Communications - Scientific Letters of the University of Zilina, 2012 (vol. 14), issue 4A
Articles
Experimental and Numerical Modelling of Turbulent Flow over an Inclined Backward-Facing Step in an Open Channel
Jaromir Prihoda, Pavel Zubik, Jan Sulc, Milan Sedlar
Communications - Scientific Letters of the University of Zilina 2012, 14(11):6-12 | DOI: 10.26552/com.C.2012.4A.6-12 
The contribution deals with the experimental and numerical modelling of the turbulent flow over an inclined backward-facing step in an open water channel. The modelling was carried out in the wide range of the Froude number covering the subcritical, supercritical and near critical regimes as well. The study was concentrated particularly on the development of flow separation and on changes of free surface. Numerical results obtained using the commercial software ANSYS CFX 12.0 for the two-equation SST model and the EARSM model were compared with experimental data obtained by means of PIV and LDA measuring techniques.
Definition of Boundary Conditions for the Numerical Model of Transient Temperaturefield of a Concast Steel Slab
Frantisek Kavicka, Josef Stetina, Bohumil Sekanina, Milos Masarik
Communications - Scientific Letters of the University of Zilina 2012, 14(11):13-17 | DOI: 10.26552/com.C.2012.4A.13-17
The solidification and cooling of a continuously cast billet, slab or cylinder (generally a concasting) is a very complicated problem of 3-D transient heat and mass transfer. This paper is focused on the derivation of boundary conditions, i.e. the values of the heat transfer coefficient (HTC) on all boundaries of the concasting machine (CCM). The definition of boundary conditions is the most difficult part of the investigation of the thermokinetics of this process. The boundary conditions in the numerical model of the temperature field of the concasting are defined as the heat transfer by convection. This HTC includes the so-called reduced convection...
Transfer of Low-Potential Heat from a Deep Borehole by Means of a Heat Pipe and Heat Pump
Milan Malcho, Jozef Jandacka, Martin Vantuch, Andrej Kapjor, Stanislav Gavlas
Communications - Scientific Letters of the University of Zilina 2012, 14(11):18-22 | DOI: 10.26552/com.C.2012.4A.18-22
The paper presents a design of equipment for the usage of low-potential heat collected from rock in a deep borehole by means of heat pipes (HP) and a description of its realization. The borehole model together with a CFD simulation of heat transfer from rock to a soil probe enabled dimensioning of a heat exchanger on the condenser part of the heat pipe and choice of a suitable heat pipe charge. Part of the paper deals with the processing of measurement results gathered from the comparative experimental equipment requiring forced and unforced circulation(heat pipe) of the working substance on the primary side of the heat pump.
Causes and Solution of Aperiodicity of Supersonic Flow Field Downstream of a Profile Cascade
Martin Luxa, Jaroslav Synac, Pavel Safarik, David Simurda
Communications - Scientific Letters of the University of Zilina 2012, 14(11):23-28 | DOI: 10.26552/com.C.2012.4A.23-28
The paper deals with problems of transonic flow field periodicity downstream of a profile cascade with finite number of blades. The origin of phenomena generating this aperiodicity in transonic wind tunnel is described. The solution of this problem by means of adjustable perforated tailboard is documented by results of optical measurements (interferometry, schlieren methods) on a turbine profile cascade consisting of six blades only. The conditions of formation of limit load regime are described.
Calculation of Maximum Water Content in Various Natural Gases by Using Modified Peng-Robinson Equation of State
Jan Rajzinger
Communications - Scientific Letters of the University of Zilina 2012, 14(11):29-35 | DOI: 10.26552/com.C.2012.4A.29-35
During the expansion of natural gas in the pipeline, the gas cools down due to the Joule-Thomson effect and due to the interaction between pipeline system and its surroundings to the conditions at which gas is saturated by water vapors, and gas is not able to keep excess humidity and its condensation and hydrate formation will occur. For that reason it is necessary to identify the critical parts of the pipeline systems, maximum water content in the gas, to choose appropriate physical model, and apply appropriate solutions which also will considerably influence energy consumption requirements. For the physical model development, method based on equality...
Optimization of Construction Parameters of a Floor Convector
Andrej Kapjor, Milan Malcho, Jozef Jandacka, Jozef Huzvar, Tomas Gressak
Communications - Scientific Letters of the University of Zilina 2012, 14(11):36-41 | DOI: 10.26552/com.C.2012.4A.36-41
The paper deals with optimization of construction parameters of a floor convector with natural convection. Intensification of heat transfer from the floor convector with an objective to achieve the highest possible thermal performance of the floor convector was performed by CFD method and criterion equations at boundary conditions, like ambient temperature 20ºC and the temperature of the heating medium 75/65°C in thermostatic chamber.
Influence of the Mixing Grid Position on the Coolant Flow at the Outflow Part of the Nuclear Reactor Fuel Assembly Physical Model and Validation of CFD Model
Frantisek Urban, Lubor Kucak, Jozef Bereznai, Marian Pulmann
Communications - Scientific Letters of the University of Zilina 2012, 14(11):42-46 | DOI: 10.26552/com.C.2012.4A.42-46
In the year 2007 an experimental device with a physical model of the VVER 440 nuclear reactor fuel assembly was installed in the laboratory of The Institute of Power Engineering of the Faculty of Mechanical Engineering of the Slovak University of Technology in Bratislava. Within the framework of the reconstruction a third measuring plane was added to the original two, new combined probes were designed with temperature resistant sensors instead of the thermocouples, and piezoelectric pressure sensors for each probe were used. In this article we investigate the influence the mixing grid positioning has on smoothing the temperature profile in the fuel...
Complex Effects of Free Stream Turbulence and Surface Roughness on the Transitional Intermittency
Ondrej Hladik, Pavel Jonas, Oton Mazur, Vaclav Uruba
Communications - Scientific Letters of the University of Zilina 2012, 14(11):47-52 | DOI: 10.26552/com.C.2012.4A.47-52
The comparison of transitional intermittency distributions in boundary layers on smooth and rough surface is presented. The conditional analysis of the instantaneous wall friction is applied on time records made in transitional boundary layers originating on smooth surface or surface covered by sand paper. Special procedures of measurement and evaluation are applied that allowed evaluations of turbulent and nonturbulent zone averages.
Performance Parameters of a Closed Loop Thermosyphon
Patrik Nemec, Milan Malcho, Martin Smitka, Jozef Matusov
Communications - Scientific Letters of the University of Zilina 2012, 14(11):53-57 | DOI: 10.26552/com.C.2012.4A.53-57
Devices composed of output electronic components produce waste heat that has to be dissipated to the environment. Heat removal by means of forced convection air cooling is quite often insufficient and new alternatives for cooling of output electronic components are being looked for. One option is the use of a closed loop thermosyphon. Theclosed loop thermosyphonis a simple and reliabledevice providing heat transfer. The paper deals with the cooling of an output electronic component by means of this device. It describes a design and construction of the device to provide heat removal from the electronic component, measurement of dependences of performance...
Investigation of High Speed Cavitation Flows
Robert Olsiak, Branislav Knizat, Marek Mlkvik
Communications - Scientific Letters of the University of Zilina 2012, 14(11):58-62 | DOI: 10.26552/com.C.2012.4A.58-62
The paper presents an investigation of cavitating flow behind a micro-orifice. Experiments were performed with a device making visualization of a cavitation cloud possible. Two micro-orifices of the sizes 0.58 mm and 0.35 mm were tested at different cavitation numbers and at different flow rates. Parameters of the cavitation cloud collapse on a smooth surface were found. The experimental device, experiment methodology and some results are presented. Results of visualization are compared with CFD simulations.
Effects of Leading Edge Protuberances on the Aerodynamic Performance of 2D Cambered Airfoil
Abeer Mukhtar, Humera Golandaz, Young Hwan Kim
Communications - Scientific Letters of the University of Zilina 2012, 14(11):63-67 | DOI: 10.26552/com.C.2012.4A.63-67
The pressure and lift coefficients were measured for airfoils with leading edge protuberances in a subsonic wind tunnel and compared with the performance of a baseline NACA 4415 airfoil with a straight leading edge. These protuberances of 2D airfoil mimic the leading edge configuration of the flippers of a humpback whale. Wavelengths were varied from 25% to 50% of the chord length whilst amplitude was varied from 5% to 12% of the chord length of the baseline. Surface flow visualisation using kaolin was also performed to further analyze the flow patterns and surface swirls and vortices were observed. The presence of protuberances enhanced the stall...
Analysis of the Energy Mix of Slovakia in the Context of Improvement of Quality of the Environment
Barbara Hlavnova, Milena Svitekova, Jana Simonidesova
Communications - Scientific Letters of the University of Zilina 2012, 14(11):68-72 | DOI: 10.26552/com.C.2012.4A.68-72
The contribution is focused on the trends of energy balance of Slovakia that is the key indicator determining the consumption of fuelbased energy sources accepting the principles of sustainable development and improving the quality of the environment. It refers to the development of pollutants release from the energy industry in the production of electricity and thermal energy referring to the potential use of RES in the energy mix the composition of which is in direct interaction not only with the development of the environmental load of the components of the environment but also with energy security and energy independence of Slovakia.
Special Approach for Thermal Modelling Fibre-Reinforced Composites with Larger Aspect Ratio
Milan Zmindak, Pavol Novak
Communications - Scientific Letters of the University of Zilina 2012, 14(11):73-78 | DOI: 10.26552/com.C.2012.4A.73-78
Fundamental solutions and their derivatives located along fibre axes are presented to simulate the interactions of matrix and reinforcing elements in composite materials, when the primary field is a scalar function temperature in heat conduction. The inter-domain continuity is specified in discrete points on fibres boundaries. Intensities of the source functions are defined by 1D NURBS (Non-Uniform Rational Basis Spline) and computed in LS (Least Square) sense in the fibres. The inter-domain continuity equations have to be completed by balance equations (energy, equilibrium, etc.) in order to obtain temperature in centre of each fibre. Gradients of...
Fatigue Performance of 6063-T66 Aluminum Alloy under Combined Cyclic Loading of Parent and Welded Specimens
Peter Kopas, Milan Saga, Milan Uhricik
Communications - Scientific Letters of the University of Zilina 2012, 14(11):79-84 | DOI: 10.26552/com.C.2012.4A.79-84
The article deals with the determining of fatigue lifetime of structural materials during multiaxial cyclic loading. The aim of this investigation is to evaluate the influence on fatigue behavior of the welded aluminum alloy EN AW 6063-T66. Experimental device was designed for combined bending - torque loading. The experimental part was focused on the modeling of combined biaxial loading and determining the number of cycles to fracture in the region of low-cycle fatigue. In-phase loading can be treated fairly well using the conventional hypotheses (von Mises or Tresca) on basis of nominal, structural or local strains or stresses. Based on the experimental...
A New Filling Material for Cold Sleeve
Pavol Novak, Milan Zmindak
Communications - Scientific Letters of the University of Zilina 2012, 14(11):85-89 | DOI: 10.26552/com.C.2012.4A.85-89
The aim of this paper is to review material properties of a new filling material for cold sleeve. The first experimental measurements were performed on samples to determine required material properties. Then the structural analysis was performed for a pressurized pipe with insufficiently welded root and installed cold sleeve. The case of depressurized pipes that could cause a breach of cohesion between the filling material and surface of pipe or sleeve with the usage of cohesive finite elements was simulated.