Universität Stuttgart
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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 The morphology of ice structure in a parallel plate channel(1991) Weigand, Bernhard; Beer, HansAn experimental study has been performed to investigate the ice formation phenomena of water flow between two horizontal cooled parallel plates. A detailed and comprehensive investigation of the morphology of the ice-structure is given. It is shown that the different shapes of ice-layers can be classified with the help of a Θc - ReD diagramm. The regions for which a certain type of ice-layer occures are clearly separated.Item Open Access A numerical and experimental study of wavy ice structure in a parallel plate channel(1992) Weigand, Bernhard; Beer, HansThe paper presents a numerical model for predicting steady-state ice formation inside a cooled, parallel plate channel. The study takes into account the strong interactions occuring between the turbulent flow, the shape of the ice and the heat transfer at the ice-water interface, which lead to the formation of wavy ice layers. The presented model is found to be able to predict realistic variations of the ice layer thickness for a wide range of Reynolds numbers and cooling parameters. The numerical results were verified by comparing with own measurements and good agreement was found.Item Open Access A numerical and experimental study of wavy ice-structure in an asymmetrically cooled parallel-plate channel(1992) Weigand, Bernhard; Beer, HansIce formation of flowing water in a pipe or a channel, whose wall is kept at a uniform temperature below the freezing temperature of the water, is a basic engineering problem. It Introduces many practical problems, such as pressure drop, diminution of flow rate and sometime, breakage of the pipe as a result of flow blockage by ice. The phenomenon of freezing of flowing water involves interactions between the turbulent flow, the shape of the ice layer and the heat transfer at the ice-water interface. Under certain conditions these interactions result in an instabilily of the ice layer. This instability is caused by the strong laminarization of the turbulent flow due to converging ice layers in the entrance region of the cooled channel and results in a wavy ice structure. Wavy ice layers with one wave, occuring in a parallel.plate channel subjected to symmetrically oooled walls were investigated experimentally by Seld et al. and by Weigand and Beer. More recently Weigand and Beer were able to predict numerically the shape of wavy ice layers with one wave occuring in a symmetrically cooled channel. Wavy ice layers in a parallel-plate channel with one wave in the case of asymmetrically cooled walls were investigated experimentally by Tago et al. and by Weigand and Beer. No numerical calculation of asymmetric wavy freezing fronts was done in the past. Therefore, the subject of this paper is the presentation of a numerical model for calculating steady state ice layers with one wave in the entrance region of an asymmetric cooled channel. The method is based on a work performed by Weigand and Beer. The given numerical study is supported by a detailed experimental investigation.Item Open Access Computations of a film cooled turbine rotor blade with non-uniform inlet temperature distribution using a three-dimensional viscous procedure(1994) Weigand, Bernhard; Harasgama, Sriwickrama P.A numerical investigation of film cooling on a turbine rotor blade has been carried out. The computations were performed with a 3D-Navier-Stokes code utilizing an unstructured solution adaptive grid methodology. The code uses a low Reynolds number k-epsilon model for prescribing the Reynolds stresses. The results show that there is a significant interaction between the coolant flow and the secondary flow near the hub and the tip of the turbine blade. It was observed that, by blowing on the pressure side of the blade some of the cooling air was transported through the tip gap of the blade to the suction side of the blade where the coolant flow interacts with the secondary flow field.Item Open Access Resolving breakup in flash atomization conditions using DNS(2019) Loureiro, Daniel Dias; Reutzsch, Jonathan; Kronenburg, Andreas; Weigand, Bernhard; Vogiatzaki, KonstantinaFlash 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.Item Open Access Fluid flow and heat transfer in an axially rotating pipe: the rotational entrance(1992) Weigand, Bernhard; Beer, HansThe complex interactions between turbulence and rotation in the rotational entrance region of a pipe, rotating about its axis, are examined. By assuming, a universal tangential velocity profile and with the use of a modified mixing length theory, the development of the axial velocIty profile and the heat transfer coefficient along the rotational entrance length are calculated. The theoretical results are compared with experimental flndings of Reich.