Universität Stuttgart
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Item Open Access Fluid flow and heat transfer in an axially rotating pipe subjected to external convection(1992) Weigand, Bernhard; Beer, HansThe effects of external insulation and tube rotation on the heat transfer to a fluid flowing inside a tube are examined by analysis. The turbulent flow is assumed to be hydrodynamically fully developed. Heat transfer was found to be strongly suppressed by tube rotation. It is shown that the significance of external insulation on the Nusselt number increases with growing rotation rate of the pipe.Item Open Access 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 Wärmeübertragung in einem axial rotierenden, durchströmten Rohr im Bereich des thermischen Einlaufs. T. 2, Einfluß der Rotation auf eine laminare Strömung(1989) Weigand, Bernhard; Beer, HansDer Einfluß der Rotation auf das Temperaturprofil und die Wärmeübergangszahl einer laminaren Rohrströmung im Bereich des thermischen Einlaufs wird theoretisch untersucht. Es wird angenommen, daß das Geschwindigkeitsprofil voll ausgebildet ist. Die Rotation hat einen destabilisierenden Einfluß auf die Laminarströmung, die umschlägt und turbulent wird. Aufgrund der Anfachung der Turbulenz durch die Rotation verbessert sich die Wärmeübertragung mit steigender Rotations-Reynoldszahl und die thermische Einlauflänge nimmt beträchtlich ab.Item Open Access Particle image velocimetry measurements in accelerated, transonic wake flows(2022) Richter, Judith; Alexopoulos, Charalampos; Weigand, BernhardThis paper reports on particle image velocimetry (PIV) measurements in compressible accelerated wake flows generated by two different central injector types, which are mounted in a convergent-divergent nozzle. The injectors differ by the extent of their trailing edge located either in the subsonic (injector A) or supersonic flow region (injector B). In addition, the undisturbed nozzle flow without injector is studied as a reference case. The PIV results reveal typical wake flow structures expected in subsonic (injector A) and supersonic (injector B) wake flows. They further show that the Reynolds stresses Rexxand Reyysignificantly decay in all three cases due to the strong acceleration throughout the nozzle. Interestingly, in the case of injector A, the flow stays non-isotropic with Reyy>Rexxalso far downstream in the supersonic flow region. These measurements were motivated by the lack of velocity data needed to validate numerical simulations. That is why this paper additionally contains results from (unsteady) Reynolds-averaged Navier-Stokes ((U)RANS) simulations of the two wake flows investigated experimentally. The URANS simulation of the injector A case is able to accurately predict the entire flow field and periodic fluctuations at the wake centerline. However, in the case of injector B, the RANS simulation underestimates the far wake centerline velocity by about 4%.Item Open Access Heat transfer and solidification of a laminar liquid flow in a cooled parallel plate channel: the stationary case(1991) Weigand, Bernhard; Beer, HansA simple numerical model is presented to predict the steady-state ice layers on the cooled walls inside a parallel plate channel for arbitrary entrance velocity profiles. The effect of two different entrance velocity distributions (a parabolic velocity distribution and a slug flow) on the shape of the ice-layers are examined. The quality of an approximative solution given in literature was checked by comparing with the numerical results. For the case of a fully developed parabolic velocity distribution at the entrance of the cooled channel the results are compared with experimental findings of Kikuchi. A generally good agreement was found.Item Open Access The effect of patterned micro-structure on the apparent contact angle and three-dimensional contact line(2021) Foltyn, Patrick; Restle, Ferdinand; Wissmann, Markus; Hengsbach, Stefan; Weigand, BernhardThe measurement of the apparent contact angle on structured surfaces is much more difficult to obtain than on smooth surfaces because the pinning of liquid to the roughness has a tremendous influence on the three phase contact line. The results presented here clearly show an apparent contact angle variation along the three phase contact line. Accordingly, not only one value for the apparent contact angle can be provided, but a contact angle distribution or an interval has to be given to characterize the wetting behavior. For measuring the apparent contact angle distribution on regularly structured surfaces, namely micrometric pillars and grooves, an experimental approach is presented and the results are provided. A short introduction into the manufacturing process of such structured surfaces, which is a combination of Direct LASER Writing (DLW) lithography, electroforming and hot embossing shows the high quality standard of the used surfaces.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 Solidification of flowing liquid in an asymmetric cooled parallel-plate channel(1992) Weigand, Bernhard; Beer, HansAn experimental study has been performed to investigate the ice-formation phenomena of water flow between two horizontal parallel plates, with the lower wall kept at a temperature below the freezing temperature of the liquid. A detailed and comprehensive investigation of the morphology of the ice-structure is given.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.