Enerji Sistemleri Mühendisliği Bölümü Yayın Koleksiyonu

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Isı transferinde ısı kayıplarının yapay sinir ağları yöntemi ile incelenmesi
(Doç. Dr. Fikret TÜRKAN, 2017) Çırak, Bekir; Korcak, Sabit
Bu çalışmada binalarda ısıtma için kullanılan kalorifer borularının yalıtımlı ve yalıtımsız durumları için iki ayrı yapay sinir ağı (YSA) modeli ve bina duvarlarının yalıtımlı ve yalıtımsız durumları için iki ayrı YSA modeli olmak üzere toplamda 4 ayrı model kullanılmıştır. Bu durumlar için tasarlanan YSA modellerinde 3 katmanlı ileri beslemeli ve geri yayılımlı bir model şekli tercih edilmiştir. Gizli katmanda sigmoid transfer fonksiyonu, çıkış katmanında ise doğrusal transfer fonksiyonu kullanılmıştır. YSA ağ topolojisi olarak geri yayılımlı YSA topolojisi tercih edilmiş ve veriler normalize edilerek ağa sunulmuştur. Ağdan elde edilen sıcaklık değerleri gerçekte ölçülen sıcaklık değerleri ile mukayese edilmiş ve sonuçların birbirlerine çok yakın ve yeterli hassasiyette olduğu görülmüştür. Bu şekilde 4 farklı iç model için YSA metodunun kullanımı, modellerin açıklayıcılık ve tahmin etme gücünü artırmıştır.
An investigation of magnetoresistivity properties of an Y3Ba5Cu8Oy bulk superconductor
(Elsevier Science BV, 2015) Kütük, Sezai; Bolat, Süleyman; Terzioğlu, Cabir; Altıntaş S.P.
A bulk ceramic superconducting sample was fabricated using the standard solid state reaction (SSR) method in the stoichiometric ratio of Y:Ba:Cu:O (3:5:8). The electrical and microstructural properties of the sample were investigated by means of magnetoresistivity (R-T) and micrograph measurements, respectively. Critical temperatures (T-c) such as onset and offset, irreversibility magnetic field (H-irr), upper critical magnetic field (H-c2), and activation energy (U-0) were determined from the R-T under various values of applied magnetic field. The findings obtained were compared with a standard Y:Ba:Cu:O(1:2:3) superconducting sample fabricated by the SSR method. T-c(onset) was 95.83 K for the Y358 sample and 94.86 K for the Y123 sample without the magnetic field. T-c(offset) decreased from 92.87 to 74.08 K for the Y358 sample and from 82.83 to 32.44 K for the Y123 sample when the magnetic field ranged from 0 to 5 T. U-0 values of Y358 and Y123 samples without the magnetic field were respectively 1.1486 and 0.3166 eV. These results indicate that the Y358 sample could provide significant benefits for high temperature superconductivity applications in future.
Investigation of superconducting and normal-state properties of the filled-skutterudite system PrPt4Ge12-xSbx
(Amer Physical Assoc, 2016) Jeon, Inho; Huang, Kevin; Yazıcı, Duygu; Kanchanavatee, Noravee; White, B. D.; Ho, Pei-Chun; Jang, Sooyoung; Pouse, Naveen; Maple, M. Brian
We report a study of the superconducting and normal-state properties of the filled-skutterudite system PrPt4Ge12-xSbx. Polycrystalline samples with Sb concentrations up to x = 5 were synthesized and investigated by means of x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements. We observed a suppression of superconductivity with increasing Sb substitution up to x = 4, above which no signature of superconductivity was observed down to 140 mK. The Sommerfeld coefficient, gamma, of superconducting specimens decreases with increasing x up to x = 3, suggesting that superconductivity may depend on the density of electronic states in this system. The specific heat for x = 0.5 exhibits an exponential temperature dependence in the superconducting state, reminiscent of a nodeless superconducting energy gap. We observed evidence for a weak "rattling" mode associated with the Pr ions, characterized by an Einstein temperature Theta(E) similar to 60 K for 0 <= x <= 5; however, the rattling mode may not play any role in suppressing superconductivity.
Structure, magnetization, specific heat, and microwave properties of KxFe2-ySe2
(IOP Publishing Ltd, 2016) Yazıcı, Duygu; Başaran, Ali Cemil; Ramirez, Juan Gabriel; Schuller, Ivan K.; Maple, M. Brian
Temperature-dependent magnetization, specific heat, and magnetic field modulated microwave spectroscopy (MFMMS) measurements were performed on single crystals of KxFe2-ySe2. Magnetization measurements yield a superconducting transition temperature (T-c) of similar to 30 K, with a diamagnetic shielding fraction of nearly 90%. Specific heat measurements revealed a 'jump' at T-c, Delta C/T vertical bar T-c, of about 6.8 +/- 1 mJ mol(-1) K-2, consistent with bulk superconductivity in KxFe2-ySe2. Moreover, MFMMS measurements detect the superconductivity of KxFe2-ySe2. with a peak with an onset at T-c(mu) similar to 28 K, close to the values of T-c determined from the magnetization and specific heat measurements. The presence and the shape of the low temperature MFMMS signal could be ascribed to a complex dissipation mechanism and percolative superconductivity.
Experimental determination of cooling performance on heat sinks with cone-jet electrospray mode
(Elsevier - Division Reed Elsevier India Pvt Ltd, 2021) Kabakuş, Absüssamed; Yakut, Kenan; Özakın, Ahmet Numan; Yakut, Rıdvan
In this study, electrospray cooling characteristics for smooth surface heat sink and finned surface heat sink were investigated. An experimental study was carried out using ethanol for 7 different heat fluxes in the cone-jet mode, in which a stable and continuous droplet diameter is produced. In the experiments, 7 kV voltage, 20 mm nozzle-to-substrate distance, a stainless steel nozzle with 0.61 mm inner diameter (d(i)) and 0.45-0.60 ml/min flow rates were used. Due to that the two flow rate values are very close to each other, no difference in the formation of electrospray was observed, but since the amount of fluid sent to the heat sink is higher, at 0.60 ml/min flow rate, 15-44% better a cooling was achieved under different heat fluxes compared to 0.45 ml/min flow rate. In addition, finned heat sink applied for the first time in electrospray cooling provided approximately 1.3-1.6 times better cooling than smooth surface heat sink. The effect of electrospray dripping on cooling in a finned heat sink is expressed by enhancement ratio (ER). Additionally, the change in fin enhancement ratio (FER), which denotes the enhancement of cooling in finned heat sink in comparison to that in finless heat sink, was scrutinized at different surface temperatures. As a result, as a distinction from the studies on improving heat transfer with electrospray cooling, it was suggested that finned surface heat sinks, which were not used before, can be used as an effective parameter to further enhance the heat transfer.
Spatial characteristics of ecological and health risks of toxic heavy metal pollution from road dust in the Black Sea coast of Turkey
(Elsevier, 2021) Yeşilkanat, Cafer Mert; Yaşar, Kobya
The purpose of this study was to investigate the toxic heavy metal (Cd, Cr, Cu, Ni, Pb, Zn) concentrations of dust samples from the Black Sea coastal road in the north of Turkey. The Black Sea coastal road was represented by 50 sampling sites. The highest average concentrations were identified in Ordu for Cd (0.4 mg kg?1), Cu (132 mg kg?1), and Zn (457 mg kg?1), Samsun for Cr (59 mg kg?1) and Ni (46 mg kg?1), and Artvin for Pb (59 mg kg?1). The analysis of the environmental pollution risk levels of heavy metals was evaluated with the geo-accumulation index (Igeo), pollution assessment indexes (CF and NIPI) and pollution load indexes (PLI and PLIzone). The mean values of Igeo were found in the following decreasing order: Zn (1.2) > Cu (0.5) > Cd (0.2) > Pb (?0.4) > Cr (?0.6) = Ni (?0.6). According to pollution assessment indexes (CF and NIPI), and pollution load indexes (PLI and PLIzone), there were relatively high levels of environmental pollution in Samsun, Ordu, and Artvin provinces, compared to other provinces in the study area. Furthermore, it was identified that the significant contribution of the monomial potential ecological risk (Ei) originated from Cu in Rize (57.6%), Artvin (46.6%), Giresun (40.2%), Pb in Trabzon (31.1%), and Cd in Ordu (71.8%) and Samsun (62.6%). The mean potential ecological risk (PER) values of Ordu and Samsun were categorized as considerable risk. The health risks of toxic heavy metals in road dust for adults and children were evaluated in two different categories, as non-carcinogenic (hazard index, HI), and carcinogenic health risks (total cancer risk, TCR), considering the three possible exposure pathways. In the study area, the mean of HI and TCR for children and adults were found in the negligible non-carcinogenic health risk (HI < 1) and the lower cancer risk range (10?6 < TCR ? 10?4), respectively.
Determining and mapping of natural and artificial radioactivity from moss samples in Artvin (Turkey)
(Parlar Scientific Publications, 2020) Kobya, Yaşar; Yeşilkanat, Cafer Mert; Taşkın, Halim; Karahan, Gürsel
In this study, moss samples which have radiologic indicator characteristic were collected from 80 stations in Artvin to determine the natural (Ra-226, Th-232, and K-40) and artificial (Cs-137) radioactivity levels. Natural and artificial radioactivity results were observed that range activity values for Ra-226, Th-232, K-40 and Cs-137 reach the levels of <1.3 - 31.6 Bq kg(-1), <0.7 - 51.7 Bq kg(-1), 56.3 - 1100 Bq kg(-1) and <0.5 - 535 Bq kg(-1), respectively. Average values of radioactivity results were determined as 6.5 Bq kg(-1), 6.9 Bq kg(-1), 366.9 Bq kg(-1) and 26.8 Bq kg(-1) for Ra-226, Th-232, K-40 and Cs-137, respectively. Pre-determined radiologic levels in the soil samples in the study area were compared with these results. Furthermore, obtained results were evaluated with similar studies in the literature. The estimated values of unmeasured fields to determine the radiologic distribution of the study area were determined with Ordinary Kriging method, and radiologic maps were created for each radionuclide by conducting interpolated value estimation.
Spatial interpolation and radiological mapping of ambient gamma dose rate by using artificial neural networks and fuzzy logic methods
(Elsevier SCI LTD, 2017) Yeşilkanat, Cafer Mert; Kobya, Yaşar; Taşkın, Halim; Çevik Uğur
The aim of this study was to determine spatial risk dispersion of ambient gamma dose rate (AGDR) by using both artificial neural network (ANN) and fuzzy logic (FL) methods, compare the performances of methods, make dose estimations for intermediate stations with no previous measurements and create dose rate risk maps of the study area. In order to determine the dose distribution by using artificial neural networks, two main networks and five different network structures were used; feed forward ANN; Multi-layer perceptron (MLP), Radial basis functional neural network (RBFNN), Quantile regression neural network (QRNN) and recurrent ANN; Jordan networks (JN), Elman networks (EN). In the evaluation of estimation performance obtained for the test data, all models appear to give similar results. According to the cross-validation results obtained for explaining AGDR distribution, Pearson's r coefficients were calculated as 0.94, 0.91, 0.89, 0.91, 0.91 and 0.92 and RMSE values were calculated as 34.78, 43.28, 63.92, 44.86, 46.77 and 37.92 for MLP, RBFNN, QRNN, JN, EN and FL, respectively. In addition, spatial risk maps showing distributions of AGDR of the study area were created by all models and results were compared with geological, topological and soil structure.
Investigation of structural, electronic, elastic, optical and dynamical properties of Ga1-xAlxN alloys
(IOP Publishing Ltd, 2019) Özdemir, Utkan; Korcak, Sabit; Gültekin, Arzuhan; Öztürk, Mustafa Kemal
In this study, structural, optical, electronic, thermodynamic, dynamic and elasticproperties of Ga1-xAlxN alloys have been investigated using the combination of ab initio method based on Density Functional Theory (DFT). Generalized Gradient Approximation (GGA) has been used as exchange correlation. Components ofelastic constants have been used in order to calculate Poisson Ratio, Bulk Modulus, Shear Modulus, Compressibility, Kleiman Parameter, CouchyPressure. Refractive Constants, Extinction Coefficient, Absorption Coefficient, Energy Loss Function and Conductance Values have been obtained from dielectric constants using Kramers-Kronig relations. The properties of ternary alloys have been obtained with reference to known characteristics of binary alloys. The alloys have semiconductor properties with direct band transmission whose band gaps was determined as 2.19, 2.62 and 3.43 eV while Al contents were x = 0.25, x = 0.5 and x = 0.75respectively. They form cubic structure with 3 elastic constants for x = 0.75 and 0.25, and tetragonal structure with 6 elastic constants for x = 0.50. Ga0.5Al0.5N and Ga0.75Al0.25N alloys are elastic whereas Ga0.25Al0.75N is brittle. Compressibility varies depending on increasing x where its maximum is 0.0053 1 /GPa, at x = 0.25. Real parts of Refraction Constants and Dielectric Constants graphs have been found to be have similar trend. Characteristic peak of Lose Function is 19.84 eV for x = 0.75 that is the highest Plasmon Frequency. When Al amount increases, the distance between the maximum and minimum points in the phonon dispersion curves increases so the conductivity decreases. Finally, alloys with large band gaps are suitable semiconductors for optoelectronic devices such as diodes and laser.
Superconducting and normal state properties of the systems La1-xMxPt4Ge12 (M = Ce, Th)
(Amer Physical Soc, 2016) Huang, Kevin; Yazıcı, Duygu; White, Bryan D.; Jeon, Inho; Breindel, Alexander J.; Pouse, Naveen; Maple, M. Brian
Electrical resistivity, magnetization, and specific heat measurements were performed on polycrystalline samples of the filled-skutterudite systems La1?xMxPt4Ge12 (M = Ce, Th). Superconductivity in LaPt4Ge12 was quickly suppressed with Ce substitution and no evidence for superconductivity was found down to 1.1 K for x > 0.2. Temperature-dependent specific heat data at low temperatures for La1?xCexPt4Ge12 show a change from power-law to exponential behavior, which may be an indication for multi-band superconductivity in LaPt4Ge12. A similar crossover was observed in the Pr1?xCexPt4Ge12 system. However, the suppression rates of the superconducting transition temperatures, Tc(x), in the two systems are quite disparate, indicating a difference in the nature of superconductivity, which is conventional in LaPt4Ge12 and unconventional in PrPt4Ge12. In comparison, a nearly linear and smooth evolution of Tc with increasing Th was observed in the La1?xThxPt4Ge12 system, with no change of the superconducting energy gap in the temperature dependence of the specific heat, suggesting similar types of superconductivity in both the LaPt4Ge12 and ThPt4Ge12 compounds.
Multiband superconductivity in the correlated electron filled skutterudite system Pr1-xCexPt4Ge12
(Amer Physical Soc, 2016) Zhang, S.; Singh, Yogesh P.; Adhikari, Rupendra Bahadur; Zhang, S.; Huang, Kevin; Yazıcı, Duygu; Jeon, Inho; Maple, M. Brian; Dzero, Maxim; Almasan, Carmen C.
Studies of superconductivity in multiband correlated electronic systems have become one of the central topics in condensed-matter and materials physics. In this paper, we present the results of thermodynamic measurements on the superconducting filled skutterudite system Pr1-xCexPt4Ge12 (0 <= x <= 0.2) to investigate how substitution of Ce at Pr sites affects superconductivity. We find that an increase in Ce concentration leads to a suppression of the superconducting transition temperature from T-c similar to 7.9 K for x = 0 to T-c similar to 0.6 K for x = 0.14. Our analysis of the specific-heat data for x <= 0.07 reveals that superconductivity must develop in at least two bands: the superconducting order parameter has nodes on one Fermi pocket and remains fully gapped on the other. Both the nodal and nodeless gaps decrease, with the nodal gap being suppressed more strongly upon Ce substitution. Ultimately, the higher-Ce-concentration samples (x > 0.07) display a nodeless gap only.
Influence of milling parameters on particle size of ulexite material
(Elsevier Science BV, 2016) Kütük, Sezai
Commercial raw ulexite (U) materials were milled from 0 h to 8 h by using mechanical milling technique for var-ious initial powder size (?3mm;?75?m), ball to powder ratio (5:1; 10:1), and ball size (10 mm; 5 mm). Par-ticle size, morphology, elemental, and crystal structure measurements of the milled powders were performed. Inthe particle size distribution analysis, the smallestd50andd10values were respectively found as 8.846?mand790 nm for U_5m powder (obtained from that initial powder with?3 mm in size was milled by balls with5 mm in size) at 0.5 h. In the morphology analysis, the microstructure of the U_5m powder was observed tobe more homogeneous by means of milling process. In the elemental analysis, it was deduced that the U_5mpowder is not a pure compound. In the crystal structure analysis, it was determined that the crystal structureof the U_5m powder is exponentially deteriorated with increasing the milling time, and then it has become anamorphous structure at the end of 8 h. The crystalline size of the U_5m powder is reduced to 15.5 nm after 4 hof milling. The results of this study are considered to be useful for future nano studies and industrial applicationsof ceramic ulexite compound, which is a boron mineral.
Biomonitoring of trace elements and gross alpha/beta radioactivity levels in human teeth: A case study from Istanbul, Turkey
(Elsevier Science BV, 2018) Taşkın, Halim; Kobya, Yaşar; Tokgöz, Murat; Bassarı, Asiye; Yeşilkanat, Cafer Mert
The purpose of this study is to determine the amount of various trace elements and the concentration of grossalpha and gross beta radioactivity in human teeth and alsofind out their possible effects on humans. Fifty-sixtooth samples were collected from female and male subjects of four different age groups, all of whom reside inIstanbul (0–18, 19–30, 31–45, 45+). In addition, the variation of trace elements such as Al, Ba, Ca, Cl, F, Fe, K,Mg, Na, P, S, Si, Sr and Zn for the age groups of both genders was examined by trace element analysis. The meangross alpha and gross beta concentrations were observed in the range of 31.0–47.8 Bq/kg and 71.2–89.2 Bq/kgrespectively for the female subjects and 52.7–82.1 Bq/kg and 114.0–154.4 Bq/kg respectively for the malesubjects. The radioactivity level was higher in the tooth samples of the male subjects than those of the femalesubjects. The results, when compared with similar studies, suggest that the concentrations of both gross alphaand gross beta radioactivity in the female subjects were quite smaller than those in the male subjects.Furthermore, it was observed that the concentrations of trace elements were generally the highest in the agegroups of 31–45 and 45+, which are around the time when the body functions in humans start slowing down.Thefindings of study would be useful for creating a biomonitoring database, which has not yet been done in thisarea of study.
Evaluation and mapping of radionuclides in the terrestrial environment and health hazard due to soil radioactivity in Artvin, Turkey
(Springer Heidelberg, 2018) Taşkın, Halim; Yeşilkanat, Cafer Mert; Kobya, Yaşar; Çevik, Uğur
In this study, the radiological characterization of top soil in the Artvin region, the assessment of relevant health risks, and obtaining the distribution of radiological parameters in soil of the region by means of the geostatistical method were aimed. Mean concentration values of Ra-226, Th-232, K-40, and Cs-137 radionuclides in 117 soil samples were determined by gamma spectrometric analysis as 42.26, 27.57, 390.42, and 14.87Bq/kg, respectively. According to the results obtained, the radium equivalent activity (Ra-eq), the internal radiation hazard (H-in), the external radiation hazard (H-ex), and the gamma representative level (I-r) were found to be 111.75Bq/kg, 0.41, 0.30, and 0.81, respectively, for the region. In addition, mean absorbed dose rate, annual effective dose equivalent, and excessive lifetime cancer risk values were calculated as 54.79nGy/h, 0.067mSv/year, and 0.00023, respectively, for the province. In addition, it was determined that all radiological parameters obtained for the region were lower than the worldwide averages. Finally, in order to estimate the activity for unsampled points and to determine the radiation distribution, the interpolations for each radionuclide were done through ordinary kriging method; the spatial distribution maps were created using these estimations, and it was seen that the distribution of the radionuclides around the region was quite compatible with both the calculated dose rates and cancer risk distribution in the region.
Radiochemical characterization of mineral waters in the Eastern Black Sea Region, Turkey
(Springer, 2011) Kobya, Yaşar; Damla, Nevzat; Çevik, Uğur; Kopya, Ali İhsan
This study has evaluated the levels of natural radionuclides and chemical components of mineral waters in the Eastern Black Sea Region (Turkey). The mean activity concentrations of (226)Ra, (232)Th, (137)Cs, (40)K, gross alpha and gross beta were found as 129, 33, 28, 714, 125 and 170 mBq L (-aEuro parts per thousand 1), respectively. Due to consumption of mineral waters, the radiological impact of them on the inhabitants was calculated by taking the annual intake into account through ingestion of aforementioned radionuclides. The estimated effective doses from mineral water were found to be 13.20 mu Sv year (-aEuro parts per thousand 1) ((226)Ra), 2.74 mu Sv year (-aEuro parts per thousand 1) ((232)Th), 0.13 mu Sv year (-aEuro parts per thousand 1) ((137)Cs) and 1.62 mu Sv year (-aEuro parts per thousand 1) ((40)K). The overall contribution of these radionuclides to the committed effective dose from a year's consumption of mineral water in the region is therefore estimated to be only 17.69%, which is in concordance with the recommended WHO value (100 mu Sv year (-aEuro parts per thousand 1)). The chemical analysis results showed that these waters contain Na, Al, P, Cl, K, Ca, V, Mn, Fe, Ni, Cu and Zn elements. These values were evaluated and compared with the internationally verified values. This study provides important information for consumers and authorities because of their internal radiochemical exposure risk from mineral water intake.
Vacancy transfer probabilities from K to L shell for low atomic number elements at 5.96 keV
(Polish Acad Sciences Inst Physics, 2010) Ertuğral, Bilal; Baltas, Haralambos; Çelik, Ahmet; Kopya, Yaşar
Vacancy transfer probabilities from K to L shell were measured using IK?/IK? intensity ratios for six elements in the atomic region 16 ? Z ? 22. The K X-rays from the targets excited by 5.96 keV (1.85 GBq 55Fe filtered annular source) incident photon were obtained using a high resolution PGT model Si(Li) detector. Theoretical values were calculated using radiative and radiationless transition rates of these elements. The measured values of ?KL were compared with the theoretical ones. Reasonable agreement was typically obtained between present and theoretical values.

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