Yazar "Kurtulmus, Nazim" seçeneğine göre listele

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    A review of hydrodynamics and heat transfer through corrugated channels
    (Pergamon-Elsevier Science Ltd, 2019) Kurtulmus, Nazim; Sahin, Besir
    Ever-increasing consumption of limited energy sources forces researchers and engineers to produce more efficient energy systems in order to use energy sources effectively. Enhancing the heat transfer rate with an acceptable pressure drop is an important parameter to produce more compact heat exchangers which are used in air-conditioning systems, chemical reactors, thermal power plants and others. Nowadays many engineering techniques such as surface modifications, swirl flow creators, flow conditioners, additives, etc. are implemented to enhance the heat transfer performance of energy systems. Researchers are still interested in the implementation of these techniques to improve the performance of energy systems further although the current literature has many experimental and numerical research data. Even though the corrugated channel is a passive technique to augment heat transfer, researchers have been insisting on trying to get further improvement by implementing one or more passive/active techniques to corrugated channels. The objective of the present work is to gather available research data which particularly focus on the flow characteristics and heat transfer rates through the corrugated channels. The design parameters, practical limitations and conclusions of energy systems, obtained previously are presented in tabular forms and the necessary discussion is also provided.
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    Active and passive wake control of a circular cylinder using rotation and surface texturing
    (Pergamon-Elsevier Science Ltd, 2026) Aksoy, Muharrem Hilmi; Kurtulmus, Nazim; Ispir, Murat; Goktepeli, Ilker
    Flow structures over circular cylinders have been widely investigated due to their practical importance and the complexity of wake interactions. In this study, combined passive and active flow control strategies have been examined for rotating cylinders with surfaces having dimples/protrusions at a Reynolds number as Re = 2 x 103. Particle Image Velocimetry (PIV) measurements have been conducted for rotation rates ranging from alpha = 0 to alpha = 1.26 to explore modifications in wake topology for different circumferential angles ((3) between 15 degrees and 60 degrees. For the stationary bare cylinders, symmetric recirculation regions including counter-rotating eddies have been observed, whereas rotation caused the separated region to contract, shifted and lost its symmetry. Dimples/ protrusions, even without rotation, altered shear layer development with effects varying according to the circumferential angle. When rotation was applied, the modified surfaces produced notable changes compared with that of the bare cylinder, including reduced negative-velocity regions and more rapid momentum recovery. The time-averaged vorticity distributions indicated the diminished vortex intensity, while the periodic organization of the shedding became less distinct at higher rotation rates. Turbulence kinetic energy and Reynolds stress correlations presented that surface modifications, particularly at (3 = 30 degrees, 45 degrees and 60 degrees, enhanced shear layer energization, delayed flow separation and disrupted wake symmetry at alpha >= 0.84. The combination of rotation and surface texturing is therefore effective in reshaping wake dynamics and attenuating large scale vortex structures, offering practical implications for flow control applications.
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    AN INDUSTRIAL VAPOR ABSORPTION AIR CONDITIONING APPLICATION
    (Turkish Soc Thermal Sciences Technology, 2017) Kurtulmus, Nazim; Horuz, Ilhami
    In this study, the application of VAR system to an industrial company is investigated. For this purpose, the company which requires an air-conditioning system to its office building was chosen. After determining this company had an industrial furnace, this research aimed to design the VAR system utilizing waste heat from this furnace flue gases to air-condition the office building. Firstly, the physical properties and heat quantity of the flue gases were determined and the cooling load of the office building was calculated. It was found out that the flue gases had enough heat capacity to drive the VAR system. Next, single effect VAR system was introduced and the thermodynamic, energy and exergy analysis were made. The COP of the VAR system is calculated to be 0.64 and the highest exergy destruction was obtained at the generator which is 37.19kW. Finally, the application of the VAR system to this industrial company was analyzed in detail. The capital, maintaining and operating costs of the VAR system were analyzed and compared with alternative systems.
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    Experimental investigation of flow past circular cylinders with dimpled and protruded surface modifications using PIV
    (Elsevier Sci Ltd, 2026) Kurtulmus, Nazim; Ispir, Murat; Aksoy, Muharrem Hilmi; Goktepeli, Ilker
    This study examines the impact of surface modifications, including dimpled/protruded formations, on the flow characteristics of circular cylinders in free-stream flow, aiming to enhance passive flow control. Experiments have been conducted in an open-water channel using a Particle Image Velocimetry (PIV) system to obtain detailed velocity field data and turbulent statistics. Circular cylinders with in-line dimple/protrusion arrangements have been fabricated via 3D printing and their performance was evaluated at Reynolds numbers of Re = 4000 and Re = 6000. Furthermore, the angle of dimple/protrusion configured in an in-line arrangement around the circumference of the cylinder varied between beta = 15 degrees and beta = 60 degrees. The results indicate that the formation of dimples/protrusions on the cylinder surface is an efficient tool that substantially alters wake characteristics, including reduced backflow intensity and delayed vortex interactions. Compared to that of the bare cylinder, configurations with beta = 45 degrees exhibited the most notable improvements in wake recovery and reduction in crossstream velocity fluctuations. Furthermore, normalized Reynolds shear stress distributions revealed a marked decrease in magnitude and a reduced affected region for modified cylinders, contributing to potential drag reduction and lowering the forces influencing the body.
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    Experimental investigation of flow structure and heat transfer characteristics for confined cylinders
    (Pergamon-Elsevier Science Ltd, 2022) Kurtulmus, Nazim
    In this work, the heat transfer characteristics and the flow structures for confined cylinders are presented. The aspect ratio of the confined cylinders was varied between 0.4 and 1; however, the ratio between the projected height of cylinders to the confinement height was kept constant as 0.25 in this work. The experiments were conducted to determine heat transfer rates for Reynolds numbers ranging from 200 to 1000. The results indicated that the highest heat transfer rate was achieved for the confined cylinder having the aspect ratio of 1. Both heat transfer rates and predicted drag coefficient reduce as the aspect ratio of the cylinders decreases. A particular concern is revealing the relations between flow physics and the heat transfer mechanisms by quantifying and examining instantaneous velocity patterns to state instantaneous data and time-mean data through confined cylinders using the technique of PIV for the different Reynolds numbers such as 360, 500, 600 and 750. According to the quantitative data obtained from the PIV system, the acceleration of the fluids particles due to the conservation of mass in the cross-streamwise direction when the flow encounters the blockage is the main consequence of confinement on the flow characteristics over the cylinders.
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    Experimental investigation of pulsating flow structures and heat transfer characteristics in sinusoidal channels
    (Pergamon-Elsevier Science Ltd, 2020) Kurtulmus, Nazim; Sahin, Besir
    In the present work, hydrodynamics and heat transfer characteristics in the sinusoidal channel are investigated experimentally for both steady and pulsating flow conditions. The experiments for heat transfer investigations were performed under a constant heat flux in the range of Strouhal number, St 0.11 <= St <= 2.07for the Reynolds number, in the range of 4 x 10(3) <= Re <= 7 x 10(3). After seeing the improvement of heat transfer with employing pulsation to the working fluid the hydrodynamics of pulsating flow was analyzed by considering the pulsating flow characteristics such as the time-averaged streamlines topology, (Psi), streamwise velocity distribution, < u >, cross-streamwise velocity distribution, < v >, and turbulent Reynolds stress, (u'v') over bar /U-2 using instantaneous flow data measured by the Particle image velocimetry (PIV) system. The results revealed that pulsating flow is highly effective for the lower turbulent flow case in the sinusoidal channel. As the Reynolds number increases, the effect of Strouhal number, St becomes less effective. There is an optimum Strouhal number,St value for different Reynolds numbers, Re to reach the maximum enhancement compared to steady flow cases.. The entrainment between the core flow and recirculating flow enhances the heat transfer rates in a steady flow. But the pulsating flow forces the recirculating flow zones in the diverging-converging section of the channel wave to exchange the flud from the core flow region further and that is an additional mechanism to upgrade the rate of heat transfer comparing to the steady flow cases.
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    Flow control over a circular cylinder using vortex generators: Particle image velocimetry analysis and machine-learning-based prediction of flow characteristics
    (Pergamon-Elsevier Science Ltd, 2023) Okbaz, Abdulkerim; Aksoy, Muharrem Hilmi; Kurtulmus, Nazim; Colak, Andac Batur
    Controlling the flow around circular cylinders is crucial to mitigate vortex-induced vibrations and prevent structural damage in a range of applications, such as marine and offshore engineering, tall buildings, long-span bridges, transport ships, and heat exchangers. In this study, we aimed to control the turbulent flow structure around a circular cylinder by placing vortex generators (VGs). We examined the flow structure using particle image velocimetry (PIV). This enabled quantitative data acquisition, intuitive flow visualization, and drag coefficient determination from PIV data. We developed artificial neural network (ANN) models that successfully predict both mean and instantaneous flow characteristics for different scenarios. Our findings show that using VGs elongated the wake and increased vortex formation lengths while reducing velocity fluctuations and the drag coefficient. A minimum drag coefficient of 0.718 was achieved with VGs oriented at alpha = 60 degrees & beta = 60 degrees, reducing the drag by 35.3% compared to the bare cylinder. The drag coefficient exhibited a substantial inverse correlation with both wake and vortex formation lengths. This study is significant for controlling flow structures, providing detailed insights into the near-wake region, and highlighting the potential applications of machine learning in fluid dynamics.
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    Flow past two tandem square cylinders placed in parallel walls
    (Pergamon-Elsevier Science Ltd, 2022) Kurtulmus, Nazim
    In this work, pulsating and non-pulsating flow structures past two tandem square cylinders placed in parallel walls was investigated. The variation of the distance between the cylinders and the frequency of flow pulsation was also examined. The Particle Image Velocimetry (PIV) method was applied to reveal hydrodynamic characteristics such as streamlines patterns, , the velocity vector distribution, the vorticity field, , the normalized streamwise velocity contours, < U/U-avg >, and turbulent statistics such as streamwise velocity fluctuations, u(rms), cross-streamwise velocity fluctuations, v(rms), and turbulent Reynolds stress, u ' v ' over line /U-0(2) distributions for non-pulsating flow cases. In addition, instantaneous and phase averaged flow data over the flow field is presented in order to evaluate the influence of flow pulsation on the quantitative analyses. The experiments were performed at Re=500. According to the results, the confinement profoundly affects the flow regime past two tandem cylinders. The alteration from the reattach flow to the co-shedding vortex flow regime take place at a lower gap ratio. Moreover, flow pulsation considerably contributes to mainstream penetration into the gap between cylinders. The mechanism of the fluid interactions near the wall and mainstream is highly dependent on the ratio of L/D and pulsation frequency
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    Heat transfer and flow characteristics in a sinusoidally curved converging-diverging channel
    (Elsevier France-Editions Scientifiques Medicales Elsevier, 2020) Kurtulmus, Nazim; Zontul, Harun; Sahin, Besir
    The objective of this work is to present the thermal performance characteristics and to examine the hydrodynamic structure of the fluid which improves the rate of heat transfer in parallel with the penalty of pressure drop by means of the time-averaged streamlines topology, , streamwise velocity distribution, , vorticity concentration, and turbulent Reynolds stress, (u'v') over bar /U-2 for the sinusoidal wavy channel. A wide range of experiments were performed for Reynolds numbers, Re ranging from 4 x 10(3) to 1 x 10(4) in order to determine the heat transfer rate and the friction factor, f with varying the channel height expansion/contraction ratio, M = H-min/H-max such as 0.5, 0.35 and 0.28. The results revealed that a significant heat transfer enhancement was achieved with a considerable penalty of pressure drop. The highest thermal performance factor, TPF was obtained as 1.46 for M = 0.5. Numerical simulations were conducted to confirm the experimental results for the same parameters. The Shear Stress Transport k-w (SST k-w) turbulence model was used to perform numerical analyses. After ensuring the consistency of experimental thermal performance results with numerical predictions, the Particle image velocimetry (PIV) system was utilized for investigating the flow physics in the sinusoidally curved converging-diverging channel for all M values at Re = 4 x 10(3) where the TPF is maximum.
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    Impact of dimpled surfaces on the hydrodynamic flow patterns of circular cylinders
    (Springer, 2026) Kurtulmus, Nazim; Ispir, Murat; Aksoy, Muharrem Hilmi; Goktepeli, Ilker
    The motivation of this study is the evaluation of the dimpled patterns on a cylinder surface in terms of controlling the turbulent flow structures. To reach this goal, the cylinders with dimples have been manufactured by utilizing a high-resolution 3D printer and these cylinders have been subjected to cross-flow in a water tunnel and the flow data have been acquired by Particle Image Velocimetry (PIV) at Reynolds number (Re) values of 4000 and 6000. The dimple formation angle (beta) on the cylinder surface texture varied between 15 degrees and 60 degrees, with the dimples arranged to be in-line formation around the entire cylinder circumference. Vortex formation length for bare cylinder was nearly Lf = 2.75 at Re = 4000. Its slight decrement has been obtained by increasing beta. However, it suddenly reached 3.15 at beta = 45 degrees. Its trend at Re = 4000 was not obvious at Re = 6000. It was 2.4 and remained approximately constant as beta increased. The results revealed that dimpled surfaces could become an effective tool to control the flow characteristics over the cylinders, resulting in reducing the undesired dynamic forces. The vortices shed from the sides of the cylinders with dimple surfaces interact to a lesser extent, leading to an elongation of the shear layer compared to that of a bare cylinder. Moreover, the severity of Reynolds shear stress attenuated and the high-intensity cluster formations have been delayed in a particular case, such as beta = 45 degrees at Re = 4000.
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    Investigation of convective heat transfer and flow hydrodynamics in rectangular grooved channels
    (Pergamon-Elsevier Science Ltd, 2021) Zontul, Harun; Hamzah, Hudhaifah; Kurtulmus, Nazim; Sahin, Besir
    An experimental and numerical investigation is conducted for convective heat transfer and flow characteristics in a channel with rectangular grooved top and bottom walls. The study is performed between 2 x 103and 6.5 x 103 of Reynolds numbers. Heat transfer experiments are conducted by applying constant heat flux to the channel upper and lower walls. The averaged Nusselt number, Nu, is presented as an indicator of heat transfer. In the experiments, 1.9-2.4 times larger Nu values were obtained by using grooved channels instead of straight channels. Pressure measurements show that the corrugated channel causes an increase in pressure loss. The Particle Image Velocimetry, PIV method is employed to reveal the flow hydrodynamics and its relation with convective heat transfer. Using the ability of the PIV method to provide instantaneous flow data, interactions between the recirculation flow bubbles in the grooves and the mainstream are observed. In the numerical part of the study, the k-epsilon turbulence model is used, and predicted results are found to be consistent with experiments. The validated numerical method is used to investigate the effect of groove aspect ratio on heat transfer and pressure drop.
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    PIV Investigation of Flow Structures Around Rotating Circular Cylinders with Dimpled Surfaces
    (Springer, 2026) Goktepeli, Ilker; Ispir, Murat; Kurtulmus, Nazim; Aksoy, Muharrem Hilmi
    Flow around rotating cylinders with dimples has been evaluated at a Reynolds number of Re = 2000. It has been determined since the rotational influence was more effective for lower Re values. Angular dimple distributions have been considered as 15 degrees <= beta <= 60 degrees. Different rotation rates have also been evaluated from alpha = 0 to alpha = 1.26 as lower rotation rates for the current Re were significant to suppress the vortex shedding. Particle Image Velocimetry (PIV) results have been compared by those of a bare cylinder. The wake regions have been shrunk due to dimpled surfaces. Excluding the case of alpha = 0.42 for a bare cylinder (4.5%), the regions had tendency to approach the bodies. The highest displacement (-78.9%) to the cylinder has been attained for beta = 15 degrees with alpha = 1.26 as observed. The lowest value (-5.6%) has been obtained by alpha = 0.42 for beta = 45 degrees by approaching the cylinder. For the influence of passive flow control, the cases without rotation have been regarded and beta = 60 degrees was more dominant. For the wake lengths, the changes for F and S locations have also been exhibited in the wake regions as these points affecting the length of recirculation bubble were substantial. As interpreted from the vorticity results, peak values were more dominant in the cylinder wakes. Since the rotation rates were increased, these regions tended to shrink. It is related to the presence of fluctuations in the wake regions. The displacements of maximum turbulence kinetic energy values have been shown for the level of turbulence intensity. Turbulent intensity has been enhanced in the regions closer to the bodies except the stationary cases of beta = 30 degrees and beta = 45 degrees. Nevertheless, the forced rotation was more effective.