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ÖZDEMİR, MEHMED RAFET

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ÖZDEMİR

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MEHMED RAFET

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Now showing 1 - 9 of 9

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.
Open Access
A REVIEW OF SINGLE-PHASE AND TWO-PHASE PRESSURE DROP CHARACTERISTICS AND FLOW BOILING INSTABILITIES IN MICROCHANNELS
(2018-09-29) ÖZDEMİR, MEHMED RAFET
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.
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.
Open Access
EXERGO-ECONOMIC ANALYSIS OF MICROCHANNELS IN SINGLE-PHASE FLOW
(YILDIZ TECHNICAL UNIV, 2018-06-30) ÖZDEMİR, MEHMED RAFET; Ozdemir, M. R.; Sozbir, O. R.; Yilmaz, M.
With the increase of energy demand, many researchers tried to develop scientific approaches in order to design more efficient and environmentally friendly energy systems. Exergo-economic (thermoeconomic) analysis of a system or device is an efficient tool for evaluating the system in terms of the thermodynamic and economic aspects. In this parametric study, exergo-economic analysis of rectangular copper plain microchannels under single-phase flow conditions were investigated using de-ionised water. The exergo-economic performance was evaluated based on the relative cost difference and unit cost per product exergy tools. The channel aspect ratio effect on the unit cost per product exergy and relative cost difference was examined using three microchannel test sections with the same channel hydraulic diameter (D-h = 0.56 mm) and length (L = 62 mm) but different aspect ratios beta = 0.5, 2.56 and 4.94) under single-phase flow conditions. The results showed that the exergo-economic performances of the three microchannel test sections decreased as the net heat input increased over the experimental range. Moreover, the exergo-economic performance of test section 2 (beta = 4.94) was found to be greater than the exergo-economic performances of test sections 1 and 3 (beta = 0.5 and 2.56) at fixed flow rate and fixed net heat input case.
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 ÖZDEMİR
Son 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.
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 Authors
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
Open Access
FLOW BOILING HEAT TRANSFER IN A RECTANGULAR COPPER MICROCHANNEL
(YILDIZ TECHNICAL UNIV, 2016) ÖZDEMİR, MEHMED RAFET; Ozdemir, Mehmed R.; Mahmoud, Mohamed M.; Karayiannis, Tassos G.
Flow boiling characteristics of de-ionized water were tested experimentally in a rectangular copper single microchannel of 1 mm width, 0.39 mm height and 62 mm length. De-ionized water was supplied to the microchannel at constant inlet temperature (89 degrees C) and constant inlet pressure (115 kPa). The mass flux ranged from 200 to 800 kg/m(2)s and the heat flux from 56 to 865 kW/m(2). The heat transfer rate data are presented as plots of local heat transfer coefficient versus vapour quality and distance along the channel. Flow visualization was also conducted using a high-speed, high-resolution camera. The results indicate that unstable flow boiling occurred starting at boiling incipience for all mass flux values. The local heat transfer coefficient depends on heat flux only at very low heat and mass fluxes. At high mass flux, there is no heat flux effect with little dependence on vapour quality after the entry region. The mass flux effect was more complex.