Defect and Diffusion Forum Vol. 453

Abstract: In this work we investigate the effect of chromium on the self‑diffusion of Fe and Cr in binary bcc FeCr alloys using ab initio (density‑functional) calculations. While diffusion data for FeCr systems exist, most are limited to selected compositions or rely on semi-empirical parameterisations, and a consistent ab initio assessment of vacancy-mediated Fe and Cr transport across the dilute to intermediate Cr range is still lacking. We also analyse the influence of chromium concentration on the lattice parameter of these alloys. The lattice parameter shows a clearly non‑linear dependence on Cr content: it increases steeply with chromium addition up to about 8 at.% and then remains almost constant up to the highest considered concentration of 25 at.%. The vacancy formation energy on Cr sites decreases markedly from 2.27 eV to 1.95 eV as the chromium content is raised from 7.4 at.% to 26 at.%. In contrast, the vacancy formation energy on Fe sites exhibits only a small increase between 7.4 at.% and 11 at.% Cr and then remains nearly unchanged up to 26 at.% Cr. These results indicate that chromium additions strongly facilitate the formation of vacancies on Cr sites and therefore are expected to enhance Cr self‑diffusion, whereas the self‑diffusion of Fe is only weakly sensitive to composition in the studied range. Based on the calculated activation energies, Arrhenius‑type temperature dependences were constructed for the Fe and Cr diffusion coefficients in five model alloys: Fe-7.41 at. % Cr, Fe-11.11 at. % Cr, Fe-18.52 at. % Cr and Fe-25.93 at. % Cr. The resulting diffusion coefficients are in good agreement with available experimental and theoretical data, which supports the reliability of the present ab initio-based approach for describing vacancy‑mediated transport in α‑FeCr alloys. Combining the calculated vacancy formation energies with literature data on vacancy migration energies allows us to construct Arrhenius‑type estimates for Fe and Cr self‑diffusion coefficients in α‑FeCr alloys, which are relevant for modelling microstructural evolution in ferritic steels.

Abstract: Composite material systems involving impinging flame jets on flat surfaces are critical in applications requiring precise thermal management and energy efficiency. A prominent example is Teflon-coated steel cookware, where the Teflon layer not only provides a nonstick surface but also influences thermal performance. Optimal coating thickness is essential: excessive thickness can reduce heat transfer efficiency, while insufficient thickness may compromise adhesion and durability. This study investigates the thermal interaction between a premixed flame jet and a Teflon-coated steel substrate using high-fidelity simulations in OpenFOAM. A conjugate heat transfer approach captured the coupled heat fluxes between the flame, steel substrate, and Teflon layer. Teflon thicknesses ranging from 0.01 to 0.20 mm were systematically analyzed to evaluate their effect on heat transfer performance. Simulation results enabled the development of a thermal efficiency model as a function of Teflon thickness, achieving a high correlation (R² = 0.9923). The proposed model offers quantitative guidance for optimizing coating thickness, providing a practical tool for the design and manufacturing of thermally efficient cookware.

Abstract: Rhodium plating is currently in appreciable demand not only for automotive as in catalyst converter but also for jewelry industry due to its silvery-white appearance, allergy-friendly, durable, scratch and tarnish-resistant. In accordance with circular economy, to assure efficient reuse and deviating from primary resource reliance, rhodium recovery is therefore of significance as the secondary resources. This research investigated comparative study of rhodium recovery from the plating solution via three techniques: cementation, chemical precipitation and electrowinning. The fresh plating solution prepared from rhodium concentrate and additive solutions was diluted to 0.1, 0.2 and 0.4 g/L of rhodium and used as the equivalent spent plating solutions. It was found that both cementation using zinc powder and chemical precipitation using sodium hydroxide and ammonium hydroxide did not yield notable recovery and gave low purity recovered products. Electrowinning has shown be more effective among the three techniques. For electrowinning, the diluted solutions were used as electrolyte, while the current density was controlled at 0.08-0.14 A/cm² for 2-24 h. Rhodium could be obtained at the cathode, giving the average rhodium content of 94.02 wt.% and the purity of greater than 98%. At the greater rhodium concentration of 0.4 g/L, deposition of rhodium metal was uniform appearing as clusters of particles on the cathode surface where nucleation and growth are competitive.

Abstract: Surface roughness plays a major factor in energy conversion, which impacts both heat transfer and energy losses. The word “Roughness” is defined due to variations in height of the surface arising from geometry or waviness, which directly impacts the thermal–hydraulic performance of heat exchangers. In this research, the study aims to determine the impact of surface roughness on heat transfer characteristics in a double-pipe heat exchanger. The numerical simulation was conducted using ANSYS CFD Fluent for one smooth surface and four rough surfaces with sand grain roughness values ranging from 0.5 mm to 2 mm, applied to the inner pipe wall boundary. The result proved that increasing the roughness surface will have an effect in the heat transfer coefficient and Nusselt number, which was calculated mathematically for the data derived from the outlet temperature of the Numerical Simulation. In addition to these, higher roughness also has a major effect on pressure drop and heat loss within the system. This study demonstrates that controlled application of surface roughness can significantly improve the thermal performance of heat exchangers, providing both economic and ecological benefits for industrial applications.

Abstract: The movement of gases and liquids in a minichannel with a T-junction during two-phase flow is also important in fields like medicine, chemistry, and thermal management. The dynamics of the influence of the bend radius on the downstream pressure difference are not well comprehended. This paper examines the statistical characterisation of the pressure drop downstream of a horizontal T-junction in a minichannel, considering the observed flow patterns. The geometric parameters of the T-junction involved are variations of the ratio of the bend radius to the hydraulic diameter (r/D_h = 0.5, 0.7 and 1.0). Air and water are used as test fluids, with superficial velocities ranging from 0.59 to 2.96 m.s^-1 for air (J_g) and 0.63 to 3.19 m.s^-1 for water (J_l). The pressure sensors are used to measure pressure drop signals (ΔP_2-3), which are recorded by a data collection device at 1000 Hz. A high-speed camera is also used to record the flow and verify the flow regime. There are six downstream flow regimes identified, and they include: Bubbles, Bubble to Slug, Slug, Elongated Slug, Churn to Elongated Slug, and Churn. These flow patterns are characterised using statistical, spectral, and nonlinear analysis methods. The findings suggest that as the bend radius increases, the amplitude of fluctuations also increases and the probability distribution becomes wider. However, there is a possibility that larger bend radii decrease chaos levels, resulting in a characteristic regime pattern. Additionally, there are artificial neural networks (ANN) that utilise wavelet energy variance as input, achieving a classification accuracy of 85.5%. The ordered association between statistical characterisation and regime classification through ANN is useful in comprehending the impact of the instability due to the bend radius in multiphase flow. These results complement the basic knowledge and predictive modelling of pressure drops in minichannels with horizontal T-junctions.

Abstract: A hydraulic conveying system utilizing a jet pump is commonly employed for the transportation of coal rejects from the PCCB (pulverized coal combustion boiler) unit. This study applies CFD (computational fluid dynamics) combined with the DEM (discrete element method) to simulate solid-liquid two-phase flow in the jet pump. The coal reject particles were modeled as perfect spheres with a diameter 55 mm a mass flow rate of 5 kg/s. The hydraulic jet pump was supplied with water at a pressure of 12 bar and a flow velocity of 3 m/s. The diameter of nozzle was varied at 26-, 36-, 46-, 56-, and 66 mm to evaluated the effects on particle velocity, residue buildup, and the flow characteristics. The simulation results indicate that increasing the nozzle diameter leads to lower particle velocities and decreases the amount of coal reject residue in the jet pump. The nozzle diameter also influences to the multiphase flow behaviors in the outlet pipe.

Abstract: Natural circulation is a passive cooling mechanism that is attractive for application in thermal engineering systems and advanced nuclear reactors because it is simple, reliable, and does not require pumps or external power sources. However, detailed studies on the thermal characteristics and energy balance of medium-scale experimental facilities remain limited, even though such investigations are essential to validate system performance and support the development of numerical models. This study aims to analyze the performance of the FASSIP-06 Ver.3 facility, which is configured as a rectangular loop with a height of 3.4 m and a width of 0.85 m, operated under single-phase conditions with heater power variations ranging from 750 to 1550 W. Experimental results show that increasing heater power leads to a rise in fluid temperature along the loop until a quasi steady-state condition is achieved. The temperature distribution demonstrates a clear gradient between the hot leg and cold leg, with the highest temperature observed at the heater outlet and the largest drop at the cooler inlet. Heat loss analysis indicates that the major contributions occur at the cooler, the BRT, and the visualization window, while other sections are relatively well insulated. Energy balance evaluation shows that the difference between heater input power and total measured heat loss consistently remains below ±10%, indicating that the system achieves thermal equilibrium.

Abstract: Increasing demand for reliable and passive thermal management in modern energy systems, particularly in nuclear reactors, has elevated interest in natural circulation loops. Among the influencing factors, loop geometry and heating power are critical in natural circulation systems. This study investigates the effect of heating power on heat transfer in a rectangular VHVC natural circulation loop with an enlarged upper elbow radius of 350 mm. The analysis was conducted using CFD under steady-state conditions, employing a pressure-based solver with the realizable k-epsilon turbulence model and energy equation to simulate buoyancy-driven flow. Three power inputs 750 W, 1100 W, and 1540 W were applied to evaluate their effect on temperature distribution and energy absorption. Results show that increasing the heating power enhances buoyancy forces, leading to higher mass flow rate and stronger natural circulation within the loop. The fluid temperature difference between the heating and cooling sections rises with power input, which directly increases the convective heat transfer coefficient. Consequently, the obtained Nusselt number increased from 25.69 at 750 W to 31.23 at 1540 W. This finding confirms that higher heating power significantly improves the loop heat transfer performance, providing insight into the optimization of passive cooling systems in nuclear safety applications.

Abstract: This study examines the enhancement of Photovoltaic Thermal (PVT) systems through the application of nanofluids containing hexagonal boron nitride (Al2O3) nanoparticles. PVT systems, which integrate photovoltaic cells with thermal collectors, offer a dual-function solution by generating both electricity and heat, thereby maximizing the utilization of solar energy. The research specifically focuses on optimizing the thermal and electrical efficiencies of PVT systems by adjusting two critical parameters: the inlet velocity of the nanofluids and the concentration of Al₂O₃ nanoparticles. Computational simulations were performed using ANSYS Fluent software to analyze the impact of these variables on temperature distribution within the systems. The simulations revealed that both higher inlet velocities and increased nanoparticle concentrations lead to significant improvements in system performance. The most notable gains were observed at a nanoparticle concentration of 0.05% and an inlet velocity of 0.08 m/s, where thermal efficiency reached 74.80%, and electrical efficiency increased to 14.43%. The study confirms that the enhanced thermal conductivity of nanofluids due to the presence of Al₂O₃ nanoparticles plays a pivotal role in improving heat transfer and cooling processes. This optimization leads to the photovoltaic cells operating at more efficient temperatures, thus elevating both the output and overall efficiency of the PVT systems. The findings suggest that carefully controlled adjustments to the nanofluid properties can effectively optimize PVT systems, making them a more viable and efficient solution for simultaneous heat and electricity production from solar energy.

Abstract: Studying turbulent mixing, stress redistribution, pressure losses, secondary flows, and energy dissipation enhances industrial process efficiency, improves equipment durability, minimizes operational costs, optimizes fluid transport systems, supports design and promotes energy conservation. However, little is known about the influence of outlet geometry on turbulence characteristics (i.e. turbulent kinetic energy, turbulent intensity, effective viscosity , and effective thermal conductivity) in air, water, and kerosene flowing through Y-shaped copper ducts featuring regular, converging, and diverging outlets. A hybrid geometric configuration incorporating angular inlets and asymmetric outlets enables detailed analysis of turbulent mixing, stress redistribution, pressure losses, and secondary flow development in complex internal flow regimes. Numerical simulations were conducted using ANSYS Fluent 2023 R2, employing the shear stress transport (SST) turbulence model for accurate resolution of adverse pressure gradients and boundary-layer effects. High-quality meshing, grid independence validation, and robust solver configurations ensured numerical reliability and convergence. The results demonstrate that outlet geometry significantly influences turbulence intensity, effective viscosity, and thermal conductivity across different working fluids. For air, increasing inlet velocity enhances turbulent kinetic energy and turbulence intensity at the regular outlet, while the diverging outlet exhibits peak turbulence at higher cold-fluid velocities. In water flows, the converging outlet shows substantial turbulence growth under increased velocity conditions, highlighting the role of geometric restriction in enhancing mixing and energy dissipation. For kerosene, the regular outlet achieves maximum effective thermal conductivity due to improved fluid interaction and turbulence under elevated velocity conditions, whereas the diverging outlet exhibits lower viscosity as a consequence of geometric flow dispersion. Overall, the findings underscore the critical role of outlet configuration in determining turbulence behavior and thermal-fluid transport characteristics.