Browsing by Author "Kiyak, B"

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    Analysis of solidification of phase change material flowing through a channel with backward step: Effects of step curvature
    Öztop, HF; Kiyak, B; Biswas, N; Selimefendigil, F; Cosanay, H
    Thermal energy storage (TES) allows to the conservation of energy, enhancing the overall system efficiency and balancing the supply and demand of energy. This article presents a computational analysis of the solidification process of melted paraffin wax phase change material (PCM) through a partially heated backward-facing step channel partially cooled from the bottom and top. Melted PCM enters through the left opening of the step channel and leaves through the right opening. The study examines two distinct cases: considering sharp stepped or angular corner, and stepped or streamlined corner (of radius r = 20 mm) with the same inlet flow and boundary conditions. A comprehensive numerical model is developed and solved using the finite volume-based computational approach. The overall thermal performance of the model during the solidification process is evaluated for the different Reynolds numbers (Re = 20 and 40) and temperature differences (Delta T = 20 and 25 degrees C) for the different time steps. Furthermore, the effect of backward step curvature on the solidification process is also analyzed. The results revealed that all the parameters (Re, Delta T, and curvature) affect the melted PCM flow structure as well as the solidification process inside the channel. Changing the sharp step corner into a streamlined one at the bottom of the channel led to the lowest lesser flow separation and faster solidification process. Therefore, a well-designed streamlined step corner and its shape could be used to increase the discharging speed of thermal energy storage units meaningfully. Furthermore, such designs, the placement of the heater and cooler location, and step curvature are the guiding factors for controlling the performance of the energy storage. With the change in the curvature, the energy efficiency can be increased and the solidification time is lowered by at least 5 %.
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    Effects of geometrical configurations on melting and solidification processes in phase change materials
    Kiyak, B; Oztop, HF; Biswas, N; Selimefendigil, F
    Phase-change materials (PCMs) are recognized for their effective role in thermal energy storage systems, offering the potential to balance fluctuations between energy supply and demand by absorbing, storing, and releasing heat during phase transitions. The performance of such systems, however, is heavily influenced by the melting and solidification behaviors of the PCM, which depend on the system's geometric configuration and heat source placement. In this research, a comprehensive numerical analysis is conducted to examine the thermal behavior of a PCM-filled energy storage unit, with particular focus on the effects of heater and cooler positioning (bottom, side, and top) and the influence of two distinct geometrical shapes (square and circular). A finite volume-based computational technique is employed to solve the governing equations, and the numerical outcomes are validated against experimental data from in-house tests. The investigation covers both the melting and solidification stages for two geometries under different Grashof numbers (Gr), which quantify the strength of natural convection within the system. The results reveal that heating from the side offers the quickest melting performance, especially for the circular configuration at lower Gr values, where a melting rate 240% higher than bottom heating is observed. On the other hand, top heating proves to be the least efficient. For solidification, the best performance is obtained using a square geometry with a top cooling configuration, resulting in a solidification rate 15.4% faster than the bottom cooling scenario. This study highlights the critical role of geometric shape and heater/cooler positioning in enhancing the thermal response of PCM systems. The findings suggest that side heating combined with circular geometries is optimal for accelerating the melting process, while square geometries with top cooling enhance the solidification rate. Future research areas include exploring more complex geometries, varied boundary conditions, and different PCM materials with distinct thermal characteristics.
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    Effects of fin shapes and orientations with cyclic heating and cooling on melting and solidification of PCM-filled closed space
    Kiyak, B; Öztop, HF; Biswas, N; Cosanay, H; Selimefendigil, F
    Phase-change materials (PCMs) offer an effective way to store and release thermal energy to balance the supply and demand for energy. Both the melting and solidification processes have a major impact on how effectively energy storage works and also it is affected by the thermal conditions of the heating or cooling source. Thermal energy storage systems using (PCMs are often limited by slow melting and solidification rates. The current work explores a novel strategy of cyclic heating and cooling for improving the PCM melting and solidification process combined with variations in fin shapes and orientations, to address these inefficiencies. The fins are heated and cooled following cyclic heating and cooling pattern for three different cycle periods (CP) with same amplitude. As a result, PCM is subjected to cyclic heating and cooling. The finite volume method is employed to analyze the impact of cyclic heating-cooling cycles on PCM performance. An analysis is also conducted on the impact of the relative shape of fins-that is, flat, concave, and convex, positions-vertical and horizontal-on the melting and solidification process under three different cycle periods. By applying a finite volume-based computational approach, the numerical model is solved. It is observed that the overall thermal performance of PCM-based energy storage is modulated by the cyclic heating-cooling arrangements. With this, melting time is reduced by 47.1 % compared to horizontal fin arrangement. When the fin pair is arranged vertically (theta = 0 degrees), with the increase in the cycle period to CP3, the amount of stored energy (during the heating cycle) is about 24.7 %. Similarly, the amount of stored energy recovery (during the cooling cycle) is about 43.6 %. When the fin pair is arranged horizontally (theta = 90 degrees), the amount of energy stored is up to 10 % due to the increase in the cycle periods. Similarly, the amount of stored energy recovery (during the cooling cycle) is about 38.5 %. An improved fin designs, combined with cyclic heating-cooling strategies, present an effective solution to enhance PCM-based thermal energy storage systems.
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    Geometry curvature influence on melting and solidification performance in nano particle added phase change material to storage energy
    Kiyak, B; Biswas, N; Öztop, HF; Selimefendigil, F
    This research explores the combined impact of geometric curvature and nanoparticle-enhanced phase change materials (PCMs) on the performance of thermal energy storage (TES) systems. Two geometrical configurations-curved and straight-are analysed under nine distinct heating and cooling setups, using paraffin-based PCM (RT18HC) mixed with copper (Cu) nanoparticles at concentrations ranging from 0 % to 4 %. The finite volume-based numerical simulations technique utilized an enthalpy-porosity method to investigate the melting and solidification behavior during a 450 min thermal cycle. The results quantitatively demonstrate that curved geometries significantly improve natural convection, enhancing the heat transfer during melting and solidification process. For example, curved geometries, combined with 4 % Cu nanoparticle-enhanced PCM, significantly improved heat transfer, reducing phase change times by 35 % and 47 % compared to straight configurations. Additionally, the Curved Right design achieved up to a 50 % improvement in energy discharge efficiency. The enhanced natural convection within the curved structures reduces melting times by up to 46.2 %, outperforming the conduction-driven heat transfer in straight geometries. The reduced thermal gradients and uniform phase changes process allows rapid thermal cycling. In contrast, pure PCM without nanoparticles shows slower melting and prolonged solidification rate, which highlights its thermal limitations. The present study reveals that the combination of curved geometries with nanoparticle-enhanced PCMs significantly improves the efficiency of TES by accelerating the phase transitions through enhanced free convection and thermal conductivity. These understandings contribute to the optimization of energy storage designs for industrial and renewable energy-based applications.
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    Effects of cooler shape and position on solidification of phase change material in a cavity
    Öztop, HF; Kiyak, B; Biswas, N; Selimefendigil, F; Cosanay, H
    Background: For balancing the imbalance between the energy supply and demand, phase-change materials (PCMs) provide an efficient means in terms of thermal energy storage and release. The performance of the energy storage is primarily dependent on the melting as well as the solidification process of the storage medium. Faster charging or discharging of the thermal energy is a primary concern for any thermal energy storage unit. On this background, the present study explores the novel approach for enhancing the solidification process of PCM considering the effects of cooler shape (namely semi-circular, triangular, and rectangular) and their position (namely top, side, and bottom) in a molten PCM-filled enclosure. The middle portion of the cooler wall is curved; whereas the remaining cooler wall is straight maintaining the same cooler wall length. Methods: To analyze the solidification process, the involved transport equations are solved numerically following a finite volume-based computational approach using Ansys Fluent solver in conjunction with the appropriate boundary conditions. The computational model is generated for all the geometry comprising different shapes, as well as positions of the cooler wall. The third-order upwind scheme (QUICK) technique is utilized to discretize the momentum and energy equations. This scheme is well capable to accurately capture the gradients in the temperature and flow domains. Furthermore, the semi-implicit pressure-linked equation (SIMPLE) technique is utilised to address the pressure-velocity coupling. The resolved data are then saved as selective variables (U, V, and theta), which undergo post-processing to produce a local thermo-fluid flow field and extract average data. Significant findings: The shape, as well as the position of a cooler, dictates the solidification process in an energy storage system. Thermal energy storage with a triangular-shaped cold wall positioned at the top could be opted as an appropriate design approach of an efficient energy storage system compared to a semi-circular or rectangular-shaped cooler model. The shortest solidification time of PCM occurs when the cooler wall is positioned at the top. The top position of the cooler having a triangular shape with higher Grashof number (Gr) values leads to a faster solidification process. Some ideas for possible future research areas in this field are provided after a comprehensive examination.