Browsing by Author "Koşar, A."

Now showing 1 - 9 of 9

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Boiling heat transfer enhancement of magnetically actuated nanofluids
(AIP, 2013) Şeşen, M.; Tekşen, Y.; Şahin, B.; Şendur, K.; Mengüç, Mustafa Pınar; Koşar, A.; Mechanical Engineering; MENGÜÇ, Mustafa Pınar
Nanofluids offer a potential breakthrough as next-generation heat transfer fluids since they offer exciting new possibilities to enhance heat transfer performance compared to pure liquids. A major drawback for using nanofluids in practical applications is difficulty in maintaining their stability due to deposition on surfaces. In this study, we propose and experimentally investigate a magnetic actuation scheme to avoid this deposition. Two-phase heat transfer characteristics of the designed system have been experimentally investigated with magnetic actuation and compared to the results without magnetic actuation. Two phase average heat transfer enhancement observed with the suggested system was 17%. The average single phase enhancement is found as 29% with magnetic actuation. It was observed that magnetically actuated nanoparticles neither form any clusters nor precipitate after the experiments.
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Effect of electrostatic stabilization on thermal radiation transfer in nanosuspensions: Photo-thermal energy conversion applications
(Elsevier, 2018-04) Al-Gebory, Layth Wadhah Ismael; Mengüç, Mustafa Pınar; Koşar, A.; Şendur, K.; Mechanical Engineering; MENGÜÇ, Mustafa Pınar; Al-Gebory, Layth Wadhah Ismael
Solar thermal collectors are among the most important photo-thermal energy conversion systems. Effectiveness of these systems is measured by the ability of working fluid to absorb incident radiative energy. Although nanosuspensions are considered very promising for this purpose, there is a concern about their stability and their long-term use. Electrostatic and steric stabilization methods are among the two approaches used for colloidal suspensions. In thermal applications, electrostatic stabilization is usually preferred; especially in high temperature applications. The aim of this study is to investigate, both experimentally and numerically, the effect of electrostatic stabilization on the thermal radiation transfer mechanisms in TiO2 and Al2O3 nanosuspensions. The experimental section covers nano suspensions preparation and characterization, where the effects of electrostatic stabilization (pH and zeta potential values) on the increasing effective particle size due to agglomeration behaviour are explored. The numerical part covers the estimation of radiative properties and thermal radiation transfer based on the average particle agglomerate size obtained from the particle size distributions in the experimental part. The radiative properties are assessed using the single scattering approximation technique based on the Lorenz-Mie theory. The thermal radiation transfer is obtained by solving the radiative transfer equation by the discrete ordinate method. The results show remarkable stability behaviour under the effect of the pH value for the two nanosuspensions types. The effect of the different particle agglomerate size shows a considerable enhancement in the radiative properties specifically in the UV/Vis spectrum, which has a significant impact on the thermal radiative transfer phenomena, due to the solar spectrum. It is also shown that nanosuspensions with different particle agglomerate sizes have a significant effect on the volumetric radiative heat flux, where the radiative energy losses decrease in comparison to those of pure water. (C) 2017 Elsevier Ltd. All rights reserved.
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The effect of nanoparticle type and nanoparticle mass fraction on heat transfer enhancement in pool boiling
(Elsevier, 2017-06) Karimzadehkhouei, M.; Shojaeian, M.; Şendur, K.; Mengüç, Mustafa Pınar; Koşar, A.; Mechanical Engineering; MENGÜÇ, Mustafa Pınar
Determining the heat transfer performance with nanofluids is of cardinal importance in the utilization of nanofluids in thermal systems. This study presents an experimental investigation on nucleate pool boiling heat transfer of TiO2 nanoparticles/water and CuO nanoparticles/water nanofluids on a flat heater plate and aims to reveal the effect of mass fraction of nanoparticles in these nanofluids for attaining the maximum enhancement in pool boiling heat transfer. The effect of mass fraction on boiling heat transfer characteristics was studied for mass fractions varying from 0.001% to 0.2% for the heat flux range between 48.7 and 134.9 kW/m2. The experimental results showed that the heat transfer performance was improved when TiO2 nanoparticles were added to pure water, as base fluid. However, the amount of enhancement was highly dependent on mass fraction. It was realized that the lowest mass fraction (0.001%), namely the dilute TiO2 nanoparticles/water nanofluid, has the largest enhancement (around 15%). A further increase in mass fraction still augments heat transfer compared to pure water, however, the amount of enhancement decreased with mass fraction. Furthermore, the performed visualization showed that the addition of nanoparticles into the base fluid, increased the number of nucleation sites, and the bubbles had a more spherical shape along with a decrease in their size. For CuO/water nanofluids, heat transfer was enhanced at mass fractions larger than 0.001%. This enhancement could be more than 35% for the mass fraction of 0.2 wt.%. This study clearly indicates that the nanoparticle mass fraction corresponding to the best performance is highly dependent on the type of nanoparticle.
Open Access
Entropy generation analysis of laminar flows of water-based nanofluids in horizontal minitubes under constant heat flux conditions
(MDPI AG, 2018-04) Karimzadehkhouei, M.; Shojaeian, M.; Sadaghiani, A. K.; Şendur, K.; Mengüç, Mustafa Pınar; Koşar, A.; Mechanical Engineering; MENGÜÇ, Mustafa Pınar
During the last decade, second law analysis via entropy generation has become important in terms of entropy generation minimization (EGM), thermal engineering system design, irreversibility, and energy saving. In this study, heat transfer and entropy generation characteristics of flows of multi-walled carbon nanotube-based nanofluids were investigated in horizontal minitubes with outer and inner diameters of ~1067 and ~889 µm, respectively. Carbon nanotubes (CNTs) with outer diameter of 10–20 nm and length of 1–2 µm were used for nanofluid preparation, and water was considered as the base fluid. The entropy generation based on the experimental data, a significant parameter in thermal design system, was examined for CNTs/water nanofluids. The change in the entropy generation was only seen at low mass fractions (0.25 wt.% and 0.5 wt.%). Moreover, to have more insight on the entropy generation of nanofluids based on the experimental data, a further analysis was performed on Al2O3 and TiO2 nanoparticles/water nanofluids from the experimental database of the previous study of the authors. The corresponding results disclosed a remarkable increase in the entropy generation rate when Al2O3 and TiO2 nanoparticles were added to the base fluid.
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Experimental and numerical investigation of inlet temperature effect on convective heat transfer of γ-Al2O3/Water nanofluid flows in microtubes
(Taylor & Francis, 2019-06-15) Karimzadehkhouei, M.; Sadaghiani, A. K.; Motezakker, A. R.; Akgönül, S.; Ozbey, A.; Şendur, K.; Mengüç, Mustafa Pınar; Koşar, A.; Mechanical Engineering; MENGÜÇ, Mustafa Pınar
Nanofluids are the combination of a base fluid with nanoparticles with sizes of 1–100 nm. In order to increase the heat transfer performance, nanoparticles with higher thermal conductivity compared to that of base fluid are introduced into the base fluid. Main parameters affecting single-phase and two-phase heat transfer of nanofluids are shape, material type and average diameter of nanoparticles, mass fraction and stability of nanoparticles, surface roughness, and fluid inlet temperature. In this study, the effect of inlet temperature of deionized water/alumina (Al2O3) nanoparticle nanofluids was both experimentally and numerically investigated. Nanofluids with a mass fraction of 0.1% were tested inside a microtube having inner and outer diameters of 889 and 1,067 µm, respectively, for hydrodynamically developed and thermally developing laminar flows at Reynolds numbers of 650, 1,000, and 1,300. According to the obtained numerical and experimental results, the inlet temperature effect was more pronounced for the thermally developing region. The performance enhancement with nanoparticles was obtained at rather higher Reynolds numbers and near the inlet of the microtube. There was a good agreement between the experimental and numerical results so that the numerical approach could be further implemented in future studies on nanofluid flows.
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Heat transfer enhancement with actuation of magnetic nanoparticles suspended in a base fluid
(AIP, 2012) Şeşen, M.; Tekşen, Y.; Şendur, K.; Mengüç, Mustafa Pınar; Öztürk, H.; Yağcı Acar, H. F.; Koşar, A.; Mechanical Engineering; MENGÜÇ, Mustafa Pınar
In this study, we have experimentally demonstrated that heat transfer can be substantially increased by actuating magnetic nanoparticles inside a nanofluid. In order to materialize this, we have utilized a miniature heat transfer enhancement system based on the actuation of magnetic nanoparticles dispersed in a base fluid (water). This compact system consists of a pool filled with a nanofluid containing ferromagneticnanoparticles, a heater, and two magnetic stirrers. The ferromagnetic particles within the pool were actuated with the magnetic stirrers. Single-phase heat transfer characteristics of the system were investigated at various fixed heat fluxes and were compared to those of stationary nanofluid (without magnetic stirring). The heat transfer enhancement realized by the circulation of ferromagneticnanoparticles dispersed in a nanofluid was studied using the experimental setup. The temperatures were recorded from the readings of thin thermocouples, which were integrated to the heater surface. The surface temperatures were monitored against the input heat flux and data were processed to compare the heat transfer results of the configuration with magnetic stirrers to the heat transfer of the configuration without the magnetic stirrers.
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Pool boiling critical heat flux in dielectric liquids and nanofluids
(Elsevier, 2011) Arık, Mehmet; Koşar, A.; Bostanci, H.; Bar-Cohen, A.; Mechanical Engineering; ARIK, Mehmet
With the recent advances in consumer and power electronics, efficient thermal management of high heat flux components has taken on new urgency. While its inherent advantages have made air cooling the method of choice for the vast majority of electronic systems, the relatively poor thermophysical properties of air limit its ability to meet today's more demanding thermal requirements. Therefore, researchers have continued to investigate liquid cooling techniques such as microchannel forced convection, jet impingement, and pool boiling heat transfer. Although water has superior thermal properties, concern over its dielectric strength and chemical activity have directed attention to the use of the inert and dielectric perfluorocarbons as alternative cooling fluids, with particular emphasis on harnessing boiling and evaporative heat transfer processes. Passive pool boiling with these dielectric fluids, including the effects of subcooling, pressure, length scale, mixtures, surface enhancements, and nanoadditives, has been investigated by a large number of researchers. The present study focuses on the critical heat flux for pool boiling of these dielectric liquids, representing the upper limit of the nucleate pool boiling regime, and provides a review and summary of the work reported in more than 100 archival papers by research groups from around the globe.
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Pressure drop and heat transfer characteristics of nanofluids in horizontal microtubes under thermally developing flow conditions
(Elsevier, 2015-10) Karimzadehkhouei, M.; Yalçın, S. E.; Şendur, K.; Mengüç, Mustafa Pınar; Koşar, A.; Mechanical Engineering; MENGÜÇ, Mustafa Pınar
This study presents pressure drop and heat transfer characteristics of water based nanofluids with TiO2 and Al2O3 nanoparticles of various mass fractions in horizontal smooth hypodermic microtubes with an outer diameter of ∼717 μm and an inner diameter of ∼502 μm over a wide variety of Reynolds numbers under hydrodynamically fully developed and thermally developing conditions. For this purpose, TiO2 and Al2O3 nanoparticles of 20 nm average solid diameters were added to deionized water to prepare nanofluids with mass fractions of 0.01–3 wt.%, and prepared nanofluids were characterized by standard methods such as Dynamic Light Scattering (DLS), Scanning Electron Microscopy (SEM), and zeta potential measurements. Experimental friction factor coefficients were predicted within ±10% and are in good agreement with existing analytical predictions, while experimental heat transfer coefficients were predicted within ±15% with existing correlations for single phase flow. Our results show that there is no considerable heat transfer enhancement for Re < 1000. A consistent enhancement in heat transfer was observed (for average heat transfer coefficient up to 25%), once Reynolds number goes beyond 1500. At low Reynolds numbers, flow is mainly laminar. However, at higher Reynolds numbers, flow starts to transition to turbulent flow, when heat transfer enhancement is also observed. Under these conditions, the enhancement in heat transfer increases with mass fraction.
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Subcooled flow boiling heat transfer of γ-Al2O3/water nanofluids in horizontal microtubes and the effect of surface characteristics and nanoparticle deposition
(Elsevier, 2017) Karimzadehkhouei, M.; Sezen, M.; Şendur, K.; Mengüç, Mustafa Pınar; Koşar, A.; Mechanical Engineering; MENGÜÇ, Mustafa Pınar
In this study, subcooled flow boiling heat transfer characteristics of nanofluids were investigated at micro scale. For this purpose, the effect of γ-Al2O3 (gamma-alumina) nanoparticles with an average solid diameter of 20 nm was considered. In the experiments, various mass fractions were considered in horizontal smooth stainless steel microtubes with inner and outer diameters of ∼502 µm and ∼717 µm, respectively, at mass fluxes of 1200 and 3400 kg m−2 s−1. Nanoparticles were added to distilled water (base fluid) at five mass fractions (low mass fractions 0.05 wt% and 0.2 wt%; high mass fractions 0.5 wt%, 1 wt% and 1.5 wt%). According to our results, subcooled flow boiling heat transfer coefficients for nanofluids with low mass fractions were nearly the same as those of the pure water. However, heat transfer deteriorated for nanofluids with high mass fractions. Observations of dynamic light scattering measurements for low and high mass fractions before and after the experiments revealed that agglomeration of nanoparticles is an important parameter in deterioration of heat transfer at higher concentrations. Besides, Scanning Electron Microscopy images of microtube inner surfaces showed that deposition of nanoparticles and agglomerated nanoparticles on the inner surface of the microtubes also contributed to the heat transfer deterioration at high mass fractions. Generally, the deterioration in heat transfer beyond a specific mass fraction value was linked to the disturbance in the stability of suspended nanoparticles and deposition of nanoparticles upon boiling.