Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Article Citation - WoS: 11Citation - Scopus: 13Effect of Cnt Incorporation on Pan/Ppy Nanofibers Synthesized by Electrospinning Method(TÜBİTAK, 2020) İnce Yardımcı, Atike; Tanoğlu, Metin; Yılmaz, Selahattin; Selamet, YusufIn this study, carbon nanotubes (CNTs) added polyacrylonitrile/polypyrrole (PAN/PPy) electrospun nanofibers were produced. Average diameters of the nanofibers were measured as 268 and 153 nm for 10 and 25 wt% of PPy contents, respectively. A relatively higher strain to failure values (23.3%) were observed for the low PPy content. When as-grown CNTs (1 and 4 wt%) were added into the PAN/PPy blends, disordered nanofibers were observed to form within the microstructure. To improve the interfacial properties of CNTs/PAN/PPy composites, CNTs were functionalized with H2SO4/HNO3/HCl solution. The functionalized CNTs were well dispersed within the nanofibers and aligned along the direction of nanofibers. Therefore, beads formation on nanofibers decreased. The impedance of the nanofibers was found to decrease with the PPy content and CNT addition. These nanofibers had a great potential to be used as an electrochemical actuator or a tissue engineering scaffold.Article Citation - WoS: 3Validation on of Local Thermal Equilibrium and Uniform Pressure Assumptions for an Isobaric Adsorption Process in an Adsorbent Bed(Türk Isı Bilimi ve Tekniği Derneği, 2016) Gediz İliş, Gamze; Mobedi, Moghtada; Ülkü, SemraBu çalışmanın amacı, adsorbent yatakta ısı ve kütle transferini analiz etmek için kullanılan yerel ısıl denge ve sabit basınç yaklaşımı varsayımların geçerliliğini araştırmaktır. İçerisinde silika jel partikülleri içeren bir yatak ile su kabı olan bir deney düzeneği tasarlanmış ve adsorpsiyon sürecinde yatağın içinde farklı yerlerde yerel sıcaklık ve basınç ölçülmüştür. Ayrıca, sabit basınç yaklaşımı ve yerel ısıl denge varsayımlara dayalı ısı ve kütle transferi denklemleri çözülmüştür. Sayısal sonuçlar, ilgili deneysel sonuçlarla karşılaştırılmış ve aralarında oldukça iyi bir uyum tespit edilmiştir. Gerçekleştirilen karşılaştırmaya dayanarak, incelenen yatak için iki önemli sonuç şu şekildedir: a) katı madde ve su buharı arasında yerel ısıl denge bulunmaktadır, b) bir yatak içinde parçacıklar arası kütle transferi direnci ihmal edilebilir düzeyde olup konsantrasyonunun ve sıcaklığın belirlenmesi için sabit basınç yaklaşımı geçerlidir. Ayrıca, bu çalışmada sunulan deneysel sonuçlar diğer araştırmacıların sayısal çalışmalarının geçerliliğini doğtulamak için değerli veriler sağlayacaktır.Article Citation - WoS: 25Citation - Scopus: 26Lowering the Sintering Temperature of Solid Oxide Fuel Cell Electrolytes by Infiltration(Elsevier Ltd., 2019) Sındıraç, Can; Çakırlar, Seda; Büyükaksoy, Aligül; Akkurt, SedatA dense electrolyte with a relative density of over 95% is vital to prevent gas leakage and thus the achievement of high open circuit voltage in solid oxide fuel cells (SOFCs). The densification process of ceria based electrolyte requires high temperatures heat treatment (i.e. 1400-1500 degrees C). Thus, the minimum co-sintering temperatures of the anode-electrode bilayers are fixed at these values, resulting in coarse anode microstructures and consequently poor performance. The main purpose of this study is to densify gadolinia doped ceria (GDC), a common SOFC electrolyte, at temperatures lower than 1400 degrees C. By this aim, an approach involving the infiltration of polymeric precursors into porous electrolyte scaffolds, a method commonly used for composite SOFC electrodes, is proposed. By infiltrating polymeric precursors of GDC into porous GDC scaffolds, a reduction in the sintering temperature by at least 200 degrees C is achieved with no additives that might affect the electrical properties. Energy dispersive x-ray spectroscopy line scan analyses performed on porous GDC scaffolds infiltrated by a marker solution (polymeric FeOx precursor in this case) reveals a homogeneous infiltrated phase distribution, demonstrating the effectiveness of polymeric precursors.Article Citation - WoS: 55Citation - Scopus: 60Effect of Regeneration Temperature on Adsorption Equilibria and Mass Diffusivity of Zeolite 13x-Water Pair(Elsevier Ltd., 2016) Sayılgan, Şefika Çağla; Mobedi, Moghtada; Ülkü, SemraThe adsorption equilibrium and mass diffusivity of zeolite 13X-water pair for different adsorption and regeneration temperatures were determined by a homemade volumetric system. The isotherms of the zeolite 13X-water pair were obtained by collecting pressure versus time data and applying ideal gas law. The effective diffusivity of the pair was calculated by using long term analytical solution of mass diffusivity based on Fick's law. The experimental study showed that the adsorption capacity of zeolite 13X-water pair was 23% (kg/kg), 21% (kg/kg) and 19% (kg/kg) when the adsorption temperature was 35, 45 and 60 °C respectively for the desorption temperature of 90 °C. Furthermore, the adsorption capacity increased from 22% (kg/kg) to 24% (kg/kg) when the desorption temperature was increased from 90 °C to 150 °C. It was observed that the present adsorption equilibrium results were compatible with the reported results in the literature. The mass diffusivity of the pair was found in the range of 4 × 10-9-6 × 10-8 m2/s for the long time period when the initial adsorptive pressure was 2000 Pa. The effective mass diffusivity depends on concentration and it was decreased with increasing adsorbate concentration.Article Citation - WoS: 11Citation - Scopus: 11Comparison of Uniform and Non-Uniform Pressure Approaches Used To Analyze an Adsorption Process in a Closed Type Adsorbent Bed(Springer Verlag, 2013) Gediz İliş, Gamze; Mobedi, Moghtada; Ülkü, SemraHeat and mass transfer in an annular adsorbent bed filled with silica gel particles is numerically analyzed by uniform and non-uniform pressure approaches. The study is performed for silica gel-water pair, particle radius from 0.025 to 1 mm and two bed radii of 10 and 40 mm. For uniform pressure approach, the energy equation for the bed and the mass transfer equation for the particle are solved. For non-uniform pressure approach, the continuity and Darcy equations due to the motion of water vapor in the bed are added, and four coupled partial differential equations are solved. The changes of the adsorbate concentration, pressure, and temperature in the bed throughout the adsorption process for both approaches are obtained and compared. The obtained results showed that the particle size plays an important role on the validity of uniform pressure approach. Due to the interparticle mass transfer resistance, there is a considerable difference between the results of the uniform pressure and non-uniform pressure approaches for the beds with small size of particles such as 0.025 mm.Article Citation - WoS: 4Citation - Scopus: 4Effects of Catalyst Precursor Type and Preparation Conditions, and Solvent Type on Activity and Selectivity of Pt/Sio2 Catalyst in Citral Hydrogenation(Walter de Gruyter GmbH, 2011) Depboylu, Can Okan; Yılmaz, Selahattin; Akkurt, SedatIn this study, citral hydrogenation reaction in liquid phase over silica gel supported Pt catalysts was investigated. It was desired to hydrogenate carbonyl group selectively to produce valuable unsaturated alcohols, namely geraniol and nerol. Pt/SiO2 catalysts were prepared by impregnation method. The effects of parameters investigated in the present study included Pt precursor type (hexachloroplatinic acid (HCLPA), platinum II acetylacetonate (PAA)), catalyst activation temperature (350°C and 450°C), catalyst activation without calcination, catalyst washing with 0.1 M NaOH and solvent type (ethanol, 2-pentanol). The catalysts activities and selectivities were affected by the type of precursor and activation temperature. The maximum citral conversion (89.50 percent) was achieved at lower activation temperature (350°C) with PAA precursor based catalyst. It was observed that higher activation temperature provided lower citral conversion but higher selectivity to unsaturated alcohols; increasing the activation temperature to 450°C decreased citral conversion to 26.10 percent. But selectivity to unsaturated alcohols, geraniol and nerol, increased from 7.06 to 54.60 percent. Catalyst washing and 2-pentanol prevented acetal formation. Catalyst activation without calcination gave lower citral conversion (20.84 percent) and selectivity to unsaturated alcohols (30.00 percent). Copyright © 2011 The Berkeley Electronic Press. All rights reserved.Article Citation - WoS: 23Citation - Scopus: 25Microcalorimetric Investigation of Water Vapor Adsorption on Silica Gel(Springer Verlag, 2011) Demir, Hasan; Mobedi, Moghtada; Ülkü, SemraWater vapor adsorption on silica gel was investigated using Tian-Calvet-type microcalorimetry. Differential heat of adsorption data was obtained. The setup of microcalorimetry was used volumetric system to determine adsorption isotherms of water vapor-silica gel. The Langmuir model was used in the interpretation of the adsorption data. The Clausius-Clapeyron diagram was also given. Effective mass diffusivity of water vapor in the silica gel particle as a function of temperature was also determined. The silica gel, which was degassed under vacuum at 10 -7 mbar and 120 °C for 18 h, was found to adsorb 0.6, 0.98, 1.1, 1.4, 2, 3.5, 11, 13, and 14 wt% water vapor at 120, 110, 100, 90, 75, 60, 40, 35, and 30 °C, respectively. The diffusivities of water vapor inside the silica gel for short- and long-range periods were described using kinetics data as a function of temperature in the Arrhenius form.Article Citation - WoS: 61Citation - Scopus: 66The Use of Metal Piece Additives To Enhance Heat Transfer Rate Through an Unconsolidated Adsorbent Bed(Elsevier Ltd., 2010) Demir, Hasan; Mobedi, Moghtada; Ülkü, SemraThe effects of metal piece additives on effective thermal conductivity and diffusivity of an unconsolidated adsorbent bed in which adsorbent is silica gel were investigated. The metal piece additives were copper, brass, aluminum and stainless steel with two different sizes as 1.0-2.8 mm and 2.8-4.75 mm. The effective thermal conductivity and diffusivity of the mixed bed were predicted by comparison of the experimental results with the solution of dimensionless heat conduction equation for the bed. The performed experiments showed that the addition 15wt% of aluminum pieces with sizes between 1.0 and 2.8 mm enhances the effective thermal diffusivity and conductivity of a pure silica gel bed by 157% and 242%, respectively. © 2010 Elsevier Ltd and IIR.Article Citation - WoS: 62Citation - Scopus: 64Effects of Porosity on Heat and Mass Transfer in a Granular Adsorbent Bed(Elsevier Ltd., 2009) Demir, Hasan; Mobedi, Moghtada; Ülkü, SemraIn the present study, the mechanism of heat and mass transfer in an annulus adsorbent is handled. The heat and mass transfer equations for the adsorbent bed and the mass balance equation for the adsorbent granules are numerically solved to obtain the distributions of temperature, pressure, adsorptive density and adsorbate concentration in the adsorbent bed. The study is performed for the silica gel-water pair and for three different values of porosity as 0.1, 0.2 and 0.3. The distributions of temperature and adsorbate concentration are considerably influenced from the bed porosity. The adsorption period increases with the increase of the porosity value. The porosity affects the pressure and adsorptive density distributions at the beginning of the process and after a relatively short time, the averages of these dependent variables approach to the final equilibrium state.Article Citation - WoS: 29Citation - Scopus: 31Cure Kinetics of Epoxy Resin-Natural Zeolite Composites(Springer Verlag, 2008) Cansever Erdoğan, Beyhan; Seyhan, Abdullah Tuğrul; Ocak, Yılmaz; Tanoğlu, Metin; Balköse, Devrim; Ülkü, SemraThe cure kinetics of epoxy resin and epoxy resin containing 10 mass% of natural zeolite were investigated using differential scanning calorimetry (DSC). The conformity of the cure kinetic data of epoxy and epoxy-zeolite system was checked with the auto-catalytic cure rate model. The results indicated that the hydroxyl group on the zeolite surface played a significant role in the autocatalytic reaction mechanism. This group was able to form a new transition state between anhydride hardener and epoxide group. The natural zeolite particles acted as catalyst for the epoxy system by promoting its curing rate.