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Öğe A three-dimensional mathematical model for absorption in a metal hydride bed(PERGAMON-ELSEVIER SCIENCE LTD, 2002) Aldas, K; Mat, MD; Kaplan, YHeat and mass transfer, fluid flow and chemical reactions in a hydride bed are numerically investigated with a general purpose PHOENICS code. Hydride formation takes place faster near the cooled boundary walls and slower around the core region of the bed. It is found that fluid flow affects the temperature distribution in the system, however, it does not significantly improve the amount of hydrogen absorbed. (C) 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.Öğe A two phase model for electrochemical systems(SPRINGER, 2005) Mat, MD; Aldas, K; Veziroglu, TN; Sammes, N; Smirnova, A; Vasylyev, OTwo phase flow is encountered in many electrochemical systems and play vital role on system efficiency, species transport, velocity distribution etc. A two phase flow model which accounts specific nature of liquid and gaseous phase is developed. The model applied to water electrolysis in an electrochemical cell. Transport equations are solved numerically for both phases with allowance for inter - phase mass and momentum Exchange. Liquid and gaseous phase distributions velocities, current density distribution are calculated under various working conditions. It is found that gas layer accumulation on the electrode surface decreases the active reaction area and adversely affects the reaction rate.Öğe A two-phase flow model for hydrogen evolution in an electrochemical cell(PERGAMON-ELSEVIER SCIENCE LTD, 2004) Mat, MD; Aldas, K; Ilegbusi, OJHydrogen evolution, flow field and current density distribution in an electrochemical cell are investigated with a two-phase flow model. The mathematical model involves solutions of transport equations for the variables of each phase with allowance for inter-phase transfer of mass and momentum. The buoyancy force generated due to density difference between two phases modifies flow profile and increases fluid velocity at the vicinity of the electrode. The current density decreases over the electrode mainly because of the decrease in effective conductivity of electrolyte. It is found that the hydrogen generation significantly increases at higher electrolyte flow by reducing the residence time of bubbles over the electrode. The predicted results satisfactorily agree with data available in the literature. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.Öğe An improved correlation for heat transfer coefficient of two-phase flow in a vertical tube(BEGELL HOUSE, INC, 1999) Altinisik, K; Mat, MD; Aldas, K; Kaplan, Y; Karakoc, F; Dincer, I; Ayhan, TA correlation is developed using Lockhard-Martinelli parameters for predicting the heat transfer coefficient during the two phase flow of water-vapor system. An upward flow in a vertical circular pipe at atmospheric conditions is considered. The constants in the correlation have been obtained after 2800 measurements. The accuracy of correlation is found to be 96.5 using t test. Delta T-h(tp) and h(tp) variations are also investigated using measured data.Öğe Application of a two-phase flow model for hydrogen evolution in an electrochemical cell(ELSEVIER SCIENCE INC, 2004) Aldas, KA two-phase mathematical model is applied to numerical investigation of gas evolution in a vertical electrochemical cell. The model comprises transport equations derived from first principles for each phase. Void fraction distribution for both hydrogen and oxygen gases velocity of gas and liquid phases, concentration and current density distribution are calculated. It is found that gas fraction increases towards to the top of the cell. Gas phase confined regions at the vicinity of the electrodes. Gas release significantly affects the velocity profile in the system. The large density differences between two-phase generate flow which modify velocity profile. Gas release is enhanced at high current density however; gas layer accumulated on the electrode surface decreases the active reaction area which adversely affects the reaction rate. (C) 2003 Elsevier Inc. All rights reserved.Öğe Application of a two-phase flow model for natural convection in an electrochemical cell(PERGAMON-ELSEVIER SCIENCE LTD, 2005) Mat, MD; Aldas, KA two-phase mathematical model is applied to natural convection in an electrochemical cell. The model solves transport equations for both phases with an allowance of interphase mass and momentum exchange. The effect of current density and bubble size on the gas release rate, velocity field and void fraction distribution are investigated in a range of parameter. The flow in the system was generated due to the density difference between gas and liquid phases. It is found that both current density and bubble size significantly affect the gas release rate and velocity field. At an intermediate current density two circulation patterns form at the vicinity of the free surface. The circulations rotating opposite directions enhance lateral diffusion of gas phase. The gas evolution is enhanced with higher current density and lower bubble diameters. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.Öğe Experimental and theoretical analysis of particle distribution in particulate metal matrix composites(ELSEVIER SCIENCE SA, 2005) Aldas, K; Mat, MDParticle distribution in particulate metal matrix composites was investigated experimentally and theoretically. SIC reinforced Pb 20%Sn alloy mechanically was stirred and cast into a cylindrical mold by an inert gas pressure. Particle distribution in the cast is determined at several locations by sectioning and microscopic investigation techniques. The particle fraction is found to decrease along the axis of the mold. The regions close to the boundary wall are determined as the possible clustering areas. A mathematical model is developed to characterize flow of mixture of molten metal alloy and ceramic particles. The distribution of particles is estimated from the trajectories of representative particles with the Lagrangian point of view. The numerical results are reasonably suited with experimental data. (c) 2004 Elsevier B.V. All rights reserved.Öğe Investigation of three-dimensional heat and mass transfer in a metal hydride reactor(JOHN WILEY & SONS LTD, 2002) Mat, MD; Kaplan, Y; Aldas, KA mathematical model for three-dimensional heat and mass transfer in metal-hydrogen reactor is presented. The model considers three-dimensional complex heat, and mass transfer and chemical reaction in the reactor. The main parameter in hydriding processes is found to be the equilibrium pressure, which strongly depends on temperature. Hydride formation enhanced at regions with lower equilibrium pressure. Hydriding processes are shown to be two dimensional for the system considered in this study. Effects of heat transfer rate and R/H (radius to height) ratio on hydride formation are investigated. Hydride formation increases significantly with larger heat transfer rate from the boundary walls, however after a certain heat transfer rate, the increase in formation rate is found to be not significant, due to the low thermal conductivity of the metal-hydride systems. The estimated results agree satisfactorily with the experimental data in the literature. Copyright (C) 2002 John Wiley Sons, Ltd.Öğe Thermal non-linear stresses in an adhesively bonded and laser-spot welded single-lap joint during laser-metal interaction(ELSEVIER SCIENCE SA, 2003) Apalak, MK; Aldas, K; Sen, FIn this study the transient thermal analysis of an adhesively bonded and laser-spot welded joint was carried out based on a thermal model developed for the laser-spot welding of multi-layered sheets using a pulsed Nd:YAG laser. In the thermal stress analysis the material non-linear properties of adhesive and sheets were considered using the non-linear finite element method. The keyhole formation and temperature distributions around the keyhole were analysed for the different sheet materials, i.e. aluminium, steel and titanium, used widely in the automotive and aerospace applications. The laser beam causes small heat-affected zones in the sheets and adhesive layer. As a result non-uniform temperature and thermal strain distributions arise in the vicinity of the keyhole. However, thermal strains are incompatible along the adhesive-sheet interfaces since the adhesive and sheets have different thermal and mechanical properties. Consequently, non-uniform stress distributions were observed around the keyhole causing permanent deformations in the adhesive and sheets. Especially, the free ends of the adhesive-upper sheet and the adhesive-lower sheet interfaces were subjected to considerable plastic strains. The free ends of the adhesive-plate interfaces corresponding to the keyhole boundaries arise as most critical zones; therefore, the first crack initiation in the adhesive layer can be expected from these zones. The residual stresses around the keyhole due to plastic strains would contribute the fast crack growth along the adhesive-plate interfaces when the laser beam-metal interaction ceases. (C) 2003 Elsevier Science B.V. All rights reserved.
