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1.
Meshless modeling of thermo-mechanics of low-frequency electromagnetic direct chill casting
Boštjan Mavrič, 2017, doctoral dissertation

Abstract: The aim of this dissertation is to devise a meshless model describing the thermomechanical phenomena, which occur during DC casting of aluminium alloys under the influence of electromagnetic stirring. The thermoemchanical phenomena are important, because they can cause several type of defects, which can significantly deteriorate the quality of the resulting billet. The two most important of them are the hot tearing, which causes cracks to appear in the mushy zone, and the porosity, which demonstrates itself as micrometer sized voids in the microstructure of the billet. To calculate the stresses and strains, a computational model of the stationary state of the process, stated in axial symmetry, is formulated. It uses Eulerian formulation by fixing the computational domain to the mold of the casting device allowing the material to move through the computational domain. The stresses are calculated from the stress equilibrium equations. The small strain approximation is used to consider the three contributions to strain. The strain consists of the thermal strain, which is caused by the inhomogeneous thermal profile in the billet, the viscoplastic strain, which is caused by the irreversible deformation because of the large stresses occurring in the billet, and the elastic strain. The spatial discretization of the governing equations is performed by local radial basis function collocation method (LRBFCM) and the temporal discretization is achieved by the method of lines with implicit Euler formula. The method used for spatial discretization uses radial basis functions augmented by monomials to approximate the solution values on localized stencils. This approximation is used to construct the discretization coefficients of the differential operators present in the model. A flexible framework for formulation of multiphysics problems is developed to use the obtained discretization coefficients to construct the temporal discretization of the governing equations. The node arrangement, on which the spatial discretization is performed, was generated by a point-repel algorithm. The performance of the method is tested on several benchmark test cases. The accuracy of the discretization is estimated by comparing the analytic and the numerical solution to several stationary problems in thermomechancis. Of special interest is the performance of the method with respect to the choice of the shape parameter, which determines the spatial scale of the radial basis functions. Besides this, the dependence of the condition number of the interpolation matrix on the shape parameter is studied. The condition number is found fit to replace the condition number as the shape-determining free parameter of the method. The implementation of the solver of time dependent problems is tested on problem of thermoelasticity, which couples the thermal transport with the elastic waves. The results of the problem are compared with the finite element method, showing good agreement of the two methods. The results are also compared with the results obtained by meshless local Petrov-Galerkin method and the proposed local collocation method demonstrated significantly better solution quality in the studied case. The performance of the solver used to solve the system of nonlinear equations given by the viscoplastic constitutive equations is estimated on a quasi zero-dimensional problem. The results are found to match perfectly. Solution of a more complicated problem is obtained with the proposed method and the finite-element method, both methods giving practically the same solution, although some serious limitations of the chosen finite element solver are exposed during the selection of the problem parameters. Finally, the devised method is applied to the problem of DC casting of aluminium alloys. The thermomechanical model relies on a model of heat and mass transfer to obtain the input fields needed in the solver. The required fields are: temperature, pressure, liquid
Found in: ključnih besedah
Summary of found: ...appear in the mushy zone, and the porosity, which demonstrates itself as micrometer sized voids...
Keywords: thermomechanics, viscoplasticity, aluminium alloys, direct-chill casting, electromagnetic stirring, hot tearing, porosity, meshless methods, local collocation method, radial basis functions, shape parameter
Published: 28.06.2017; Views: 4044; Downloads: 144
.pdf Fulltext (21,30 MB)

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Natural sporopollenin microcapsules composites from cellulose and chitosan
Mladen Franko, Chieu D. Tran, Mark Mitmoen, Dorota Korte, Hanna Budasheva, Patricia Cazón, 2021, published scientific conference contribution abstract

Found in: ključnih besedah
Keywords: CS:CL biocomposites, beam deflection spectrometry, natural sporopollenin, porosity
Published: 31.01.2022; Views: 469; Downloads: 8
URL Fulltext (0,00 KB)
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4.
Absolute Porosity Analysis in Carbon Allotropic Nanofluids: A Sankar–Swapna Model Approach
SANKARARAMAN S, SREEJYOTHI S, SWAPNA MOHANACHANDRAN NAIR SINDHU, 2020, original scientific article

Abstract: Porous materials have gained significant attention in recent years as a class of material exhibiting interesting chemical and physical properties. The existing methods of porosity analysis have limitations that prevent absolute porosity measurement. Hence, a technique independent of surface physical properties alone can give the absolute porosity of the material. The porosity greatly influences the thermal diffusivity of a material. The manuscript is the first report of employing the Sankar–Swapna model for analyzing the porosity variations in carbon allotropic nanofluids. The model helps not only in getting information about the absolute porosity variations among samples, but also suggests morphological modifications through the thermal diffusivity study using the sensitive single-beam thermal lens technique. The variations in thermal diffusivity and absolute porosity values are also correlated to morphological modifications based on the theoretical model and thereby proposing this as a surrogate method for absolute porosity analysis.
Found in: ključnih besedah
Keywords: absolute porosity, Sankar–Swapna model, thermal diffusivity, thermal lens, thermal conductivity
Published: 04.07.2022; Views: 74; Downloads: 0
.pdf Fulltext (1,96 MB)

5.
Characterization of smart and biocompatible materials based on chitosan:cellulose composites containing sporopollenin exine capsules
Mladen Franko, Dorota Korte, Patricia Cazon Diaz, Hanna Budasheva, 2022, published scientific conference contribution abstract

Found in: ključnih besedah
Summary of found: ...an:cellulose composites, biocompatible materials, beam deflection spectrometry, porosity determination, structural characterisation, thermal properties...
Keywords: chitosan:cellulose composites, biocompatible materials, beam deflection spectrometry, porosity determination, structural characterisation, thermal properties
Published: 30.06.2022; Views: 112; Downloads: 4
.pdf Fulltext (84,71 KB)

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