p and 6p manifold desorb at very different timescales, separated by 2 orders of magnitude (~100 ps and ~1 ps for 5p and 6p respectively). This is in good agreement with experimental results where the desorption behaviour of photo-excited Rb atoms is determined using a femtosecond pump-probe scheme. However, in our simulations excitation to the 5pPi_{3/2}-state results in a surface-bound RbHe exciplex, contrary to the experimental case where the RbHe exciplex desorbs from the droplets surface. Introducing spin-relaxation from Pi_{3/2} to Pi_{1/2} into the simulations, the RbHe exciplex is able to desorb from the droplet's surface, which resolves this contradiction. The second aspect concerns a purely theoretical investigation that is inspired by recent work of Gomez and Vilesov et al., where quantised vortices were visualised by doping He nanodroplets with silver atoms, subsequently "soft landing" them on a carbon screen. Electron-microscope images show long filaments of silver atom clusters that accumulated along the vortex cores. Also the formation of quantum-vortex lattices inside nanodroplets is evidenced by using X-ray diffractive imaging to visualise the characteristic Bragg patterns from xenon (Xe) clusters trapped inside the vortex cores. First, head-on collisions between heliophilic Xe and a He nanodroplet made of 1000 He atoms are studied. The results are then compared with the results of a previous study of an equivalent kinematic case with cesium (Cs), which is heliophobic. Xe acquires a "snowball" of He around itself when it traverses the droplet and much more kinetic energy is required before Xe is able to pierce the droplet completely. When it does, it takes away some He with it, contrary to the Cs case. Next, collisions between argon (Ar)/Xe and pristine superfluid He nanodroplets are performed for various initial velocities and impact parameters to determine the effective cross-section for capture. Finally, the simulations are then repeated for droplets hosting a single quantised vortex line. It is observed that the impact of the impurities induces large bending and twisting excitations of the vortex line, including the generation of helical Kelvin waves propagating along the vortex core. We conclude that Ar/Xe is captured by the quantised vortex line, although not in its core. Also we find that a He droplet, hosting a 6-vortex line array whose cores are filled with Ar atoms, results in added rigidity to the system which stabilises the droplets at low angular velocities. Our simulations involving droplets hosting quantum vortices open the way to further investigations on droplets hosting an array of vortices, involving multiple impurities

The objective of the research was to investigate proton-coupled electron transfer (PCET) reactions involving antioxidants to gain insight into the detailed mechanisms of glutathione (GSH), Trolox, and α-tocopherol (α-TOH). PCET reactions are complex redox reactions that transfer electrons and protons sequentially or in concert. These reactions are ubiquitous in natural or artificial processes that produce electrochemical energy that is extractable as electricity or as chemical fuels of high energy content. Examples of processes based on PCET are photosynthesis, respiration, nitrogen fixation, carbon dioxide reduction, redox fuel cells, and artificial photosynthesis. Antioxidants were selected as a PCET model to understand the coupling between proton transfer (PT) and electron transfer (ET) in order to elucidate structure-reactivity relationships under different experimental conditions. PCET reactions were studied with a set of electrochemical techniques to propose a preliminary mechanism that could be validated with digital simulations matching the electrochemical response. In some cases, other analytical techniques were used to aid in the system characterization. This thesis presents the results and discussion of the effects of oxidant-base pairs on the mediated oxidation of GSH, the -2e-/-H+ process of Trolox in aqueous and nonaqueous solvents with various pH values, and the particle collision electrolysis of α-tocopherol in oil-in-water emulsion droplets on an ultramicroelectrode. Ultimately our goal was to determine the kinetic and thermodynamic factors that control PCET reactions so that they can be applied in designing artificial systems for the production of energy using more abundant reagents with lower cost and better yields.

Cette thèse de doctorat est dédiée au développement de modèles physiques pour l’étude des systèmes d’aspersion de gouttelettes d’eau en milieu réactif d’hydrogène-air pré-mélangée dans les centrales nucléaires. Des modèles d’ordre réduit sont développés pour décrire l’évaporation des gouttelettes d’eau dans la flamme, la dispersion des nuages de particules après le passage des ondes de choc et l’évolution de l’échelle caractéristiques de turbulence avec la présence d’un jet d’eau. Une nouvelle méthodologie est proposée pour évaluer les effets de l’évaporation par l’aspersion sur la propagation de la flamme d’hydrogène turbulente à l’intérieur d’un volume fermé et un modèle simple est développé pour la quantification de la décélération de la vitesse laminaire avec l’évaporation des gouttelettes à l’intérieur de la flamme. Également, un modèle analytique est proposé pour la prédiction de la dispersion de nuage de particule après le passage d’une onde de choc en s’appuyant sur le one-way formalisme avec une extension afin de prédire l’apparition d’un pic de densité du nombre de particules en utilisant le two-way formalisme. En ce qui concerne la modulation de la turbulence induite par les particules, un modèle simple est utilisé pour l’estimation des échelles intégrales de la turbulence induites par l’injection de nuage des particules. Ces modèles numériques développés peuvent être couplés pour être mis en œuvre dans les simulations numériques à grande échelle de l’effet du système d’aspersion sur les explosions accidentelles d’hydrogène dans les centrales nucléaires
This PhD dissertation is dedicated to develop simple models to investigate the effect of water spray system on the premixed hydrogen-air combustion in the nuclear power plants. Specific simple models are developed to describe the water droplet evaporation in the flame, particle cloud dispersion after the shock wave passage, and turbulence length scale evolution with the presence of a water spray. A methodology is proposed to evaluate the spray evaporation effects on the propagation of the turbulent hydrogen flame inside a closed volume and a simple model is developed for the quantification of the laminar velocity deceleration with the droplets evaporation inside the flame. An analytical model is proposed for the prediction of particle cloud dispersion after the shock passage in the one-way formalism and another analytical model is dedicated to describe the spray-shock interaction mechanism and predict the appearance of a particle number density peak using the two-way formalism. A review of the important criteria and physical modelings related to the particle-induced turbulence modulation is given and a mechanistic model is used for the estimation of the turbulent integral length scales induced by the injection of particle clouds. These developed numerical models can be coupled to implement in the large-scale numerical simulations of the spray system effects on the accidental hydrogen explosions in the nuclear power plants

博士
國立交通大學
機械工程系所
94
Abstract Collision dynamics between two nanoscale argon droplets with the same diameter of ~10nm under vacuum and pressurized environment is simulated using a parallelized cellular molecular dynamics (PCMD) simulation code. Simulation results show that the collision dynamics between two droplets can be very complicated, which strongly depends upon the magnitude of the background pressure, the relative inertia (or collision velocity) and impact parameter. These phenomena include bouncing, direct coalescence, stretching coalescence, stretching separation and shattering. Regime maps at different background pressures are constructed for the first time to the best knowledge of the author. Analysis of snapshots of molecular distribution, fragment size distribution, surface tension on droplet surfaces and energy transfer process during collision are used to explain the complicated collision dynamics. The research is divided into two phases, which is described as follows. In the first phase, a PCMD code is developed on memory-distributed parallel machines (e.g., PC-cluster system) by taking advantage of link-cell data structure, which is often used for fast search in constructing the Verlet list. Dynamic spatial domain decomposition using multi-level graph-partitioning technique is employed to enforce the load balancing among processors. A simple threshold scheme (STS), in which workload imbalance is monitored and compared with some threshold value during the runtime, is proposed to decide the proper time for repartitioning the domain. Results show that the parallel efficiency using one million L-J atoms reaches 57%, 35% and 65%, respectively, for condensed, vaporized and supercritical test cases at 64 processors of HP clusters at NCHC. In the second phase, the above developed PCMD code using L-J (12-6) potential is used to study the collision dynamics between two nanoscale droplets under vacuum and pressurized environments. Test conditions will include variations of the impact parameter (0-8 nm), relative velocity of droplets (20-1500 m/s), background gas pressure (0, 0.055 and 0.55 atm; ¥, 2312.3 and 216.9) and the background gas temperature is 216K. Observed phenomena can be categorized as bouncing, direct coalescence, stretching coalescence, stretching separation and shattering. Distributions of these regimes, as a function of relative velocity and impact parameter, are constructed for the first time for different background gas pressures. The simulation results under vacuum condition show that disruption, fragmentation and shattering can be easily observed at higher relative velocities, while direct coalescence can only be found at lower relative velocities. However, with the existence of background gas, disruption and fragmentation can only be observed at higher velocities than those under vacuum conditions. Bouncing at very low velocity (10-30 m/s) can be clearly observed under pressurized environments, which coincides with previous findings. In addition, stretching coalescence is observed for the first time at intermediate relative velocity and impact parameter under pressurized environment. Effects of the relative velocity, impact parameter and ambient pressure to the collision process are discussed in detail using the concept of the separable rotational energy and the vibration energy of the largest cluster during collision.

碩士
臺灣大學
機械工程學研究所
98
The droplets collision dynamics with different surfactant and viscous fluids are investigated. The high speed camera and reliable measuring methods facilitate the experimental procedures. Depending on the two independent factors of Weber number and impact parameter, the consequences of droplets collision can be categorized into five distinct regimes: (Ⅰ) coalescence after minor deformation, (Ⅱ) bouncing, (Ⅲ) coalescence after substantial deformation, (Ⅳ) reflective separation and (Ⅴ) stretching separation, and it reveals that the viscosity, surfactant types and concentrations all have great influences on regime diagrams, especially the finding of the extensive bouncing regime and the retardant occurrence of separation regime. It is because the Marangoni effect which is motivated by the surfactant gradient could manifestly affect droplet interfacial behaviors. However, these induced transformations will dominate over the microscopic situations such as droplet bouncing but become trivial in the macroscopic conditions such as droplet separation phenomena.

Binary droplet collision plays an important role in nature and in many technical processes involving sprays. The modeling of the collision outcomes, namely bouncing, coalescence, separation after temporary coalescence, and spatter (also called ‘shattering’ and ‘splashing’), establishes the basis for the investigation of the atomization processes on larger length scales. The aim of this thesis is to develop numerical methods that are employed in the prediction of the collision outcomes and the numerical investigation of the phenomena in binary droplet collisions which affect the collision outcomes. The in-house code Free Surface 3D (FS3D), which is based on the Volume of Fluid (VOF) method, is employed for the numerical simulations. The numerical investigations are restricted to head-on collisions. Spatter occurs at high energetic collisions, resulting in a thin liquid lamella that ruptures artificially in standard numerical simulations. In order to simulate spatter, an improved lamella stabilization algorithm has been developed and extensively validated. By means of properly chosen white noise disturbances of the initial velocity field, the instability of the rim of the collision complex is triggered and the spatter is successfully reproduced in the simulations. Very good agreements between the simulation results and the experiments are achieved. Based on the simulation results, the development of the rim instability is considered as an amplification of disturbances via a signal amplification system that is subdivided into three sequential connected subsystems. It is confirmed that the development of the rim instability in the linear phase of the instability can be predicted by the Rayleigh-Plateau instability theory. The influence of the droplet viscosity is studied numerically and it is shown that the collision outcome tends to be spatter when the droplet viscosity is reduced. This dependency decreases with the decrease of the droplet viscosity. The droplet viscosity influences the development of the rim instability mainly through varying the geometrical evolution of the rim. A successful elucidation of the mechanism of rim instability builds the foundation for the prediction of the occurrence of spatter and the prediction of the size distribution of the secondary droplets arising in spatter. The investigation of the mechanism of the rim instability in the context of binary droplet collisions is of general importance because the ejection of secondary droplets from an unstable rim also emerges in collisions of a droplet on a solid substrate or on a liquid film. Binary droplet collisions result in bouncing or coalescence at relatively small Weber numbers. The simulations of bouncing and coalescence have been successfully conducted by switching the boundary conditions on the collision plane. The simulation results are in good agreement with corresponding experiments. However, the simulations are not predictive because the collision outcome must be specified in advance. The difficulty of the prediction of bouncing versus coalescence lies in the fact that the thin gas film between the colliding droplets cannot be resolved in feasible simulations and that a physically meaningful coalescence criterion is missing in the numerical method. In order to facilitate the predictive simulation, a multi-scale simulation concept has been developed. In addition to the main solver FS3D, which solves the flow on the macroscopic scale, the multi-scale simulation concept consists of three parts: (1) A sub-grid-scale (SGS) model is integrated within the main solver FS3D. (2) Coalescence is numerically suppressed before a suitable coalescence criterion is contingently satisfied. (3) A numerical coalescence criterion is applied. Based on the lubrication theory, the SGS model is derived which accounts for the rarefied flow effect. The SGS model is implemented in FS3D and extensively validated. For the integration of the SGS model, the pressure in the gas film, which is solved by the SGS model, applies as a pressure boundary condition on the collision plane. Employing the first intersection of PLIC-surfaces with the collision plane as coalescence criterion, the collision outcome in the simulation can be both bouncing and coalescence. The predicted collision outcome, however, depends on the grid resolution. Employing zero gas film thickness (in algorithm tolerance) as coalescence criterion, the simulations result only in bouncing. It is shown that various possible corrections of the velocity field, which decides the transport of the liquid phase, have not led to a meaningful prediction of the transition between coalescence and bouncing. Further developments, e.g. the volume-averaged Volume of Fluid (VA-VOF) method, which takes into account the velocity difference within a computational cell, shall be implemented in future work to increase the accuracy of the transport of the fluid phase. By means of the multi-scale simulation it is qualitatively shown that the collision outcome tends to be coalescence at higher rarefaction in the gas phase.

碩士
國立臺灣大學
機械工程學研究所
96
In this study, we investigate the phenomena of droplet-droplet collision with different Ohnesorge number in a head-on collision condition, i.e., the impact number is zero. It aims at observing the phenomena of droplet collision and discussing the physical phenomena of that under a head-on situation. In addition, we try to find a different way from the original one to generate high-speed droplets. Instead of using a compressor to inject a liquid jet and then cutting it off into several droplets by a rotating disk, we simply let the accelerating air move through a tube and carry the droplets into high speed. One of the main goals of this experiment is to take clear photographs whether in lower Weber number or in higher one. The other is to achieve a relative high Weber number about 5100. On the other hand, we discuss how Ohnesorge number influences the droplet collision. Break-up phenomenon is observed in high-speed droplet collision. The Z-We and We-W figures which illustrate the trends of break-up diameter, break-up boundary, and splattering boundary are also obtained in this thesis, where Z indicates the Ohnesorge number and W is the non-dimensional parameter of the diameter of droplet with respect to the break-up diameter. Keywords: droplet collision, high-speed droplet, impact number, Ohnesorge number, Webber number

碩士
國立臺灣大學
機械工程學研究所
97
An investigation in droplet-film collision is experimentally conducted. The phenomena of a drop impinging upon a liquid layer are categorized into five characteristics including absorption, central jet, secondary droplet, multiple droplets, and closed crown. Four regime thresholds are defined to segregate these five characteristics. By changing the viscosity and the surface tension of the fluids, a peninsular trend is summarized, which describes the behaviour of these specific thresholds. The sheeting-splashing threshold is postponed when the viscosity of the fluid is increased, and is advanced if the surface tension of the solution is decreased. Moreover, the material effect on the droplet-film collision is discussed. The turning point of each threshold arises with higher Weber number when the aluminum plate is replaced for the acrylic plate.

碩士
國立臺灣大學
機械工程學研究所
106
Identical droplet-droplet collisions were studied, with emphasis on the criterion for boundary transition between regimes of rotational separation and regimes of coalescence. Based on the confirmatory experimental results and the numerical simulation outcomes, two types of motion in the regimes of rotational separation were found to co-exist：region of interaction and the region outside of the region of interaction. The former is oscillating; the latter is rotating. The above two motions achieve good coherence that can overcome the bonding forces at critical time is the fragmentation criterion. A physical model is established and found to agree well with the experimental data. Further analyzing the criterion for reflexive separation and stretching separation, we based the physical model of the rotation separation on head-on reflexive separation and stretching separation. We constructed two new physical models for these two regimes after making some modifications. Both physical models and experimental results are in good agreement. By means of the physical models we established, we observed that the critical time of head-on reflexive separation is the very time while the region of interaction is stretching. On the other hand, the critical time of rotation separation is the second times while the region of interaction is stretching. This outcome is consistent with the previous study that rotational separation is second reflexive separation.

碩士
國立臺灣大學
機械工程學研究所
90
This article is targeting on a PZT driven droplet generator to proceed the structure design, analysis and performance tests, as well as the observation of micro-droplet impacting on a plate. The structure characteristics of the droplet generator are simulated by finite analysis software ANSYSTM. And, the droplet formulating processes are simulated by MEMS analysis software MEMCADTM. After that, the designed PZT driven droplet generator is fabricated and set up. To compare and verify the results of experiment and analysis, the performance of the whole system and the characteristics of formulating droplet are experimentally tested. As to the micro-droplet impacting on a plate, there have been plenty of related literatures. However, restricted by the experiment and observation equipments, few of the past studies conducted the micrometer scale droplet research. In terms of the application of high technological industries, the size of the droplet is getting more and more minute. Therefore, the understanding of dynamic characteristics of micro droplet is of great urgency. This research used the PZT driven droplet generator to produce micro-droplet, took the observation of the micro-droplet impacting on a plate and probed into its observed results. This research integrated the research method of droplet generator equipments in the former part and the characteristic of the micro-droplet formulated by the equipments in the latter part to build up a complete procedure for design, analysis, and test of the micro-droplet generator. As a result, they can help the application of the equipments in all industries and research fields to meet all kinds of needs.

碩士
國立臺灣大學
機械工程學研究所
96
The boundaries of droplet collision phenomena are defined by Weber number and Impact parameter, namely(I) coalescence after minor deformation, (II) bouncing, (III) coalescence after substantial deformation, (IV) coalescence followed by separation for near head-on collisions, and (V) coalescence followed by separation for off-centre collisions. According to earlier investigation, bouncing collision of water is not observed at 1 atm. Air. In this experiment, the surfactant, Triton X100, is dissolved in water in order to change the surface tension. Observing its collision phenomena, we can find bouncing collision and the boundaries of collision is changed. The influence of the surface tension on the boundaries of droplet collision will be discussed in this experiment.

碩士
國立臺灣大學
機械工程學研究所
91
In the study we investigated the phenomena of liquid-droplet impact the solid surface and displayed a certain appearance of the semicircle liquid film on the solid surface after impact. It was generated single steady liquid droplet by means of Ink-jet printing method. After liquid droplet detaches from generator and impacted through around air at room temperature and pressure. The kind of liquid droplet was distilled water and solid surface of impact was plastic. The method of study was we changed single liquid droplet size and impacted solid surface by different velocity. At the same time, we used a hemispherical liquid film was formed after liquid droplet impact solid surface, then fixed first liquid droplet size （static hemispherical droplet）and steady. To make changes in velocity and size of liquid droplet which impinging hemispherical liquid film (which size was greater, equal, and smaller). Therefore, first liquid droplet may regard as a hemispherical liquid film, and observes behavior of liquid droplet impacted hemispherical liquid film on the solid surface, and the outcome compared with model of single liquid droplet impacted on the solid surface. Primary parameters on experiment were size and impact velocity of liquid droplet. Finally, the sum and substance of experiment provided interaction of liquid droplet on the solid surface. Besides the result, in the experiment we found single liquid droplet impacted the solid surface, which produced a condition that had air bubbles were encompassed in hemispherical liquid film and made qualitative analysis.

碩士
國立臺灣大學
機械工程學研究所
99
An experimental investigation of binary droplet collision dynamics was conducted, with emphasis on the off-center separation outcomes and surfactant effect on collision behavior of hydrocarbon fuels. First, six kinds of hydrocarbon fluid and Span 80/hydrocarbon fluid were used in the experiment. The results show that collision outcomes of hydrocarbon droplets exhibit six distinct regimes, namely (I) coalescence after minor deformation, (II) bouncing, (III) coalescence after substantial deformation, (IV) reflective separation, (V) stretching separation and, (VI) second reflexive separation. Then we base on a basic droplet collision model with some assumption properly to explain the second reflexive separation. The other part of the study, we will analyze the critical Weber number of the Span 80 /hydrocarbon and silicone oil /hydrocarbon to prove that viscosity is the only factor affected by Span 80. Finally, we conclude that higher viscosity will promote droplet bouncing but become trivial as it reaches a critical value.

Research Doctorate - Doctor of Philosophy (PhD)
Droplet-particle collision interaction in a flowing gas stream is one of the major phase interaction phenomena in a wide class of multiphase process applications such as spouted bed coating, fluid catalytic cracking unit, fluid coking process for bitumen upgrade process etc. that govern the process performance to a significant extent. Such interactions are manifestations of complex hydrodynamics involving competing interplay of various forces e.g. viscous, capillary, inertial and gravity coupled with simultaneous heat and mass transport process which involves further complexity of phase change. Depending on the size ratio of the droplet-particle pair, relative velocity, physical properties, temperature difference, surface roughness and hydrophobicity; a number of different outcomes are possible which presumably affect the associated transport phenomena to a significant extent. For instance, inefficient contact of atomized feed droplets and hot catalyst particles adversely affects the desired product yield in a fluid catalytic cracking unit. Motivated by a dearth of knowledge in this field, the present research aims at investigating some of these interaction mechanisms with specific focus on the single droplet-particle system to broaden the mechanistic understandings using both non-invasive optical technique (high speed imaging) and numerical modelling wherever applicable. Based on the droplet-particle size ratio (Δ), there different systems were studied: Δ < 1, Δ > 1 and Δ ~ 1. For Δ < 1 system, normal collision behaviour of different fluids namely water, isopropyl alcohol and acetone was studied on a highly thermally conductive stationary brass particle surface in the temperature ranging from normal atmospheric condition (20 °C) to film boiling regime (250 °C - 350 °C) at different impact velocities (0.34 - 1.67 m/s) of droplets using high speed imaging technique. With increasing impact velocity (Weber number), three distinct outcomes were noted – deposition or wetting behaviour at normal atmospheric condition to nucleate boiling regime and then rebound and disintegration in the film boiling regime. In the impact dynamics, broadly two distinct phases were observed – inertia-dominated spreading or advancing phase and surface tension dominated recoiling or receding phase below disintegration limit and only spreading phase above this limit. Specifically in the film boiling regime, a critical Weber number range was determined wherein the transition from rebound to disintegration regime occurs. Two important parameters were quantified – maximum wetted contact area and droplet-particle contact time that governs the collision induced heat transfer process. Using image analysis, maximum spreading parameter was quantified to characterise the wetted area and correlated with impact Weber numbers using a general functional form. Also an analytical expression was suggested to determine this parameter on spherical surface based on an energy balance approach which showed reasonable agreement with experimental measurements. Also determined were a spreading kinetics of two functional forms – power law and recovery type exponential; both of which predicted the spreading trend well. Measured contact times were observed to have inverse dependency on the impact Weber number. Below limit of disintegration, a power law form of contact time as a function of Weber number was obtained which predicted the trend well and also confirmed a general form of contact time in film boiling regime with little dependency on the system characteristics. No such functional form could be established in the disintegration regime wherein contact time appears to be almost independent of Weber number. A 3D computational fluid dynamics (CFD) model based on volume of fluid (VOF) method was developed using the FLUENT platform to simulate the droplet deformation behaviour during impact for Δ < 1. It was shown that the temporal evolution of complex droplet shapes depends critically on the contact angle boundary condition and use of dynamic contact angle improves the prediction compared with static contact angle. It was observed experimentally that in film boiling regime, contact angle hysteresis was minimal due to presence of the insulating vapour film at solid-liquid interface which enhances surface hydrophobicity. Also the effect of contact line velocity and surface temperature on the contact angle variation was found to be insignificant. Based on this observation, it was further shown that in film boiling regime, CFD model could predict the dynamics based on a static contact angle in the limit of super-hydrophobicity and a free slip wall boundary condition to account for the vapour film which reasonably agreed with the experiments both qualitatively and quantitatively. Also studied experimentally for the Δ < 1 system, was in-flight collision interactions between a number of small droplets and a larger particle at different droplet (Weber number ≈ 3.0 – 26) and particle (Reynolds number ≈ 14 – 46) impact velocities where both were in the moving state. The droplets were observed to undergo inelastic collision resulting into complete deposition onto the particle surface in lower Weber number cases and forming a thin film in the higher Weber number cases. The measured film thickness normalized by particle radius was found to be in the range of 0.033 -0.314. Also during collision, significant deflection in the particle trajectory was noted especially at higher droplet velocity. However the angle of deflection was observed to decrease when the relative velocity between droplet and particle was decreased. A force balance model was developed accounting for the impact behaviour during collision to predict the particle trajectory and velocity. The model predicted outcomes were in good agreement with the experimental measurements when the angle of deflection was small however larger deviations were noted when angle of deflections were relatively large. The deviations were attributed to a number of factors such as uncertainty in droplet size due to in-flight coalescence, loss of droplet momentum due to coalescence on particle surface and intricate rotational motion during impact which were not completely accounted in the model. For a Δ > 1 system, collision interactions between a small particle and a larger stationary supported spherical cap droplet were investigated at different particle impact velocities (Weber number ≈ 1.4 - 33). Two outcomes were noted – particle capture or retention at interface and penetration through the droplet interface. A one dimensional model was developed based on force balance approach to predict these collision outcomes. Effect of different competing forces namely gravity, virtual mass, buoyancy, drag, capillary and pressure were analysed. Among others, the capillary force was noted to have dominating effect however effect of the drag force was also observed to be significant when impact velocity was increased. The earlier mentioned CFD model was modified to include the effect of particle motion utilizing a dynamic meshing technique. Using a static contact angle and no-slip wall boundary conditions, the CFD model predicted outcomes were in reasonable agreement with the high speed visualizations and force balance model predictions. Also investigated for the Δ > 1 system was the collision interactions between a small particle and a stationary liquid film confined in a capillary tube using different diameters of particle and impact velocities since the complete penetration behaviour of the impacting particle could not be studied with a supported droplet. Three different outcomes were noted based on the impact Weber number - particle capture/retention at top interface; particle capture or retention at bottom interface and complete penetration through both interfaces. A criterion was developed based on the energy balance approach to predict the collision outcomes. In complete penetration cases, the particle was observed to entrain a certain amount of liquid mass with it which was explained by the end-pinching mechanism of ligament breakup. A model based on the energy balance approach was developed to quantify this liquid mass carryover. Also an empirical correlation was obtained to correlate the liquid mass carryover and the particle Bond number. A sensitivity analysis on the predictions of CFD model was performed using different contact angle boundary conditions – advancing, static and receding contact angle which could only predict a specific instance of the outcome well and not the overall outcome. The simulated particle trajectory and velocity were compared with the experimental measurements. Also the contributions of pressure force and viscous force predicted by the CFD model were analysed to explain the collision outcomes wherein pressure force was found be higher than the corresponding viscous force by at least an order of magnitude. The Δ ~ 1 system was studied experimentally for normal collision between an impacting droplet and a stationary particle where two outcomes were noted – deposition in lower Weber number cases and film formation in higher Weber number cases. Also studied here was the interaction behaviour in higher Weber cases involving heat transfer. The film was observed to rupture when the film reached a limiting thickness due to intense vaporization involving nucleate boiling at the apex point of the particle. This system was also studied computationally using a coupled level-set and volume of fluid (CLSVOF) CFD model using different combinations of droplet-particle size ratio, impact Weber number and impact parameter (collision angle). Three distinct outcomes were noted – deposition, ripping and coating and skirt scattering in the increasing order of impact Weber number which reasonably agreed with the predictions of LBM model previously reported by Gac and Gradon (2014). In both ripping and coating and skirt scattering outcomes, separation of ligaments was explained by the end-pinching mechanism. It was shown that all outcomes are governed by a competition between surface energy and kinetic energy and while higher surface energy favours deposition, higher kinetic energy leads to separation. Force analysis reveals that both pressure force and viscous force increases when impact Weber number is increased. Rise of these forces occur specifically in the early phase of interaction followed by a sharp decay in higher Weber number cases indicating separation of liquid ligament from particle surface. Also investigated was the effect of impact parameter (collision angle) which was shown to be critical for droplet-particle interaction. Increase in impact parameter leads to significant decrease in contact area of liquid-solid interface which consequently results in inefficient momentum transfer. This was evident by the corresponding decrease in the magnitude of both force and strain rate. In continuation with this study, a droplet-particle collision induced heat transfer model was developed applicable for FCC environment. The model computes the heat transfer coefficient based on a conductive heat transfer mechanism involving wetted contact area and droplet-particle contact time based on the interaction mechanism of Δ < 1 system. The model also includes volume fraction of droplets and solid particles to account for the multiphase environment. The developed heat transfer sub-model was incorporated into a lumped parameter vaporization model to compute vaporization times for typical gas-oil feed droplets. Computed vaporization times were compared with the predictions of other heterogeneous vaporization models reported in literature and found to be in reasonable agreement. Finally, a multi-particle system involving injection of an acetone jet in a bubbling fluidized bed of Geldart A-B particles was briefly studied to understand the collision interactions of droplets in a multi-particle environment involving vaporization. Several interesting phenomena like jet breakup into multiple droplets, re-suspension of solid particles due to vapour explosion, droplet shape deformation, coalescence, levitation and nucleate boiling were noted. Also evolution of the vapour concentration profile in freeboard region was visualized using Schlieren imaging technique. Numerically, a two-way coupled Eulerian-Lagrangian CFD model was developed using the FLUENT platform to simulate the jet vaporization process using user defined source terms for heat transfer between the liquid jet and the bed. Simulated vaporization phenomenon qualitatively agreed with experimental observations and captured the key feature of the vaporization process indicating diffusing nature of vapour concentration profile from bed surface to bulk. The CFD model also indicated significant reduction of local bed temperature which indirectly explained the presence of particle agglomerates found in the experiment.

碩士
國立臺灣大學
機械工程學研究所
103
The viscous and nanoparticle effect on droplet-droplet collision behavior at ambient conditions were studied experimentally. Glycerol solutions and silica nanofluids were used as working liquid providing viscosities in the range from 0.93 to 15.94 mPa‧s. The droplet image and collision history were recorded on a video recorder by using a strobe light synchronized with the droplet generator with various phase differences. The collision outcomes in terms of Weber number and the impact parameter could be categorized into six distinct regimes: (I) coalescence after minor deformation, (II) bouncing, (III) coalescence after substantial deformation, (IV) reflexive separation, (V) stretching separation and, (VI) rotational separation. It is shown that, by varying the viscosity of the glycerol solution through its concentration, the border between bouncing and coalescence were shifted toward lower impact parameter until bouncing appears on head-on with increasing viscosity and verified empirical correlation for the onset of reflexive separation for high viscosity fluids. Furthermore, it was found the collision behavior of 1% weight percentage concentration nanofluids was the same with water and separation occurred at higher Webber number with increasing concentration.

博士
國立臺灣大學
應用力學研究所
103
Digital microfluidics attracts much attention for its prospective applications to revolutionize biological laboratory procedures by allowing efficient assays with great versatility, small sample consumption and short detection duration. Droplets collision, colalescence and mixing behavior with different viscosities and surface tensions are the basic and important research in the development process of digital microfluidics. The aim of this study is to buildup the performance of bio-chemical detection device using droplet-based microfluidics. We manipulated the droplet on the self-assambled textured surface and investigated the different droplet coalescence profile, internal flow field inside the coalesced droplet, and mixing behavior inside the coalesced droplet caused by different characteristic (viscosity and surface tension) of fluids. We also investigated the difference of fluid mixing and reaction inside the droplet, and we show a simple and maneuverable method of digital microfluidics to modulate a biochemical reaction with a ternary droplet collision using a simple chemical reaction and DNA fluorescence resonance-energy transfer (FRET) test. We utilized micro-PIV and confocal microscopy to measure the coalescence process, internal flows, and mixing patterns of droplets with different viscosities and surface tensions after a head-on collision between a moving droplet and a stationary droplet on a wettability gradient surface. The results indicate that the mixing is driven sequentially by interior convection and diffusion once the two droplets touch each other; the convection endures less than 100 ms but dominates more than 60 % of the mixing. For the collision of droplets of identical surface tension, the surface tension affects the coalescence behavior; for the collision of droplets with distinct surface tension, the coalescence behavior and mixing quality depend on the colliding arrangement of stationary and moving droplets. We also used a high-speed camera to observe the color changing reaction inside a coalesced droplet. Compare to the traditional dye-mixing test, the chemical reaction inside the coalesced droplet facilitated the mixing of two counter-reactive fluids and was more than hundred times as efficient as the unreactive fluids mixing inside the coalesced droplet. Instead of mixing, chemical reaction inside a coalesced droplet is worth attention to the applications of digital microfluidic open-system. In droplet coalescence process, the characteristic of fluids and the ratio of volumes of two droplets caused different droplet coalescence profile especially the necking-curvature which affects the shape of the material interface between the two droplets in an initial phase. Capsules are used to protect, control and deliver drugs to the specific tissue. In recent year, multilayer microcapsules and nanocapsules are under review as multifunctional delivery systems. In this study, we also show a simple and maneuverable method to modulate the bio-chemical reaction for digital microfluidics on the surface by ternary droplet collision. The coalescence behavior and mixing quality are significantly concerned with the arrangement and configuration of different droplets on a droplet-based microfluidic system. This work significantly contributes to the understanding of droplet mixing and reaction in droplet-based microfluidic systems. Instead of mass transfer and mixing, chemical reaction inside a coalesced droplet is worth attention for digital microfluidic open-system. This work illustrates a correlation between the growth and evolution of chemical reaction and the profile (necking-curvature) of a coalesced droplet, which is also a significant reference in droplet-based microfluidic systems for biochemical use. Furthermore, the moduration of initial time and initial point of reaction inside the coalesced droplet is greater development potential on bio-chimical detection and cell-drug interaction test specifically.

碩士
國立臺灣大學
機械工程學研究所
97
This study focuses on numerical simulation for binary droplets collision. We compare the numerical results with the experimental ones by front-tracking method and volume of fluid (VOF) method. Front-tracking method is controlled artificially by prescribing the rupture of the inter-drop film in multiphase flow, and the simulation results are consistent with the experimental ones. In high-speed droplet collision, the surface tension force damping option in VOF increases the viscosity in the vicinity of the interface, which damps the capillary wave effect that is invariably generated at the interface by the surface tension. In the view of physics, the droplets are not supposed to rupture in the separation regime. Therefore, if the surface tension force damping option is selected, the droplet does not rupture during the collision. The simulation condition of head-on collision of binary equal-size droplets is set. Water and tetradecane are used to be the liquid phases to predict the phenomena of droplet collision under the atmosphere. We examine the feasibility of droplet collision simulation with VOF in CFD-ACE solver. The 3D simulation results of high-speed droplet collision are obtained by VOF.

碩士
崑山科技大學
機械工程研究所
99
The dynamic behaviors of alcohol fuel droplets impinging on the hot surface were investigated. The alcohol fuel were made from 85% to 97% of unlead gasoline and various concentration of alcohol , which were 3%，5%，10% and 15% of mathanol, ethanol or butanol. Alcohol Fuel droplets impinging on a hot surface were tested under a smooth surface, two altitudes (10cm and 15cm), two kinds of hot plate temperatures (100℃ and 200℃). The observation was achieved by using high speed digital camera. When the surface temperature was at 100℃, the experimental results showed that the phenomenon of fuel droplets were spread, recoiling, split, shock, boiling then stable. At 200℃, the fuel droplets were more intense and became spread, boiling, evaporating, splashing, and jumping up from the surface. We also found that M3, E3 and B3 had more strong evaporated phenomena than the others when the surfface temperature was at 200℃. At high surface temperature, the viscosity and surface tension of the alcohol fuel decreased and the diffusive diameter of droplets increased. At last, we found that the evaporating phenomena happened on B15 but M15 and E15 when the surface temperature reached 200℃. Comparing the physical properties of M15, E15 and B15, we found that butanol had higher energy density, low heat of vaporation and high specific energy as well. The low heat of evaporation might be the reason why B15 was easy to get evaporating. With the three physical properties, butanol might become better additive instead of ethanol or be the better energy substitute.

The injector faceplate of a liquid propellant rocket engine is comprised of numerous single-element atomizers to inject propellants into the engine thrust chamber. The sprays from these single-element atomizers interact and mix, and then develop a combined spray. The present study investigates the characteristics of combined spray from a multi-injector assembly discharging three identical hollow cone swirl sprays arranged in an equilateral triangular configuration. The experiments are carried out in a spray test facility using water as the experimental liquid for different values of pressure drop (ΔPl) across the atomizers. The images of combined spray, captured using the technique of backlighted shadowgraphy, are used to deduce quantitative details of spray interaction behavior, and laser-based optical diagnostic systems (Phase Doppler Interferometry, and Spraytec) are used to record droplet characteristics of the combined spray. A mechanical patternator is used to describe the evolution of liquid mass distribution of the combined spray at different values of axial distance (Z) from the atomizer exit. The interaction process between the individual sprays influences spray width and liquid sheet breakup characteristics of the combined spray, particularly for sprays with low ΔPl. The interaction zones of the combined spray are marked by three lobes of high liquid mass flux, which develop asymmetry in the spray cross section perpendicular to the spray axis. It is showed quantitatively that the level of asymmetry in the combined spray decreases with increase in Z. The analysis of droplets characteristics of the combined spray reveals the presence of droplet coalescence for sprays with low ΔPl and droplet shattering for sprays with high ΔPl, which highlights droplets collision effects caused by the interaction and mixing of individual sprays in multi-injector thrust chamber.

The concern with the environment led the human being to develop new alternative fuels to reduce pollution and mitigate the emission of greenhouse gases. The air transport sector and the burning of fossil fuels are responsible for a huge portion of the pollution. Therefore, introducing new sustainable ways to provide energy, such as biofuel, is of major importance. However, in order to make these new energy sources more efficient and safer, it is necessary to carry out studies related to the injection of fuel into the combustion chambers, and the impact of droplets. This study focuses on an experimental investigation of a single droplet impact onto a heated solid surface. The main purpose of this work is to analyse the influence of wall temperature on the impact morphology of a single droplet and observe the possible outcomes. To do so, in these experimental tests, Jet Fuel and HVO (Hydroprocessed Vegetable Oil) mixtures were used. The fluids tested were: water (as a control group), 100% Jet A-1, 75% Jet A-1 and 25% NExBTL, 50% Jet A-1 and 50% NExBTL, and 100% NExBTL. The present work studies the impact outcomes depending on the working fluids and the wall temperature. The impact energy was kept constant. Therefore, the Weber number in this experiment was set to W e = 320 by varying the droplet diameter or the impact velocity. Furthermore, different wall temperatures were chosen, that vary from Tw = 25ºC to Tw = 330ºC, to seek for every possible impact phenomenon and characterise the impact morphology. The impact dynamics were captured using a high-speed digital camera and the images were digitally processed. It was possible to observe the heat regimes for all fluids, as well as two additional regimes for the mixtures of 75% jet fuel - 25% HVO and 50% jet fuel - 50% HVO.
A preocupação com o ambiente levou o ser humano a desenvolver novos combustíveis alternativos para reduzir a poluição e mitigar a emissão de gases de efeito de estufa. O setor de transporte aéreo e a queima de combustíveis fósseis é responsável por grande parte da poluição. Por conseguinte, introduzir novas formas sustentáveis de fornecer energia, como os biocombustíveis, é de elevada importância. Contudo, de modo a tornar estes novos meios de energia mais eficientes e seguros, é necessário realizar estudos relativos à injecção de combustíveis nas câmaras de combustão e ao impacto de gotas. Este estudo é focado numa investigação experimental sobre o impacto de gotas numa superfície sólida quente. O principal objectivo deste trabalho é analisar a influência da temperatura da superfície na morfologia do impacto de uma única gota e observar os possíveis resultados. Para isso, nestes ensaios experimentais foram utilizadas misturas de Jet Fuel e HVO (Óleo Vegetal Hidroprocessado). Os fluidos utilizados foram: água (como grupo de controlo), 100% Jet A-1, 75% Jet A-1 e 25% NExBTL, 50% Jet A-1 e 50% de NExBTL, e 100% NExBTL. Estas misturas seguem os requisitos da aviação civil, no qual têm que conter um mínimo de 50% de jet fuel. O presente trabalho estuda os efeitos de impacto de uma gota em função da temperatura da superfície para diferentes fluidos. A energia de impacto foi mantida constante. Portanto, o número de Weber nesta experiência foi fixado em W e = 320, tendo variado ou o diâmetro da gota ou a velocidade de impacto. Além disso, foram escolhidas diferentes temperaturas da superfície, que variam entre Tw = 25ºC e Tw = 330ºC, para procurar obter cada fenómeno de impacto e caracterizar a morfologia do mesmo. As dinâmicas de impacto foram capturadas utilizando uma câmara digital de alta velocidade e as imagens foram processadas digitalmente. Foi possível observar os regimes de calor para todos os fluidos, bem como alguns adicionais para as misturas de 75% jet fuel - 25% HVO e 50% jet fuel - 50% HVO.

碩士
國立交通大學
機械工程系所
95
In this thesis, Parallelized cellular molecular dynamics (PCMD) to simulate two droplets consist of Helium or Xenon in nanoscale and adopt the L-J (12-6) potential to discuss the behavior and effects when two droplets collide in vacuum. In the simulation, parameters which influence the behavior of collision primarily involve the relative velocity between droplets, the impact parameter and the material we use. The simulation in this context sets the relative velocity of helium atom range from 250 m/s to 750 m/s, the relative velocity of xenon atom range from 250 m/s to 2250 m/s, and the impact parameters all range from 0 to 8.75 nm. By the way of visualization program “pvwin” we can observe several behavior of simulation as follow: Direct Coalescence, Stretching Coalescence, Stretching Separation, and Shattering. The greater the relative velocity and impact parameters are, the more obvious separation and rotation the droplets display after collision. Furthermore differences in material will affect the degree of shattering after collision. And we can compare the results with literature before to study the behavior and the change of energy in different molecular weights after the collision of droplets in the collision of droplets in nanoscale.

We propose a mathematical procedure to derive a stochastic parameterization for the bulk warm cloud micro-physical properties of collision and coalescence. Unlike previous bulk parameterizations, the stochastic parameterization does not assume any particular droplet size distribution, all parameters have physical meanings which are recoverable from data, all equations are independently derived making conservation of mass intrinsic, the auto conversion parameter is finely controllable, and the resultant parameterization has the flexibility to utilize a variety of collision kernels. This new approach to modelling the kinetic collection equation (KCE) decouples the choice of a droplet size distribution and a collision kernel from a cloud microphysical parameterization employed by the governing climate model. In essence, a climate model utilizing this new parameterization of cloud microphysics could have different distributions and different kernels in different climate model cells, yet employ a single parameterization scheme. This stochastic bulk model is validated theoretically and empirically against an existing bulk model that contains a simple enough (toy) collision kernel such that the KCE can be solved analytically. Theoretically, the stochastic model reproduces all the terms of each equation in the existing model and precisely reproduces the power law dependence for all of the evolving cloud properties. Empirically, values of stochastic parameters can be chosen graphically which will precisely reproduce the coefficients of the existing model, save for some ad-hoc non-dimensional time functions. Furthermore values of stochastic parameters are chosen graphically. The values selected for the stochastic parameters effect the conversion rate of mass cloud to rain. This conversion rate is compared against (i) an existing bulk model, and (ii) a detailed solution that is used as a benchmark. The utility of the stochastic bulk model is extended to include hydrodynamic and turbulent collision kernels for both clean and polluted clouds. The validation and extension compares the time required to convert 50\% of cloud mass to rain mass, compares the mean rain radius at that time, and used detailed simulations as benchmarks. Stochastic parameters can be chosen graphically to replicate the 50\% conversion time in all cases. The curves showing the evolution of mass conversion that are generated by the stochastic model with realistic kernels do not match corresponding benchmark curves at all times during the evolution for constant parameter values. The degree to which the benchmark curves represent ground truth, i.e. atmospheric observations, is unknown. Finally, among alternate methods of acquiring parameter values, getting a set of sequential values for a single parameter has a stronger physical foundation than getting one value per parameter, and a stochastic simulation is preferable to a higher order detailed method due to the presence of bias in the latter.
Graduate
0725 0608 0405
davidc@uvic.ca

碩士
國立臺灣大學
機械工程學研究所
93
Experiment applies Ink-Jet Printing method to generate dual free droplet streams and collided along their path as they fall. Then the collided droplets will fall into the combustion chamber freely and some collision states and burning characteristics are investigated. We use n-hexadecane, methanol, ethanol, and propanol in the first part of this experiment, and then use hexadecane and ethanol-MTBE(or beneze)-premixture in the second part. Then we will compare the results of these two parts. According to our experiment results, the combustion states of non-mixable n-hexadecane/methanol droplets is always micro-explosion in adhesive mode. The occurrence of micro-explosion will increase burning rate. In discussion on the n-hexadecane and methanol, we find strong micro-explosion and the zone of micro-explosion in 40%~90% in n-hexadecane volume ratio probably. In the second part of this experiment, we find that if we join the MTBE or beneze of the suitable volume ratio, can increase burning state. But if we join too much MTBE or beneze, it will make immiscible droplets become to multiple miscible droplets.

碩士
國立臺灣大學
機械工程學研究所
92
Experiment applies Ink-Jet Printing method to generate dual free droplet streams and collided along their path as they fall. Then the collided droplets will fall into the combustion chamber freely and some collision states and burning characteristics are investigated. We use diesel fuel, methanol, and water in the first part of this experiment, and use diesel fuel and water-methanol-premixture in the second part. Then we will compare the results of these two parts. According to the experiment results, the combustion states of water and diesel fuel are water droplet micro-explosion and extinction. The probability of the occurrence of micro-explosion will increase when the water droplet size decrease. In discussion on the diesel fuel and methanol we find strong and unstable micro-explosion. In the second part of this experiment, we find the difference of the concentration of water-methanol-premixture will affect the droplet collision and burning state.

碩士
臺灣大學
機械工程學研究所
95
Experiment applies Ink-Jet Printing method to generate dual free droplet streams and collided along their path as they fall. Then the collided droplets will fall into the combustion chamber. We can observe the characteristics of combustion : ignition delay、burning time、burning rate、flame shrinking and micro-explosion. We use diesel fuel, bio-diesel, and water in the first part of this experiment. In different droplet size, oxygen volume fraction observe the characteristics of combustion and micro-explosion. Use bio-diesel and methanol, ethanol in the second part. Then we can observe the characteristics of premixture and collision. According to the experiment results, the combustion states of water and bio-diesel are water droplet micro-explosion and extinction. The probability of the occurrence of micro-explosion will increase when the water droplet size decrease. Then find flame shrinking in this experiment. The methanol, ethanol and bio-diesel collision、combustion、micro-explosion stability better than diesel.

The spacetime dimensions of the particle source in proton-lead collisions at √sNN = 5.02 TeV are measured with the ATLAS detector at the Large Hadron Collider. Femtoscopic measurements are made from correlation functions built with charged pions identified by their ionization energy loss. The measured HBT radii that represent the source dimensions are presented differentially as a function of centrality, transverse momentum, and rapidity. The effect of jet fragmentation on the two-particle correlation function is studied, and a method using opposite-charge pair data to constrain its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair transverse momentum. A correlation of the radii with the local charged-particle density dN/dy is demonstrated. The scaling of the extracted radii with the mean number of participating nucleons is also used to compare a parameterization of an initial-geometry model that allows for fluctuations in the proton cross-section. The cross-term R_ol is measured as a function of rapidity, and a nonzero value is observed that agrees with hydrodynamic predictions. The HBT radii are also shown for central events in intervals of azimuthal angle relative to the 2nd-order event plane, pair transverse momentum, and flow vector magnitude, where the correlation functions are corrected for the event plane resolution. Significant modulations of the transverse HBT radii R_out, R_side, and R_os are observed. The orientation of this modulation is the same as that in heavy-ion collisions, in which they are attributed to hydrodynamic evolution from an elliptic initial geometry. The sign and transverse momentum dependence of these modulations are consistent with a hydrodynamic evolution of a short-lived medium.