CONICET | Buscador de Institutos y Recursos Humanos

Computational science is now indispensable to the solution of complex problems in every sector,from traditional science and engineering domains to such key areas as national security, public health,and economic innovation. Advances in computing and connectivity make it possible to develop com-putational models and capture and analyze unprecedented amounts of experimental and observationaldata to address problems previously deemed intractable or beyond imagination. Yet, despite the greatopportunities and needs, universities and the National government have not effectively recognized thestrategic significance of computational science in either their organizational structures or their researchand educational planning. In the past century, scientific research has been extraordinarily successful inidentifying the fundamental physical laws that govern our material world. At the same time, the advancespromised by these discoveries have not been fully realized, because the real-world systems governed bythese physical laws are extraordinarily complex. Computer-based simulation provides a means of solvingthe mathematical equations and predicting the behavior of complex systems that can only be describedempirically at present. One of the area of major contributions is that related with Computational Fluid Dynamics (CFD), inparticular outstanding research problems in multiphase flow have arisen in last decades with particularrelevance to energy technologies and to chart out a roadmap for solving such problems. This advance-ment has achieved the organization of several meetings or workshops, one of them, the Workshop onMultiphase Flow Research constitute a basis for technological and academic research. In the last edi-tion of such workshop it was ensure that by 2015 multiphase science-based computer simulations playa significant role in the design, operation, and troubleshooting of multiphase flow devices in fossil fuelprocessing plants. Nowadays the main topics in this area are: (1) Dense gas-solids flows and granular flows, (2) Dilute gas-solids flows, (3) Liquid-solids and gas-liquid flows, and (4) Computational physics and applications. Multiphase flow is prevalent in fossil fuel processes, appearing in the form of gas-solids, gas-liquid,and gas-liquid-solids systems. These systems are notoriously difficult to design and scale up. The volumefraction of different phases can vary from low to high within a short length scale. The flows invariablyspan multiple time and length scales and pose enormous computational and experimental challenges. Forexample, the granular flow in a fluidized bed may range from incompressible to hypersonic, while thegranular media may undergo a phase change similar to a gas-to-solid transition, all within the same reac-tor. The volume fraction, stress, and energy typically fluctuate spatially and temporally with amplitudescomparable to the mean. The interaction of the phases with boundaries is often complex and poorlyunderstood. Because multiphase flows may not exhibit a clear separation among the spatial and temporalmicro-, meso-, and macro- scales, advanced multiscale theories may be needed to analyze them. There was general agreement the importance of considering the challenges involved in building highlyefficient, near-zero emission fossil energy plants in the next 20 years. Addressing the challenges involvedin new power generation only would lead to great economic benefits. If the applications to chemical,petroleum, mineral, consumer products, and pharmaceuticals industries are considered, the potentialeconomic benefit is enormous. On the other hand it became clear that the envisioned computational fluid dynamic (CFD) simulationswould require a set of software tools, not one tool, for several reasons. First, there is a need for ahierarchy of models based on different levels of details, which in the order of decreasing levels of detailsare direct numerical simulation (DNS), discrete element modeling (DEM), continuum model, coarse-grained continuum model, and various reduced order models. For industrial applications it may bepossible to use only coarse-grained continuum models (for design) or reduced order models (for processsimulation and control), and the more detailed models (DNS, DEM, and continuum) are necessary forvalidating or for generating closures for the less detailed models. Second, although desirable, it is notcertain that the same software can be used for describing different types of flows (gas-solids, gas-liquid,gas-liquid-solids) or even for different flow regimes within a particular type of flow (dense and dilute ingas-solids flow; bubbly flow, plug flow, stratified flow, and slug regimes in gas-liquid flow). Third, thereare different numerical techniques such as finite volume, finite element, volume of fluid, and LatticeBoltzmann that need to be explored. Fourth, the distribution of the models could be an open or closedsource. Open source software is necessary for facilitating peer review and promoting scientific advances.Commercial (often closed source) software is preferred by industry because of the availability of usersupport and consulting services. Also the deployment must be staged so that the industry gets usablesoftware tools (not fully accurate) early on while the university and scientific researchers are continuallyrefining the tools. Finally, there is a need for several preprocessing tools for setting up simulations andpostprocessing tools for analyzing, visualizing, and data mining output results. While there is no need for all of the above software tools to be subsumed in monolithic software,researchers need the capability to compose novel hybrid or multiscale models or new applications bylinking different software tools: 1) link multiphase flow models at different levels of details (e.g., DEMwith continuum model); or 2) link multiphase models with models that consider other physical phenom-ena (e.g., stress analysis) or larger physical systems (e.g., process simulation). Therefore, the softwaretools are best developed as components with well-defined interfaces that can be linked using frameworksoftware. There is a need for a standard set of simulation cases (based on analytical solutions and physicaland numerical experimental data) that can be used by university and commercial software developers toverify (that the mathematical model is expressed and solved correctly by the software) and validate (thatthe mathematical model correctly describes reality) computational software. The desired turnaround time of computer simulations was determined to be about nine hours, so thatthe simulation results can be routinely used for design. Certain benchmark industrial problems need tobe specified. This is one of the major challenges for the next decade, make computational simulation inmultiphase flow adequate to the real need of designers. With all this introduction in mind this lecture pretends only to show some of the research lines thatmay arise from these needs. Historically multiphase researcher have adopted as problem one well defined in terms of flow regime,bubbly flows, drop atomization and break up, liquid film, disperse flows, among other. Even thoughthere is a lot to work for this specific problems the new paradigm establishes the simulation of transi-tions and also the situations where several different regime coesxist. In this work we put emphasis onhow to simulate multiphase flows where disperse flow and separated flow come together, for example avery frequently industrial problem like annular flow. Annular flow is a multiphase system formed by aliquid film flowing at walls with a disperse phase formed by majorily a gas phase and liquid drops diluteon it with a mechanism of entrainment and deposition that keep the system dynamics. To simulate thiscomplex system we need first to have an accurate simulation of separated flows, next being capable ofsimulate Kelvin-Helmholtz instabilities and drop entrainment before put together in one real problem.Therefore, the aim of this first stage of this project is to predict in the best way flow in heterogeneousmedia, i.e. flows formed by different fluids with different properties that put in contact at the interface atleast two different fluids where normally either surface tension effects or viscosity and/or density jumpsand this situation makes the pressure physicallly discontinuous. We can demonstrate that a discontinuousdiscretization for the pressure is better than a continuous one regarding mass conservation at the interfaceand therefore this is our first contribution. To finalize this part the method used to treat discontinuitiesis presented. The following step introduces here is the analysis of Volume of Fluid (VOF) and Level Set(LS) with its inherent advantages and disadvantages and the possibility to couple both of them in a betterhybrid method to capture the interface. Moreover, a particle based method for capturing the interfaceembedded in an Eulerian approach is also analyze as a future reserach line. On the other hand this workestablishes a first approximation to solve annular flow using a separated flow approximation for the liquidfilm with a one phase gas flow at the core with empirical correlations for drop entrainment and depo-sition following the papers of Fukano, Ishii, and others. An improvement in this analysis may be donereplacing single phase gas flow with a disperse eulerian multiphase flow. To finish the presentation sometrends about Direct Numerical Simulation (DNS) modeling of interfaces, following Kelvin-Helmholtzinstabilities with drop break-up are presented. Future results go in the direction of enhancing empiricalcorrelations with CFD results to put all this expertise in 0D/1D process simulators. Finally some words of acknowlege to lot of people involved in this project along the last 10 years notmentioned explicitly for brevity reasons. Here a summary of the contents is presented: state of the art and introduction to the subject definition of the physical and mathematical modeling of the problem presentation of computational methodologies employed to solve the problem, numerical approxi- mations and software used discontinuous pressure to solve discontinuities between different fluids a brief discussion of how to capture accurately the interface a presentation of very different industrial problems solved at present and some critical point of view for detecting research lines some future perspective and conclusions

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