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Öğe A two-stage polynomial chaos expansion application for bound estimation of uncertain FRFs(Academic Press Ltd- Elsevier Science Ltd, 2023) Kara, Murat; Ferguson, Neil S.Polynomial Chaos Expansion (PCE) is a method for analysing uncertain vibratory structures with lower computational effort. It may simply be described as a curve fitting method with orthogonal basis terms, where the polynomial type, dimension and order are predefined for the uncertain responses. However, the polynomial order in PCE must be very high to accurately estimate statistical moments of the frequency response function in resonance regions of lightly damped and uncertain structures. To solve this issue different transformation techniques are reported in the literature, where implementations of PCE produce higher accuracy with a lower order polynomial. However, these transformations lose the attraction for using PCE, since they require some additional mathematical operations and, mostly, they present high accuracy if the higher orders of polynomials are again of interest. In this study, an efficient approach is presented for the upper bound estimation of the uncertain frequency response functions (FRFs) via PCE with lower order terms without performing any transformation. Rather than one-stage application of PCE for the desired response of an uncertain problem, the approach comprises a two-stage application of the classical PCE, i.e. first for the natural frequencies and then for the FRF calculations. As an example application of the approach, a thin beam for two different uncertainty cases is considered, namely local and global uncertainty. The local and global input uncertainties are generated by variability of lumped masses added at the boundary and Young's modulus, respectively. The FRF bounds are compared with extensive experimental and numerical Monte Carlo simulations, showing that low order polynomials are sufficient to calculate the bounds accurately with the technique described.Öğe Analytical and experimental investigation of the rotary inertia effects of unequal end masses on transverse vibration of beams(MDPI, 2023) Bilge, Habibullah; Morgül, Ömer KadirIn this study, the transverse vibration of free-free slender beams with two unequal end masses attached were studied. The effects of the rotary inertia of the end masses on the free vibration of the beam were investigated. An exact frequency equation and the boundary conditions were obtained by using the Euler-Bernoulli beam theory and Hamilton's principle. Natural frequencies and mode shapes of the beams in transverse vibrations were calculated for various combinations of physical and geometrical parameters, such as mass ratios, the distances between the attachment point and the center of the masses, etc. The effects of an increase in the rotational inertia of the end masses and different mass ratios on the natural frequencies and mode shapes of the beam are presented. It is shown that the increase in the rotational inertia of the end masses had a greater effect at low frequencies of the beam. In addition, experimental tests were performed to validate the obtained analytical results. A good agreement was obtained between the analytical and experimental results. The main scope of this study was to reveal the effects of the rotary inertia of the end masses on the dynamic behavior of the beam. Thus, the aim is to contribute to the understanding of the properties of the end mass and the effect of rotary inertia on the dynamics of end-mass-attached structures. Furthermore, the results obtained from this research are helpful for designing end-mass-attached structures, such as micromechanical sensors, energy harvesters, and Stockbridge-type dynamic absorbers.Öğe Bending performance of the AuxOcta multi-cellular beam structure(Elsevier Science Ltd, 2023) Kahraman, Mehmet Fatih; Genel, KenanAuxetic cellular tubes are potential candidates for several engineering applications requiring resistance to bending due to the enormous demand for lightweight and high mechanical performance. Interestingly, we found that the preferential buckling mode can change the direction of deformation in the beam cross-section by applying negative Poisson's ratio cells in the lateral direction and is an effective modifying region. The bending response and energy absorption of multi-cellular tube with auxetic cells (Aux) and novel design tubes with auxetic and octagonal cells (AuxOcta) have been investigated with numerical and experimental systematically. These unique auxetic structures are produced by direct metal laser sintering (DMLS) with 316L stainless steel. The bending results show that the Aux tube indicates gradual local deformation in the loading region, while the cells of the AuxOcta tube beam are homogeneously deformed in the loading region. Moreover, AuxOcta structure is optimized to improve load-carrying and energy-absorbing capacities. This optimized AuxOcta structure (AuxOcta-G) offers superior bending performance from the test results. Compared to the Aux structure, the improvement in the specific load carrying (SLC) capacity of the AuxOcta-G structure above is 15% for 5 mm displacement, while the improvement in the specific energy absorption (SEA) approximately reaches 16%. The Finite Element Method (FEM) results showed that AuxOcta and AuxOcta-G structures offer a preferred behavior with a wide displacement range. This study provides a baseline for future investigation of hybrid beam design with a negative Poisson cross-section.Öğe Annealing-induced modifications on structural, surface chemical bonding, and electrical characteristics of p-NiO/n-TiO2 heterostructure(Springer, 2023) Kaya, Şenol; Soykan, Uğur; Sunkar, Mustafa; Karaboğa, Seda; Doğan, Muhsin Uğur; Terzioğlu, Rıfkı; Yıldırım, Gürcan; Terzioğlu, CabirThe influences of annealing temperatures on the electrical characteristics of a p- NiO/n-TiO2 heterojunction diode were thoroughly investigated, taking into account changes in microstructure, morphology, and surface chemistry of the p-NiO/n-TiO2 films, which were deposited on an insulating SiO2/ Si layer. During different annealing processes, considerable stress variations were observed in the p-NiO/n-TiO2 films due to the crystalline evolution of p-NiO and n-TiO2. Notably, the crystallization of the TiO2 layer, which serves as the intermediary between the back contact materials and NiO, led to the evident formation of grain structures. As the annealing temperature increased, the surface roughness also grew from 5.4 to 8.7 nm. At an annealing temperature of 500 degrees C, the formation of a parasitic NiTiOx phase was observed, particularly at the interface between NiO and TiO2. Conversely, the study also revealed that annealing temperature played a significant role in the rectifying behavior, barrier potential, and ideality factor of the diode. Among the various annealing processes, the most favorable results were achieved after annealing at 400 degrees C. At this temperature, the diode demonstrated the lowest ideality factor of 1.89, accompanied by superior rectifying behavior and a barrier potential of 0.70 eV. The findings clearly indicate that any alterations in the surface chemistry and microstructure of the film directly impact the diode's characteristics. Thus, optimizing the annealing temperature becomes crucial for enhancing the performance of the p-NiO/n-TiO2 heterojunction diode.Öğe Contribution of vanadium particles to thermal movement of correlated two-dimensional pancake Abrikosov vortices in Bi-2223 superconducting system(Elsevier, 2023) Ülgen, Asaf Tolga; Erdem, Ümit; Yıldırım, Gürcan; Türköz, Mustafa Burak; Turgay, TahsinThis article breaks new ground in understanding of variation in the magnetic strength performance, flux pinning and energy dissipation mechanism of polycrystalline bulk Bi1.8Sr2.0Ca2.2Cu3.0Oy (Bi-2223) superconducting materials added with the different vanadium concentration level (0.0 <= x <= 0.30) under the magnetic field strengths applied up to 5 T for the first time. We provide the sophisticated and phenomenological discussions on the magnetoresistivity measurement results in three main sections along the paper. All the findings show that the increase of both the vanadium concentration in the crystal structure and external magnetic field strength damages significantly the magnetic strength performance, vortex dynamics, flux pinning ability and vortex lattice elasticity of bulk Bi-2223 superconducting ceramics. The vanadium addition promotes thermally the movement of correlated two-dimensional (2D) pancake Abrikosov vortices between the in-plane Cu-O-2 layers in the valance band, vortex lattice elasticity, vortex dynamics, distance for interlayer Josephson couplings and flux pinning centers and the theoretical computations confirm the remarkable degradation in the formation of super-electrons in the Bi-2223 crystal system. Thus, the vanadium addition is anticipated to be one of the best selectable materials to examine the differentiation in the thermal movement of correlated 2D Pancake Abrikosov vortices in the bulk Bi-2223 superconducting system. (c) 2022 The Author(s). Published by Elsevier Espana, S.L.U. on behalf of SECV. This is anopen access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).Öğe Development of modulation, pairing mechanism, and slip system with optimum vanadium substitution at Bi-sites in Bi-2212 ceramic structure(Elsevier Science Sa, 2023) Ülgen, Asaf Tolga; Okur, Semih; Erdem, Ümit; Terzioğlu, Cabir; Yıldırım, Gürcan; Turgay, TahsinPresent study focuses extensively on the change in electrical, superconducting and microhardness parameters with partial substitution of trivalent V+3 impurities replacing Bi+3 ions in Bi-2212 ceramic compound with the aid of dc electrical resistivity and microhardness test measurements. Experimental findings, calculation results, and phenomenological discussions provide that the optimum vanadium substitution level is found to be x = 0.01 in the Bi2.0-xVxSr2.0Ca1.1Cu2.0Oy (Bi-2212) ceramic system for the highest conductivity, crystallinity quality, superconducting, and mechanical performance features depending on the decreased microscopic structural problems. All the findings are wholly verified by scanning electron microscopy (SEM) and X-Ray diffraction (XRD) analyses. The dc electrical measurements indicate that the optimum vanadium ions support the pairing mechanism for the formation of new polaronic states in the clusters of microdomains, and hence expand superconducting energy gap due to the enhancement of amplitude part of pair wave function in the spin-density wave systems. The excess vanadium content degrades all the basic thermodynamics and quantum mechanical quantities mentioned due to the stress-induced phase transformation. Numerically, the Bi-2212 advanced ceramic matrix prepared by the optimum vanadium impurity is noticed to present the smallest residual resistivity value of 0.08 m & omega; cm, room temperature resistivity value of 8.84 m & omega; cm, and broadening degree of 0.36 K. Similarly, the ceramic material is found to possess the highest residual resistivity ratio of 3.05, carrier concen-tration number of 0.153041, critical transition offset and onset value of 84.66 K and 85.02 K, respectively. Besides, the microhardness findings reveal that the same compound with the least sensitivity to the applied test loads exhibits the largest Hv value of 4.799 GPa, Young's moduli of 393.303 GPa, yield strength of (0.969 GPa), and elastic stiffness coefficient of 15.5574 (GPa)7/4 under the applied test load of 0.245 N. The XRD in-vestigations show that the presence of optimum vanadium impurity supports the formation of a high super-conducting phase, c-axis length, and average crystallite size. All the findings are morphologically confirmed by the SEM images. It is found that the crystallographically best crystallinity quality and view of surface morphology is observed for the optimum vanadium substitution level. All in all, new higher properties for the conductivity, crystallinity quality, surface morphology, superconducting, and microhardness parameters based on the optimum vanadium replacement encourage the Bi-2212 crystal system to use in much more application places.Öğe Evolution of basic features of Bi1.8Pb0.4Sr2Ca2.2Cu3Oy ceramic systems with NiO impurity(Springer, 2023) Kara, Emre; Doğan, Muhsin Uğur; Kaya, Şenol; Terzioğlu, Rıfkı; Yıldırım, Gürcan; Terzioğlu, CabirIn the current work, we have investigated the role of different nickel oxide (NiO)(x) impurity addition levels (0 & LE; x & LE; 1.2) on the fundamental crystallographic, morphological, electrical, magnetic, critical current density, and superconducting features of Bi1.8Pb0.4Sr2Ca2.2Cu3Oy (Bi-2223) by the temperature-dependent electrical resistivities (& rho;-T), X-ray diffraction data (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and magnetic hysteresis (M-H) measurements for the first time. The NiO-added Bi-2223 ceramics have been produced by the standard solid-state reaction method at 840 & DEG;C for 36 h in the air atmospheric conditions. EDX investigations have demonstrated that there appear no extra contamination and additional phases for any anionic or cation-including phases. Besides, the Ni impurity concentration has been observed to increase systematically depending on the impurity amount. Thus, the NiO impurities have been added to the ceramic matrix as requested. Similar findings have been confirmed by the XRD examinations. The experimental results have pointed out the multiple substitution reaction of Ni-sites with the Bi-sites, Ca-sites, and Cu-sites in the Bi-2223 system. Moreover, the experimental results have shown that all the fundamental features are strongly dependent upon the NiO addition level due to the variation of Cu-O-2 interlayer bonding forces, grain alignment distributions, mobile carrier concentrations in the in-plane Cu-O-2 layer x(2)-y(2) bands, and coupling probabilities between the adjacent layers in the crystal structure. Similarly, the induction of new impurity phase formations, grain boundaries, modulation of Bi-O double layers, spin fluctuations, inhomogeneities, structural defects, and non-recoverable structural problems have also triggered the decrease in the main characteristic quantities. On this basis, the microstructural morphology and related crystallographic features have been noted to degrade with the impurity. Furthermore, the presence of NiO impurity has resulted in a decrement in the flux-pinning centers due to the increase in the lower resistance regions against the flux drifts in the bulk crystal system. All in all, the addition of NiO impurity in the Bi-2223 ceramic matrix is not a good idea for the improvement in the fundamental features of the Bi-2223 superconducting structure.Öğe Solid particle erosion and scratch behavior of novel scrap carbon fiber/glass fabric/polyamide 6.6 hybrid composites(Wiley, 2023) Koçoğlu, Hürol; Korkusuz, Orkan Baran; Özzaim, Pelin; Kodal, Mehmet; Altan, M. Cengiz; Sınmazçelik, TamerThis study investigated the tribological performance of hybrid composites composed of scrap carbon fiber (CF), glass fabric (GF), and polyamide 6.6 (PA6.6) through an innovative approach for reusing scrap CFs in high-value composite structures. The experimental setup included CF/GF/PA6.6 hybrid composite laminates with varying CF contents and surface-modified GFs, as well as PA6.6 sheets and GF/PA6.6 composite laminates. Solid particle erosion and scratch tests were conducted to assess the influence of scrap CF hybridization and GF surface modification on the tribological properties of the composites. The results demonstrated that neat PA6.6 sheets exhibited the lowest erosion rate, while the incorporation of CF and GF reinforcements had a detrimental effect on erosion resistance. The highest erosion rate was observed within the impact angle range of 15 degrees -30 degrees for pure PA6.6 sheets, whereas for composite laminates, it occurred within the range of 30 degrees-45 degrees. In contrast, CFs positively affected scratch hardness despite their negative impact on erosion resistance. Additionally, the silane treatment of GFs, which enhanced interfacial strength, improved the erosion resistance and scratch hardness of GF/PA6.6 composite laminates without CF. Profilometer-based topographic analysis revealed a correlation between the average surface roughness of the eroded surfaces and the weight loss resulting from solid particle erosion.Öğe Experimental investigation into efficiency of SiO2/water-based nanofluids in photovoltaic thermal systems using response surface methodology(Pergamon-Elsevier Science Ltd, 2022) Geliş, Kadir; Çelik, Ali Naci; Özbek, Kadir; Özyurt, ÖmerPhotovoltaic thermal systems (PVT) are systems that can convert solar energy into electricity and thermal energy simultaneously. In this study, the effect of nanofluids on the electrical and thermal efficiency of PVTs was investigated using the Response Surface Methodology (RSM). In the experimental study presently undertaken, SiO2 nanoparticles were suspended in deionized water, which was used as base fluid in 3 different volumetric ratios (0.1-0.2-0.3). A mathematical model has been developed to calculate the thermal and electrical efficiency of the PVT system using the RSM approach. In the RSM method, the flow rate of the nanofluid, the nanofluid volumetric concentration, and the solar radiation were determined as independent variables, and their effects on the thermal and electrical efficiency of PV/Ts were statistically investigated. The model presently developed was validated based on the analysis of variance (ANOVA). The most influential parameters affecting the electrical and thermal efficiencies have been found as radiation, flow rate and volumetric concentration, respectively. The variance between the mathematical models developed and the experimental results was measured in terms of the coefficient of determination (R-2), which was between 0.85% and 1.91% for the electrical efficiency and between -6.34% and 1.06%. for the thermal efficiency. As a result, the mathematical models developed for the electrical and thermal efficiencies of the PV/T system has been successfully verified based on the experimental outcomes.Öğe The effect of sintering parameters and MgO ratio on structural properties in Al7075/MgO composites: A review(Springer, 2023) Pul, Muharrem; Erdem, Ümit; Türköz, Mustafa Burak; Yıldırım, GürcanIn this experimental study, Al7075 matrix composites reinforced with different proportions of MgO were produced by powder metallurgy method. Different sintering temperatures and times were applied in the powder metallurgy production process. In the second stage of the experimental study, firstly, the porosity and hardness measurements of the composite materials were made. Then, microstructure images were taken with SEM and optical microscope, and XRD analyzes were performed. Using the obtained data, the effects of different MgO ratios and different sintering parameters on the structural properties of composite materials were evaluated. As the sintering temperature increased, the density of the composite structure increased and then decreased again. Accordingly, the amount of porosity first decreased and then increased again. Significant size growth occurred in all samples sintered at 600 degrees C. This change was associated with the high amount of porosity in the same samples. A more stable microstructure was obtained from the samples sintered at 550 degrees C. Thus, it can be said that the presence of excess MgO particles in the system causes the material quality to deteriorate due to increased microscopic structural problems, wetting rates, intergranular interaction problems between adjacent layers, recovery mechanism and entanglement of voids, and dislocations. Therefore, the ideal rate, time and temperature value for MgO addition should be carefully determined. As a result, it was seen that the sintering temperature of 550 degrees C gave the most suitable results. The sintering time strengthened the phase volume of the Al7075 alloy, making the compound more stable.Öğe Stochastic prediction of natural frequencies of laminated composite beams by using a high-order statistical moment based approach(Elsevier Science Ltd, 2022) Kara, Murat; Seçgin, Abdullah; Baygün, Tuba; Akyol, Çağrı GökhanStatistical Moment (SM) based modelling is a quite straightforward approach in stochastic modelling of uncertain structures. However, the method still has deficiencies including determination of SMs of natural frequencies of vibratory structures, and it has not been tested yet for realistic structures. This study aims such verification by employing high-degree statistical moments in stochastic equations. In this respect, SM approach is applied for two different uncertainty cases. In the first case, uncertain parameters are experimentally reproduced from the batch of laminated composite beams. Then, those uncertainties are fed to SM equations used in finite element model to obtain descriptive statistical quantities (mean, variance, skewness, and kurtosis) of stochastic natural frequencies. Next, Pearson model is utilized to obtain probability density function of the natural frequency by using standardized SMs. Beside this, uncertain fundamental natural frequency of fifty samples of composite beams is measured by experimental modal tests. All SM based predictions and modal test results are also compared with numerical Monte Carlo Simulations. The latter case examines composite beams having non normal uncertain thickness. Since the results are in good harmony with each other, it is concluded that high order SM based approach may effectively be used in uncertainty modelling of realistic structures.Öğe Development of novel flow fields for pem fuel cells: Numerical solution and experimental validation(Begell House Inc, 2022) Geliş, Kadir; Şahin, Bayram; Yurtcan, Ayşe BayrakçekenIn the present study, the main purpose is to design flow channels with less pressure drop and higher performance compared to single serpentine flow channels which are found in the literature for PEM fuel cells. Within the scope of the present study, a numerical and experimental research was conducted on the design of the flow channels on bipolar plates. The fuel cell with a serpentine flow field was experimentally tested under conditions of 70 degrees C temperature, 1 atm pressure, 100% humidification, and 0.25 L/min anode/cathode flow rate, and analyzed numerically. This way, a numerical model verified with experimental data was obtained. Four models (Models 1-4) with unique flow fields were designed and numerically analyzed to compare with the verified numerical model. The flow field percentages (channel to rib ratio) of the 5 models (1 serpentine-type model + 4 new models) designed were fixed at a value of approximately 55.4%. For all designs, the channel width was set to 1.5 mm and the channel depth was set to 1 mm. Results indicate that the experimental data obtained were in accordance with the numerical results with an error margin of 5.3%. Based on the numerical analysis results at 0.6 V, current density increased by 23.9% in Model 1, 26.9% in Model 2, and by 13.8% in Model 3 compared to the reference model while a 12% decrease was observed in Model 4.Öğe Evolution of operable slip systems, lattice strain fields and morphological view of Bi-2223 ceramic system with optimum NiO addition(Elsevier, 2023) Mercan, Ali; Kara, Emre; Doğan, Muhsin Uğur; Kaya, Şenol; Terzioğlu, Rıfkı; Erdem, Ümit; Yıldırım, Gürcan; Terzioğlu, CabirThe current work extensively reveals the influence of different nickel oxide (NiO) impurity addition levels on the morphological, microstructural, key mechanical performance, and mechanical characteristic properties of Bi1.8Pb0.4Ca2.2Sr2Cu3Oy (Bi-2223) ceramics using scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and Vickers micro-indentation (Hv) hardness measurements. It was observed that the addition of NiO impurity in the Bi-2223 crystal structure affected seriously the fundamental characteristic features. In the case of the optimum NiO concentration level of x = 0.1, the Bi-2223 materials exhibited the best crystallinity quality and coupling strengths between the adjacent layers, the most uniform surface view, and the densest, and the smoothest crystal structure. Similarly, the compound was noted to possess the hardest, highest mechanical strength, durable tetragonal phase, resistance toward failure by fatigue, and elastic recovery properties. Besides, it was observed that the characteristic Bi-2223 superconducting phase fraction and stabilization of the tetragonal crystal system reached the maximum level for the optimum concentration. Moreover, optimum NiO particles brought about a considerable increase in the number of operable slip systems, surface residual compressive force regions, and lattice strain fields. Correspondingly, the mobility of defects was blocked significantly depending on the preference of defects through transcrystalline regions. Additionally, optimum addition strengthened the typical indentation size effect due to the improvement of the recovery mechanism. In this regard, the NiO-added sample exhibited the least response to the applied loads. Thus, the Bi-2223 sample with the optimum NiO concentration was found to present the highest hardness parameter of 0.496 GPa, greatest elastic deformation value of 16.493 GPa, largest stiffness value of 1.044 MN/m, and smallest contact depth of 5.849 mu m. On the other hand, after the optimum concentration level of x = 0.1, there appeared serious increase in problems including internal defects, impurity residues, microscopic structural problems, and connection problems between the grains. All experimental findings were theoretically supported by semi-empirical mechanical methods. To sum up, the addition of NiO particles was noticed to increase the potential application areas of Bi-2223 ceramicÖğe Mechanical and fatigue behavior of resistance spot welded dual-phase and twinning-induced plasticity steel joints(Asce-Amer Soc Civil Engineers, 2022) Doruk, Emre; Fındık, Fehim; Pakdil, MuratThe new generation high-strength steel sheet has been widely used to reduce the vehicle weight. In this study, the mechanical, metallurgical, and fatigue behavior of the resistance spot-welded joints of high strength steel sheets were experimentally investigated. The Dual-Phase (DP) 800 and Twinning-Induced Plasticity (TWIP) 1000 steel sheets were joined together in three different nugget diameters. The welding current was parameterized to create three different nugget diameters. Hence, four groups of welded combining combinations were obtained. The tensile shear test was applied to the samples to study the effects of weld nugget diameter and different welded joint combination in terms of the joint strength. For long fatigue life tests, six test samples were prepared for each test series; in total, 24 tests have been conducted, in which S-N curves of the samples were extracted. Finally, the effect of different nugget diameters on the fatigue life of the welded joint combinations was investigated. (C) 2022 American Society of Civil Engineers.Öğe Improvement in deformation degree of Zr surface-layered Bi-2223 ceramics by diffusion annealing temperature(Elsevier Sci LTD, 2023) Mercan, Ali; Terzioğlu, Rıfkı; Doğan, Muhsin Uğur; Kaya, Şenol; Erdem, Ümit; Yıldırım, Gürcan; Terzioğlu, Cabir; Varilci, AhmetThis study investigated the effects of different annealing temperatures (650 degrees C <= T <= 840 degrees C) on the surface morphological and mechanical performance properties of Zr surface-layered Bi-2223 materials with scanning electron microscopy (SEM) images, Vickers microhardness (Hv) measurements, and semi-empirical mechanical approaches. It was observed that the ceramic compound exposed to 650 degrees C annealing temperature exhibited the superior performance features due to the enhancement in the deformation degree. This is because the Zr ions behaved as the nucleation centers to prevent the propagations of cracks and dislocations throughout the main matrix depending on the decrease in the degree of granularity and distributions of crystal structure problems over a wider area. Similarly, the SEM pictures indicated that the diffusion mechanism increased the random distributions of the thinner plate-like granular structures (serving as nucleation centers), leading the decrease in the coupling problems between the grains. Among the materials, the highest surface densification was observed for the compound exposed to 650 degrees C. Namely, surface morphological analysis showed a strong correlation be-tween microstructure and mechanical performances. Further, the zirconium ions were found to decrease in the non-recoverable stress concentration sites, crack-initiating defects, and dislocations in the ceramic system. Accordingly, the sensitivity to the applied test load was noted to decrease dramatically. Shortly, crack growth size and velocity were observed to be more easily under control. Correspondingly, the Zr ions delayed consid-erably the beginning points of saturation limit (load-independent) regions for the bulk Bi-2223 superconducting materials. Additionally, the Zr ions led to the change in the mechanical characteristic behavior from typical indentation size effect to reverse indentation size effect. Lastly, the microindentation hardness measurements were semi-empirically analyzed by the different models. According to the comparison, Hays-Kendall mechanical model was noted to provide the closest parameters to the load-independent microhardness results.Öğe Experimental investigation on heat transfer of Al2O3-MWCNT/water-based hybrid nanofluids in double-pipe mini heat exchanger: Design of experiments using RSM(Begell House Inc, 2022) Geliş, Kadir; Akyürek, Eda FeyzaHeat exchangers are industrial equipment used to transfer energy from one medium to another at different temperatures. In the present study, the effects of using hybrid nanofluids in a double-pipe mini heat exchanger (DPMHE) on heat transfer and pressure drop performance were investigated using the response surface methodology (RSM). The DPMHE used in the present study differs from the minichannels widely applied in the literature in terms of heating mechanism and design. The Al2O3-MWCNT nanoparticles used in the experimental study were suspended into the deionized water used as the base fluid at three different volumetric ratios (0.1, 0.2, and 0.3) and three mixture ratios (75:25, 50:50, and 25:75) to prepare the hybrid nanofluids. The effects of volumetric concentration, the MWCNT percentage, and the Reynolds number Re, which were determined as input parameters in the response surface methodology, on the Nusselt number Nu and the friction factor f were analyzed statistically. The most effective parameters for the Nusselt number are Re, MWCNT percentage, and volumetric concentration, respectively, while the most effective parameters for f are Re and volumetric concentration, respectively. When the Nu number values calculated with experimental data and those calculated through the mathematical model obtained with RSM were compared, it was found that the error rate ranged between 0.37% and 1.84%, while this rate was in the range of 0.33% and 2.05% for the f values. In conclusion, the Nu and f values calculated using the mathematical model and the data from the verification experiments are very close to each other, and the error rate between them is at an acceptable level.Öğe Refinement of some basic features of Zr surface-layered Bi-2223 superconductor with diffusion annealing temperature(Springer, 2022) Orhan, Emre; Kara, Emre; Kaya, Şenol; Doğan, Muhsin Uğur; Terzioğlu, Rıfkı; Yıldırım, Gürcan; Terzioğlu, CabirThis study aims to investigate the influences of diffusion annealing temperatures on structural, morphological, electrical, and superconducting features of Zr surface-layered Bi-2223 ceramics. The present study also covers an in-depth understanding of correlations between disorders and transition temperatures. The Zr diffusion is carried out via an annealing process between 650 and 840 degrees C. The observed results depict that the Zr ions can easily diffuse into the deeper level of Bi-ceramics and possible Zr/Bi substitution has occurred due to the driving force of high thermal energy. Besides, it is found that the Zr diffusion improves the general crystallinity quantities of Bi-2223 ceramic up to 800 degrees C annealing temperature. In addition, better intergranular couplings with a smoother plate-like structure are extensively observed in surface morphology for the samples annealed at 800 degrees C. Significant refinements of both basic electrical resistivity, hole carrier densities, and critical temperatures with narrow transitions are also obtained for the Zr surface-layered Bi-2223 ceramics after the 800 degrees C annealing process. The obtained improvements in critical fundamental features can be attributed to the optimum pairing mechanism, best crystal structure quality, ideal Cu-O-2 interlayer coupling strengths, and enhanced interaction between adjacent superconductive layers. Besides, the first-order derivative of electrical resistivity versus temperature graphs indicates that the best annealing temperature enables to triggers to stabilize the superconductivity in the homogeneous regions. It can be concluded that the Zr impurity diffusion at 800 degrees C is promising for the improvement in the basic features of Bi-2223 superconducting systems for future applications in superconductor technology.Öğe Multi-objective optimization of a photovoltaic thermal system with different water based nanofluids using Taguchi approach(Pergamon-Elsevier Science Ltd, 2023) Geliş, Kadir; Özbek, Kadir; Özyurt, Ömer; Çelik, Ali NaciIn this study, the effect of different nanofluids on the electrical and thermal efficiencies of photovoltaic thermal (PVT) systems was investigated experimentally and the variables affecting the efficiency were optimized by the Taguchi method. Experiments were carried out under laboratory conditions on the PVT system presently developed. In this study, a novel partitioned cooler block in the form of a rectangular prism was designed and used. Thus, the contact between the cooler block and the back surface of the PV panel and the associated PVT efficiency were increased. The following physical characteristics have been identified as the independent vari-ables for the experiments carried out: SiO2/Water, Al2O3/Water and CuO/Water as different types of nanofluid, 0.1, 0.2 and 0.3 as volumetric concentration, 0.55, 1.1 and 1.65 lpm as volumetric flow rate and 300, 600 and 900 W/m2 as irradiance level. The reliability of the measurements made during the experiments was ascertained through the uncertainty analysis performed. The maximum electrical efficiencies of the PVT system for SiO2/ Water, Al2O3/Water, and CuO/Water nanofluids were found to be 20.69 %, 21.18 %, and 20.77 %, and the maximum thermal efficiencies to be 57.06 %, 63.01 %, and 66.49 %, respectively. As the optimization method for determining the most optimum combination of variables, Taguchi analysis has been employed. As a result of this analysis, it has been shown that the most effective variables on the electrical efficiency of the PVT system are the irradiance, flow rate, volumetric concentration, and nanofluid type, respectively. On the other hand, regarding the thermal efficiency of the PVT system, the most effective variables are the irradiance, flow rate, nanofluid type, and volumetric concentration, respectively.Öğe Optimization of external wall insulation thickness in buildings using response surface methodology(Springer, 2022) Özbek, Kadir; Geliş, Kadir; Özyurt, ÖmerBuildings account for one-third of the world's energy consumption. Reducing this consumption is only possible by making buildings more energy efficient. One of the most efficient methods for increasing the energy efficiency of buildings is thermal insulation. Fuel consumption and therefore emission values can be reduced by achieving adequate thermal insulation of buildings. In this study, the optimum insulation thicknesses for cities in the various climatic regions of Turkey were determined using statistical methods. Insulation thickness, thermal conductivity, heating degree-days (HDD), and fuel type were determined as variable parameters, and the optimum insulation thickness and total heating cost for cities in four different climate zones were determined using the response surface method (RSM). The effect ratios for each parameter on total costs were also reviewed and analyzed using the RSM method. Mathematical models have been developed that estimate the total cost of natural gas, coal, and fuel oil based on thermal insulation thickness, thermal conductivity, and heating degree days. With the mathematical models presented in the study, dependent parameters (total heating cost) can be obtained as a function of independent parameters (fuel type, thermal conductivity of insulation material, and HDD). The models provide a calculation of direct costs for different types of fuels and provide a basis for various research. As a result, the optimum insulation thicknesses for Izmir (HDD: 1781), Istanbul (HDD: 2531), Ankara (HDD: 3303), and Erzurum (HDD: 5393) are 0.059 m, 0.066 m, 0.075 m, and 0.080 m, respectively; reductions in annual total costs were found to be 40.7%, 39.7%, 41.9%, and 50.1%, respectively.Öğe Experimental and numerical analysis of vibration-based hybrid energy harvesting from non-classical beam structures(Elsevier Science Inc, 2022) Bolat, Fevzi ÇakmakIt is known that the displacement and stress on beams will directly affect the energy harvesting performance. For this purpose, structural changes are made on the beam elements. Behaviours of these beam structures are investigated by designing different non-classical beam geometries. This study examines vibration-based electromagnetic and piezoelectric energy harvesting from the non-classical beam structures under various conditions. Before analysing the non-classical beam structure, studies are performed for a classical beam for electromagnetic and piezoelectric energy harvesting studies. In the electromagnetic energy harvesting analysis, the effects of magnitude of magnetic field and the distance between the coil and the permanent magnet on the harvesting performance are studied by free vibration experiments. In this regard, a neodymium permanent magnet is attached to the vibrating aluminium beam endpoint. During the free vibration period, the coil and the magnet interact closely. As a result, a voltage output is obtained by inducing an electric field on the coil side. Secondly, voltage outputs of piezoelectric energy harvesting structures are experimentally evaluated for a classical beam and used for the verification of numerical simulations. Using the verified numerical simulations, new types of beam structures so-called non-classical beams are designed to obtain higher voltage outputs. Finally, by introducing the phenomenon of continuous vibration with the effect of air, the hybrid energy performance is analyzed for the proposed structure. The continuous vibration is created by the air-flow on the square galloping geometry.
