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 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 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 theoretical and experimental investigation of smooth- and wavy ice layers in laminar and turbulent flow inside an asymmetrically cooled parallel-plate channel(1993) Weigand, Bernhard; Beer, HansThe present paper shows the adaption of the numerical model originally developed by Weigand and Beer [14] for calculating steady-state ice layers inside an asymmetrically cooled parallel-plate channel. The investigation shows the characteristics in ice formation behaviour due to asymmetrically cooled walls. Further, a simple analytical model is presented for calculating smooth ice layers in turbulent flow. The study is supported by own measurements of the freezing fronts inside an asymmetrically cooled channel. A comparison between theoretical calculations and measurements shows generally good agreement.Item Open Access An analytical study on the mechanism of grouping of droplets(2022) Vaikuntanathan, Visakh; Ibach, Matthias; Arad, Alumah; Chu, Xu; Katoshevski, David; Greenberg, Jerrold Barry; Weigand, BernhardThe condition for the formation of droplet groups in liquid sprays is poorly understood. This study looks at a simplified model system consisting of two iso-propanol droplets of equal diameter, Dd0, in tandem, separated initially by a center-to-center distance, a20, and moving in the direction of gravity with an initial velocity, Vd0>Vt, where Vt is the terminal velocity of an isolated droplet from Stokes flow analysis. A theoretical analysis based on Stokes flow around this double-droplet system is presented, including an inertial correction factor in terms of drag coefficient to account for large Reynolds numbers (≫1). From this analysis, it is observed that the drag force experienced by the leading droplet is higher than that experienced by the trailing droplet. The temporal evolutions of the velocity, Vd(t), of the droplets, as well as their separation distance, a2(t), are presented, and the time to at which the droplets come in contact with each other and their approach velocity at this time, ΔVd0, are calculated. The effects of the droplet diameter, Dd0, the initial droplet velocity, Vd0, and the initial separation, a20 on to and ΔVd0 are reported. The agreement between the theoretical predictions and experimental data in the literature is good.Item Open Access The freeze-shut of a convectively cooled parallel plate channel subjected to laminar internal liquid flow(1993) Weigand, Bernhard; Ruß, GeraldThe paper presents an approximative solution for the time dependent development of the ice layers at the cooled walls inside a parallel plate channel. The upper and the lower wall of the channel are cooled by an uniform external convection. By assuming a constant pressure drop across the channel, the freeze-shut of the planar channel could be calculated approximately. It was found out that the origin of the freezing fronts moves upstream during the ice layer growth. Furthermore a simple criterion is presented to predict whether a given system will lead to blockade.
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