Browsing by Author "Öztop H.F."

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    Fuzzy-based estimation of mixed convection heat transfer in a square cavity in the presence of an adiabatic inclined fin
    (2012) Selimefendigil F.; Öztop H.F.
    In this study, heat transfer from a square cavity in the presence of a thin inclined adiabatic fin is estimated using inputs-outputs generated from a CFD code with a fuzzy based identification procedure. The Reynolds number based on cavity length is 300 and the Richardson number is varied between 1 and 30. The top and bottom walls of the cavity are kept at constant temperature while the vertical walls are assumed to be adiabatic. The fin height, fin inclination angle, and Richardson number are considered as the input and the spatial averaged Nusselt number is taken as the output for the fuzzy model. Two data sets are used. One data set which contains 45 cases is used for estimation and another data set which contains 10 cases (not used in estimation) is used for validation purposes. The predictions using fuzzy model compare well with the CFD computations. © 2012 Elsevier Ltd.
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    Numerical analysis of laminar pulsating flow at a backward facing step with an upper wall mounted adiabatic thin fin
    (2013) Selimefendigil F.; Öztop H.F.
    The effect of an upper wall mounted adiabatic thin fin on laminar pulsating flow in a backward facing step has been investigated numerically. Study is performed for different Reynolds numbers (based on the step height) in the range of 10 and 200 and for the expansion ratio of 2. The working fluid is air with the Prandtl number of 0.71. The governing equations are solved with a general purpose finite volume based solver, FLUENT. The effects of various pertinent parameters, Reynolds number, fin length and pulsating frequency on the fluid flow and heat transfer characteristics are numerically studied. It is observed that fin alters the flow field and thermal characteristics. In the steady flow case, heat transfer enhancement is obtained with the installation of the fin on the upper wall and increases with increasing fin length and increasing Reynolds number. Heat transfer enhancement of 188% is obtained for fin length of Lf=1.5H at Reynolds number of 200. In the pulsating flow case, time-spatial averaged Nusselt number along the bottom wall downstream of the step normalized with spatial averaged Nusselt number in the steady flow case versus excitation Strouhal number shows a resonant type behavior; first an increase in the value is seen up to St=0.05, then a decrease is seen with the increasing values of the frequency of the pulsation for the case without fin. Adding a fin shifts the maximum value of the normalized Nusselt number from St=0.05 to St=0.1. Compared to steady flow with no-fin case, adding a fin is not advantageous for heat transfer enhancement in pulsating flow. © 2013 Elsevier Ltd.
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    Identification of forced convection in pulsating flow at a backward facing step with a stationary cylinder subjected to nanofluid
    (2013) Selimefendigil F.; Öztop H.F.
    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 φ 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. © 2013 Elsevier Ltd.
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    Numerical investigation and dynamical analysis of mixed convection in a vented cavity with pulsating flow
    (2014) Selimefendigil F.; Öztop H.F.
    In the present study, numerical investigation of pulsating mixed convection in a multiple vented cavity is carried out for the range of parameters; Reynolds number (500≤Re≤2000), Grashof number (104≤Gr≤106), Strouhal number (0≤St≤2). The governing equations are solved with a general purpose finite volume based solver. The effects of various parameters on the fluid flow and heat transfer characteristics are numerically studied. It is observed that the flow field and heat transfer rate are influenced by the variations of Reynolds, Grashof and Strouhal numbers. Furthermore, recurrence plot analysis is applied for the analysis of the time series (spatial averaged Nusselt number along the vertical wall of the cavity) and for a combination of different parameters, the systems are identified using recurrence quantification analysis parameters including recurrence rate, laminarity, determinism, trapping time and entropy. © 2013 Elsevier Ltd.
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    Numerical study and identification of cooling of heated blocks in pulsating channel flow with a rotating cylinder
    (2014) Selimefendigil F.; Öztop H.F.
    A numerical study of pulsating channel flow with heated blocks in the presence of an adiabatic rotating cylinder is performed. The governing equations are solved with a finite volume based commercial solver. The effects of pulsating frequency, Reynolds number and cylinder rotation angle on the fluid flow and heat transfer characteristics from the surface of the heated blocks are numerically studied. It is observed that the flow field and heat transfer rate are influenced by the variations of these parameters. Furthermore, nonlinear models are created to identify the dynamics of the heat transfer for each of the surface of the blocks using system identification. © 2014 Elsevier Masson SAS. All rights reserved.
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    Effect of a rotating cylinder in forced convection of ferrofluid over a backward facing step
    (2014) Selimefendigil F.; Öztop H.F.
    In this study, numerical analysis of the heat transfer enhancement and fluid flow characteristics of a rotating cylinder under the influence of magnetic dipole in the backward facing step geometry is conducted. The governing equations are solved with a finite element based commercial solver. The effects of Reynolds number (10≤Re≤200), cylinder rotation angle (-75≤Ω≤75) and strength of the magnetic dipole (0≤γ≤16) on the heat transfer characteristics are studied for backward facing step flow. It is observed that the length and size of the recirculation zones can be controlled with magnetic dipole strength and cylinder rotation angles. As the Reynolds number increases, local Nusselt number increases and number of peaks in the presence of the magnetic field decreases. The effect of cylinder rotation on the local Nusselt number distribution is more pronounced at low Reynolds number. © 2013 Elsevier Ltd. All rights reserved.
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    Soft computing methods for thermo-acoustic simulation
    (Taylor and Francis Inc., 2014) Selimefendigil F.; Öztop H.F.
    In the present study, soft computing methods are employed for thermoacoustic simulation. A ducted Burke-Schumann diffusion flame is used as the heat source for a horizontal duct. First, a dynamic model is constructed from the input-output data sets (velocity forcing - heat release) generated from the Burke-Schumann flame using Comsol. An efficient and cheap model of heat source is obtained using dynamic fuzzy identification. The full thermoacoustic system is simulated in a time domain with the Galerkin method using the identified heat source model. Finally, dynamic neural networks are utilized for obtaining a dynamic fit for a set of operating conditions for the acoustic velocity at the heater location. The overall agreement between the outputs of the soft computing tools (fuzzy and neural network tools) with the Comsol and Galerkin solver is found to be satisfactory. © 2014 Taylor & Francis Group, LLC.
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    MHD mixed convection of nanofluid filled partially heated triangular enclosure with a rotating adiabatic cylinder
    (Taiwan Institute of Chemical Engineers, 2014) Selimefendigil F.; Öztop H.F.
    MHD mixed convection of Cu-water nanofluid filled triangular enclosure with a rotating cylinder is investigated numerically. A partial heater is added on the left vertical wall of the cavity and the right inclined wall is kept at constant temperature. Other walls of the triangular cavity and cylinder surface are assumed to be adiabatic. The governing equations are solved using the finite element method. The effects of the Grashof number, Hartmann number, angular rotational speed of the cylinder and volume fraction of the nanoparticle on fluid flow and heat transfer are investigated numerically. The second law of thermodynamics is also applied to the flow and heat transfer corresponding to different combinations of parameters. It is observed that with increasing the Hartmann number the total entropy generation, local and averaged heat transfer decrease. Averaged Nusselt number increases with the Grashof number. Averaged heat transfer and total entropy generation increase with increase in the angular rotational speed of the cylinder. 50.4% and 37.4% of heat transfer enhancements are obtained for ω= 20 and ω= -20 compared to motionless cylinder ω= 0. Heat transfer and total entropy generation increase as the solid volume fraction of nanoparticle increases. © 2014 Taiwan Institute of Chemical Engineers.
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    POD-based reduced order model of a thermoacoustic heat engine
    (Elsevier Ltd, 2014) Selimefendigil F.; Öztop H.F.
    In this study, numerical simulation of a thermo-acoustic heat engine is performed and a reduced order model of the system based on the Proper Orthogonal Decomposition method is obtained. The governing equations are solved with a finite volume-based solver. The reduced order model is constructed using the snapshots when the system reaches the limit cycle. A quadratic polynomial type ODE system with 11-modes is constructed from the Galerkin projection for the high fidelity CFD computations of the coupled thermo-acoustic system. © 2014 Elsevier Masson SAS. All rights reserved.
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    Pulsating nanofluids jet impingement cooling of a heated horizontal surface
    (2014) Selimefendigil F.; Öztop H.F.
    In this study, a numerical study of pulsating rectangular jet with nanofluids is presented. The aim of this work is to numerically investigate the effects of various parameters such as pulsating frequency, Reynolds number, nanoparticle volume fraction on the fluid flow and heat transfer characteristics. The unsteady Navier-Stokes and energy equations are solved with a commercial finite volume based code. It is observed in the steady case, adding nanoparticles increases the peak value of the Nusselt number at the stagnation point and spatial-averaged Nusselt number along the impingement plate. Heat transfer enhancement up to 18.8% is obtained for particle volume fraction of 6% at Reynolds number of 200. In the pulsating flow case, the combined effect of pulsation and inclusion of nanoparticles is not favorable for the augmentation of the stagnation point Nusselt number at Re = 200, = 1%, 3% and at Re = 400, = 1%, 3% when compared to the steady case. © 2013 Elsevier Ltd. All rights reserved.
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    Forced convection of ferrofluids in a vented cavity with a rotating cylinder
    (Elsevier Masson SAS, 2014) Selimefendigil F.; Öztop H.F.
    In this study, numerical investigation of the forced convection of ferrofluid in a square cavity with ventilation ports in the presence of an adiabatic rotating cylinder is carried out. The governing equations are solved with a finite element based solver. The effects of Reynolds number (20 ≤ Re ≤ 400), angular rotational speed of the cylinder (-500 ≤ Ω ≤ 500), strength and location of the magnetic dipole (0 ≤ γ ≤ 250), (0.2 ≤ a ≤ 0.8, -0.8 ≤ b ≤ -0.2) on the flow and thermal fields are numerically studied. It is observed that the length and size of the recirculation zones can be condtrolled with magnetic dipole strength and angular rotational speed of the cylinder. When the magnetic dipole is closer to the bottom wall of the cavity, flow is accelerated towards the bottom wall with larger influence area. The increasing values of the angular rotational speed of the cylinder in the clockwise direction enhance the heat transfer. © 2014 Elsevier Masson SAS. All rights reserved.
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    Numerical study of MHD mixed convection in a nanofluid filled lid driven square enclosure with a rotating cylinder
    (Elsevier Ltd, 2014) Selimefendigil F.; Öztop H.F.
    In this study, a numerical study of MHD mixed convection nanofluid filled lid driven square enclosure was performed. The bottom wall of the cavity is heated and the top wall is kept at constant temperature lower than that of the heater. Other walls of the square enclosure and cylinder surface are assumed to be adiabatic. The governing equations are solved with finite element method. The influence of the Richardson number (0.001≤Ri≤10), Hartmann number (0≤Ha≤50), angular rotational speed of the cylinder (-10≤Ω≤10) and solid volume fraction of the nanoparticle (0≤φ≤0.05) on fluid flow and heat transfer are numerically investigated. It is observed that 17% of heat transfer enhancement is obtained for Ri = 10 when compared to flow at Ri = 1. Averaged heat heat transfer decreases with increasing Hartmann number and 14.2% of heat transfer enhancement is obtained for Ω=-10 compared to motionless cylinder case at Ω=0. When the solid volume fraction of nanoparticle is increased, heat transfer increases. © 2014 Elsevier Ltd. All rights reserved.
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    Natural convection of ferrofluids in partially heated square enclosures
    (Elsevier, 2014) Selimefendigil F.; Öztop H.F.; Al-Salem K.
    In this study, natural convection of ferrofluid in a partially heated square cavity is numerically investigated. The heater is located to the left vertical wall and the right vertical wall is kept at constant temperature lower than that of the heater. Other walls of the square enclosure are assumed to be adiabatic. Finite element method is utilized to solve the governing equations. The influence of the Rayleigh number (104≤Ra≤5×10 5), heater location (0.25H≤yh≤0.75H), strength of the magnetic dipole (0≤γ≤2), horizontal and vertical location of the magnetic dipole (-2H≤a≤-0.5H, 0.2H≤b≤0.8H) on the fluid flow and heat transfer characteristics are investigated. It is observed that different velocity components within the square cavity are sensitive to the magnetic dipole source strength and its position. The length and size of the recirculation zones adjacent to the heater can be controlled with magnetic dipole strength. Averaged heat transfer increases with decreasing values of horizontal position of the magnetic dipole source. Averaged heat transfer value increases from middle towards both ends of the vertical wall when the vertical location of the dipole source is varied. When the heater location is changed, a symmetrical behavior in the averaged heat transfer plot is observed and the minimum value of the averaged heat transfer is attained when the heater is located at the mid of vertical wall. © 2014 Elsevier B.V.
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    Numerical Study and POD-Based Prediction of Natural Convection in a Ferrofluids-Filled Triangular Cavity with Generalized Neural Networks
    (Taylor and Francis Ltd., 2015) Selimefendigil F.; Öztop H.F.
    The effects of a magnetic dipole source on the natural convection of ferrofluids in a triangular cavity are studied. A partial heater is added to the left vertical wall of the cavity while the right vertical wall is kept at the constant temperature. A magnetic dipole source is placed outside the cavity close to the heater. The governing equations of a coupled multi-physics system are solved with a commercial solver using the finite element method. Computations are performed for different ranges of parameters: Rayleigh number (104 ≤ Ra ≤ 106), strength of the magnetic dipole (0 ≤ γ ≤ 8), horizontal and vertical location of the magnetic dipole (-2.5H ≤ a ≤ -0.5H, 0.2H ≤ b ≤ 0.8H). It is observed that the interaction between natural convection and ferrofluid convection under the influence of magnetic dipole affects the flow and thermal field in such the triangular enclosure. The external magnetic field acts in such a way to decrease local heat transfer in some locations and increase it in others for certain combinations of flow parameters and therefore it can be used as a control parameter for fluid flow and heat transfer. Furthermore, an interpolation method based on Proper Orthogonal Decomposition and Generalized Neural Networks is proposed to predict the thermal performance of the system. This approach gives satisfactory results in terms of local and averaged heat transfer values. © 2015 Taylor & Francis Group, LLC.
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    Numerical investigation and reduced order model of mixed convection at a backward facing step with a rotating cylinder subjected to nanofluid
    (Elsevier Ltd, 2015) Selimefendigil F.; Öztop H.F.
    In the present study, numerical investigation of mixed convection at a backward facing step with a rotating cylinder subjected to nanofluid is conducted and a reduced order model of the system is obtained by using the proper orthogonal decomposition method. The governing equations are solved with a finite element based commercial solver. The effects of various pertinent parameters, Reynolds number, cylinder angular velocity and nanofluid volume fraction on the fluid flow and heat transfer characteristics are numerically studied. It is observed that flow field and thermal patterns change for different parameters and heat transfer enhancement is obtained for some combinations of parameters. Length averaged Nusselt number plots indicate that there is almost a linear increase in the heat transfer enhancement with increasing Reynolds number and nanoparticle volume fraction. Heat transfer enhancement is obtained for cylinder angular velocities of Ω. = -. 4.5 and Ω. = 1.5. A reduced order model of the system with proper orthogonal decomposition method is obtained and it provides accurate results when compared to high fidelity CFD model of the system. © 2014 Elsevier Ltd.
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    Influence of inclination angle of magnetic field on mixed convection of nanofluid flow over a backward facing step and entropy generation
    (Elsevier, 2015) Selimefendigil F.; Öztop H.F.
    In this paper, numerical study of laminar forced convection of nanofluid flow over a backward facing step for different inclination angles of magnetic field is performed. The bottom wall of the channel downstream of the step is isothermally heated and the other walls of the channel are assumed to be adiabatic. Finite element method was used to solve the governing equations. The influence of the Reynolds number (between 20 and 200), Hartmann number (between 0 and 50) and solid volume fraction of the nanoparticle (between 0 and 0.04) on the fluid flow and heat transfer are numerically investigated for different orientation angles of the magnetic field. It is observed the averaged heat transfer increases as the Reynolds number increases and this effect is more pronounced with higher values of inclination angle of the magnetic field. As the value of the Hartmann number decreases for horizontally aligned magnetic field and volume fraction of the nanoparticles increases averaged and local enhancement of heat transfer are observed. For the inclined and vertical magnetic field, suppression of the recirculation zone behind the step is observed as the value of Ha increases which results in heat transfer enhancement. The total entropy generation ratio increases with increasing values of Reynolds number, solid volume fraction of nanoparticles and decreasing values of Hartmann number for horizontally oriented magnetic field. © 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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    Natural convection and entropy generation of nanofluid filled cavity having different shaped obstacles under the influence of magnetic field and internal heat generation
    (Taiwan Institute of Chemical Engineers, 2015) Selimefendigil F.; Öztop H.F.
    In this study, natural convection in a nano-fluid filled cavity having different shaped obstacles (circular, square and diamond) installed under the influence of a uniform magnetic field and uniform heat generation was numerically investigated. The cavity was heated from below and cooled from the vertical sides while the top wall was assumed to be adiabatic. The temperatures of the side walls vary linearly. The governing equations were solved by using Galerkin weighted residual finite element formulation. The numerical investigation was performed for a range of parameters: external Rayleigh number (104 ≤ RaE ≤ 106), internal Rayleigh number (104 ≤ RaI ≤ 106), Hartmann number (0 ≤ Ha ≤ 50), and solid volume fraction of the nanofluid (0 ≤ ϕ ≤ 0.05). It is observed that the presence of the obstacles deteriorates the heat transfer process and this is more pronounced with higher values of ReE. Averaged heat transfer reduces by 21.35%, 32.85% and 34.64% for the cavity with circular, diamond and squared shaped obstacles compared to cavity without obstacles at RaI = 106. The effect of heat transfer reduction with square and diamond shaped obstacles compared to case without obstacle is less effective with increasing values of Hartmann number. Second law analysis was also performed by using different measures for the normalized total entropy generation. © 2015 Taiwan Institute of Chemical Engineers.
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    Mixed convection in a two-sided elastic walled and SiO2 nanofluid filled cavity with internal heat generation: Effects of inner rotating cylinder and nanoparticle's shape
    (Elsevier, 2015) Selimefendigil F.; Öztop H.F.
    In this study, mixed convection in a cavity with volumetric heat generation and filled with nanofluid having an inner rotating cylinder and two flexible side walls is numerically investigated. The top wall of the cavity is at constant cold temperature while the bottom wall is kept at hot temperature. Two flexible side walls and the surface of the inner rotating cylinder are accepted as adiabatic. The finite element formulation is used to solve the governing equations. The Arbitrary-Lagrangian-Eulerian method is used to describe the fluid motion with the flexible side walls of the cavity in the fluid-structure interaction model. The influence of external Rayleigh number, elastic modulus pair of the flexible side walls, angular rotational speed of the cylinder, internal Rayleigh number and nanoparticle volume fraction on the fluid flow and heat transfer are numerically simulated by using different solid nanoparticle shapes (spherical, cylindrical, brick and blade type). It is observed that the local and averaged heat transfer enhances as the external Rayleigh number, nanoparticle volume fraction and absolute value of the angular rotational velocity of the cylinder increase and as the internal Rayleigh number decreases. The elastic modulus of the side walls can be used to control the fluid flow and heat transfer inside the cavity. Utilizing cylindrical nanoparticles gives the best performance in terms of heat transfer enhancement. © 2015 Elsevier B.V. All rights reserved.
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    Effects of phase shift on the heat transfer characteristics in pulsating mixed convection flow in a multiple vented cavity
    (Elsevier Inc., 2015) Selimefendigil F.; Öztop H.F.
    In the current study, numerical investigation of pulsating mixed convection in a multiple vented cavity with phase shift is carried out for the range of parameters; Richardson number (0.25. ≤. Ri. ≤. 4), phase shift (0. ≤. ϕ. ≤. π) and Strouhal number is fixed at 1. The governing equations are solved with a general purpose finite volume based solver. The effects of Ri number and phase shift parameters on the fluid flow and heat transfer characteristics are numerically studied. It is observed that the flow field and heat transfer enhancement are influenced by the variation of these parameters. Furthermore, recurrence plot analysis is applied for the analysis of the time series (spatial averaged Nusselt number along the vertical wall of the cavity) and for a combination of different parameters, the systems are identified using recurrence quantification analysis parameters including recurrence rate, laminarity, determinism, trapping time and entropy. © 2014 Elsevier Inc..
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    Mixed convection of ferrofluids in a lid driven cavity with two rotating cylinders
    (Elsevier B.V., 2015) Selimefendigil F.; Öztop H.F.
    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 ≤ Ω ≤ 400), magnetic dipole strengths (0 ≤ γ ≤ 500), angular velocity ratios of the cylinders (0.25≤Ωi/Ωj≤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 Ω = −400 compared to motionless cylinder case. Increasing the angular velocity ratio from Ω2/Ω1=0.25 to Ω2/Ω1=4 brings about 91.7 % of heat transfer enhancement. © 2015 Karabuk University