Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Article Citation - WoS: 3Citation - Scopus: 3Computation Time Reduction of Pcm Melting Process by Changing Modeling Parameters(Taylor & Francis, 2022) Demirkıran, İsmail Gürkan; Çetkin, ErdalThis study can be considered as a helpful reference for whom endeavor to boost the computation efficiency of the PCM melting process. Researchers sacrifice accuracy to decrease computation time since computational fluid dynamics (CFD) solutions of PCM melting processes require comparatively very long time, i.e., from hours to days or weeks, depending on the system geometry. The present study compares the approaches recommended in the literature in terms of their influence on computation time reduction and accuracy. A horizontally finned tube LHTES unit is modeled in 2-D space using ANSYS Fluent, the most common commercial CFD software for the considered problem in the literature. The outcomes obtained from the attempts to boost the computation efficiency are as follows: adaptive time step size approach causes 72% enhancement in computation time (from 90 hours to 25 hours), frozen flux algorithm and constant thermophysical properties have almost no influence on computation time. Even though low convergence criteria and neglecting natural convection reduces computation time drastically, the errors in accuracy are not in acceptable level.Article Citation - WoS: 7Citation - Scopus: 10The Computational Approach To Predicting Wear: Comparison of Wear Performance of Cfr-Peek and Xlpe Liners in Total Hip Replacement(Taylor & Francis, 2022) Alpkaya, Alican Tuncay; Mihçin, ŞenayWear on articulating bearing surfaces is a key factor causing revision in total hip replacement (THR). Wear debris that releases particles from bearing surfaces might result in adverse soft tissue reactions requiring revision surgeries. In this study, a comprehensive computational wear model based on the Archard wear equation was performed to investigate the wear performance under a three-dimensional (3D) physiological gait cycle, mimicking a normal walking condition (5 million cycles). The study shows that the accuracy of the model is highly dependent on the mesh convergence, the wear fraction, and the scaling factor. The simulations were run to provide a vast amount of detail for the reproducibility of the work. Cobalt chromium (CoCr) on cross-linked polyethylene (XLPE) and CoCr on carbon-fiber-reinforced polyether ether ketone (CFR-PEEK) prototype models were created in silico. The volumetric wear rates for CoCr-on-XLPE were calculated as 0.2989 (Formula presented.) for CoCr head and 21.0271 (Formula presented.) for XLPE liner, while for CoCr-on-CFR-PEEK they were 0.3484 (Formula presented.) for CoCr head and 1.8476 (Formula presented.) for CFR-PEEK liner. When compared to in vivo and in vitro studies, the wear patterns of these two prototypes are consistent with those of the conventional polyethylene liners in the literature. Although the volumetric wear rate of the CFR-PEEK liner is about 11 times lower than the counterpart of XLPE in MoP implants, the wear rate of CoCr was higher when compared to its use with XLPE. Therefore, CFR-PEEK articulating against orthopa\edic metals may not be as good an alternative as XLPE, due to higher indicative metallic wear. This detailed computational wear modeling methodology could be utilized in design improvements of implants.Article Citation - WoS: 3Citation - Scopus: 4Environmental Assessment of Transparent Conductive Oxide-Free Efficient Flexible Organo-Lead Halide Perovskite Solar Cell(Taylor & Francis, 2020) Sarıaltın, Hüseyin; Geyer, Roland; Zafer, CeylanPerovskite solar cells (PSCs), one of the third-generation photovoltaic (PV) technologies, have recently become a very popular topic in photovoltaic research. This technology, which is a candidate for commercialization in the future, needs to be evaluated from an environmental point of view. The amount of electricity consumption is the most important factor that directly determines the environmental impact values of photovoltaic cell manufacturing. Transparent conductive oxide (TCO) coated glass is one of the major contributors to electricity consumption in PSC architecture. It is therefore useful to investigate the environmental profile of TCO coated glass-free PSC architecture with conventional PVs. One of the solutions to this issue is manufacturing PSC on a flexible substrate. Flexible PVs are considered to be one of the most promising candidates for mass production with its advantages of low-temperature manufacturing, higher efficiency with a lower weight, portability, and compatibility with a roll to roll fabrication. In this work, we show that the environmental impacts of a representative PSCs with a flexible substrate. While the energy payback time (EPBT) of the flexible PSC is already competitive with commercial PVs, the device must reach a 25-year cell lifetime for its global warming potential (GWP) to reach a reasonable range.Article Citation - WoS: 17Citation - Scopus: 18Electric Field Controlled Heat Transfer Through Silicon and Nano-Confined Water(Taylor & Francis, 2019) Yenigün, Onur; Barışık, MuratNanoscale heat transfer between two parallel silicon slabs filled with deionized water was studied under varying electric field in heat transfer direction. Two oppositely charged electrodes were embedded into the silicon walls to create a uniform electric field perpendicular to the surface, similar to electrowetting-on-dielectric technologies. Through the electrostatic interactions, (i) surface charge altered the silicon/water interface energy and (ii) electric field created orientation polarization of water by aligning dipoles to the direction of the electric field. We found that the first mechanism can manipulate the interface thermal resistance and the later can change the thermal conductivity of water. By increasing electric field, Kapitza length substantially decreased to 1/5 of its original value due to enhanced water layering, but also the water thermal conductivity lessened slightly since water dynamics were restricted; in this range of electric field, heat transfer was doubled. With a further increase of the electric field, electro-freezing (EF) developed as the aligned water dipoles formed a crystalline structure. During EF (0.53 V/nm), water thermal conductivity increased to 1.5 times of its thermodynamic value while Kapitza did not change; but once the EF is formed, both Kapitza and conductivity remained constant with increasing electric field. Overall, the heat transfer rate increased 2.25 times at 0.53 V/nm after which it remains constant with further increase of the electric field.Article Citation - WoS: 11Citation - Scopus: 11A Numerical Study on Determination of Volume Averaged Thermal Transport Properties of Metal Foam Structures Using X-Ray Microtomography Technique(Taylor & Francis, 2018) Çelik, Hasan; Mobedi, Moghtada; Nakayama, Akira; Özkol, ÜnverVolume averaged thermal transport properties of two metal foams with 10 and 20 PPI are obtained by using microtomography technique. The digital 3D structures of samples are generated in computer environment. The governing equations are solved for the entire domain and the volume averaged technique is used to determine interfacial heat transfer coefficient, longitudinal and transverse thermal dispersion conductivity. The study is performed for the pore scale Reynolds number from 100 to 600. The obtained results are within the ranges of the suggested correlations in literature. The present study supports the correlations suggested by Calmidi and Mahajan (2000) and Zhang et al. (2016).Article Citation - WoS: 35Citation - Scopus: 38Heat Transfer Enhancement in a Microchannel Heat Sink: Nanofluids And/Or Micro Pin Fins(Taylor & Francis, 2020) Coşkun, Turgay; Çetkin, ErdalHere, we show that overall thermal conductance in a rectangular microchannel heat sink can be maximized with the combination of nanofluids and micro pin fins. We uncover the effect of micro pin fins and nanofluids both separately and simultaneously in order to uncover their effect on the thermal conductance (i.e., thermal resistance). Both nanofluids and micro pin fins decrease the overall thermal resistance due to increase in the average thermal conductivity of the flow system. In addition, they increase the heat transfer surface area of the solid interacting with the fluid. However, the pumping power (pressure drop) increases in both methods due to the increase in the resistances to the fluid flow. The results document what should be the nanoparticle volume fraction mixed into the base fluid and the micro pin fin volume in order to minimize thermal resistance. If the thermal conductivity of the nanoparticles and micro pin fins are the same, the thermal conductance becomes the maximum with 4% and 0.14% volume fractions for the nanofluid and micro pin fins, respectively. This result shows that inserting micro pin fins and using nanofluids with a given volume fraction ratio maximize the overall thermal conductance. © 2019, © 2019 Taylor & Francis Group, LLC.