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Sebastijan Kovačič, 2011, dissertation

Abstract: A new class of polyHIPE materials has been prepared using high internal phase emulsions (HIPEs) with monomers in both phases. Resulting materials, namely hybrid polyHIPE materials, are obtained consisting of hydrophobic matrix (consisted of styrene cross-linked with DVB or dicyclopentadiene) filled with hydrophilic polymer gel (polyacrylic acid or polyNIPAM) and exhibit morphology changes according to pH and temperature of the surrounding medium. Our focus with regards to the production of hybrid polyHIPE materials was the responsiveness of such materials used for flow control. This property is beneficial for controlling the flow of the solution through the monolithic polymers. Furthermore, polyHIPE materials have also been prepared by using ring opening metathesis polymerisation of monomers, such as dicyclopentadiene and norbornene. Obtained materials have Young’s moduli in the range of hundred times higher than standard polyHIPE materials with the same level of porosity which represents a very important improvement in the development of highly porous cellular polymeric materials.
Keywords: emulsions, high internal phase emulsions, polymers, polyHIPEs, hybrid, hybrid polyHIPE materials, ROMP, flow-through, styrene, DVB, acrylic acid, NIPAM
Published: 04.05.2011; Views: 2315; Downloads: 208
.pdf Full text (3,71 MB)

BEM simulation of compressible fluid flow in an enclosure induced by thermoacoustic waves
Leopold Škerget, Jure Ravnik, 2009, original scientific article

Abstract: The problem of unsteady compressible fluid flow in an enclosure induced by thermoacoustic waves is studied numerically. Full compressible set of Navier-Stokes equations are considered and numerically solved by boundary-domain integral equations approach coupled with wavelet compression and domain decomposition to achieve numerical efficiency. The thermal energy equation is written in its most general form including the Rayleigh and reversible expansion rate terms. Both, the classical Fourier heat flux model and wave heat conduction model are investigated. The velocity-vorticity formulation of the governing Navier-Stokes equations is employed, while the pressure field is evaluated from the corresponding pressure Poisson equation. Material properties are taken to be for the perfect gas, and assumed to be pressure and temperature dependent.
Keywords: compressible fluid flow, boundary element method, thermoacoustic waves, velocity-vorticity fomulation
Published: 31.05.2012; Views: 1362; Downloads: 64
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Fast single domain-subdomain BEM algorithm for 3D incompressible fluid flow and heat transfer
Jure Ravnik, Leopold Škerget, Zoran Žunič, 2009, original scientific article

Abstract: In this paper acceleration and computer memory reduction of an algorithm for the simulation of laminar viscous flows and heat transfer is presented. The algorithm solves the velocity-vorticity formulation of the incompressible Navier-Stokes equations in 3D. It is based on a combination of a subdomain boundary element method (BEM) and single domain BEM. The CPU time and storage requirements of the single domain BEM are reduced by implementing a fast multipole expansion method. The Laplace fundamental solution, which is used as a special weighting function in BEM, is expanded in terms of spherical harmonics. The computational domain and its boundary are recursively cut up forming a tree of clusters of boundary elements and domain cells. Data sparse representation is used in parts of the matrix, which correspond to boundary-domain clusters pairs that are admissible for expansion. Significant reduction of the complexity is achieved. The paper presents results of testing of the multipole expansion algorithm by exploring its effect on the accuracy of the solution and its influence on the non-linear convergence properties of the solver. Two 3D benchmark numerical examples are used: the lid-driven cavity and the onset of natural convection in a differentially heated enclosure.
Keywords: boundary element method, fast multipole method, fluid flow, heat transfer, velocity-vorticity fomulation
Published: 31.05.2012; Views: 1349; Downloads: 53
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Influence of water scale on thermal flow losses of domestic appliances
Danijela Urbancl, Darko Goričanec, 2007, original scientific article

Abstract: Research results of how the precipitated water scale on heaters of small domestic appliances influences the consumption of electricity are presented. It shows that the majority of water scale samples are composed of aragonite, calcite and dolomite and that those components have an extraordinary low thermal conductivity. Also, the results show that at 2 mm thick deposit, depending on the chemical composition of water scale, the thermal flow is reduced for 10% to 40%; consequently, the consumption of electricity significantly increases.
Keywords: electricity, heat transfer, heat flow, heat exchanger, water scale
Published: 31.05.2012; Views: 1272; Downloads: 44
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Laminar flow of a shear-thickening fluid in a 90° pipe bend
Jure Marn, Primož Ternik, 2006, original scientific article

Abstract: The non-Newtonian fluid flow in a sharp 90 curved pipe is studied numerically to obtain the pressure loss coefficient prompted by disagreement between the existing empirical correlations and results obtained by computer codes. This disagreement results from presumption of fully developed flow throughout the curvature (correlations) while the actual flow is partially developed for the Newtonian and sharp 90 curved bend non-Newtonian flows, and fully developed for slightly bent 90 curvature non-Newtonian flow. The Quadratic model is employed to accommodate the shear-thickening behavior of an electrostatic ash and water mixture. Numerical results are obtained for different values of Reynolds number. Finally, results for local pressure loss coefficient are compared with values obtained for the Power law rheological model.
Keywords: shear-thickening fluid, fluid dynamics, rheological model, pipe bend flow, numerical modelling, pressure losses
Published: 30.05.2012; Views: 1188; Downloads: 58
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3D multidomain BEM for solving the Laplace equation
Matjaž Ramšak, Leopold Škerget, 2007, original scientific article

Abstract: An efficient 3D multidomain BEM for solving problems governed by the Laplace equation is presented. Integral boundary equations are discretized using mixed boundary elements. The field function is interpolated using a continuous linear function while its derivative in a normal direction is interpolated using a discontiuous constant function over surface boundaey elements. Using amultidomain approach, also known as the subdomain technique, sparse system matrices similar to FEM are obtained. Interface boundary conditions between subdomains leads to an over-determined system matrix which is solved using a fast iterative linear least square solver. The accuracy and the robustness of the developed algorithm is presented on a scalar diffusion problem using simple cube geometry and various types of meshes. The efficiency is demonstrated with potential flow around a complex geometry of a fighter airplane using a tetrahedral mesh with over 100.000 subdomains on a personal computer.
Keywords: fluid mechanics, aerodynamics, multidomain boundary element method, Laplace equation, mixed boundary elements, potential flow
Published: 31.05.2012; Views: 1398; Downloads: 64
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The influence of rotating domain size in a rotating frame of reference approach for simulation of rotating impeller in a mixing vessel
Matej Zadravec, Sani Bašič, Matjaž Hriberšek, 2007, original scientific article

Abstract: This paper presents simulation of rotating impeller in a mixing vessel by means of Computational Fluid Dynamics (CFD). A special emphasis is devoted to the study of influence of the choice of numerical model for simulation of rotation of impeller when mixing a Newtonian fluid in a vessel equipped with Rushton impeller, and operating under turbulent flow conditions. In order to determine the best simulation approach experimental validation of the selected problem is done by means of Particle Image Velocimetry (PIV) system. When using the rotating frame of reference approach, the stirring vessel geometry has to be split into a stationary and rotating part, and the questionarises where to position the interface between both regions in order to avoid numerical errors, originating in numerical approximations at the interface. To answer this question, a comparison between the CFD based numerical results and experimental results, was made.
Keywords: fluid mechanics, turbulent flow, Computational Fluid Dynamics, Particle Image Velocimetry, rotating frame of reference
Published: 31.05.2012; Views: 1273; Downloads: 36
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Selection of the economic objective function for the optimization of process flow sheets
Zorka Novak-Pintarič, Zdravko Kravanja, 2006, original scientific article

Abstract: This paper highlights the problem of selecting the most suitable economic optimization criteria for mathematical programming approaches to the synthesis, design, and optimization of chemical process flow sheets or their subsystems. Minimization of costs and maximization of profit are the most frequently used economic criteria in technical papers. However, there are manyother financial measures which can lead to different optimal solutions if applied in the objective function. This paper describes the characteristics ofthe optimal solutions obtained with various optimization criteria like the total annual cost, the profit, the payback time, the equivalent annual cost, the net present worth, and the internal rate of return. It was concluded that the maximization of the net present worth (NPW) with a discount rate equal to the minimum acceptable rate of return (MARR) is probably the most appropriate method for the optimization of process flow sheets or their subsystems. Similar or equal solutions can be obtained by simpler criteria of minimum equivalent annual cost or maximum profit if the annual investment cost is calculated by using the MARR instead of the straight-line depreciation method.These criteria represent a thorough compromise between quantitative andqualitative measures, because they consider the absolute terms of future cash flows of investments equally important as profitability through the life cycle of the project. The uncertainty related to the value of the MARR was considered by the generation of Pareto optimal solutions for the NPW and by the stochastic analyses of two design example problems.
Keywords: chemical processing, optimization, mathematical programming, flow sheets
Published: 31.05.2012; Views: 1179; Downloads: 43
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Velocity vorticity-based large eddy simulation with the bounadr element method
Jure Ravnik, Leopold Škerget, Matjaž Hriberšek, 2006, published scientific conference contribution (invited lecture)

Abstract: A large eddy simulation using the velocity-vorticity formulation of the incompressible Navier-Stokes equations in combination with the turbulent heat transfer equation is proposed for the solution of the turbulent natural convection drive flow in a 1:4 enclosure. The system of equations is closed by an enthropy-based subgrid scale model.The Prandtl turbulent number is used to estimate turbulent diffusion in the heat transfer equation. The boundary element method is used to solve the kinematics equation and estimate the boundary vorticity values. The vorticity transport equation is solved by FEM. The numerical example studied in this paper is the onset of a turbulent flow regime occuring at high Rayleigh number values ▫$(Ra=10^7-10^10)$▫. The formation of vortices in the boundary layer is observed, along with buoyancy driven diffusive convective transport. Quantitative comparison with the laminar flow model and the worh of other authors is also presented in terms of Nusselt number value oscillations.
Keywords: fluid mechanics, incompressible viscous fluid, turbulent flow, velocity vorticity formulation, finite element method, large eddy simulation
Published: 31.05.2012; Views: 1267; Downloads: 18
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