1. Fast single domain-subdomain BEM algorithm for 3D incompressible fluid flow and heat transferJure Ravnik, Leopold Škerget, Zoran Žunič, 2009, izvirni znanstveni članek Opis: 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. Ključne besede: boundary element method, fast multipole method, fluid flow, heat transfer, velocity-vorticity fomulation Objavljeno: 31.05.2012; Ogledov: 918; Prenosov: 5 Polno besedilo (0,00 KB) |
2. Influence of water scale on thermal flow losses of domestic appliancesDanijela Doberšek, Darko Goričanec, 2007, izvirni znanstveni članek Opis: 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. Ključne besede: electricity, heat transfer, heat flow, heat exchanger, water scale Objavljeno: 31.05.2012; Ogledov: 889; Prenosov: 20 Polno besedilo (0,00 KB) |
3. Response functions and thermal influence for various multiple borehole configurations in ground coupled heat pump systemsMetka Pešl, Darko Goričanec, 2007, izvirni znanstveni članek Opis: Ground coupled heat pump (GCHP) utilizes the immense renewable storage capacity of the ground as a heat source or sink to provide space heating, cooling, and domestic hot water. GCHP systems are generally comprised of watersource heat pumps and ground heat exchangers (GHEs). Consisting of closedloop of pipes buried in boreholes, ground heat exchangers (GHEs) are devised for extraction or injection of thermal energy from/into the ground. Despite the low energy and lower maintenance benefits of ground-source heat pump systems, little work has been undertaken in detailed analysis. Many models, either numerical or analytical, have been proposed to analyze the thermal response of vertical heat exchangers that are used in ground coupled heat pump systems (GCHP). In both approaches, most of the models are valid after few hours of operation since they neglect the heat capacity of the borehole. In this paper, we present for three various multiple borehole configurations a comparison between g-functions, which will be calculated after an analytical model of final line source and g-functions, obtain with numerical model derived from the work of Eskilson. A case study is presented to show how the ground temperature changes with time for various multiple borehole configurations. Ključne besede: geothermal heat exchanger, heat transfer, heat conduction, thermal influence Objavljeno: 31.05.2012; Ogledov: 807; Prenosov: 9 Polno besedilo (0,00 KB) |
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5. The combined analysis of phonon and electron heat transfer mechanism on thermal conductivity for nanofluidsJurij Avsec, 2008, izvirni znanstveni članek Opis: The paper features the mathematical model representing the analytical calculation of phonon and electron heat transfer analysis of thermal conductivity for nanofluids. The mathematical model was developed on the basis of statistical nanomechanics. We have made the detailed analysis of the influence of temperature dependence on thermal conductivity for nanofluids. On this basis are taken into account the influences such as formation of nanolayer around nanoparticles, the Brown motion of solid nanoparticles and influence of diffusive-ballistic heat transport. The analytical results obtained by statistical mechanics are compared with the experimental data and they show relatively good agreement. Ključne besede: statistical nanomechanics, phonons, electron heat transfer, nanofluids, thermal conductivity, thermodynamic properties, mathematical model, statistical nanomechanics Objavljeno: 31.05.2012; Ogledov: 923; Prenosov: 3 Polno besedilo (0,00 KB) |
6. Simulation of unsteady fluid flow and heat transfer by BEMJure Ravnik, Leopold Škerget, 2011, objavljeni znanstveni prispevek na konferenci Opis: A boundary element method based solver for simulation of unsteady laminar viscous flow and heat transfer in three-dimensions has been developed. The algorithm solves the incompressible Navier- Stokes equations written in velocity-vorticity form and coupled with energy equation. Buoyancy is modelled within the Boussinesq approximation. The solver has recently been adapted for simulation of unsteady phenomena. The governing set of equations consists of the kinematics equation and transport equations for vorticity and temperature. Velocity and temperature boundary conditions are known, vorticity boundary conditions are calculated during the simulation by single domain BEM applied on the kinematics equation. The transport equations are solved by a domaindecomposition BEM approach, which yields sparse integral matrices and enables simulation using large computational grids. Rayleigh-Benard convection is used for a test case. In this case fluid is heated flow below, thus making it highly unstable. Already at low driving temperature of the bottom wall, the fluid becomes unstable. Vortices are formed above the hot bottom wall and travel up by buoyancy forces. Flow exhibits oscillatory behaviour, which at higher temperatures leads to chaotic and turbulent flow. The phenomenon was simulated using different driving temperatures of the bottom wall, observing the change of flow characteristics for steady to unsteady oscillatory regime and at even higher temperature to chaotic behaviour. Time series of heat flux and field functions were examined by phase portraits to determine the flow regime. A grid independence study and time step analysis was performed to asses the algorithms capability of simulation of unsteady behaviour. Ključne besede: boundary element method, heat transfer, unsteady fluid flow, transport equations Objavljeno: 01.06.2012; Ogledov: 706; Prenosov: 2 Polno besedilo (0,00 KB) |
7. Heat conduction in closed-cell cellular metalsMatej Vesenjak, Zoran Žunič, Andreas Öchsner, Matjaž Hriberšek, Zoran Ren, 2005, izvirni znanstveni članek Opis: The purpose of this research was to describe the thermal transport properties in closed-cell cellular metals. Influence of cell size variations with different pore gases has been investigated with transient computational simulations. Heat conduction through the base material and gas in pores (cavities) was considered, while the convection and radiation were neglected in the initial stage of this research. First, parametric analysis for definingthe proper mesh density and time step were carried out. Then, two-dimensional computational models of the cellular structure, consisting of the base material and the pore gas, have been solved using ANSYS CFX software within the framework of finite volume elements. The results have confirmed theexpectations that the majority of heat is being transferred through the metallic base material with almost negligible heat conduction through the gas in pores. The heat conduction in closed-cell cellular metals is therefore extremely depended on the relative density but almost insensitive regarding tothe gas inside the pore, unless the relative density is very low. Ključne besede: heat transfer, cellular metal materials, porous materials, closed cells, gas fillers, computational simulations Objavljeno: 01.06.2012; Ogledov: 818; Prenosov: 4 Polno besedilo (0,00 KB) |
8. Computational fluid dynamics by boundary-domain integral methodLeopold Škerget, Matjaž Hriberšek, G. Kuhn, 1999, izvirni znanstveni članek Opis: A boundary-domain integral method for the solution of general transport phenomena incompressible fluid motion given by the Navier-Stokes equation set is presented. Velocity-vorticity formulation of the conservations is employed. Different integral representations for conservation field functions based on different fundamental solutions are developed. Special attention is given to the use of subdomain technique and Krylov subspace iterative solvers. The computed solutions of several benchmark problems agree well with available experimental and other computational results. Ključne besede: fluid mechanics, fluid dynamics, numerical methods, boundary domain integral method, viscous fluid, heat transfer, diffusion-convective solution Objavljeno: 01.06.2012; Ogledov: 797; Prenosov: 4 Polno besedilo (0,00 KB) |
9. NUMERICAL MODELING OF THE IMMERSION QUENCHING PROCESS USING AN EULERIAN MULTI-FLUID MODELING APPROACHRok Kopun, 2014, doktorska disertacija Opis: Optimization of heat transfer characteristics in automotive industries is one of the more important factors leading to reductions in fuel consumption and lower emissions values. Efficient heat treatment techniques, like immersion quenching, have been introduced in order to replace heavier metals with lower weight alloys (aluminum), which results in vehicle weight reduction and consequently improved fuel consumption and emission values. Immersion quenching is one of the more important heat treatment techniques introduced in automotive industries, where heat transfer plays a vital role in determining the structure and mechanical properties of the material (e.g. the cylinder head). This presented PhD thesis deals with the development and validation of improved computational methodology in order to simulate the heat transfer characteristics of the immersion quenching cooling process, as implemented within the commercial computational fluid dynamics (CFD) code AVL FIRE®. The boiling phase change process between the heated part and a sub-cooled liquid domain is handled by using the Eulerian multi-fluid modeling approach, where each phase is treated as interpenetrating continua. While for the fluid domain mass, momentum and energy equations are solved within the content of the multi-fluid modeling approach, where only the energy equation is solved to predict the thermal field within the solid region. Result comparisons between the measurements and corresponding numerical simulations using the variable Leidenfrost temperature with additional interfacial forces showed very good agreement. The comparisons were performed on an aluminum step-plate test piece with different thicknesses along its length and on a simplified aluminum cylinder head structure. The specimens were submerged within the liquid domain at different pool temperatures and different solid part orientations, where the solid temperature histories predicted by the numerical model correlated very well with the provided measurement data. Ključne besede: CFD, Multiphase boiling, heat transfer, quenching, Leidenfrost temperature, interfacial forces, results comparison Objavljeno: 14.08.2014; Ogledov: 613; Prenosov: 86 Polno besedilo (6,07 MB) |
10. Analysis of two-stage high- temperature heat pump efficiencyG. Tadić, Milovan Jotanović, Jurij Krope, Darko Goričanec, 2012, izvirni znanstveni članek Opis: The paper shows the potential benefit of using low temperature heat sources with high temperature pumps. Two-stage heat pumps with flash unit and heat exchanger are described. To determine the characteristic parameters of the twoheat pumps a computer program was designed, with which we could determine the dependence of COP and compressor pressure ratios from evaporating temperature. An economic analysis of the justification of the use of heat pumps was made. Ključne besede: heat transfer, high temperature heat pumps, computer programme, economic analysis Objavljeno: 10.07.2015; Ogledov: 402; Prenosov: 7 Polno besedilo (0,00 KB) |