Universität Stuttgart

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Institute: search.filters.UBSInstitut.Institut für Thermodynamik der Luft- und RaumfahrtPublication Type: search.filters.UBSTyp.Konferenzbeitrag

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Now showing 1 - 4 of 4
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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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    A comparison between transient heat transfer measurements using TLC and IR thermography
    (2017) Brack, Stefan; Poser, Rico; Wolfersdorf, Jens von
    Narrowband thermochromic liquid crystals (TLCs) and infrared thermography (IRT) are compared in the context of spatially resolved and transient heat transfer measurements. For accurate measurements the TLC coating was calibrated with a stationary method before the experiment. The IRT camera was in-situ calibrated with a surface thermocouple. A good agreement on temperature was achieved for both methods. The TLC data as a single-point-in-time measurement was evaluated for a time-independent heat transfer coefficient hTLC. The surface temperature history measured with the IRT camera enables an additional evaluation for a time-dependent hIR(t). In the case of one-dimensional heat conduction situations and late TLC indications, hTLC and hIR(t) agree well after the first 10 s of the experiment. An investigation of the heat transfer in the wake region of a vortex generator illustrated the influence of lateral conduction. This effect is taken into account by an analytical-empirical correction method for the TLC data. For the IRT data an evaluation method based on an analytical solution of the three-dimensional heat conduction equation is presented. Equally to the one-dimensional case the evaluation methods considering the lateral conduction effects agree well after 10 s while the other methods deliver erroneous results.
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    ItemOpen Access
    A comparison between transient heat transfer measurements using TLC and IR thermography
    (2016) Brack, Stefan; Poser, Rico; Wolfersdorf, Jens von
    Narrowband thermochromic liquid crystals (TLCs) and infrared thermography (IRT) are compared in the context of spatially resolved and transient heat transfer measurements. For accurate measurements the TLC coating was calibrated with a stationary method before the experiment. The IRT-camera was in-situ calibrated with a surface thermocouple. A good agreement on temperature was achieved for both methods. The TLC data as a single point measurement was evaluated for a time-independent heat transfer coefficient ℎTLC. The surface temperature history measured with the IRT-camera enables an additional evaluation for a time-dependent ℎCF(t). In the case of one-dimensional heat conduction situations and late TLC indications ℎTLC and ℎCF(t) agree well after the first 10 s of the experiment. An investigation of the heat transfer in the wake region of a vortex generator illustrated the influence of lateral conduction. This effect is not taken into account by any method and leads to greater differ-ences between ℎTLC and ℎCF(t).