Person: ÖZDEMİR, MEHMED RAFET
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ÖZDEMİR
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MEHMED RAFET
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Publication Open Access Flow Boiling of Water in a Rectangular Metallic Microchannel(TAYLOR & FRANCIS INC, 2021-03-26) ÖZDEMİR, MEHMED RAFET; Ozdemir, Mehmed Rafet; Mahmoud, Mohamed M.; Karayiannis, Tassos G.The article presents the experimental results of flow boiling of water in single rectangular microchannels. Three rectangular copper microchannels having the same hydraulic diameter (0.56 mm) and length (62 mm) but different aspect ratios (width/height, 0.5, 2.56, and 4.94) were investigated using de-ionized water as the working fluid. The experiments were conducted over the experimental range of mass flux 200-800 kg/(m(2)s), heat flux 4-1350 kW/m(2) and inlet subcooling of similar to 14 K. The results showed that the channel with smaller aspect ratio exhibited better heat transfer performance up to certain heat fluxes (similar to 480-500 kW/m(2)), whilst the effect of channel aspect ratio became insignificant for higher heat fluxes. The flow boiling patterns were observed and the main flow regimes were bubbly, slug, churn, and annular flow. Flow reversal was also observed that caused a periodic flow in the two microchannels having smaller aspect ratio. A comparison of the experimental results with widely used macro and micro-scale heat transfer correlations is presented. The macro-scale correlations failed to predict the experimental data while some micro-scale correlations could predict the data reasonably well.Publication Metadata only Influence of piston bowl geometry on combustion and emission characteristics(SAGE PUBLICATIONS LTD, 2019) ÖZDEMİR, MEHMED RAFET; Sener, Ramazan; Ozdemir, Mehmed R.; Yangaz, Murat U.Together with the global energy concerns, the norms are getting stringent to prevent the emission threat. There are on-going studies on systems working with both fossil and renewable energy sources aiming to create more efficient and less emissive processes and devices. Accordingly, a set of numerical simulations was performed to examine the effect of the bowl shape of a piston on the performance behaviour, emission rates and combustion characteristics in a four-cylinder, four strokes, water-cooled compression ignition engine using n-heptane (C7H16) as fuel. Six different piston bowl geometries, five from the literature and proposed one, were utilized having different length-to-diameter ratio, curvature and sidewall radius. The study was conducted at 1750 r/min engine speed and a constant compression ratio with a full performance condition. The intake and exhaust valves have been considered as closed during the analysis to provide the variation of crank angle from 300 CA to 495 CA. The results showed that the piston bowl geometry has a significant impact on the rate of heat release, in-cylinder pressure, in-cylinder temperature, and emission trends in the engine. Among the piston bowl geometries studied, design DE and design DF exhibited better combustion characteristics and relatively lower emission trends compared to other designs. The observed rate of heat release, in-cylinder pressure and in-cylinder temperature magnitudes of these two geometries was higher in comparison to other geometries. Moreover, the trade-off for NOx emission was also observed higher for these piston bowl designs.Publication Open Access A REVIEW OF SINGLE-PHASE AND TWO-PHASE PRESSURE DROP CHARACTERISTICS AND FLOW BOILING INSTABILITIES IN MICROCHANNELS(2018-09-29) ÖZDEMİR, MEHMED RAFETPublication Open Access EFFECT OF THE GEOMETRICAL PARAMETERS IN A DOMESTIC BURNER WITH CRESCENT FLAME CHANNELS FOR AN OPTIMAL TEMPERATURE DISTRIBUTION AND THERMAL EFFICIENCY(YILDIZ TECHNICAL UNIV, 2019-12-02) ÖZDEMİR, MEHMED RAFET; Sener, Ramazan; Ozdemir, Mehmed R.; Yangaz, Murat U.Domestic cookers are common tools of house appliances in the world and they have significant share in global energy consumption. Therefore, a small amount of improvement in efficiency would result in a huge drop in total energy and resource activity. This study aims at presenting numerically the thermal efficiency of a domestic burner with crescent-shaped flame channels by changing the distance from the cooker to the burner head and the diameter of the burner. The energy efficiency parameter was evaluated analyzing temperature distribution along the bottom surface of the cooker and unburnt HC, CO and NO emissions. Simulations have been carried out with methane as fuel for three different diameter and distance parameters. The results showed that the temperature on the surface and the emission values of unburnt CO, NO and HC decreased with increasing the cooker diameter and distance parameter.Publication Metadata only Experimental studies on ferrofluid pool boiling in the presence of external magnetic force(PERGAMON-ELSEVIER SCIENCE LTD, 2018) ÖZDEMİR, MEHMED RAFET; Ozdemir, Mehmed Rafet; Sadaghiani, Abdolali K.; Motezakker, Ahmad Reza; Parapari, Sorour Semsari; Park, Hyun Sun; Acar, Havva Yagci; Kosar, AliThe past decade has witnessed rapid advances in thermal-fluid applications involving nanoparticles due to existing heat transfer enhancements. The main challenges in working with nanoparticles are clustering, sedimentation and instability encountered in many studies. In this study, magnetically actuated Fe3O4 nanoparticles were coated with a fatty acid and dispersed inside a base fluid (water) in order to avoid clustering, sedimentation and instability as well as to improve the thermal performance. Boiling heat transfer characteristics of the ferrofluids were experimentally investigated with magnetic actuation and compared to the results without magnetic actuation. Nanoparticle mass fraction was the major parameter. Boiling heat transfer coefficient of the magnetically actuated system was found to be significantly higher compared to the case without magnetic actuation. The results showed that boiling heat transfer coefficient was not sensitive to the nanoparticle mass fraction.Publication Metadata only Energy, Exergy and Exergo-Economic Characteristics of Hydrogen Enriched Hydrocarbon-Based Fuels in a Premixed Burner(TAYLOR & FRANCIS INC, 2021) YILMAZ, İLKER TURGUT; Ozdemir, Mehmed Rafet; Yangaz, Murat Umut; Yilmaz, Ilker TurgutThe enrichment of conventional fuels by hydrogen is quite popular owing to positive environmental aspects compared to the main fuel. In most of the studies, the entropy analysis was not examined in detail. In this research, exergy (second law) analysis of hydrogen-enriched methane, natural gas, propane, LPG, and biogas were analyzed using the numerical model in a premixed burner due to their wide usage in the industry as well as in the household appliances. In the numerical model, the discrete ordinates radiation and Realizable k- epsilon turbulence models were coupled with the premixed combustion model. The rate of increase for hydrogen of each fuel is considered as 10%. The premixed burner exhibited better performance in terms of energy and exergy with the rise of hydrogen enrichment. The most improvement was observed at biogas tests. The results showed that the chemical composition of the base fuel is a significant parameter that affects the exergy and energy capability of the burner due to chain reactions of radicals in the base fuel with hydrogen. It can be concluded that the hydrogen enrichment of biogas in premixed burners is promising in comparison with other tested fuels from both energy and exergy perspective.Publication Metadata only Design and implementation of minichannel evaporator for electronics cooling(SPRINGER, 2021) ÖZDEMİR, MEHMED RAFET; Sokucu, Mehmet Harun; Ozdemir, Mehmed RafetThe present study elucidates the design and experimentation of a minichannel evaporator in an R134a vapour compression refrigeration system for electronics cooling applications. In the current study, a calculation module was developed to design a minichannel evaporator to keep the surface temperature of the chip below a certain value for reliable operation conditions in electronic cooling applications. In the calculation module, the conventional-scale heat transfer correlation was used to predict the surface temperature of the chip. On the other hand, the conventional-scale and microscale pressure drop correlations were tested to assess the pressure drop in the minichannel evaporator. The proposed calculation module was verified using experimental tests for different heat loads. It was found that the proposed calculation model predicted very well the experimental data of the surface temperature of the chip for all heat input. The calculation module with micro-scale pressure drop correlation predicted well the experimental pressure drop data in the minichannel evaporator for all heat loads. Moreover, the effects of the degree of subcooling, superheating degree and condensation temperature on the surface temperature of the chip and pressure drop in the minichannel evaporator were investigated to determine optimum operating conditions at different cooling capacities using the calculation module. The results showed that the increase in the degree of subcooling enhances the performance of the minichannel evaporator. On the other hand, the lower degree of superheating and condensation temperature yielded better performance for the minichannel evaporator. The feasibility of the results for electronic cooling applications is discussed based on the findings.Publication Open Access A review on laminar-to-turbulent transition of nanofluid flows(2022-11-01) ÖZDEMİR, MEHMED RAFET; Subaşı A., ÖZDEMİR M. R. , Estelle P.Nanofluids have emerged as powerful instruments in heat transfer applications due to their improved thermophysical properties. Additionally, many heat transfer equipments are started to be operated within the range of transitional flow regions in the advances in thermal management enhancement techniques. However, up to date, the friction factor and heat transfer coefficient features of nanofluids within the transitional flow regions and the effect of nanoparticle addition into the base fluid on the laminar-to-turbulent transition characteristics are still not understood clearly with contradictory published results. At this point, this paper comprehensively reviews the studies dealing with the nanofluid flow within the transitional flow regions for internal flow applications. After the presentation of applications of nanofluid flow in the transitional flow regions, the nanofluid properties such as nanoparticle type and concentration and base fluid type in the reviewed studies are given in detail. The pressure drop and heat transfer features of nanofluid flow within the transitional flow regions are distinctly identified and discussed for internal flows. The effect of the nanoparticle addition into the liquid on the transition onset is discussed with results from different research groups. A complete evaluation, challenges and further studies are proposed based on available results in the literature.Publication Metadata only Pool boiling heat transfer of ferrofluids on structured hydrophilic and hydrophobic surfaces: The effect of magnetic field(ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER, 2020) ÖZDEMİR, MEHMED RAFET; Sadaghiani, A. K.; Rajabnia, H.; Celik, S.; Noh, H.; Kwak, H. J.; Nejatpour, M.; Park, H. S.; Acar, H. Y.; Misirlioglu, I. B.; Ozdemir, M. R.; Kosar, A.The combined effect of external magnetic field and surface modification on boiling heat transfer of ferrofluids was investigated in this study. Experiments were performed on suspensions of Fe3O4 nanoparticles (volume fraction of 0.025% vf%) with and without presence of magnetic field on structured (surfaces with artificial cavities) hydrophilic and hydrophobic surfaces. Surface related effects such as the hole diameter, pitch size and surface wettability on boiling heat transfer were revealed using the high speed camera system. According to the obtained results, application of magnetic field enhanced boiling heat transfer. The effect of magnetic field was more pronounced on surfaces with larger pitch sizes. Magnetic field promoted bubble nucleation on the superheated surfaces by generating an additional force via Fe3O4 nanoparticles, resulting in enhanced bubblebubble interactions and coalescence. Furthermore, the surfaces with the larger cavity diameter performed better in terms of heat transfer. Scanning Electron Microscopy (SEM) images showed that as the cavity diameter decreased, deposited nanoparticles tended to completely fill the cavities on hydrophilic surfaces and thus deteriorate boiling heat transfer. On hydrophobic surfaces, deposition of nanoparticles led to a biphilic surface, thereby enhancing boiling heat transfer. As the cavity size increased, smaller portion of the cavities was filled with nanoparticles, and nucleation could still occur from the nucleation sites.Publication Metadata only A new distributor selection methodology for refrigeration cycles(2023-01-01) ÖZDEMİR, MEHMED RAFET; Hacımusalar H. A., ÖZDEMİR M. R., Sökücü M. H., DALKILIÇ A. S.The pressure gradient analysis is important to be able to observe the homogeneity of the refrigerant and to choose the correct distributor equipment in refrigeration cycles. This research proposes a new method for determining the two-phase pressure gradient accurately compared to existing methods. Two-phase pressure gradient determination approach of Chisholm was modified by integrating the flow pattern map of Thome and Cioncolini. The thermophysical properties of refrigerant 404A were obtained through EES software and the proposed method was coded in the same software. The two-phase pressure gradient results of the proposed method were verified using the experimental data for the refrigerant 404A and compared with the commonly used methods and correlations. It was found that the proposed method predicted the experimental data of R404A better compared to other methods and correlations. The proposed method accurately determines the two-phase pressure drop under superheating and subcooling conditions for various tube wall thicknesses.