04 Fakultät Energie-, Verfahrens- und Biotechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5
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Item Open Access DNS of multiple bubble growth and droplet formation in superheated liquids(2018) Loureiro, Daniel Dias; Reutzsch, Jonathan; Dietzel, Dirk; Kronenburg, Andreas; Weigand, Bernhard; Vogiatzaki, KonstantinaFlash 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.Item Open Access Sparse-Lagrangian PDF modelling of silica synthesis from silane jets in vitiated co-flows with varying inflow conditions(2020) Neuber, Gregor; Kronenburg, Andreas; Stein, Oliver T.; Garcia, Carlos E.; Williams, Benjamin A. O.; Beyrau, Frank; Cleary, Matthew J.This paper presents a comparison of experimental and numerical results for a series of turbulent reacting jets where silica nanoparticles are formed and grow due to surface growth and agglomeration. We use large-eddy simulation coupled with a multiple mapping conditioning approach for the solution of the transport equation for the joint probability density function of scalar composition and particulate size distribution. The model considers inception based on finite-rate chemistry, volumetric surface growth and agglomeration. The sub-models adopted for these particulate processes are the standard ones used by the community. Validation follows the “paradigm shift” approach where elastic light scattering signals (that depend on particulate number and size), OH- and SiO-LIF signals are computed from the simulation results and compared with “raw signals” from laser diagnostics. The sensitivity towards variable boundary conditions such as co-flow temperature, Reynolds number and precursor doping of the jet is investigated. Agreement between simulation and experiments is very good for a reference case which is used to calibrate the signals. While keeping the model parameters constant, the sensitivity of the particulate size distribution on co-flow temperature is predicted satisfactorily upstream although quantitative differences with the data exist downstream for the lowest coflow temperature case that is considered. When the precursor concentration is varied, the model predicts the correct direction of the change in signal but notable qualitative and quantitative differences with the data are observed. In particular, the measured signals show a highly non-linear variation while the predictions exhibit a square dependence on precursor doping at best. So, while the results for the reference case appear to be very good, shortcomings in the standard submodels are revealed through variation of the boundary conditions. This demonstrates the importance of testing complex nanoparticle synthesis models on a flame series to ensure that the physical trends are correctly accounted for.Item Open Access Fully-resolved simulations of coal particle combustion using a detailed multi-step approach for heterogeneous kinetics(2019) Tufano, Giovanni Luigi; Stein, Oliver T.; Kronenburg, Andreas; Gentile, Giancarlo; Stagni, Alessandro; Frassoldati, Alessio; Faravelli, Tiziano; Kempf, Andreas M.; Vascellari, Michele; Hasse, ChristianItem Open Access Modeling of scalar mixing in turbulent jet flames by multiple mapping conditioning(2009) Vogiatzaki, Konstantina; Cleary, Matthew J.; Kronenburg, Andreas; Kent, JohnItem Open Access Single-shot two-dimensional multi-angle light scattering (2D-MALS) technique for nanoparticle aggregate sizing(2021) Martins, Fabio J. W. A.; Kronenburg, Andreas; Beyrau, FrankThe two-dimensional multi-angle light scattering (2D-MALS) technique has been extended for single-shot size measurements of soot aggregates in flames. Six cameras are used for instantaneous acquisition of the elastic scattering from the aggregates at different directions between 10 to 90∘ of a laser light sheet. Two diluted ethylene (50 and 60% by volume of C2H4 fuel diluted with inert N2) coflow laminar diffusion flames with little flickering are used as proof of concept. Results of instantaneous, average and fluctuating 2D fields of the effective radii of gyration, which are expected to characterize the size of the aggregates, compare well with the literature, demonstrating the applicability of the proposed sizing method to weakly unsteady combustion processes.