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 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.Publication Metadata only Combustion performance of hydrogen-enriched fuels in a premixed burner(TAYLOR & FRANCIS LTD, 2020) ÖZDEMİR, MEHMED RAFET; Yangaz, Murat Umut; Ozdemir, Mehmed Rafet; Sener, RamazanPremixed burners have been widely used in many applications for both industrial and household appliances. For this reason, it is very important to enhance the combustion and emission efficiency of premixed burners because of their strong position in the global dimension. On the other hand, the addition of hydrogen to various fuels has been a research topic in the last decade due to its environmental and economic positive effects. In this study, the effect of hydrogen addition to different gaseous fuels at different rates has been investigated in a premixed burner. A numerical parametric study has been carried out using a commercial CFD code. The gaseous fuels namely; methane, propane, LPG and natural gas have been enriched by different hydrogen addition rates with a 10% increment. Moreover, the results of pure hydrogen have been presented. The results reveal that the combustion efficiency for all fuels has been affected positively with hydrogen enrichment except for certain gas compositions. Furthermore, this study has shown that the hydrogen addition in general, decreases the unburnt HC and CO emissions.Publication Open Access Demir Bazlı Nano-Sıvının Tek-Faz ve Havuz Kaynama Isı Transferi_x000D_ Şartlarında Ekserji Analizi(2020-06-30) ÖZDEMİR, MEHMED RAFET; Mehmed Rafet ÖZDEMİRSon yıllarda termal-sıvı uygulamaları artan ısı akısını karşılamak için en sık kullanılan yöntemlerden biri olmaya başlamıştır.Bu uygulamalardan en popüler olanlarından biri sıvıya nano-parçacık karıştırarak ısı transfer hızını arttırmaya çalışmaktır.Teoride kabul gören bu yöntem için farklı araştırma gruplarından farklı sonuçlar gelmekle birlikte kesin bir yargıya henüz tamolarak ulaşıldığı söylenemez. Sıvıya nano-parçacık eklemenin en zorlu yanı, birçok çalışmada belirtildiği gibi nanoparçacıklarınyüzey üzerinde kümelenmeye ve çökelmeye meyilli olması ve bu durumun olması halinde ısı transferine negatifetki yapmasıdır. Bu özelliklerinden ötürü nano-parçacıkların sistem üzerinde kararsız davranış oluşturduğu da bazı çalışmalardarapor edilmiştir. Bu çalışmada ana sıvı olarak suya Fe3O4 nano-parçacıkları eklenen sistemin ekserji analizi yapılmıştır. Buradaen önemli nokta, sistemin manyetik kuvvete maruz bırakılması olup bu sayede çökelme ve kümelenmeye fırsat verilmeyecekolmasıdır. Bu çalışmada literatürden farklı olarak sistemin tek-fazlı akış ve havuz kaynama şartlarındaki verimi ekserjetik verimüzerinden değerlendirilecektir. Sonuçlar saf su, su-Fe3O4 nano-sıvısı ve manyetik kuvvet altındaki su-Fe3O4 nano-sıvısışeklinde sunulup ekserji yıkım oranları karşılaştırılmıştır.Publication Open Access Thermodynamic and mathematical analysis of geothermal power plants operating in different climatic conditions(Elsevier Ltd, 2022-02) ÖZDEMİR, MEHMED RAFET; Başaran T., Çetin B., Özdemir M.R.This study presents the thermodynamic and mathematical analysis of the different cooling systems for geothermal power plants at various climatic conditions. The existing binary geothermal power plant data, located in Turkey, were processed in the commercial software Ebsilon Professional to model the power plant and cooling systems. The dry-bulb temperature and relative humidity values from data obtained from local meteorological stations in the analyzed region are used as variables in the model. The reference air-cooled cooling system, the wet-tower cooling system, the additional dry cooling system and three different hybrid cooling systems are modelled and analyzed separately in the reference geothermal power plant model. Accordingly, mathematical equations are developed to evaluate the power production and water consumption characteristics of cooling systems on the thermodynamic performance of geothermal power plants depending on weather conditions. The power production and water consumption of geothermal power plants can be calculated using hourly ambient temperature and relative humidity data at different climatic conditions with these equations. Considering water reserves in the area, operation periods of the hybrid cooling systems can be determined. Moreover, power plant operators can compare the performance and water consumption characteristics of hybrid cooling systems for different conditions with these equations. © 2021 The AuthorsPublication Open Access Numerical optimization of gas cooler geometry in transcritical refrigeration cycles(2023-01-01) URKUT, AHMET FURKAN; ÖZDEMİR, MEHMED RAFET; URKUT A. F., Karcı E. O., ÖZDEMİR M. R.Traditional halocarbon – based refrigerants tend to considerably increase global warming and ozone depletion factors. Therefore, CO2 is fast becoming a key instrument as a natural refrigerant which was widely applied and attracted the consideration of the research community. The gas cooler is a critical component in the CO2 transcritical refrigeration system and plays a major role in the performance due to the determination of operating pressure consequently power consumption. In this research, the performance characteristics of a CO2 gas cooler having wavy fin geometry, which is currently employed in industries including air conditioning, automotive and aviation, was determined experimentally in a calorimetric test room. The experimental results was used as benchmark data to validate the three – dimensional numerical model. Laminar model and realizable k - ɛ turbulent model were employed for analyses. Moreover, the second order upwind scheme was considered to discretize momentum and energy equations. Accordingly, a multi-objective optimization process has been performed employing Response Surface Method (RSM) to determine the optimum wavy fin geometry in CO2 transcritical refrigeration system. Four geometrical parameters namely longitudinal pitch, half transverse pitch, tube outer diameter, and fin pitch of the gas cooler were optimized. According to results, the new optimized CO2 gas cooler exhibited lesser pressure drop and higher heat transfer capacity in comparison with the tested gas cooler geometry used in the industry. It was appeared that the overall heat transfer coefficient enhancement is between 5.4 – 12.2 % while pressure drop decreases about 175.08 – 188.58 % for three different inlet velocities