Chemnitz, Germany
Chemnitz, Germany

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Spille-Kohoff A.,CFX Berlin Software GmbH | Preuss E.,TU Berlin | Bottcher K.,Leibniz Institute for Crystal Growth
International Journal of Heat and Mass Transfer | Year: 2012

In Böttcher [K. Böttcher, Numerical solution of a multi-component species transport problem combining diffusion and fluid flow as engineering benchmark, Int. J. Heat Mass Transfer 53 (1-3) (2010) pp. 231-240], one of us described an implementation of ternary multi-component species transport by diffusion and convection and presented benchmark cases and simulation results for these cases computed with software ENTWIFE. In this paper, an implementation of the Stefan-Maxwell equation into the ANSYS CFX software is described in combination with a procedure to numerically calculate the ordinary multi-component diffusion coefficients in gases at any total number of components. Finally, the benchmark for ternary mixtures is extended to a quarternary one. A comparsion to the benchmark results of Böttcher revealed an implementation mistake in Böttcher's paper leading to wrong results in case of largely different molar masses. The correct benchmark results are presented here and furthermore compared to two different kinds of effective binary diffusion approaches in multi-component gas mixtures. © 2012 Elsevier Ltd. All rights reserved.

Schnick M.,TU Dresden | Fuessel U.,TU Dresden | Hertel M.,TU Dresden | Spille-Kohoff A.,CFX Berlin Software GmbH | Murphy A.B.,CSIRO
Frontiers of Materials Science | Year: 2011

Current numerical models of gas metal arc welding (GMAW) are trying to combine magnetohydrodynamics (MHD) models of the arc and volume of fluid (VoF) models of metal transfer. They neglect vaporization and assume an argon atmosphere for the arc region, as it is common practice for models of gas tungsten arc welding. These models predict temperatures above 20 000 K and a temperature distribution similar to tungsten inert gas (TIG) arcs. However, current spectroscopic temperature measurements in GMAW arcs demonstrate much lower arc temperatures. In contrast to TIG arcs they found a central local minimum of the radial temperature distribution. The paper presents a GMAW arc model that considers metal vapor and which is in a very good agreement with experimentally observed temperatures. Furthermore, the model is able to predict the local central minimum in the radial temperature and the radial electric current density distributions for the first time. The axially symmetric model of the welding torch, the work piece, the wire and the arc (fluid domain) implements MHD as well as turbulent mixing and thermal demixing of metal vapor in argon. The mass fraction of iron vapour obtained from the simulation shows an accumulation in the arc core and another accumulation on the fringes of the arc at 2000 to 5000 K. The demixing effects lead to very low concentrations of iron between these two regions. Sensitive analyses demonstrate the influence of the transport and radiation properties of metal vapour, and the evaporation rate relative to the wire feed. Finally the model predictions are compared with the measuring results of Zielińska et al. © Higher Education Press and Springer-Verlag Berlin Heidelberg 2011.

Schildhauer M.,CFX Berlin Software GmbH | Spille-Kohoff A.,CFX Berlin Software GmbH
Progress in Computational Fluid Dynamics | Year: 2014

The numerical simulation of fluid-structure interaction requires a simulation tool that can calculate both fluid flow and structure deformation and takes into account the results of one code (i.e., forces and deformations) in the setup of the other (as motion and load). This article shows a segregated (also: partitioned) approach that uses commercial, highly developed software for each task: ANSYS CFX for fluid dynamics and ANSYS Mechanical for structural mechanics with the ANSYS MFX solver to couple both programs. Two examples with large deformations and strong influence on the flow field are shown: The Turek benchmark from Turek and Hron (2006) consists of a laminar incompressible channel flow around an elastic object which results in periodic oscillations of the structure. The second example is the discharge simulation of compressed air through a rapidly moving reed valve in a piston compressor where opening and closing contacts are important. Copyright © 2014 Inderscience Enterprises Ltd.

Spille-Kohoff A.,CFX Berlin Software GmbH | Hesse J.,CFX Berlin Software GmbH | Shorbagy A.E.,CFX Berlin Software GmbH
IOP Conference Series: Materials Science and Engineering | Year: 2015

Computational Fluid Dynamics (CFD) simulations have promising potential to become an important part in the development process of positive displacement (PD) machines. CFD delivers deep insights into the flow and thermodynamic behaviour of PD machines. However, the numerical simulation of such machines is more complex compared to dynamic pumps like turbines or fans. The fluid transport in size-changing chambers with very small clearances between the rotors, and between rotors and casing, demands complex meshes that change with each time step. Additionally, the losses due to leakage flows and the heat transfer to the rotors need high-quality meshes so that automatic remeshing is almost impossible. In this paper, setup steps and results for the simulation of a dry screw compressor are shown. The rotating parts are meshed with TwinMesh, a special hexahedral meshing program for gear pumps, gerotors, lobe pumps and screw compressors. In particular, these meshes include axial and radial clearances between housing and rotors, and beside the fluid volume the rotor solids are also meshed. The CFD simulation accounts for gas flow with compressibility and turbulence effects, heat transfer between gas and rotors, and leakage flows through the clearances. We show time- resolved results for torques, forces, interlobe pressure, mass flow, and heat flow between gas and rotors, as well as time- and space-resolved results for pressure, velocity, temperature etc. for different discharge ports and working points of the screw compressor. These results are also used as thermal loads for deformation simulations of the rotors. © Published under licence by IOP Publishing Ltd.

Schnick M.,TU Dresden | Fussel U.,TU Dresden | Spille-Kohoff A.,CFX Berlin Software GmbH
Welding in the World | Year: 2010

Plasma Tungsten Arc Welding (PTAW) compared with TIG Welding enables an increased welding speed, a reduced energy input per unit length and butt joint welding of plates without preparation of the welded seam due to keyhole effect. However, because of missed profound understanding of effects in plasma arcs, the indisputable advantages of this process increase demands on education and especially on the experience of developers and operators. In this paper, process parameters, properties of the plasma jet and the molten pool are derived from the numerical modelling of arc and sheath layer under consideration of the process gas properties and the effects of demixing. Basics of the model and the testing site are introduced in this paper. Additionally, influences of current intensity, plasma gas quantity, process gases and their specific properties, torch geometry on plasma jet and energy input into work piece is shown by an exemplary torch. Model and numerical results have been validated by impact pressure measurements at the surface of the work piece and penetration profiles (cross section).

The agitation of electroplating baths serves two main purposes. The first of these is to achieve the most complete possible mixing of the electrolyte components, i.e. metal ions, bath additives and breakdown products and also to achieve the greatest possible uniformity of temperature within the solution in order to obtain a controlled and uniform coating. The second purpose is to bring about a desired perturbation of the diffusion layer at the solid-solution interface of the component being plated, thereby enabling higher current densities to be used and uniformity of coating to be achieved. Using numerical flow simulation (CFD = Computational Fluid Dynamics), flow patterns can be studied and thus optimised. In this presentation, results of a 3D simulation of electrolyte flow to a three-dimensional component at a rack, using injector nozzles and taking into account hydrogen bubble formation, are described. Only by taking into account hydrogen bubble formation in the flow calculations, can be incoming flow be correctly modelled. For tertiary current distribution, current density versus potential plots for a zinc electrolyte, obtained by means of cyclic voltammetry, were used. The simulation was validated using experimentally obtained thickness data at a three-dimensional component.

Hertel M.,TU Dresden | Spille-Kohoff A.,CFX Berlin Software GmbH | Fussel U.,TU Dresden | Schnick M.,TU Dresden
Journal of Physics D: Applied Physics | Year: 2013

A numerical model of the droplet detachment of a gas-metal arc welding process is presented. The model is based on the volume of fluid method and focuses on the detailed description of the interaction between the arc and the anodic wire electrode. The influence of metal vapour on the arc plasma and the arc attachment at the wire is taken into account. The formation of metal vapour at the wire is described self-consistently as a function of the wire temperature by the help of the Hertz-Knudsen-Langmuir equation. Results are presented for a pulsed gas-metal arc welding process with a wire of mild steel and argon as the shielding gas. © 2013 IOP Publishing Ltd.

Schnick M.,TU Dresden | Fuessel U.,TU Dresden | Hertel M.,TU Dresden | Haessler M.,TU Dresden | And 2 more authors.
Journal of Physics D: Applied Physics | Year: 2010

The most advanced numerical models of gas-metal arc welding (GMAW) neglect vaporization of metal, and assume an argon atmosphere for the arc region, as is also common practice for models of gas-tungsten arc welding (GTAW). These models predict temperatures above 20 000K and a temperature distribution similar to GTAW arcs. However, spectroscopic temperature measurements in GMAW arcs demonstrate much lower arc temperatures. In contrast to measurements of GTAW arcs, they have shown the presence of a central local minimum of the radial temperature distribution. This paper presents a GMAW model that takes into account metal vapour and that is able to predict the local central minimum in the radial distributions of temperature and electric current density. The influence of different values for the net radiative emission coefficient of iron vapour, which vary by up to a factor of hundred, is examined. It is shown that these net emission coefficients cause differences in the magnitudes, but not in the overall trends, of the radial distribution of temperature and current density. Further, the influence of the metal vaporization rate is investigated. We present evidence that, for higher vaporization rates, the central flow velocity inside the arc is decreased and can even change direction so that it is directed from the workpiece towards the wire, although the outer plasma flow is still directed towards the workpiece. In support of this thesis, we have attempted to reproduce the measurements of Zielińska et al for spray-transfer mode GMAW numerically, and have obtained reasonable agreement. © 2010 IOP Publishing Ltd.

Schnick M.,TU Dresden | Fussel U.,TU Dresden | Hertel M.,TU Dresden | Spille-Kohoff A.,CFX Berlin Software GmbH | Murphy A.B.,CSIRO
Journal of Physics D: Applied Physics | Year: 2010

A computational model of the argon arc plasma in gas-metal arc welding (GMAW) that includes the influence of metal vapour from the electrode is presented. The occurrence of a central minimum in the radial distributions of temperature and current density is demonstrated. This is in agreement with some recent measurements of arc temperatures in GMAW, but contradicts other measurements and also the predictions of previous models, which do not take metal vapour into account. It is shown that the central minimum is a consequence of the strong radiative emission from the metal vapour. Other effects of the metal vapour, such as the flux of relatively cold vapour from the electrode and the increased electrical conductivity, are found to be less significant. The different effects of metal vapour in gas-tungsten arc welding and GMAW are explained. © 2010 IOP Publishing Ltd.

Schnick M.,TU Dresden | Dreher M.,TU Dresden | Zschetzsche J.,TU Dresden | Fuessel U.,TU Dresden | Spille-Kohoff A.,CFX Berlin Software GmbH
Welding in the World | Year: 2012

GMA welding is one of the most frequently applied welding techniques in industry. Particularly the joining of aluminium, high alloyed steels or titanium requires a cover of shielding gas in order to provide a low PPM concentration of oxygen. The result of the welding process depends essentially on the chemical and thermophysical properties of the process gas used. Consequently, it is necessary to be able to describe and to analyse its flow with respect to various influencing variables. However, it is very difficult to realize this during arc welding processes; a poor access is predominant due to the covered areas inside the welding torch and temperatures of up to 20 000 K cause the strong radiation of the arc and electromagnetic fields. This paper deals with experimental and numerical methods for visualization and quantification of process gas flows in arc welding and gives examples for their technical applications. Unlike previous work, the described methods consider the arc as a dynamic element which determinates the gas flow. Advanced Particle Image Velocimetry (PIV) and Schlieren measurement were used for characterization of the flow field in the direct vicinity of the arc in GTA and GMA welding. Furthermore, a numerical model including magneto-hydrodynamics and turbulence models was used for a detailed visualization of the flow in the free jet and in the hidden interior of the torch. It is based on a commercial CFD code which allows to model complex 3-D geometries of torch and workpiece design. Mixing effects and turbulence model were validated by oxygen measurements in the gas shield.

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