Sürdürülebilir Kalkınma Amaçları

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• MakaleAçık Erişim2026Scopus SJRQ2/r/munzur/1247
  Examination of the thermal and metallographic characteristics at high temperatures of natural stones (andesite) extracted from different elevations

  Işık, Erdem; Kahraman, Gökhan; Atalar, Aydın

  This study examines the high‑temperature thermal response of andesite stones extracted from two different elevations of Mount Ağrı (Türkiye) in order to improve material selection strategies for engineering applications. Thermal conductivity and density values of the samples were first measured under laboratory conditions. The specimens were then heated to 850 °C and cooled by natural convection while surface temperatures were recorded at fixed time intervals using a thermal camera. Based on these measurements, convective heat transfer coefficients were determined, and the time‑dependent thermal diffusivity and Biot numbers were subsequently calculated using the first-term approximation of the Fourier series. Metallographic examinations were conducted to correlate the thermal behavior with microstructural characteristics. Results indicate a pronounced effect of elevation on thermal performance. The sample obtained from the higher elevation exhibited substantially greater thermal diffusivity, reaching 2.93 × 10⁻5 m2 s−1 at 360 s and decreasing to 8.96 × 10⁻6 m2 s−1 at 1300 s. In contrast, the lower‑elevation andesite showed lower diffusivity values (1.83 × 10⁻5 m2 s−1 at 360 s and 5.9 × 10⁻6 m2 s−1 at 1300 s). Conversely, Biot numbers were consistently higher for the lower‑elevation sample, decreasing from 0.60 to 0.29 during the cooling process, whereas the high‑elevation sample displayed values from 0.314 to 0.165. Metallographic analysis confirmed that variations in microstructure—linked to different geological formation conditions—directly influenced these thermophysical properties. Overall, the findings demonstrate that andesite sourced at higher elevations possesses superior thermal diffusivity and more favorable heat‑transfer characteristics at elevated temperatures. These results highlight the importance of geological origin in the design, assessment, and selection of natural stone materials for high‑temperature engineering applications.

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• MakaleAçık Erişim2026Scopus SJRQ2/r/munzur/691
  Production and Thermal–Structural Characterization of Nano-Oxide-Doped Paraffin-Based Phase Change Composite Materials for Thermal Storage Applications

  Yücel, Halit Lutfi; Özdemir, Osman; Kahraman, Gökhan

  Paraffin-based composite phase change materials (PCMs) have been synthesized for thermal energy storage applications to enhance thermal conductivity and stability while maintaining latent heat storage capacity. In this study, RT42 paraffin was modified with inorganic nano-oxides (Al2O3, CuO, SiO2, and ZnO) in the presence of sodium dodecyl sulfate (SDS) as a surfactant. The incorporation of SDS improved nanoparticle dispersion and reduced agglomeration, resulting in enhanced phase-change behavior and improved structural homogeneity. Differential scanning calorimetry (DSC) analysis revealed slight variations in melting and crystallization temperatures, while the ZnO-doped composite retained up to 92% of the latent heat storage capacity of pure RT42. Among all the investigated composites, the Al2O3-doped sample exhibited the highest thermal conductivity and specific heat capacity. Thermogravimetric analysis (TGA) indicated an increase of approximately 15–20°C in the decomposition temperature, demonstrating enhanced thermal stability. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses confirmed the preservation of the crystalline structure and improved microstructural uniformity. The developed SDS-assisted nano-oxide/RT42 composites exhibit significant potential for passive thermal regulation in renewable energy and building applications.

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• MakaleYalnız künye2026Scopus SJRQ2/r/munzur/623
  Experimental ınvestigation of hybrid passive cooling strategies for photovoltaic modules using nano-enhanced PCM and graphene-coated finned structures under hot climate conditions

  Özdemir, Osman; Yücel, Halit Lutfi; Kahraman, Gökhan

  This study experimentally investigates the use of several passive cooling techniques to enhance the electrical performance of photovoltaic (PV) modules under high ambient summer temperatures in Elazığ, Turkey. Throughout the trial period of July, August, and September, a system comprising four photovoltaic panels, one designated as a reference, was developed. The first passive cooling arrangement utilized a paraffin-based phase change material (PCM) RT42HC. In the second configuration, a nano-enhanced PCM (NePCM) composite was employed, prepared by dispersing 1 mass% of Al2O3 nanoparticles into the paraffin matrix and stabilizing them with 3 mass% of sodium dodecyl sulfate (SDS) to ensure homogeneous distribution. The third, hybrid configuration integrated 14 graphene-coated parabolic pin-tipped fins mounted externally onto the aluminum enclosure housing the NePCM composite. Experimental results demonstrated that the implemented passive cooling techniques markedly decreased the panel surface temperature, thereby enhancing electrical power output and photovoltaic efficiency. The hybrid graphene-coated fin-NePCM system demonstrated superior performance, achieving a maximum surface temperature reduction of 12.65 °C, a sustained electrical efficiency increase of 6–13%, and an average daily electrical energy gain of 26.23 Wh day−1 compared to the reference module. These findings demonstrate that the integrated hybrid design is an effective and reliable passive cooling method for sustaining photovoltaic system performance under hot climate conditions.

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• MakaleAçık Erişim2025/r/munzur/6136
  Hidroelektrik Santrallerde Türbin-Jeneratör Yatak Arızaları Sonucu Oluşan Mekanik Titreşimler ve Çözüm Yöntemleri

  Kahraman, Gökhan

  Günümüzde imal edilmekte olan hidrolik türbinlerin güçlerinin ve boyutlarının çok artmış olması ciddi bir mekanik titreşim ve şaft salınımı problemini de beraberinde getirmektedir. Bilinmektedir ki mekanik titreşime (vibrasyon olayı) maruz kalan bir makine devamlı olarak çekme, basma, eğme, burma, kesme gibi değişken mekanik baskılarla sürekli olarak zorlanmaktadır. Hidrolik türbinlerin bu tür titreşim ve salınımlara maruz kalmaları işletme güvenliğini ve işletmede kalma süresini önemli derecede azaltmaktadır. Bu çalışmada, bir hidroelektrik santrale ait Francis tipi tür-binde yatak boşluğunun müsaade edilen sınırları aşması neticesinde meydana gelen, mekanik titreşimler (vibrasyon olayı) ve şaft salınımları arızası incelenmiştir. Tür-bin – Jeneratör yataklarına tekrar ayar yapıldıktan sonra oluşan şaft salınımları, arıza durumunda oluşan şaft salınımları ile kıyaslanmıştır. Sonuç olarak arıza sonrası hidrolik türbin yataklarının ölçüleri kabul edilen standartlara getirildikten sonra mekanik titreşimler ve şaft salınımları normal seviyelere gelmiştir. Mekanik ti-tre-şimden kaynaklı arızalar ve aşırı şaft salınımı sonucu oluşacak yatak sürtmesi gibi büyük arızalar engellenmiştir.

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• MakaleAçık Erişim2025Scopus SJRQ2/r/munzur/1144
  Investigation of the effect of the metallographic properties of vertical and horizontal cylinders made of ferrochrome with different contents on heat transfer by natural convection

  Kahraman, Gökhan; Taşgın, Yahya

  In this study, ferrochrome materials containing elements with different content ratios were manufactured into six-cylinder specimens, 13 mm in diameter and 16 mm in length, and heated in a furnace up to 500 °C. The heated cylinders were removed and left to cool in vertical and horizontal positions in a 16 °C environment. The temperatures were recorded using thermal cameras at certain intervals while the materials cooled down. The six specimens, whose metallographic differences were identified using SEM and XRD analysis devices, were placed vertically and horizontally, and their heat transfer by natural conviction was noted. In the calculations, each specimen’s Nusselt and Rayleigh numbers were identified based on time and then compared. The fastest cooling among the specimens occurred in the ferrochromium material with 55.93% Cr, 31.11% Fe, 11.56% C, 0.48% Si, 0.83% O, and 0.09% Al compound ratios, both in vertical and horizontal positions. Additionally, since the Nusselt number of the horizontal cylinders remained below one based on their position, their natural convection flows were not initiated. The cooling in the horizontal cylinders occurred via conduction.

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• MakaleYalnız künye2025Scopus SJRQ2/r/munzur/956
  Heat transfer and condensation analysis of the walls of a house built with natural andesite stone

  Işik, Erdem; Kahraman, Gökhan; Atalar, Aydın

  The use of natural stones in buildings is increasingly popular due to both their decorative appearance and their thermal performance. Proper use of these stones in buildings is crucial for energy conservation and preventing condensation within structural components. This study investigated heat transfer and condensation in the wall of a house made from natural andesite stone in Iğdır, Turkey, under steady-state conditions and different wall combinations. Samples were created from natural stones extracted from two different elevations in the foothills of Mount Ararat, with measured thermal conductivity values of 1.73 W/m·K (higher elevation– Haverage) and 1.31 W/m·K (lower elevation– Laverage). In the wall model using a 3-cm thick insulation board between two natural stones from the higher elevation, condensation occurred on the outer surface of the insulation layer. This can negatively impact the durability and thermal performance of the structure, and such design should be avoided. The highest heat flux of 18.27 W/m² was observed in the non-insulated wall made of higher elevation stones, while the lowest heat flux of 6.68 W/m² occurred in the model using a 3-cm insulation board between two lower elevation stones. No condensation risk was found in this configuration. The difference between the worst and best cases in terms of heat flux was approximately threefold, emphasizing the importance of proper material selection and insulation design. These findings provide guidance for energy-efficient building design using natural stone.

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• MakaleYalnız künye2024Scopus SJRQ1/r/munzur/2217
  Modeling of Optimal Power Generation in Small Hydropower Plants

  Kahraman, Gokhan

  Small hydropower plants (SHPs) have gained international attention as a reliable and versatile renewable energy source. Unfortunately, this energy source is not used efficiently. This is because these systems utilize headwaters with lower water capacity called forebays instead of large water reservoirs like dams. Since this hinders the control of electricity generation, the headwater is either wasted before it can be converted into electricity or it is exhausted very quickly and electricity generation stops after a short time. The aim of this study is to prevent the loss of electricity generation through the efficient use of SHP headwater. For this purpose, a mathematical model was created based on two SHPs operating in tandem. The mathematical model obtained was analyzed, and forebay water level simulation graphs for electricity generation at water flowrates of 2000, 4000, 6000, and 8000 kg/h were obtained. With the help of the obtained graphs, critical wicket gate openings for maximum electricity generation at flowrates of 2000, 4000, 6000, and 8000 kg/h were calculated. The results of this study are applicable to all SHPs. These results are also promising in terms of optimizing the use of a renewable energy source.

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• MakaleYalnız künye2024Scopus SJRQ2/r/munzur/2041
  Dynamic Analysis and Metallographic Examination of Linings with Different Composite Contents used in the Brakes of a Hydraulic Turbine

  Işık, Erdem; Kahraman, Gökhan

  The main aim of this study was to find ways to increase the performance and life of generator brake linings, which can lead to cost reduction and adequate downtime periods for hydroelectric power plants. First, the dynamic analysis of the linings used in hydraulic turbines was realized and their reactions were examined under different operating conditions. Second, two types of brake linings with different composite components produced in the market were examined metallographically and tested to determine both the stopping performance and durability time of the turbine-generator unit for a sufficient period of time in the vertical 150-megawatt Francis turbine with approximately 500 tons of rotary weight. As a result of these experiments, the lining with the boron component led to much better results than the other brake linings. It has been determined that the boron-containing lining makes 100 stops more than the other lining and stops 20 s earlier in one stop. Therefore, the use of a composite lining with boron content in the braking systems of hydroelectric power plants significantly reduces maintenance incidents and operation costs as it prolongs brake replacement periods in these plants. Graphical Abstract: (Figure presented.)

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• MakaleYalnız künye2024Scopus SJRQ4/r/munzur/2030
  How How Much Much CCondensation ondensation Occurs Occurs With With Various Various Insulation Insulation Thicknesses? Thicknesses?

  Kahraman, Gökhan; Isik, Erdem

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• MakaleYalnız künye2024Scopus SJRQ2/r/munzur/1826
  Evaluating the metallographic properties and performance of heat transfer by natural convection of graphene-coated glass used for heating

  Kahraman, Gökhan; Taşgın, Yahya

  In this study, a decorative electric heater was manufactured to reduce carbon emissions related to residential and office heating, and this heater was metallographically evaluated. In addition, the heating performance of the manufactured plate was investigated by examining its heat transfer capacity by natural convection. For this purpose, the surface of the tempered glass plate was coated with graphene by spraying to improve its electrical and heat conduction properties. Copper strips were attached to both sides of the plate to allow for an electric current. The plate was connected to mains electricity, and its change in surface temperature over time was measured. Using the measured temperature values, the heat transfer rate by natural convection on the plate surface was calculated. As a result of the calculations, it was determined that the graphene-coated tempered glass plate reached a heating performance of 40 watts after 2400 s of electrical energy supply. This value can be increased by increasing the heat transfer surface area. This study, taking advantage of the excellent thermophysical properties of graphene, will contribute to other studies conducted in this field.

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