04 Fakultät Energie-, Verfahrens- und Biotechnik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5

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Institute: search.filters.UBSInstitut.Institut für Technische VerbrennungAuthor: search.filters.author.Loureiro, Daniel Dias

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Now showing 1 - 3 of 3
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    DNS of multiple bubble growth and droplet formation in superheated liquids
    (2018) Loureiro, Daniel Dias; Reutzsch, Jonathan; Dietzel, Dirk; Kronenburg, Andreas; Weigand, Bernhard; Vogiatzaki, Konstantina
    Flash boiling can occur in rocket thrusters used for orbital manoeuvring of spacecraft as the cryogenic propellants are injected into the vacuum of space. For reliable ignition, a precise control of the atomization process is required as atomization and mixing of fuel and oxidizer are crucial for the subsequent combustion process. This work focuses on the microscopic process leading to the primary break-up of a liquid oxygen jet, caused by homogeneous nucleation and growth of vapour bubbles in superheated liquid. Although large levels of superheat can be achieved, sub-critical injection conditions ensure distinct gas and liquid phases with a large density ratio. Direct numerical simulations (DNS) are performed using the multiphase solver FS3D. The code solves the incompressible Navier-Stokes equations using the Volume of Fluid (VOF) method and PLIC reconstruction for the phase interface treatment. The interfaces are tracked as multiple bubbles grow, deform and coalesce, leading to the formation of a spray. The evaporation rate at the interface and approximate vapour properties are based on pre-computed solutions resolving the thermal boundary layer surrounding isolated bubbles, while liquid inertia and surface tension effects are expected to play a major role in the final spray characteristics which can only be captured by DNS. Simulations with regular arrays of bubbles demonstrate how the initial bubble spacing and thermodynamic conditions lead to distinct spray characteristics and droplet size distributions.
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    Resolving breakup in flash atomization conditions using DNS
    (2019) Loureiro, Daniel Dias; Reutzsch, Jonathan; Kronenburg, Andreas; Weigand, Bernhard; Vogiatzaki, Konstantina
    Flash boiling can occur in rocket thrusters operating in the vacuum of space when cryogenic propellants are injected into the reaction chamber that is initially at low pressure. The dynamics of this process will determine the spray breakup that will then drastically affect the mixing of fuel and oxidizer, the reliability of the ignition and the subsequent combustion process. A multiphase solver with interface capturing is used to perform direct numerical simulations (DNS) of the primary breakup of the liquid oxygen jet that is driven by homogeneous nucleation, growth, coalescence and bursting of vapour bubbles in the superheated liquid. Considering the main breakup patterns and droplet formation mechanisms for a range of conditions, we evaluate the effectiveness of the volume of fluid (VoF) with continuum surface stress (CSS) method to capture the breakup of thin lamellae formed at high Weber numbers. A grid refinement study shows convergence of the mass averaged droplet size towards a droplet diameter. The order of magnitude of the resulting diameter can be estimated based on the thermodynamic conditions.
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    Simulation and modeling of droplet formation in flash atomization of cryogenic rocket propellants
    (2024) Loureiro, Daniel Dias; Kronenburg, Andreas (Prof. Dr.)
    A critical step for rocket engine operation in the vacuum of space is the efficient atomization, mixing and ignition of the liquid propellants. In the case of cryogenic fluids the atomization process is driven by flash boiling as the fluid exits the injectors. It is well known that the flash boiling process is caused by the nucleation of microscopic vapour bubbles in the superheated liquid that drive the jet expansion and an intense phase change process. This work investigates the primary breakup mechanisms at the micro-scale that are currently not well understood, due to limited empirical data. For this, direct numerical simulations (DNS) are performed using a multi-phase solver with interface capturing, providing a level of detail not previously achieved for this type of atomization process. The ab-initio methodology relies on first computing exact solutions for spherical bubble growth in superheated liquid, capturing compressibility and interface cooling effects. This reference data is then used to calibrate the fluid properties and vaporization rate on larger scale DNS that focus on the pure fluid-mechanical processes. These simulations are able to fully capture the hydrodynamic interactions between a large number of bubbles, as they grow, deform and coalesce, leading to the breakup of the liquid matrix into a spray of small droplets. The high level of resolution requires the use of high performance computing techniques with an in-house developed DNS solver. Significant effort was also invested in the development of an efficient post-processing algorithm that captures surface area of individual droplets in addition to their volume, thus avoiding limiting assumptions of droplet sphericity that are necessary in most experimental and theoretical modelling approaches. A series of test cases with regular bubble arrays demonstrates how, by varying the thermodynamic conditions and nuclei number density, various breakup mechanisms are observed resulting in distinct droplet patterns. These are systematically correlated to a range of Weber and Ohnesorge numbers, providing a predictive model for breakup classification and droplet size estimation. These breakup patterns extend beyond common assumptions and hypotheses previously suggested in the literature. Further DNS using randomized bubble clusters confirm the initial observations and provide statistical data on the spray composition. The results obtained include droplet size distributions and time-resolved evolution of total spray surface area, across a range of Weber numbers. A droplet size estimator is proposed, uncovering a minimum for the mean droplet size that is expected for each given level of local superheat, in spite of the nuclei number density being generally unknown. The DNS data is then used to calibrate and improve on this model, from which the expected area-weighted (Sauter) mean diameter of the spray can be inferred and interpolated for various cryogenic fluids. Similarly, a DNS-calibrated model for peak surface area generation rate is proposed. The models and data provided can be used for the development of sub-grid scale models for fluid simulations in engineering applications, namely source terms for surface area generation and liquid-vapour mass fractions or transported stochastic variables for droplet size. The DNS-calibrated models suggest that for relatively high fluid temperatures, or locally high levels of superheat, droplets can be formed in the sub-micron range and at equally small time scales, which are beyond the range accessible by common experimental methods. At more conservative levels of superheat, the method suggests droplets in the one to ten micron range, which is compatible with empirical evidence. This work complements experimental studies that are often limited to measurements of the global spray characteristics and lack insight into the conditions and processes in the dense regions of the spray or near the nozzle. The level of physical detail and accuracy of the DNS method is, however, still limited by computational constraints. Nonetheless, the findings of this work provide a clear path for further refinement of the technique, as well as suggestions for further investigations, both numerical and experimental.