Numerical Analysis of a Single Droplet Impinging upon Liquid Films using the VOF Method

Over the last few decades, developing alternatives to fossil fuels has become increasingly important. Biofuels, in particular, have attracted interest not only as an alternative to expensive fossil fuels but also by providing a solution to several challenges that the modern world currently faces, which include worries regarding energy security, economic development, the need to mitigate climate change and achieving lower greenhouse gas emissions. The major objective of this dissertation is numerically simulating the vertical impact of a single droplet upon a liquid surface with the same liquid properties and surrounded by air. Four fluids were taken into account: water, 100% Jet A-1 and 75%/25% and 50%/50% of Jet A-1 and NEXBTL, respectively. Particularly, multiphase flows are complicated phenomena in nature due to the difficulty in accurately predicting the interface between the phases. The numerical model consists of solving the Navier-Stokes equations by means of the explicit Volume of Fluid (VOF) method for a 2D axisymmetric assumption. This analysis considers several parameters and models that define this phenomenon. The physical properties of the liquid and gas, which are density and viscosity, surface tension, gravity and the contact angles of the different fluids represent our physical model. For the solution approach, the Fractional Step Method (FSM) numerically solves the Navier-Stokes incompressible equations, the Geometric Reconstruction scheme tracks the interface between the liquid and the gas phase and the Continuum Surface Force (CSF) model includes the effects of surface tension. The numerical results are validated both qualitatively and quantitatively with available experimental results. Six different outcomes were numerically simulated: Prompt Splash, Crown Splash, Spreading, Jetting, Fingering and Bubbling. The numerical results for the initial four outcomes are in good agreement with the experimental results in terms of the initial impact and crown expansion. The fingering phenomenon displayed several issues concerning crown development due to the 3D nature of the instabilities formed at the crown rim. The bubbling phenomenon was not possible to numerically recreate. Information regarding the dynamics of this phenomenon is scarce and more research is required to understand the peculiarities of dome formation. It was also visualized prompt splash for the crown splash and bubbling initial stages, which did not correspond to the experimental results. There are several reasons that justify this discrepancy, such as the difficulty in capturing very tiny ejected droplets or the solution approach forcing prompt splash on some of the cases. Overall, 3D simulations are required for future analysis to accurately predict secondary atomization. Work toward the bubbling phenomenon must also be considered.Nas últimas décadas, o desenvolvimento de alternativas aos combustíveis fósseis tem-se tornado gradualmente mais relevante. Os biocombustíveis, em particular, atraem interesse, não apenas como uma alternativa ao elevado custo dos combustíveis fósseis, mas também como solução para vários desafios que o mundo moderno enfrenta atualmente. Estes desafios incluem preocupações com a segurança energética, desenvolvimento económico, a necessidade de mitigar a mudança climática e redução das emissões de gases de efeito de estufa. O principal objetivo desta dissertação é simular numericamente o impacto vertical de uma gota sobre uma superfície líquida com iguais propriedades e envolvida por ar. Foram considerados quatro fluidos: água, 100% Jet A-1 e 75%/25% e 50%/50% de Jet A-1 e NEXBTL, respectivamente. Particularmente, os modelos multifásicos são fenómenos complicados na natureza devido à dificuldade em prever, com precisão, a interface entre as diferentes fases. O modelo numérico consiste em resolver as equações de Navier-Stokes a partir do método explícito do Volume de Fluido (do inglês, Volume of Fluid) para um modelo bidimensional (2D) axissimétrico. Esta análise considera vários parâmetros e métodos para caracterizar a dinâmica do impacto de gotas. As propriedades físicas dos fluidos, a gravidade e os ângulos de contacto são utilizados para clarificar este modelo. Em termos de abordagem dos métodos de solução, o Método de Etapa Fracionária (em inglês, Fractional Step Method) resolve numericamente as equações incompressíveis de Navier-Stokes, o esquema de Reconstrução Geométrica (em inglês, Geometric Reconstruction) delimita a interface entre o líquido e a gás e o modelo de Força de Superfície Contínua (em inglês, Continuum Surface Force) inclui os efeitos da tensão superficial. Os resultados numéricos são validados qualitativa e quantitativamente com os resultados experimentais disponíveis. Seis cenários diferentes foram simulados numericamente: Prompt Splash, Crown Splash, Spreading, Jetting, Fingering e Bubbling. Os primeiros quatro fenómenos referidos estão de acordo com os resultados experimentais em termos do impacto inicial e da expansão da coroa. O fenómeno de fingering apresentou vários problemas relativos ao desenvolvimento da coroa devido à natureza tridimensional (3D) das instabilidades formadas nos limites externos da coroa. O fenómeno de bubbling não foi possível de recriar numericamente. É necessário compreender a dinâmica deste fenómeno e como, atualmente, a pesquisa atual é escassa, estudos devem ser feitos para entender as peculiaridades da formação deste tipo de cúpulas. Também foi visualizado a ocorrência de prompt splash para as fases iniciais do crown splash e do bubbling, que não correspondiam aos resultados experimentais. Há várias razões que justificam essa discrepância, como a dificuldade de capturar as gotas extremamente pequenas provenientes da atomização secundária ou os métodos de solução forçarem o splash inicial em alguns casos. Em termos de análises futuras, simulações 3D são necessárias para averiguar o número, tamanho e velocidade das gotas provenientes da atomização secundária. O fenómeno de bubbling também requer um estudo mais exaustivo no comportamento da dinâmica do impacto das gotas

Additive Manufacturing of Sn63Pb37 Component by Micro-coating

AbstractMicro-coating is a novel technology to build near-net component layer by layer, which uses a crucible and nozzle instead of a weld head and wire feeder to supply material compared with shaped metal deposition. A pneumatic system is adopted to adjust liquid metal flow rate and the layer height is controlled by the distance between nozzle and substrate. Height and width of a single channel are measured by confocal microscopy, it is found that the error between numerical results and experiment are 5.5% and 1.1%. Tensile stress vertically to the deposition layers reaches to 40.89Mpa, while tensile stress parallel to the deposition layers gives a value of 43.14Mpa. Yield stress of vertically and parallel to the layer are respectively 34.28Mpa and 35.23Mpa. Specimens exhibit better mechanical properties than casting component, whose tensile stress and yield stress are respectively 36.51Mpa and 29.25Mpa

Modeling Volcanic Ash Particle Impingement In The Hot Sections Of A Gas Turbine Engine

A computational study was conducted of axisymmetric droplet impingement on a flat surface at low droplet Reynolds numbers. The study was motivated by deposition of melted volcanic ash particles within gas turbine engines, which can pose significant safety risk for jet aircraft encountering volcanic ash clouds. The computations were performed using the combined level-set volume-of-fluid method for Reynolds numbers Re in range 0.05≤Re≤10, typical of volcanic ash impingement problems. Computational results were compared to typical assumptions for approximate droplet impact models at high Reynolds number. The computational predictions were validated using existing experimental data. The computations indicate that contact radius increases over short time in proportion to the square root of time, in agreement with short-time analytical predictions. The droplet shape was well approximated by a truncated spherical cap, which spread on the substrate surface an increasing amount as Re was increased. The axial velocity component was approximately independent of radius over most of the droplet, and the radial velocity component was observed to vary log-normally with axial distance. The dissipation rate was distributed throughout the droplet for low Reynolds numbers cases, but became increasingly localized near the contact line as the Reynolds number increased past unity

Methods for the Modeling and Simulation of Sprays and Other Interfacial Flows

University of Minnesota Ph.D. dissertation. September 2019. Major: Mechanical Engineering. Advisor: Sean Garrick. 1 computer file (PDF); xix, 172 pages.Interfacial multiphase flows involve the motion of at least two fluids separated by surface tension. Atomizing interfacial flows, colloquially known as sprays, are among the most important fluid dynamic systems because of their ubiquity; power generation, delivery of aerosolized medicines, and productive produce farming all depend fundamentally on the detailed control of sprays. Atomization remains poorly understood because of a historical and persisting inability to accurately and affordably measure the dynamics inside and near the spray orifice outlet -- it is therefore desirable to be able to numerically simulate sprays with high fidelity. This dissertation presents computational methods that aim to improve current shortcomings in the modeling and simulation of sprays. Accurately characterizing the interfacial curvature of poorly-resolved liquid structures is addressed by deriving a series of finite particle methods for computing curvature. The methods are verified in analytical curvature tests, and validated against the oscillation frequency of ethanol droplets in air. The finite particle method, leveraging dynamic length scale modification, is demonstrated to out-perform the widely-used height function approach. Tracking the location of interfaces is also addressed, for which a coupled Eulerian-Lagrangian point mass particle scheme is introduced that preserves a well-distributed particle field, can be applied to an arbitrary number of fluids, and does not limit the simulation time step. The Eulerian-Lagrangian method is demonstrated to out-perform contemporary geometric volume of fluid methods at resolutions relevant to spray simulation in a variety of analytical phase tracking tests, and is dynamically evaluated by simulating extending three-phase elliptical regions, droplet dynamics, and Rayleigh-Taylor instabilities. The Eulerian-Lagrangian method is then extended to an approach for consistently and conservatively solving multiphase convection-diffusion problems -- this extension is verified via two analytical heat transfer problems, and robustness is demonstrated by simulating heated air blast atomization. Each of these tests conserves thermal energy and preserves boundedness of the temperature field. This dissertation concludes by outlining paths for consistently and conservatively solving the multiphase Navier-Stokes equations and the multiphase large eddy simulation equations in the coupled Eulerian-Lagrangian point mass particle framework

Experimental, theoretical and computational modeling of flow boiling, flow condensation and evaporating falling films

The transition from single-phase to two-phase thermal systems in future space vehicles demands a thorough understanding of phase change methods in reduced gravity, including microgravity. In this study, phase change methods like flow boiling, flow condensation and evaporative falling-films are investigated experimentally, theoretically and computationally. The experimental part of the study consists of an investigation of the influence of inlet subcooling and two-phase inlet on flow boiling heat transfer and critical heat flux in a horizontal 2.5-mm wide by 5-mm high rectangular channel in different orientations with respect to Earth gravity using FC-72 as working fluid. High-speed video imaging is used to identify dominant interfacial characteristics for different combinations of inlet conditions and heating configurations. Gravity is shown having a dominant influence on interfacial behavior at low mass velocities, while inertia dwarfs gravity effects at high mass velocities. CHF variation between different orientations with respect to Earth gravity is large for low mass velocities and diminishes for high mass velocities. In the theoretical part of the study, a consolidated investigation of the complex trends of flow boiling CHF in a rectangular channel in both microgravity and for different orientations in Earth gravity are performed. Separate theoretical models are constructed to investigate subcooled inlet flows and saturated two-phase inlet flows. It is shown that the Interfacial Lift-off Model provides good predictions of CHF data for both gravitational environments, both single-sided and double-sided heating, and both subcooled and saturated inlet conditions. In the computational part of the study, CFD models are constructed for two separate phase change configurations. First, turbulent, free-falling liquid films subjected to evaporative heating, and second, annular flow condensation in vertical upflow configuration. Implemented in FLUENT, the models are used to predict variations of various flow and thermal parameters and compare the results with available experimental data. Energy transfer at the two-phase interface are implemented successfully with the aid of appropriate phase change models. For both phase change configurations, the CFD model was able to capture complex flow behavior observed in experiments and predict heat transfer coefficients with reasonable accuracy. Also included in this part is a comprehensive review of literature on computational modeling of various boiling and condensation applications. This part of the study is laying the groundwork for future implementation of CFD models in capturing more complicated flow boiling and CHF phenomena

A Smoothed Particle Hydrodynamics Method for the Simulation of Centralized Sloshing Experiments

The Smoothed Particle Hydrodynamics (SPH) method is proposed for studying hydrodynamic processes related to nuclear engineering problems. A problem of possible recriticality due to the sloshing motions of the molten reactor core is studied with SPH method. The accuracy of the numerical solution obtained in this study with the SPH method is significantly higher than that obtained with the SIMMER-III/IV reactor safety analysis code

Index to 1985 NASA Tech Briefs, volume 10, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1985 Tech Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

A Smoothed Particle Hydrodynamics Method for the Simulation of Centralized Sloshing Experiments

The Smoothed Particle Hydrodynamics (SPH) method is proposed for studying hydrodynamic processes related to nuclear engineering problems. A problem of possible recriticality due to the sloshing motions of the molten reactor core has been studied. The accuracy of the numerical solution obtained in this study with the SPH method is significantly higher than that obtained with the SIMMER-III/IV reactor safety analysis code

Estimation of the radioactive aerosols capture in accidental sequences of nuclear power plants

The turbulent submerged jets can be found fairly frequently in a great variety of processes, their study is essential in many industrial processes and engineering applications, such as in underwater propulsion, in metallurgical processes, in chemical processes or in the nuclear industry, among others. Within the nuclear world the submerged jets can occur in light water reactors (LWR), in both pressurized water reactors (PWRs) and boiling water reactors (BWR). These submerged jets are usually associated with complex multiphase flows, so that all processes occurring after such injection will be essentially unstable and turbulent. A hypothetical severe accident in a reactor can cause deterioration of the core, so that the fission products can escape from the core and be transported through the primary system and, finally, can be released to the surrounding environment. But if there is a volume of water in the escape pathway of aerosols, a discharge in the shape of submerged jet can occur, whether in a suppression pool (during an accident with loss of power, SBO, in a water reactor boiling BWR) or in the secondary of a steam generator (in an accidental breakage sequence tube / s in U in a steam generator, SGTR, in a pressurized water reactor, PWR). So that there is a capture of aerosols in those volumes, being reduced the amount of them that escape outside. Usually these sequences have been considered only for BWRs and for low discharge velocities, but these may also take place at higher velocities and, as mentioned previously, in PWRs. Throughout this thesis there is a contribution to a better understanding and quantification of natural mitigation processes that occur when a jet is discharged into a volume of water, so that it can be applied to discharges in suppression pools in a SBO sequence (BWRs), and inside of a steam generator during a SGTR event (PWRs). Being the central activity the expansion of SPARC90 code capabilities, so as to be able to quantify the aerosol capture that occurs when the discharge takes place at high velocity (originally the code only was developed to study discharges under globular regime, i.e., injection at low velocity). So the process followed to carry out this work can be divided into several stages. The first one focuses on the literature search for available information, in a specific way on submerged jets and, given the scarce specific information, this has been extended to the literature search of processes with phenomenologies that present analogies with submerged jets. Within this part, it has on the one hand, finding aspects of jet hydrodynamics and on the other, those related to aerosol capture processes. In a second stage, there are aspects of the implementation into the new code subroutines of the expressions found and / or developed in the previous stage. While for the third stage, remains the assessment of the capabilities and behavior of the new models implemented in the code. For this last stage, first, it has been proceeded to conduct a verification process which has been tested the code robustness. And secondly, it has been proceeded to perform a validation process, which has been carried out through the comparison of the results predicted by the code against the limited experimental data that are available under similar conditions to those of the model. Being the comparison against the experimental data satisfactory, showing a marked improvement in the code capabilities.Los chorros sumergidos turbulentos se pueden encontrar con bastante asiduidad en muy diversos procesos, siendo esencial su estudio en gran cantidad de procesos industriales y aplicaciones ingenieriles, como por ejemplo, en la propulsión submarina, en procesos metalúrgicos, en procesos químicos o en la industria nuclear, entre otros. Dentro del mundo nuclear los chorros sumergidos pueden presentarse en los reactores de agua ligera (LWR), tanto en reactores de agua a presión (PWR), como en reactores de agua en ebullición (BWR). Estos chorros sumergidos llevan asociados complejos flujos multifásicos, de forma que los procesos que tienen lugar tras dicha inyección serán esencialmente inestables y turbulentos. Un hipotético accidente grave en un reactor puede causar el deterioro del núcleo, de forma que los productos de fisión pueden escapar del núcleo y ser transportados a través del sistema primario y, finalmente, pueden ser liberados al medio ambiente circundante. Pero si existe un volumen de agua en la vía de escape de los aerosoles puede tener lugar la descarga en forma de chorro sumergido, ya sea una piscina de supresión (durante un accidente con pérdida del suministro eléctrico, SBO, en un reactor BWR) o el secundario de un generador de vapor (en una secuencia accidental con rotura de tubo/s en U en un generador de vapor, SGTR, en un reactor PWR). De forma que se tiene una captura de los aerosoles en dichos volúmenes, viéndose reducida la cantidad que escapan al exterior. Habitualmente estas secuencias se han considerado solamente para reactores BWR y para descargas a bajas velocidades, pero estas podrían tener lugar también a velocidades mayores y, como se ha dicho con anterioridad, en reactores PWR. A lo largo de esta tesis se ha contribuido a una mejor compresión y cuantificación de los procesos naturales de mitigación que se producen cuando se descarga un chorro en un volumen de agua, de forma que puede ser aplicado para descargas en piscinas de supresión en una secuencia SBO (reactores del tipo BWR), como en el interior de un generador de vapor durante una secuencia SGTR (reactores del tipo PWR). Siendo la actividad central la ampliación de las capacidades del código SPARC90, de forma que sea capaz de cuantificar la captura de aerosoles que tiene lugar cuando la descarga se produce a alta velocidad (originalmente el código solamente fue desarrollado para el estudio de descarga en régimen globular, es decir, inyección a baja velocidad). De modo que el proceso seguido para llevar a cabo el presente trabajo se puede dividir en varias etapas. La primera se centra en la búsqueda bibliográfica de la información disponible, de una forma específica relativa a chorros sumergidos y, dada la escasa información específica existente, esta se ha extendido a la búsqueda bibliográfica de procesos con fenomenologías que presentan analogías con los chorros sumergidos. Dentro de esta parte, se tiene por un lado, la búsqueda de los aspectos relativos a la hidrodinámica del chorro y, por otro, los relativos a los procesos de captura de aerosoles. En una segunda etapa se tienen los aspectos relativos a la implementación dentro de las nuevas subrutinas del código de las expresiones halladas y/o desarrolladas en la anterior etapa. Mientras que para la tercera etapa resta la evaluación de las capacidades y el comportamiento de los nuevos modelos implementados en el código. Para esta última etapa, en primer lugar, se ha procedido a llevar a cabo un proceso de verificación con el que se ha comprobado la robustez del código. Y en segundo lugar, se ha procedido a realizar un proceso de validación, el cual ha sido llevado a cabo a través de la confrontación de los resultados predichos por el código frente a los limitados datos experimentales disponibles en condiciones similares a las introducidas en el modelo. Siendo la comparación frente a los datos experimentales satisfactoria, mostrando una meEls dolls submergits turbulents es poden trobar amb prou assiduïtat en molt diversos processos, sent essencial el seu estudi en gran quantitat de processos industrials i aplicacions enginyerils, com per exemple, en la propulsió submarina, en processos metal·lúrgics, en processos químics o en la indústria nuclear, entre altres. Dins del món nuclear els dolls submergits poden presentar-se en els reactors d'aigua lleugera (LWR), tant en reactors d'aigua a pressió (PWR), com en reactors d'aigua en ebullició (BWR). Estos dolls submergits solen portar associats complexos fluxos multifàsics, de manera que els processos que tenen lloc després de la dita injecció seran essencialment inestables i turbulents. Un hipotètic accident greu en un reactor pot causar el deteriorament del nucli, de manera que els productes de fissió poden escapar del nucli i ser transportats a través del sistema primari i, finalment, poden ser alliberats al medi ambient circumdant. Però si existeix un volum d'aigua en la via de fuita dels aerosols pot tenir lloc la descàrrega en forma de doll submergit, ja siga una piscina de supressió (durant un accident amb perduda del subministrament elèctric, SBO, en un reactor d'aigua en ebullició, BWR) o el secundari d'un generador de vapor (en una seqüència accidental amb trencament de tub/s en U en un generador de vapor, SGTR, en un reactor d'aigua a pressió, PWR). De manera que es té una captura dels aerosols en els anteriorment dits volums, veient-se reduïda la quantitat d'ells que escapen a l'exterior. Habitualment aquestes seqüències s'han considerat solament per a reactors BWR i per a baixes velocitats de descàrrega, però aquestes podrien tenir lloc també a velocitats majors i, com s'ha dit amb anterioritat, en reactors PWR. Al llarg d'aquesta tesi s'ha contribuït a una millor compressió i quantificació dels processos naturals de mitigació que es produeixen quan es descarrega un doll en un volum d'aigua, de manera que pot ser aplicat per a descàrregues en piscines de supressió en una seqüència SBO (reactors del tipus BWR), com a l'interior d'un generador de vapor durant una seqüència SGTR (reactors del tipus PWR). Sent l'activitat central l'ampliació de les capacitats del codi SPARC90, de manera que siga capaç de quantificar la captura d'aerosols que té lloc quan la descàrrega es produeix a alta velocitat (originalment el codi solament va ser desenvolupat per a l'estudi de descàrrega en règim globular, és a dir, injecció a baixa velocitat). De manera que el procés seguit per dur a terme el present treball es pot dividir en diverses etapes. La primera d'elles se centra en la cerca bibliogràfica de la informació disponible, d'una forma específica relativa a dolls submergits i, donada l'escassa informació específica existent, esta s'ha estés a la cerca bibliogràfica de processos amb fenomenologies que presenten analogies amb els dolls submergits. Dins d'aquesta part, es té d'una banda, la cerca dels aspectes relatius a la hidrodinàmica del doll i, per un altre, els relatius als processos de captura d'aerosols. En una segona etapa es tenen els aspectes relatius a la implementació dins de les noves subrutines del codi de les expressions trobades i/o desenvolupades en l'anterior etapa. Mentre que per a la tercera etapa resta l'avaluació de les capacitats i el comportament dels nous models implementats en el codi. Per a aquesta última etapa, en primer lloc, s'ha procedit a dur a terme un procés de verificació amb el qual s'ha comprovat la robustesa del codi. I en segon lloc, s'ha procedit a realitzar un procés de validació, el qual ha estat dut a terme a través de la confrontació dels resultats predits pel codi enfront de les limitades dades experimentals disponibles en condicions similars a les introduïdes en el model. Sent la comparació enfront de les dades experimentals satisfactòria, mostrant una millora ostensible en les capacitats del codi.Berna Escriche, C. (2017). Estimation of the radioactive aerosols capture in accidental sequences of nuclear power plants [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90495TESI