Browsing by Author "Selimefendigil, F"

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    Nanoliquid Jet Impingement Heat Transfer for a Phase Change Material Embedded Radial Heating System
    Selimefendigil, F; Öztop, HF
    Nanoliquid impingement heat transfer with a phase change material (PCM) installed radial system is considered. The study is performed by using the finite element method for various values of Reynolds numbers (100 <= Re <= 300), height of PCM (0.25H <= h(pcm) <= 0.75H), and plate spacing (0.15H <= h(s) <= 0.40H). Different configurations using water, nanoliquid, and nanoliquid + PCM are compared in terms of heat transfer improvement. Thermal performance is improved by using PCM, while best performance is achieved with nanoliquid and PCM-installed configuration. At Re = 100 and Re = 300, heat transfer improvements of 26% and 25.5% are achieved with the nanoliquid+ PCM system as compared to water without PCM. The height of the PCM layer also influences the heat transfer dynamic behavior, while there is 12.6% variation in the spatial average heat transfer of the target surface with the lowest and highest PCM heights while discharging time increases by about 76.5%. As the spacing between the plates decreases, average heat transfer rises and there is 38% variation.
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    Effects of flexible fins on melting process in a phase change material filled circular cavity
    Akbal, Ö; Selimefendigil, F; Öztop, HF
    In this study, melting process in a phase change material (PCM) filled annular closed space by using flexible fins is analyzed. Cases with different number of flexible fins and without fin are compared in terms of phase change dynamics while two-way fluid structure interaction analysis is used. Temperature differences of 30 degrees C and 60 degrees C are considered between the walls of the circular enclosure while flexible fin numbers are varied between 0 and 10. It is observed that the melting is significantly accelerated by using the elastic fins. Due to the deflection of the elastic fins, melt process dynamics is influenced by varying fin number and its elastic modulus. Further reduction in the full phase transition occurs when the temperature difference is increased. As compared to un-finned case at temperature difference of 30 degrees C by using 10 flexible fins at Delta T = 60 degrees C, there is 87 % reduction in the complete phase transition time. The ANFIS (Adaptive Neuro-Fuzzy Interface System) based modeling approach is used for phase transition dynamics in circular enclosure equipped with flexible fins.
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    Optimization assisted divide-combine approach to model cooling of a PV module equipped with TEG by using a trapezoidal shaped hybrid nano-enhanced cooling channel and performance estimation with generalized neural networks
    Selimefendigil, F; Oztop, HF
    Innovative cooling strategies and efficient thermal management techniques are needed to increase the efficiency of photovoltaic (PV) modules. In the current work, a novel cooling channel method and computational approach is utilized for thermal management of PV module combined with thermoelectric generator (TEG) unit. The method uses an optimization assisted divide-combine computational approach while a trapezoidal wavy cooling channel is utilized. Hybrid nanofluid is used in the cooling channel. Simulations for cooling channel and PV-TEG unit are conducted by using finite element method while COBYLA algorithm is considered for optimization of trapezoidal wavy channel. It is shown that the corrugation amplitude has the largest effect on a trapezoidal wavy channel's cooling effectiveness, while the inclination angle has the least effect. The range of average Nu improvements by adjusting the trapezoidal wavy channel's amplitude, wave number, and inclination are obtained as 36%-42%, 13.5%-15%, and 2.5%-3%. The average PV-cell temperature decreases by approximately 2.7oC to 3.4oC when the cooling channel is connected to the PV-TEG unit. It also decreases by approximately 1oC to 1.3oC when the wave number is changed. The optimum corrugation height (b/H) and inclination (0) for the best cooling performance are found as (b/H, 0)=(0.5, 36) when using 3 waves and (b/H, 0)=(0.5, 13.16) when using 11 waves. The PV-cell temperature drops with optimal channel configurations with wave numbers of 3 and 11 are obtained as 4.3oC and 6oC, respectively, in comparison to the reference cooling channel (flat channel employing only pure fluid). While the PV-TEG unit is coupled with parametric simulation of the cooling channel, generalized neural network models are used to successfully estimate the PV-cell temperature and TEG power. More complex channel assemblies and consideration of multiple PV-TEG combined units can be developed using the proposed optimization-assisted divide-combine methodology.
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    Cooling of a Partially Elastic Isothermal Surface by Nanofluids Jet Impingement
    Selimefendigil, F; Öztop, HF
    Numerical study of nanofluid jet impingement cooling of a partially elastic isothermal hot surface was conducted with finite element method. The impingement surface was made partially elastic, and the effects of Reynolds number (between 25 and 200), solid particle volume fraction (between 0.01 and 0.04), elastic modulus of isothermal hot surface (between 10 4 and 10 6), size of the flexible part (between 7.5 w and 25 w), and nanoparticle type (spherical, cylindrical, blade) on the fluid flow and heat transfer characteristics were analyzed. It was observed that average Nusselt number enhances for higher Reynolds number, higher values of elastic modulus of flexible wall, smaller size of elastic part, and higher nanoparticle solid volume fraction and for cylindrical shaped particles. It is possible to change the maximum Nusselt number by 50.58% and 33% by changing the elastic modulus of the hot wall and size of elastic part whereas average Nusselt number changes by only 9.33% and 6.21%. The discrepancy between various particle shapes is higher for higher particle volume fraction.
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    Mixed convection of nanofluid filled cavity with oscillating lid under the influence of an inclined magnetic field
    Selimefendigil, F; Öztop, HF
    In this study, mixed convection of an oscillating lid-driven cavity filled with nanofluid under the influence of an inclined uniform magnetic field was numerically investigated. The cavity is heated from below and cooled from above while side walls are assumed to be adiabatic. The top wall velocity varies sinusoidally while no-slip boundary conditions are imposed on the other walls of the cavity. The governing equations was solved by Galerkin weighted residual finite element formulation. The numerical investigation was performed for a range of parameters: Richardson number (10(-1) <= Ri <= 10(2)), Hartmann number (0 <= Ha <= 60), inclination angle of the magnetic field (0 <= gamma <= 90), non-dimensional frequency of the oscillating lid (0.001 <= St <= 1) and solid volume fraction of the nanoparticle (0 <= phi <= 0.04). It is observed that the flow and thermal patterns within the cavity are affected by the variation of these parameters. The heat transfer process becomes inefficient for high Strouhal number, high Hartmann number and high Richardson number. Maximum enhancement of averaged heat transfer and the damping of the convection within the cavity due to the Lorentz forces caused by magnetic field are attained for magnetic inclination angles of gamma = 90 degrees and gamma = 60 degrees. As the solid volume fraction of nanoparticles increases averaged heat transfer enhancement of 28.96% is obtained for volume fraction of phi = 0.04 compared to base fluid. (C) 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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    Effects of an adiabatic inclined fin on the mixed convection heat transfer in a square cavity
    Selimefendigil, F; Oztop, HF
    In this study, a square cavity with two ventilation ports in the presence of an adiabatic fin placed on the bottom wall of the cavity is numerically analysed for the mixed convection case for a range of Richardson numbers (Ri = 0.1,1, 10, 30) and at Reynolds number of 300. The top and bottom walls of the cavity are kept at constant temperature while the verticals walls are assumed to be adiabatic. The effect of the fin height, inclination angle and Richardson number on the fluid flow and heat characteristics is numerically analysed. The results are presented in terms of streamlines, isotherm plots and averaged Nusselt number plots. It is observed that length and inclination angle of the fin significantly alter the streamlines and isotherms and hence the thermal performance of the system. For the best performance at different fin lengths, optimum inclination angle changes.
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    Natural convection in a flexible sided triangular cavity with internal heat generation under the effect of inclined magnetic field
    Selimefendigil, F; Öztop, HE
    In this numerical study, magnetohydrodynamics natural convection in a flexible sided triangular cavity with internal heat generation is investigated. The inclined wall of the cavity is cooled and flexible while the left vertical wall is partially heated. Galerkin weighted residual finite element method is used to solve the governing equations. The effects of pertinent parameters such as external Rayleigh number (between 10(4) and 10(6)), internal Rayleigh number (between 10(4) and 10(7)), elastic modulus of flexible wall (between 500 and 10(5)), Hartmann number (between 0 and 40) and inclination angle of the magnetic field (between 0 degrees and 90 degrees) on the fluid flow and heat transfer characteristics were numerically investigated. It was observed local and averaged Nusselt number enhance with external Rayleigh number but in the vicinity of the upper location of the heater local heat transfer deteriorates due to the inclined wall deformation with increasing external Rayleigh number. Heat transfer reduces with internal Rayleigh number and Hartmann number. Averaged heat transfer decreases 13.25% when internal Rayleigh number is increased from 10(4) to 10(7) and decreases 40.56% when Hartmann number is increased from 0 to 10. The reduction in the convection with magnetic field is effective for higher values of external Rayleigh numbers and averaged heat transfer increases with magnetic field inclination angle. (C) 2016 Elsevier B.V. All rights reserved.
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    Effects of Combined Utilization of Active Cooler/Heater and Blade-Shaped Nanoparticles in Base Fluid for Performance Improvement of Thermoelectric Generator Mounted in Between Vented Cavities
    Selimefendigil, F; Oztop, HF
    A wide range of technical applications, including solar power, waste heat recovery, electronics thermal management, and heat exchangers, employ thermoelectric generators. They can be mounted in between channels / cavities where hot and cold fluid streams exist. In this study, two novel methods of enhancing the power generation from thermoelectric generator device mounted in between vented cavities are proposed by combined utilization of active heater/cooler rectangular blocks and blade-shaped nanoparticles in base fluid. Finite element method investigation is conducted numerically for a range of hot and cold stream Reynolds numbers (250-1000), non-dimensional hot and cold block sizes (0.01-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-$$\end{document}0.4), and heating/cooling increments (0-10), with nanoparticle loading limited to 0.03. Higher values of Reynolds number results in a rise in thermoelectric generator power. When comparing the cases of lowest and highest Reynolds number combinations, a 219%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document} increase in power is achieved. The thermoelectric generator power will rise by around 27.5%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document} when the object size reaches its maximum. However, for moderate object sizes, up to 31.6%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document} reduction in power generation can be realized. Greater temperature differences result in a linearly rising power generation, with an achievable power increase of up to 22%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document}. When nanoparticle loading in the base fluid for both cavities is raised to its maximum value, the resultant power increases by around 30%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$30\%$$\end{document}. Thermoelectric generator power rises by 67.8%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document} when an active heater/cooler with nanofluid is used in vented cavities, as opposed to the reference scenario of employing no object and only water. The thermoelectric generator device's hot and cold interface temperatures are accurately estimated using the artificial neural network based method. The estimated temperature can be used as boundary condition for the solution of the governing equations in the thermoelectric generator device domain which will decrease the computational cost when dealing with very complex channel configurations.
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    A computational analysis on convective heat transfer for impinging slot nanojets onto a moving hot body
    Cosanay, H; Oztop, HF; Selimefendigil, F
    Purpose The purpose of this study is to perform computational analysis on the steady flow and heat transfer due to a slot nanojet impingement onto a heated moving body. The object is moving at constant speed and nanoparticle is included in the heat transfer fluid. The unsteady flow effects and interactions of multiple impinging jets are also considered. Design/methodology/approach The finite volume method was used as the solver in the numerical simulation. The movement of the hot body in the channel is also considered. Influence of various pertinent parameters such as Reynolds number, jet to target surface spacing and solid nanoparticle volume fraction on the convective heat transfer characteristics are numerically studied in the transient regime. Findings It is found that the flow field and heat transfer becomes very complicated due to the interaction of multiple impinging jets with the movement of the hot body in the channel. Higher heat transfer rates are achieved with higher values of Reynolds number while the inclusion of nanoparticles resulted in a small impact on flow friction. The middle jet was found to play an important role in the heat transfer behavior while jet and moving body temperatures become equal after t = 80. Originality/value Even though some studies exist for the application of jet impingement heat transfer for a moving plate, the configuration with a solid moving hot body on a moving belt under the impacts of unsteady flow effects and interactions of multiple impinging jets have never been considered. The results of the present study will be helpful in the design and optimization of various systems related to convective drying of products, metal processing industry, thermal management in electronic cooling and many other systems.
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    Analysis of hybrid nanofluid and surface corrugation in the laminar convective flow through an encapsulated PCM filled vertical cylinder and POD-based modeling
    Selimefendigil, F; Öztop, HF
    In the present work, performance assessment of a PCM filled three dimensional vertical cylinder is con-ducted under the combined effects of surface corrugation and presence of binary nanoparticles in the heat transfer fluid. The numerical simulation is performed by using finite element method with varying values of Reynolds number (100 <= Re <= 750 ), number (1 <= N <= 8) and height (H/10 <= h <= H/2 ) of rect-angular type corrugation form and volume fraction of particles (0 <= phi <= 0.02) in unsteady configuration. Thermal transport features are enhanced while charging time is reduced for higher values of Reynolds number, solid volume fraction of the binary mixture in the heat transfer fluid. Complete charging time is reduced by 57 % with increase of Reynolds number from 100 to 750 while it is reduced by 23 % when nanofluid at the highest solid volume fraction is used instead of water. However, the corrugation param-eters have reverse effects on the charging process. A computational framework for reconstruction of heat transfer fluid and PCM temperatures for the unsteady parametric configuration in the computational do-main is offered by utilizing proper orthogonal decomposition (POD) technique with 25 modes for heat transfer fluid and 75 modes for PCM. (C) 2021 Elsevier Ltd. All rights reserved.
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    Mixed convection and entropy generation of nanofluid flow in a vented cavity under the influence of inclined magnetic field
    Selimefendigil, F; Oztop, HF
    In this study, mixed convection and entropy generation in a vented cavity with inlet and outlet ports are examined under the effects of an inclined magnetic field. Galerkin weighted finite element method was used for the solution of the governing equations. The numerical simulations are performed for various values of Reynolds numbers (between 100 and 500), Hartmann number (between 0 and 50) and solid particle volume fractions of CuO nanoparticles (between 0 and 4%). Different walls and domains of the computational model are considered for the heat transfer and entropy generation analysis. It was observed that at low Reynolds number number, magnetic field has the potential to enhance the heat transfer at the highest strength while the effect of magnetic field is to reduce the convection at higher Reynolds number. The contributions of different hot walls to the overall heat transfer change considerably with the change of Hartmann number while the effect of magnetic inclination angle is marginal. Inclusion of nanoparticle results in heat transfer enhancement in the absence and presence of magnetic field and the amount of enhancement is 25-27% at the highest value of solid nanoparticle volume fraction. Different parts of the cavity contribute differently to the overall entropy generation when Hartmann number varies while the overall entropy generation first decreases and then increases when the value of Hartmann number increases. The addition of nanoparticles increases the overall entropy generation rate.
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    MHD mixed convection of Ag-MgO/water nanofluid in a triangular shape partitioned lid-driven square cavity involving a porous compound
    Selimefendigil, F; Chamkha, AJ
    In the current study, magnetohydrodynamics mixed convective flow of Ag-MgO/water hybrid nanofluid in a triangular shaped partitioned cavity involving a porous layer is numerically investigated by using the finite element method. In the numerical simulation, various effects of pertinent parameters such as Richardson number (between 0.01 and 100), Hartmann number (between 0 and 60), magnetic field inclination angle (between 0 and 90), Darcy number (between 10(-4) and 5 x 10(-2)), location of the vertex of triangular porous region (between 0.2 and 0.8 H) and hybrid nanoparticle solid volume fraction (phi(1) between 0 and 0.01, phi(2) between 0 and 0.01) on the fluid flow and convective heat transfer features are examined. It was observed that a large vortex is established below the main vortex near the upper wall for the lowest value Ri number. At the highest magnetic field strength, multi- recirculation flow pattern is seen in the right bottom corner. The average heat transfer enhances with higher values of permeability of the porous medium, magnetic field inclination angle, distance of the porous layer vertex from the hot wall and solid nanoparticle volume fraction of each particles in the hybrid nanofluid. The impact is reverse for higher values of Richardson number and Hartmann number. In the current work, significant changes in the average Nusselt number are obtained by varying the location of the porous medium. The triangular shaped porous compound can be used as an excellent tool for convective heat transfer control.
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    Cooling of double PV-TEG combined units by using a T-shaped branching channel equipped with an inclined elastic fin
    Selimefendigil, F; Oztop, HF
    Modern energy technology systems including batteries, hydrogen storage units, electronic equipments and photovoltaic (PV) modules require effective cooling methods and thermal management techniques for performance improvements and safety of operation. In this study, a novel thermal management system for double PV units is proposed by using combined effects of inclined elastic fin in the T-shaped branching cooling channel and thermoelectric generator (TEG) modules. The FEM based numerical analysis is carried out for different Reynolds numbers (between 200 to 1200), fin lengths (between 0 and H), fin tilt (between 10 and 45), and fin position (yf between-H and H) where both rigid or elastic fin configurations are considered. Cell temperature drops of 14 degrees C and 15.48 degrees C are seen in PV1 and PV2 when Reynolds number (Re) is raised from 200 to 1200 using a rigid fin while average temperatures become 2 degrees C and 0.5 degrees C higher at the highest Re when elastic finis used. Poor thermal transport is observed at the fin location of yf=-H. Fins and higher Re significantly lower the PV surface temperature of both PVs in double PV-TEG combined system. When elastic and rigid fins are used at the highest Re, the temperature of PV1 is lowered by about 14.5 degrees C and 16.7 degrees C compared to the reference configuration of the no-fin case at Re=200, while the temperature of PV2 is lowered by about 12 degrees C and 11.2 degrees C. PV's performance is estimated using artificial neural network model for different flow rates, fin lengths, and fin inclinations (both elastic and rigid scenarios).
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    PERFORMANCE PREDICTIONS OF AIR-COOLED CONDENSERS HAVING CIRCULAR AND ELLIPTIC CROSS-SECTIONS WITH ARTIFICIAL NEURAL NETWORKS
    Selimefendigil, F; Öztop, HF
    In this study, mathematical models of air cooled condensers with circular and elliptic cross-sections were developed and performances were evaluated with artificial neural networks. Air velocity, orientation angle and ambient temperature were used as the input to the neural network structure while heat transfer rate to the air was used as the output. The data sets were generated from high fidelity, computationally inefficient expensive three dimensional computational fluid dynamics simulations. It was observed that artificial neural network model replaces computational fluid dynamics model and based on the mathematical model with artificial neural network, elliptic condensers perform better in terms of heat transfer compared to circular ones.
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    Identification of forced convection in pulsating flow at a backward facing step with a stationary cylinder subjected to nanofluid
    Selimefendigil, F; Öztop, HF
    In the present study, the application of the system identification method for forecasting the thermal performance of forced pulsating flow at a backward facing step with a stationary cylinder subjected to nanofluid is presented. The governing equations are solved with a finite volume based code. The effects of various parameter frequencies (0.25 Hz-8 Hz), Reynolds number (50-200), nanoparticle volume fraction (0.00-0.06) on the fluid flow and heat transfer characteristics are numerically studied. Nonlinear system identification toolbox of Matlab is utilized to obtain nonlinear dynamic models of data sets corresponding to different nanoparticle volume fractions at frequencies of 1, 4 and 8 Hz. It is observed that heat transfer is enhanced with increasing the frequency of the oscillation, nanoparticle volume fraction and Reynolds number. The level of the nonlinearity (distortion from a pure sinusoid) decreases with increasing phi and with decreasing Reynolds number. It is also shown that nonlinear dynamic models obtained from system identification toolbox could produce thermal output (length averaged Nusselt number) as close to as output from a high fidelity CFD simulation. (C) 2013 Elsevier Ltd. All rights reserved.
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    MHD mixed convection of nanofluid in a flexible walled inclined lid-driven L-shaped cavity under the effect of internal heat generation
    Selimefendigil, F; Öztop, HF
    In this study, mixed convective flow of nanofluid in an inclined L-shaped cavity which has elastic walls is numerically analyzed under the effects of internal heat generation and magnetic field by using the finite element technique with the Arbitrary-Lagrangian-Eulerian method. Simulations are performed for different values Richardson number (between 0.03 and 30), inclination angle of the cavity (between 0 degrees and 180 degrees), Hartmann number (between 0 and 50), orientation angle of the magnetic field (between 0 degrees and 90 degrees), internal Rayleigh number (between 10(4) and 10(6)), solid nanoparticle volume fraction (between 0 and 0.04), flexible wall elastic modulus (between 104 and 108) and aspect ratio (between 0.2 and 0.7) of the L-shaped cavity. It was observed that the effects of elastic wall on the convective heat transfer features are significant for the lowest value of Richardson number and lowest values of elastic modulus while 11% of discrepancy is obtained in the average Nusselt number when cavity with elastic and rigid walls is compared. The impact of the magnetic inclination angle is significant when compared to magnetic field strength for the variation of the average Nusselt number. Cavity inclination angle has significant impacts on the variation of the average Nusselt number for water and nanofluid. A higher size of the cold wall (aspect ratio) increases the heat transfer rate while the internal Rayleigh number reduces it. Enhancement in the average Nusselt number is about 15%-19% at highest nanoparticle volume fraction of the nanofluid while the trends in the convective heat transfer features with respect to changes in the pertinent parameters are similar for water and nanofluid. (C) 2019 Elsevier B.V. All rights reserved.
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    Mixed convection of nanofluids in a three dimensional cavity with two adiabatic inner rotating cylinders
    Selimefendigil, F; Öztop, HF
    In this study, mixed convection of nanofluids in a three dimensional cavity with two inner adiabatic rotating circular cylinders were analyzed by using finite element method. Vertical surfaces are kept at constant temperature while other walls and rotating cylinder surfaces were taken as adiabatic. The three dimensional cavity was filled with water and various types of nanoparticles (Cu, Al2O3 and TiO2). The water-Cu nanofluid provided the highest heat transfer rate and at the highest value of Rayleigh number, 4% higher average heat transfer rate is obtained when compared to other particles. When cylinders rotate, depending on the rotational direction either enhancement or deterioration of average Nusselt number is observed. For the highest value of rotational speed of the cylinders, 8.5% discrepancy between the average Nusselt number is observed for the nanofluid with Cu and Al2O3 nanoparticles. For Cu-water nanofluid at the highest volume fraction as compared to base fluid, 38.10% of enhancement in the average heat transfer is obtained. A correlation for the average Nusselt number in polynomial form was developed which is a function of Rayleigh number and angular rotational speed of the cylinders. (C) 2017 Elsevier Ltd. All rights reserved.
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    Mixed convection of ferrofluids in a lid driven cavity with two rotating cylinders
    Selimefendigil, F; Öztop, HF
    Mixed convection of ferrofluid filled lid driven cavity in the presence of two rotating cylinders were numerically investigated by using the finite element method. The cavity is heated from below, cooled from driven wall and rotating cylinder surfaces and side vertical walls of the cavity are assumed to be adiabatic. A magnetic dipole source is placed below the bottom wall of the cavity. The study is performed for various values of Reynolds numbers (100 <= Re <= 1000), angular rotational speed of the cylinders (-400 <= Omega <= 400), magnetic dipole strengths (0 <= gamma <= 500), angular velocity ratios of the cylinders (0.25 <= Omega(i/) Omega(j) <= 4) and diameter ratios of the cylinders (0.5 <= Di/Dj <= 2). It is observed that flow patterns and thermal transport within the cavity are affected by variation in Reynolds number and magnetic dipole strength. The results of this investigation revealed that cylinder angular velocities, ratio of the angular velocities and diameter ratios have profound effect on heat transfer enhancement within the cavity. Averaged heat transfer enhancements of 181.5 % is achieved for clockwise rotation of the cylinder at Omega = -400 compared to motionless cylinder case. Increasing the angular velocity ratio from Omega(2)/Omega(1) = 0.25 to Omega(2)/Omega(1) = 4 brings about 91.7 % of heat transfer enhancement. (C) 2015 Karabuk University. Production and hosting by Elsevier B.V.
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    A Fuzzy-Pod Based Estimation of Unsteady Mixed Convection in a Partition Located Cavity with Inlet and Outlet Ports
    Selimefendigil, F; Öztop, HF
    In the present study, a novel approach based on Proper Orthogonal Decomposition (POD) and fuzzy clustering method is utilized to predict the flow field and heat transfer for the unsteady mixed convection in a square enclosure with two ventilation ports. An adiabatic thin fin is placed on the bottom wall of the cavity and all walls of the enclosure are kept at constant temperature. An oscillating velocity is imposed at the inlet port for a range of Strouhal numbers between 0.1 and 1. Reduced order models of the system are obtained with fuzzy-POD approach for Richardson number of 1 and 100. The estimation data set is obtained for Strouhal numbers 0.1 and 0.5, and the validation data set is obtained for Strouhal number of 0.25. A comparison of the modal coefficients obtained from the proposed approach compares well with the modal coefficients obtained by projecting the CFD data at Strouhal number of 0.25 onto the POD modes. The proposed approach is computationally efficient and the problem of numerical instability in the computation with the conventional Galerkin-POD approach can be circumvented.
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    Magnetic field effects on the forced convection of CuO-water nanofluid flow in a channel with circular cylinders and thermal predictions using ANFIS
    Selimefendigil, F; Öztop, HF
    Numerical simulation of forced convection of CuO-water nanofluid over circular cylinders within a channel was conducted under the influence of a uniform magnetic field by using the finite element method. In the channel, four circular cylinders which are arranged in a two by two matrix configuration are used and they are kept at constant temperature. Effects of various pertinent parameters of the configuration such as the Reynolds number (between 100 and 1000), Hartmann number (between 0 and 10), solid particle volume fraction (between 0 and 0.04) and horizontal distance between the circular cylinder centers (between 0.5 and 8 times of the channel height) on the fluid flow and convective heat transfer were analyzed. The potential use of magnetic field in the reduction of the wake region behind the circular cylinder and augmentation of the heat transfer in the presence of nanoparticle inclusion to the base fluid was examined. It was observed that the established secondary peaks of local Nusselt number along the hot surface for moderate and higher values of Reynolds number reduces with Hartmann number and disappears at the highest value of Hartmann number. When the Hartmann number is increased from Ha = 0 to Ha = 10, 23% of the average heat transfer enhancement is achieved for cylinder which is located in the first row and second column (some distance far away from the wake of the first column cylinder). The average Nusselt number was found to augment by about 17-20 % at the highest and lowest values of Reynolds number and Hartmann numbers for circular cylinders which are placed at (first row first column) and (first row second column). Highly accurate and fast predictions of the average Nusselt number for the circular cylinders are obtained with Adaptive-Network-Based Fuzzy Inference System (ANFIS) modeling.