Heat Transfer最新文献

This study examines the time-dependent flow, mass, and heat transmission of a hydromagnetic viscoelastic fluid, which is modeled using Walters Liquid Model B′. The fluid is introduced over a stretching, permeable sheet that is embedded in a porous medium. The effects of mass suction/blowing, heat flux, and thermal radiation are taken into account. Utilizing similarity transformations, the flow-regulated equations are transformed into a self-similar form, and MATLAB's bvp4c solver is applied to determine the numerical solution. The findings reveal that the growth of viscoelasticity, unsteadiness, magnetic influence, and permeability contribute to a rise in velocity, while these same parameters cause a reduction in fluid temperature. Suction and blowing further reduce velocity and temperature, whereas thermal radiation increases temperature, and a greater Prandtl number inhibits thermal diffusion. Furthermore, flow-dominant factors result in a reduction in species concentration. These findings pertain to industrial and biological systems where magnetic fields, thermal radiation, and surface permeability influence flow and heat transfer, including increased oil recovery, thermal treatments, and solar energy applications.

This study presents an in-depth analysis of the flow and thermodynamic behavior of Casson couple-stress fluid in a vertical microchannel under the combined influence of Hall current, uniform heat source/sink, variable thermal conductivity, Brownian motion, thermophoresis, and pollutant discharge, subject to convective boundary conditions. The governing equations are transformed using similarity variables and solved numerically via the shooting technique coupled with the Runge–Kutta–Fehlberg (4,5th) scheme. The originality of this study lies in the simultaneous consideration of non-Newtonian rheology, electromagnetic effects, variable transport properties, and pollutant dynamics, a combination rarely addressed in microfluidic studies. The thermal field demonstrates a direct sensitivity to heat generation, where the temperature rises with increasing heat source strength, while it declines when variable thermal conductivity is allowed to vary, owing to enhanced thermal diffusion. The concentration within the medium is elevated by Brownian motion and external pollutant source effects, both of which promote particle dispersion and accumulation, whereas thermophoretic forces act in the opposite manner by driving particles away from hotter regions, thereby suppressing concentration. Entropy generation responds dually to Brownian motion, thermophoresis, Casson parameter, and variable thermal conductivity, indicating the sensitive interplay of flow, mass, and thermal transport mechanisms. The Bejan number profile illustrates spatial variations in the relative contributions of heat transfer and fluid friction irreversibilities under the impact of heat source, variable thermal conductivity, Brownian motion, and thermophoresis parameters. These results provide a valuable approach to the design and optimization of microchannel systems in biomedical, chemical, and energy applications, in settings where precise heat control, mass, and pollutant transport are critical.

This study presents a coupled numerical–statistical investigation of steady, laminar, incompressible magnetohydrodynamic (MHD) free convective heat and mass transfer in an electrically conducting micropolar fluid over a semi-infinite vertical plate, incorporating double stratification, wall suction/injection, and combined thermal–solutal buoyancy effects. Micropolar behavior is modeled using Eringen's theory, which accounts for both translational and microrotational dynamics. The governing equations are reduced through Lie group similarity transformations into coupled nonlinear ordinary differential equations. These are solved using the Keller–box scheme to accurately resolve near-wall gradients and asymptotic far-field behavior. Parametric analysis reveals that increasing the micropolar coupling parameter K enhances the peak velocity (10%) and microrotation (100%) while marginally reducing the thermal and solutal fields via stronger convective removal. The magnetic parameter M suppresses velocity (up to 18%) and microrotation (40%), thickens boundary layers, and lowers Nusselt and Sherwood numbers. Thermal stratification ε₁ and solutal stratification ε₂ diminish buoyancy, lowering velocity by over 40% and 15%–20%, respectively. Suction (f₀ > 0) improves transport, increasing Nusselt and Sherwood numbers by 15%–30%, while injection (f₀ < 0) produces the opposite effect. Response surface methodology (RSM) is applied for multi-objective optimization of Nu_x, Sh_x, C_f*, and g_peak. Quadratic models (R² > 0.99, Adeq Precision > 79) capture significant linear, interaction, and quadratic effects of K, M, and f₀. The optimal solution, with overall desirability 0.751, occurs at K ≈ 1.60, M ≈ 0.96, and f₀ ≈ −0.27, yielding Nu_x = 1.3047, Sh_x = 1.1433, C_f* = 0.5001, and g_peak = 0.7087, all within the 95% prediction intervals. The integrated findings demonstrate that strategic tuning of micropolar coupling, magnetic field strength, and wall mass flux can enhance thermal and mass transport while controlling frictional and microrotational effects, offering valuable design guidance for MHD micropolar systems in energy, materials processing, and thermal management applications.

In natural circulation loops (NC loops), flow initiation is driven by the difference between the heat source and the heat sink density variations. Given that the flow rates within these loops are lower than those in forced convection channels, surface characteristics can significantly influence the flow. No heat transfer studies have been found that describe the effects of UV-irradiated PDMS surface modifications in NC loops. The study focuses on evaluating the thermal performance of surface-modified natural circulation mini-loops with a hydraulic diameter of 3 mm. The impact of surface modifications at the mini-loop heater section, under different power inputs (10, 15, and 20 W) and temperatures at the heat sink (20°C and 30°C), is investigated. The performance of 0.01 and 0.02 vol.% Al₂O₃ and SiO₂ nanofluids was measured and compared with that of D.I. water. The initial experiment involved a channel with untreated polydimethylsiloxane (PDMS) coating on the heater section (contact angle of 86.2 ± 2.7°). Subsequently, the experiment was repeated with a vacuum UV (VUV)-treated PDMS coating (contact angle of 2.7 ± 2.1°). The heat transfer coefficient within the heater section was estimated using a nonintrusive interferometric technique. The thermal performance of a 3 mm hydraulic diameter mini-loop with dilute metal oxide nanofluids was compared with deionized water (D.I. water), and the results were validated against Vijayan's correlation. Figure of Merit (FOM) analysis indicated that, for the designed mini-loop, 0.02 vol.% alumina nanofluid exhibited the best performance. At 10 W and 283 K, the surface-tuned heater section with 0.02 vol.% alumina nanofluid demonstrated a 12.42 ± 1.6% enhancement in the local heat transfer coefficient. However, the Nusselt number increment was only 5.39 ± 1.7%. The high wettability of the heater section hinders the slip flow. The rise in the thermal conductivity of the basefluid due to nanoparticle addition also reduces the Nusselt number. The surface effects being comparable with the buoyancy forces, the Reynolds number inside the loop is lower.

The present work focuses on the influence of continuous and interrupted fins on laminar natural convection above a heat sink with different fins' arrangements. The hydraulic and thermal performances of a three-dimensional model of the heat sink were simulated using ANSYS Fluent software based on the finite volume method. The dimensions of the heat sink were 305 mm in length and 100 mm in width, while the fin height was 17 mm, and the fin spacing was 9.5 mm. Various power supplies were applied to the heat sink (20, 40, 60, 80, 100, and 120 W) to investigate the effect of different ranges of heat sink performance. Different fins' configurations were tested, including straight-fin, interrupted with different angle inclinations (30°, 45°, and 60°), and staggered arrangements with an angle of inclination of 60°. The results showed that the increase in the interruption length leads to an increase in the heat transfer (HT) rate. Moreover, five interruptions with a length of 30 mm lead to an HT rate of 22.3% as compared with continuous fins. The inclination of 60° gave the optimum HT rate. Furthermore, there will be a weight reduction of 47% for the optimum HT rate. An empirical correlation was developed to relate the Nusselt number to the Rayleigh number and inclination angle for interrupted rectangular fins at angles of 90°, 60°, and 45°, providing a robust predictive tool for assessing thermal performance. Furthermore, the current numerical and experimental results demonstrated an 89.5% concordance in the average temperature difference, with a divergence of less than 4.68% from existing literature.

A fine-tuned pattern net artificial neural network (PNANN) is explored for design of porous ceramic matrix (PCM) based burner by using classification model. The PNANN is attempted to fine-tune by comparing two different performance functions: mean squared error and sum absolute error. Three different numbers of hidden neurons are also tested while using scaled conjugate gradient as training algorithm. The data for the classification model is obtained by simultaneously solving the governing equations of two phases for the porous ceramic matrix (PCM) based burner. Different values of two critical parameters are used in the numerical model. The critical parameters considered for influencing the performance of the PCM based burner are: convective coupling and extinction coefficient. Radiative heat flux is also incorporated in the numerical model by solving radiative transfer equation by discrete transfer method. Based on the difference between the temperature profiles of the two phases (solid and gas), binary array are constructed and used in the classification model. Ten different classes of data signify ten different regime of operation, each having a unique range of values for the critical parameters. All PNANNs with performance function mse are able to give correct identification above 41.3% and upto 81.1%.

This study presents an in-depth analysis of mixed convection flow through a porous medium incorporating the effects of internal heat generation and heat absorption. The novelty of this study lies in the combined evaluation of localized heat sources, chemical reactivity, and magnetohydrodynamic (MHD) influences within a single porous configuration—an approach rarely addressed collectively in prior work. The governing equations, expressed as nonlinear, dimensionless ordinary differential forms, are solved using the regular perturbation method. The results reveal the combined influence of these factors on velocity, temperature, and concentration profiles, offering insights applicable to engineering applications such as advanced cooling systems, filtration processes, fuel cell design, and biomedical devices like blood purification units.

This study investigates the thermal performance of a thermosyphon through experiments, focusing on optimizing operational parameters and understanding transient behavior. A two-phase thermosyphon finds wide applications across various engineering domains, with its internal vapor–liquid phase change and flow dynamics being crucial factors influencing heat transfer efficiency. A thorough analysis of the heat transfer mechanism in thermosyphons (2 m in length) designed for cooling nuclear fuel storage, particularly under different operating conditions, is essential for optimizing their thermal design and performance. In this study, heat loads (300–600 W), filling-ratio (40%–100%), operating pressure, and inclination angle (60°−90°) of the thermosyphon are varied to analyze its performance of heat transfer in terms of transient temperature variation and thermal resistance. The results indicate that among the tested filling-ratios, a 60% filling-ratio is optimal for efficient thermosyphon operation. Furthermore, the thermal resistance decreased with higher heat loads applied to the evaporator, demonstrating an enhanced heat transfer. The presence of non-condensable gases (NCGs) is found to increase the evaporator-to-condenser temperature difference, impeding performance; however, their removal improves startup behavior and overall heat transfer efficiency. This parametric and methodological analysis provides comprehensive insights into the role of operational conditions in thermosyphon performance. The findings can be applied to optimize passive cooling systems in electronics, energy storage, and nuclear safety applications.

This study focuses on understanding the system's response to gravity modulation with two frequency components, characterized by both sinusoidal (sine wave) and non-sinusoidal (square, triangular, and sawtooth) waveforms, on three-component convection, considering a viscoelastic fluid modelled using an Oldroyd-B fluid. We apply the Venezian approach to evaluate the Rayleigh number, its corrected form, and the wave number by deriving a five-mode Lorenz model to investigate the onset of convection. A nonlinear analysis is conducted to investigate the dynamics of heat and mass transfer by solving an extended eight-mode Lorenz model, capturing higher order interactions. The onset of convection and the transport properties were observed to be influenced by combinations of sinusoidal and non-sinusoidal waveforms. This study optimizes convection-driven systems subjected to external periodic forcing by offering a more comprehensive understanding of convective instabilities in viscoelastic fluids.