Heat Transfer最新文献

The primary purpose of our work is to investigate and quantify the influence of temperature-dependent thermophysical properties on the behavior of natural convection with a porous medium. Recognizing that the properties of real fluids vary significantly with temperature, especially under the large thermal gradients common in porous media, we aim to understand how this variation affects flow patterns and heat distribution, moving beyond simplified models that assume constant properties. To achieve this, we specially examine spontaneous convection in a viscous fluid inside a square cavity where the vertical walls undergo sinusoidal oscillation and the horizontal walls are adiabatic. Fluid flow is simulated using the Boussinesq approximation and Darcy's law. Using similarity transformations, the governing equations are transformed into dimensionless form and then numerically solved using the Peaceman–Rachford alternating direction implicit technique and Gauss–Jordan elimination. The results demonstrate that taking temperature-dependent characteristics into consideration greatly increases convection, leading to more robust rolls, improved circulation, and densely packed streamlines. The distribution of temperatures is significantly changed, with isotherms smoothing/expanding with more conduction and sharpening close to heat sources under low conduction. The shifting interaction between convection and diffusion is also reflected in the isoconcentration patterns, which seem more uniformly distributed with higher diffusivity and more localized with low diffusivity.

Freshwater scarcity is a critical challenge in arid and semi-arid regions, requiring cost-effective and sustainable desalination solutions. Hemispherical solar stills (HSS) are promising due to their high efficiency and simple design, but further improvements are needed to maximize output. This study investigates the performance enhancement of HSS through the integration of walnut shells as a thermal storage medium. Experiments were conducted on August 13, 2024, at the University of El Oued in Algeria, with testing carried out from 7:00 a.m. to 7:00 p.m. under local climatic conditions. A comprehensive 4E analysis (Energy, Exergy, Economic, Environmental) evaluated the effect of walnut shell sizes (3 and 6 mm) on productivity and sustainability. The results show that the 3 mm walnut shell configuration (HSS-WS3) achieved the highest daily yield of 7.10 L/m²/day, a 51.06% improvement compared to the traditional HSS (THSS), while the 6 mm configuration (HSS-WS6) yielded 6.40 L/m²/day (+36.17%). Energy efficiency increased by 50.50% for HSS-WS3% and 35.90% for HSS-WS6, while exergy efficiency improved by 118.30% and 70.60%, respectively. Economically, the cost per liter of distilled water decreased to $0.0051 (HSS-WS3) and $0.0056 (HSS-WS6) compared to $0.0077 (THSS), with payback periods reduced to 20 and 23 days. Environmentally, HSS-WS3 achieved the highest CO₂ mitigation of 2.049 tons over a 10-year lifespan. These findings demonstrate that walnut shells, particularly at smaller particle sizes, are effective, low-cost, and sustainable materials for enhancing solar still performance, providing a promising desalination strategy for water-scarce regions.

This study conducts a numerical analysis of entropy generation, heat, and mass transfer in a vertical channel filled with two immiscible fluids, with the plates of the channel kept under asymmetric heat distribution. The system of governing equations for momentum, energy, and diffusion is made non-dimensional using relevant variables and then solved numerically using Mathematica's ND Solve technique. The momentum, temperature, and diffusion variations are presented in graphical mode using data visualization and smoothing via interpolation in Python for better visibility of the variations. The momentum, heat, and mass transfer rates on both the plates of the channel are also made non-dimensional. The obtained values are analyzed for different variations of governing parameters, with a multiple regression correlation analysis performed and calculated $��\; ��\mathrm{R²}��$ (R-squared) values to predict the level of dependency. The other important parameter, entropy, is also calculated in the defined domain, and the obtained values are tabulated. The study found that all parameters have a significant impact. In particular, the temperature, entropy generation, and velocity are strongly affected by buoyancy forces and magnetic fields. The Grashof number and molecular Grashof number improve fluid motion and reduce entropy. Both the Reynolds number and the magnetic parameter contribute to an increase in entropy generation. Shear stress analysis reveals two flow layers affected by buoyancy and magnetic damping.

This study coupling the lattice Boltzmann method (LBM) with Artificial Neural Networks (ANNs) to explore combined convection within a moving-wall square cavity containing variably shaped heated blocks, targeting applications in thermal management of electronic components such as lithium-ion batteries. This hybrid approach significantly reduces computational cost and time while enabling accurate prediction of thermal behavior for different geometries. The enclosure is stuffed with air (Pr = 0.71) and cooled from its vertical sides by a constant cold temperature, $��\; ��T_C^{\prime }��$, while the horizontal walls are thermally insulated, with the top wall is driven at a uniform velocity, $��\; ��u_0^{\prime }��$. For Richardson numbers between 0.01 and 100, the investigation has been conducted for various geometrical shapes of the heated block under consideration, including square, rhombus, circular, horizontal, and vertical ellipses. The ANN is employed to forecast new cases, thereby reducing computational effort while also serving to validate the numerical results obtained. The findings are graphically displayed as stream function contours, temperature contours, mean fluid temperature, and average heat transfer (HT) in relation to the aforementioned controlling parameters. The results indicate that, across all Ri values, the horizontal elliptical block (HEB) achieves the highest HT rate, while the circular block (CB) exhibits the lowest. More precisely, changing the block shape from CB to the horizontal elliptical one (HEB) increases the HT rate to about 20.43% at Ri = 0.01. Finally, the ANN predictions showed excellent accuracy, with results closely aligning with numerical simulations, thereby confirming the reliability and robustness of the hybrid ANN–LBM approach.

This study numerically investigates the effect of different microchannel geometries integrated into the absorber tube on the thermal performance of a parabolic trough solar collector. Four configurations were analyzed: empty (straight pipe), circular, square, and triangular microchannels. Computational Fluid Dynamics simulations were conducted using ANSYS Fluent 19.0, adopting a Finite Volume Method with a pressure-based solver, the SIMPLE algorithm for pressure–velocity coupling, and second-order upwind discretization for momentum and energy equations. Simulations were performed across mass flow rates ranging from 0.25 to 1.0 kg/h, with an inlet fluid temperature of 25°C and a constant solar irradiance of 884 W/m². The corresponding Reynolds numbers ranged approximately from 110 to 420, indicating laminar flow across all cases. Among the configurations, the triangular microchannel exhibited the best performance. At 0.25 kg/h, it achieved an outlet temperature of 101.73°C and a thermal efficiency of 85%, compared with 92°C and 37.15% in the nonchanneled base case. At 1.0 kg/h, the triangular design reached a Nusselt number of 30.48, more than double that of the baseline (14.45), indicating superior convective heat transfer. Validation against published results showed a maximum deviation of only 2.6%, confirming the accuracy of the simulation model. The findings highlight that triangular and square microchannels significantly enhance heat transfer and thermal efficiency, particularly at low flow rates, making them promising designs for advancing solar thermal energy systems.

Drying of medicinal herbs like Justicia adhatoda is essential to maintaining their bioactive constituents, but conventional methods tend to degrade quality by subjecting them to excessive heat for long periods. To overcome this, the current research compares and assesses the drying kinetics, phytochemical retention, and optical quality of J. adhatoda leaves under cabinet tray drying (CTD) and microwave drying (MD). Since the plant has pharmacological importance, the research seeks to select a drying process that guarantees efficiency and quality retention. Experimental drying was carried out at different temperatures (45°C–75°C) for CTD and microwave powers (200–700 W) for MD. Among the 11 thin-layer models, Page's model proved to be the most reliable in predicting drying behavior for both processes (R² = 0.999) for CTD and (R² = 0.996) for MD. MD under 360 W took the shortest time, whereas 200 W maintained the maximum total flavonoids and tannin contents. Antioxidant activity was maximum at 360 W, indicating maximum retention of functional constituents under optimal microwave power. Microwave-dried samples exhibited better color preservation, demonstrating its capability to retain sensory as well as commercial acceptability. The results of this study have important consequences for the herbal processing industries, allowing scalable, high-quality drying schemes to be developed that protect the therapeutic and cosmetic worth of medicinal crops.

The design freedom of three-dimensional (3D) concrete printing (3DCP) enables complex building envelopes with integrated air cavities; however, their thermal performance in extreme climates poses a critical challenge, particularly for cavities with nonuniform geometries. This study presents a combined experimental and numerical analysis of conjugate heat transfer in a real 3DCP building envelope featuring variable-thickness air cavities under hot-arid conditions. A comprehensive methodological framework was employed: (1) in situ U value measurements on north and slanted east cavity walls (3.75 m tall; max cavity width, 900 mm) with and without interior insulation, (2) development of a robust numerical model to simulate conjugate heat transfer across laminar to turbulent flow regimes, and (3) whole-building cooling load estimation for the 168 m² structure. Results demonstrate that wide cavities (> 25 mm) develop strong buoyancy-driven circulation (Ra ≈ 10¹¹ at ΔT = 20 K), severely degrading insulating performance and yielding high U values of 2.67–3.29 W/(m² K) for uninsulated walls—approximately 30% higher cooling loads than standard construction. Crucially, parametric analysis reveals that filling cavities with low-conductivity porous media (e.g., sand and k = 0.27 W/[m K]) effectively suppresses convection, restoring thermal performance to near pure conduction levels and reducing U values by up to 82% when complemented with insulation. These findings provide essential, practical strategies for optimizing energy efficiency in future 3DCP buildings constructed in hot-arid climates.

The low thermal conductivity of phase change materials (PCMs) remains a primary constraint on the performance of latent heat thermal energy storage (LHTES) systems. This review critically analyzes the extensive body of research dedicated to overcoming this limitation through the application of various fin geometries and designs. It delves into the geometrical parameters, dimensionless numbers, and placement strategies of fins, including rectangular, annular, spiral, plate, and novel biomimetic designs, evaluating their impact on thermal behavior through both numerical and experimental investigations. The analysis demonstrates that fin configuration fundamentally dictates the balance between conductive and convective heat transfer mechanisms. While longitudinally finned structures often provide the most practical enhancement due to their manufacturing simplicity, innovative designs like tree-shaped and spiral fins offer superior thermal performance at the cost of increased complexity. Key trade-offs are identified, notably between thermal performance gains and the concomitant reduction in PCM storage volume. The review concludes by highlighting critical research gaps, particularly the need for multi-objective optimization frameworks that concurrently consider thermal performance, energy density, and economic viability, and calls for long-term stability studies to assess the practicality of advanced fin designs.