Defect and Diffusion Forum Vol. 453

Abstract: This study investigates the effect of incorporating Phase Change Material (PCM) into the collector of a Solar Chimney Power Plant (SCPP). Numerical simulations were conducted using a 2D axisymmetric geometry, considering transient, turbulent, and radiative heat transfer. The results show that the PCM-enhanced system maintains higher temperatures at the collector outlet, with an average increase of approximately 6%. This thermal regulation enhances buoyancy-driven airflow, raising the chimney base velocity from 2.5 m/s in the conventional system to 3 m/s in the PCM configuration, with a corresponding increase in mass flow rate of 0.013 kg/s. Furthermore, the collector efficiency improves significantly, reaching 23% with PCM compared to 11% without. These findings demonstrate that integrating PCM into the collector effectively boosts thermal energy retention and system performance.

Abstract: This study presents a computational investigation of melting enhancement in a triplex tube heat exchanger TTHX with rectangular fins considering RT-82 as the phase change material (PCM) and copper as nanoenhanced PCM (NEPCM). The number of fins was increased from 8 to 12, and copper nanoparticles were dispersed at a volume fraction of 2% and 3% to assess their effect on thermal energy storage TES. The evolution of the solid–liquid interface was simulated using the enthalpy–porosity formulation, and the system performance was evaluated in terms of liquid fraction, total melting time, and melting efficiency, defined as the ratio of latent heat absorbed to the input energy. Raising the number of fins from 8 to 12 reduced melting time by 30% relative to the reference case with eight fins only, indicating enhanced heat conduction and faster initial melting. The use of copper nanopcm with 3 vol% further cuts melting times by 55% compared with the same reference case due to the increase in effective thermal conductivity. Hence, in brief, moderate fin numbers coupled with 3 vol% copper rendered the fastest melting rates and highest storage efficiencies among those tested.

Abstract: Thermal management of solar panels is a major challenge, particularly in hot climates, and has attracted growing interest from both researchers and professionals in the photovoltaic (PV) sector. In this study, a two-dimensional numerical analysis was conducted to evaluate the thermal performance of an aluminum box filled with a phase change material (PCM) used for cooling a solar panel. The impact of integrating horizontal fins placed either on one side or on both sides of the PV/PCM system, was also examined to optimize heat transfer and stabilize temperature. The simulations indicate that a configuration with five fins arranged on one side, spaced 18.25 mm apart, provides effective cooling. The use of RT25 PCM maintains the surface temperature at 28 °C for 100 minutes before it gradually increases without exceeding 65 °C. Similarly, RT35 also demonstrates good thermal performance, maintaining the temperature between 35 and 43 °C for 252 minutes before eventually reaching a peak of 61 °C. These results highlight the effectiveness of the fins and the critical role of PCM selection in enhancing the thermal regulation of solar panels.

Abstract: The increasing need for efficient and sustainable thermal systems has driven the exploration of non-Newtonian models such as third-grade fluids, which capture the nonlinear, memory-dependent behavior of many real-world industrial and biological fluids. This study aims to analyze the thermal and flow behavior of a third-grade fluid under the combined influences of mixed convection (through thermal and concentration Grashof numbers), Arrhenius-type chemical reactions, solar radiation, and a transverse magnetic field, while also considering slip boundary effects and a porous channel structure. The model accounts for buoyancy-driven forces coupled with externally imposed flow, as well as temperature-sensitive reaction rates governed by the Arrhenius relation, making it highly relevant to solar energy systems, catalytic reactors, and advanced heat exchangers. The resulting non-linear partial differential equations (PDEs) describing the flow are reduced to ordinary differential equations (ODEs) using suitable similarity transformations. These ODEs, along with their boundary conditions, are numerically solved via the Galerkin Weighted Residual Method (GWRM) implemented in MATHEMATICA 11.3. The findings show that mixed convection significantly enhances heat and mass transfer, while Arrhenius chemical reactions alter the temperature and concentration fields, influencing flow stability. Moreover, strong magnetic fields and thermal radiation improve energy transport, offering key insights into optimizing third-grade fluid-based thermal systems for solar and industrial applications. Keywords: Third-grade fluids, Hybrid nanofluids, Mixed convection, Solar radiation, Solar Radiation, Magneto-hydrodynamics. Nomenclature and abbreviations list

Abstract: Transient laminar mixed convection of air (Pr = 0.71) in a vertical plane channel is numerically investigated for opposing buoyancy at Re=100. The channel walls include symmetric, discrete, isothermal heated sections applied on the external side of a central wall zone, resulting in a conjugate heat-transfer problem through the solid wall and the fluid domain. The two-dimensional unsteady Navier–Stokes and energy equations are solved using a finite-volume method on a staggered grid with SIMPLE pressure–velocity coupling. The Grashof number is varied over 10³ ≤ Gr ≤ 10⁵. Instability onset is identified using a quantitative criterion based on the growth and persistence of velocity/temperature fluctuations and a domain-integrated fluctuation energy. The effects of wall thickness Δ and heated length L_h on the critical Grashof number are reported. Increasing L_h lowers the instability threshold, whereas increasing Δ stabilizes the flow by damping thermal gradients transmitted to the fluid. The results provide a stability map and physical interpretation of the transition from steady to unsteady mixed convection at low Reynolds number.