Heat Transfer最新文献

This study presents a comprehensive linear stability analysis of dual-diffusivity convection in a bidispersive porous medium, embedding a uniform vertical throughflow and variable gravity within the framework of the Darcy–Brinkman model. Three distinct gravity variation profiles—linear, parabolic, and exponential—are systematically examined to understand their impact on convective stability. A high-order Galerkin approximation is utilized to obtain solutions to the governing eigenvalue problem. The critical Darcy–Rayleigh number is evaluated as a function of key nondimensional parameters, including the Péclet number, gravity modulation parameter, solute Rayleigh number, permeability ratio, Lewis number, Darcy number, and the interphase momentum transfer parameter. Special attention is given to the role of throughflow direction and magnitude, with both upward and downward flows analyzed across varying gravity fields. The results indicate that exponential gravity variation markedly enhances system stability through intensified gravitational stratification, while higher permeability ratios and stronger interphase momentum transfer further stabilize the system by increasing viscous dissipation and drag coupling. A U-shaped, nonmonotonic variation of the critical Rayleigh number with the throughflow parameter is observed, demonstrating the destabilizing role of moderate throughflows and the stabilizing influence of strong throughflows. Overall, the findings provide key insights into the complex interplay of hydrodynamic and buoyancy-driven mechanisms in bidispersive porous systems, with implications for thermal management, geophysical flows, and engineered porous structures. The findings provide practical insights for optimizing geothermal reservoirs, chemical reactors, and environmental systems by controlling throughflow, interporosity exchange, and gravity variations to enhance stability and transport efficiency.

Single slope solar still was built to study the impact of phase change material (PCM) on water output. Bottom along with four sides of the still are built from an aluminium sheet of 0.79 mm thickness. The base is coupled with a sinusoidal shaped basin liners (SSBL), a layer of aluminium fitted with copper tubes. The unit was fixed in a box of 19 mm thick ply to minimise heat loss from the bottom and side faces to the surroundings and completely sealed to reduce the leakage. Myristic and palmitic acid granules as PCM were filled in the copper tubes of 15 mm outer diameter. PCM changes its phase by absorbing excess energy during sunshine as a result of sensible heating till it hits the melting temperature. During the night, PCM solidifies by losing its heat of fusion to the water. The heat interaction between the glass cover and basin water was examined by the analysis of energy balance and internal heat transfer equations. The experiment was performed in three sets: without PCM, with PCM I, and with PCM II; however, the fixed quantity of 6 litres of tap water was taken to maintain 2 cm of water depth. Results showed that SSBL solar still with PCM I (myristic acid) and PCM II (palmitic acid) enhanced the overall yield by 16% and 9% in comparison to SSBL solar still without PCM, while condensate production in night with PCM I and PCM II increased by 75% and 38%, respectively, compared to the solar still output in night without PCM. Meanwhile, PCM I (myristic acid) showed the best performance, achieving the highest thermal (44.0%) and exergy (6.9%) efficiencies. It also offered the lowest cost per liter ($0.0116) and the highest sustainability index (1.076).

This study investigates the enhancement of natural convection heat transfer in a differentially heated square cavity by coupling fluid–structure interaction (FSI) with a nanofluid containing nanoencapsulated phase change material (NEPCM). This configuration is important for improving passive cooling in energy-efficient technologies, such as electronics, buildings, and electric vehicles. The novelty of this study lies in the integration of a sinusoidal oscillating flexible fin with latent-heat-enhanced nanofluids in a fully coupled FSI framework. A finite element method is used to solve the governing equations for fluid flow, heat transfer, and structural motion, with validation through grid independence tests and comparison against benchmark numerical and experimental data. Parametric studies were performed for fin oscillation amplitude (0.05–0.15), oscillation period (0.1–0.7), Rayleigh number (10⁴–10⁶), and Stefan number (0.2–0.7). Results show that increasing the amplitude to 0.15 enhances the mean Nusselt number by up to 10%; lower periods (τ_fin = 0.1) and Stefan numbers (Ste = 0.2) reduce heat transfer; higher Rayleigh numbers promote stronger convective currents and better thermal uniformity. These findings offer quantitative insights for optimizing thermally responsive structures using NEPCM fluids and flexible fins, enabling efficient heat transfer.

In this study, we employ a standard perturbation technique to analyze the onset of Rayleigh-Bénard convection in a horizontally oriented, electrically conductive fluid layer overlying a porous substrate. The system is bounded laterally by rigid, adiabatic walls and subjected to internal heat generation, a uniform vertical magnetic field, and local thermal nonequilibrium (LTNE) conditions. The analysis is conducted within the Darcy regime and incorporates various temperature gradient (TG) profiles. We investigate how different TG configurations influence the onset of convection, particularly under thermally adiabatic boundary conditions at the upper and lower surfaces. Critical Rayleigh numbers are determined by solving the corresponding eigenvalue problem for configurations with both rigid-bottom/free-top boundaries, incorporating surface tension-driven velocity boundary conditions. The study further examines the effects of parameters such as the modified porosity ratio, thermal diffusivity ratio, and heat source intensity, emphasizing their role in promoting LTNE under six distinct TG models. Perturbation analysis reveals that maximum convective activity occurs at the mid-plane for the inverted parabolic (Model-3) and Dirac delta (Model-6) temperature gradients. Magneto-convective behavior under LTNE is visualized in the presence of a vertical magnetic field and volumetric heat generation, providing insights into the thermal system's behavior under complex boundary and gradient conditions. The results align well with existing literature, supporting their relevance to thermal management in energy systems, biomedical devices, aerospace structures, electronic cooling, and geophysical processes.

Compact heat exchangers are critical components in various industries, including cooling air conditioning devices, automotive, aerospace, enabling efficient heat transfer between fluids or between fluids and solids. This study investigates the influence of fin angles (0°, 30°, 60°, and 90°) on the performance of compact heat exchangers. A combined 3D-CFD simulation and experimental analysis approach was used under a constant airflow rate of 0.05 kg/s and an inlet temperature of 294 K. Both the numerical simulations and experimental tests showed that a 60° louvered fin angle delivered the best heat transfer performance. Numerically, we found an average heat transfer coefficient of about 28.964 W/m²·K for the front side and 27.978 W/m²·K for the back side. The experimental results closely matched this, with the maximum average heat transfer coefficient reaching 28.4 W/m²·K for the front side and 26.6 W/m²·K for the back side on fin 2. The highest heat transfer coefficient values were detected on fin surfaces directly exposed to the airflow, while the backside surfaces presented notably lower heat transfer coefficients. The percentage differences for numerical and experimental results for the front side and back side are 1.98% and 5.18%. These findings underscore the importance of fin orientation in maximizing heat exchanger efficiency. This analysis provides a detailed look at heat transfer on both the front and back surfaces of the fins, offering insights that were not available in previous studies.

An experimental and numerical study has investigated the heat transfer (HT) and friction characteristics of a solar air heater (SAH) duct roughened using a rectangular S-shaped artificial roughness arrangement (inline and staggered). The thermal performance of SAH is studied with design variables such as the length of the relative roughness (d/H = 1.33), the height of the relative roughness (e/H = 0.271), and the distance between S (b/H = 0.667) remaining constant as the Reynolds number (Re) ranges from 3000 to 10,000 and the angle of attack 60°. Relative roughness of pitch range p/H (1.667, 3.33, 5, and 6.667) for inline with relative roughness of length (l/H) (0.8335, 1.666, 2.5, and 3.335) for staggered. A three-dimensional computational fluid dynamics (CFD) simulation is carried out using the CFD code, ANSYS Fluent, and the renormalization group k–ε turbulence model for solving turbulence terms in governing equations. It has been found that HT enhancement can be achieved by using an artificial roughness staggered arrangement at l/H = 0.8335 and e/D = 0.271 with an angle of attack (α) of 60°. Also offers the best thermal performance factor for the investigated range of 3.12.

The current investigation objective is to employ a semianalytical method to observe the micropolar fluid flow dynamics in the vicinity of a stretched surface through a chemical reaction of medium that is nonlinear, nonlinear radiation, heat dissipation, variable viscosity, and modified Fourier heat flux effects influenced by convective boundary conditions. The Chebyshev collocation technique is employed to solve the dimensionless ordinary differential equations, which are converted from dimensional partial differential equations using similarity transformations. The research also examines the performances of velocity, temperature, microrotation, and concentration fields due to the effects of various flow-influencing supervisory parameters within the boundary layer by representing these findings through graphical diagrams. The comparison of the couple-stress, friction factor, and mass and thermal rates has been done by the computed numerical data presented in the table. The final results show that velocity profiles decreased by increasing the microgyration factor and material parameter, whereas microrotation exhibited the opposite trend. An upsurge in the first-order slip parameter leads to fluid velocity and microrotation depreciation. The potency of the magnetic field impelled to deflate the fluid moment, but the increased porosity parameter intensified fluid velocity. The thermodiffusion effect leads to an expansion concentration field, but the chemical reaction and Schmidt number have opposite impacts. The magnetic and temperature ratio parameters are driven to intensify the skin friction and couple stress, but thermal relaxation and radiation parameters have evidenced opposite effects.

Heat pipes (HPs) are becoming increasingly popular because they are efficient heat transfer technologies. This study investigates the potential application of an HP as an air-cooling system at the gas turbine (GT) inlet as a viable solution to increase GT power output. In particular, a case study is conducted on real GT engines operating in Kerman, Iran. First, a theoretical model is constructed using the established HP technologies to design a wick structure as a thermal control element. MATLAB-based simulations are utilized for HP design and analysis to specify the impact of HP structural factors on heat transport capabilities, emphasizing the significance of operating orientation and the utilization of gravity to augment heat transport capacity. To validate the methodology, selected results from previous research are replicated and compared. The HP design is optimized to maximize thermal effectiveness, while the tube bank or heat exchanger system is optimized to minimize inlet air pressure drop. Computational results indicated that the optimal tilt angle is 15°. The proposed method shows that the heat transfer rate per HP reaches a maximum of 1.4878 kW at 40°C. Then, the case study results indicated that a net power output of up to 18.6% is achievable when utilizing HP heat exchangers, which maintains its effectiveness even at temperatures below the design threshold. Consequently, this system proves suitable for peak load conditions during hot seasons and year-round performance. This technology offers a promising new approach to GT design, with significant implications for enhancing energy efficiency and power stabilization.

Drying dates to a semi-dried, soft, preferred by most consumers, remains a challenge for small-scale processors due to limited control, longer drying time, quality degradation, and other inefficiencies of traditional methods such as open-sun drying. This study investigated the mechanical hot-air drying kinetics and characteristics and evaluated quality changes after drying of Biser dates from 57% to 30% moisture content (wet basis) at various temperatures using a state-of-the-art computer vision system (CVS). This study findings revealed that drying time decreased with increasing temperature, with the shortest drying time observed at 70°C (14.5 h) and the drying occurred predominantly in the falling rate period. Additionally, among the three thin-layer drying models investigated in this study, Page model stands out as the best fitting model to describe the mechanical hot-air drying behavior of Biser dates, having an uppermost coefficient of determinations (R²) of (0.9899–0.9984) and least standard error (SE). The effective moisture diffusivity (D_eff) followed second Fick's diffusivity model and fall between a range from 3.50 × 10⁻¹⁰ to 5.84 × 10⁻¹⁰ m²/s across the temperatures studied (50°C, 60°C, and 70°C). Higher temperatures led to greater shrinkage but helped prevent surface cracking. Notably, CVS measurements showed significant differences (p < 0.05) in shrinkage across samples, with 60°C and 70°C yielding higher volumetric shrinkage. Rehydration capacity was highest at 60°C (45.47%), followed by 70°C (34.21%) and 50°C (20.99%). Overall, drying at 70°C provided the most efficient balance between drying time and product quality, making it the optimal condition for small-scale processors already transitioning to mechanical hot-air drying. It also reduced drying period, minimized quality losses, improved operational consistency, and product standard in the shift away from traditional methods. Future research should focus on sensory evaluation to assess consumer acceptance of semi-dried Biser dates processed under these conditions.

This paper reviews how tubular solar still designs can enhance thermal output and offer a sustainable desalination solution powered by solar energy. Conventional solar stills typically produce only 2–5 L/m²/day, highlighting the need for more efficient and practical designs for widespread adoption. Studies categorize performance improvement methods into two primary approaches, with particular emphasis on phase change materials due to their demonstrated efficacy. Experimental data shows that phase change materials can improve the system energy efficiency to a maximum of 30% and boost manufacturing capacity notably while reaching production quantities greater than 6 L/m²/day within optimal operating parameters. The review demonstrates how advanced wick materials, vacuum insulation together with reflective surfaces have enhanced both thermal performance and productivity of these systems. Geographical conditions, together with climate variables, influence the success of these enhancement methods; so, specific optimization measures must be developed for different locations. Recent experimental and theoretical research synthesis delivers important pathways for future development, which proves tubular solar stills as sustainable water scarcity solutions that produce less carbon than traditional desalination approaches.