In addition to corrosion resistance and processing properties, high coating uniformity is a key quality criterion for galvanized steel sheets. Hydrodynamic gas jet wiping has proved to be an efficient method to control the coating thickness. However, the occurrence of nonuniformities is attributed to the unsteadiness of the impinging jet. For the first time, vertical surface non-uniformities resulting from the interaction of the impinging jet with the liquid coating are numerically predicted under industrial boundary conditions using the ANSYS Fluent 1 . The turbulent flow field of the compressible wiping gas is accomplished by the LES (Large-Eddy-Simulation) turbulence model, whereas the interphase between the wiping gas and the liquid coating is modeled by the VOF (Volume-of-Fluid) method. It is found that the liquid coating reacts relatively slowly to the high frequent flapping gas jet. Only, when the jet is deflected for a comparatively long period, significant waves are able to develop. The waviness predicted by the simulation model is in good agreement with experimental results. Thus, the model enables a careful study of process settings on the vertical coating uniformity characteristics. For the studied case, an increase of nozzle inclination is found to enhance the performance in terms of coating uniformity significantly.
A heat sink is a specific type of heat exchanger integrated with heat generating devicesmostly electronicsfor the sake of thermal management. In the design procedure of heat sinks, several considerations such as manufacturing cost, reliability, thermal and hydraulic performance have to be included. In the past few decades, the prevailing trend of electronics design miniaturization has led to highpower-density systems necessitating high performance cooling concepts. This paper intends to provide a comprehensive review on various employed heat transfer enhancement techniques in cooling procedures of electronics thermal management devices, with a focus on core ideas. The main motivation is to give a rapid overview on the key concepts in different high-performance cooling designs along with a quantitative comparison between the different concepts all in one reference which is missing in literature. For this, the key idea of each design is firstly categorized, and then a detailed description is provided for each case. The discussed categories consist of concepts based on channel cooling in various scales, phase transition, jet impingement, spray cooling and hybrid design. At the end, quantitative comparison is illustrated for thermal and hydraulic performance of a selection of the reviewed references covering all these different categories. Based on this comparison, an overview on thermo-hydraulic performance of the presented categories is provided, and recommendations for future studies are given based on this and the detailed review of references.
Gas-Jet wiping is a widely applied technology in continuous galvanizing mills, which enables the adjustment of the specific coating mass by an impinging turbulent gas-jet. The unsteady impingement conditions, however, are reported to cause surface non-uniformities such as waves. In this study, proper orthogonal decomposition (POD) is used to analyze an industrial gasjet wiping process. POD allows to objectively extract the most dominant flow structures (modes) and their dynamics from the impinging jet. The acquisition of the necessary temporally and spatially highly resolved flow data is done by a LES-VOF simulation model. The POD analysis shows that jet flapping and axial fluctuations of the jet core are the most dominant spatial modes in the studied case. The frequency range of the modes as well as the frequency range of the height fluctuations of the impinged film are compared in a spectral analysis. Additionally, it is shown that slight changes, for example, the absence of the thin liquid film on the impinged surface, alters the frequency range of the dominant POD modes, whereas the modes themselves remain mostly unchanged.
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