Heat Transfer最新文献

This contribution evaluates the propagation mechanics of Rayleigh waves through a piezothermoelastic orthotropic medium incorporating nonlocal elasticity theory, diffusion effects, and the three-phase-lag (TPL) model. By applying normal mode analysis, we explore how the nonlocal parameter influences wave propagation, thermal diffusion, and piezoelectric interactions within the medium. The TPL model, which accounts for time delays in heat and stress responses, is integrated to better capture the material's dynamic behavior. The results demonstrate significant variations in wave speed, attenuation, and dispersion due to the interplay of nonlocal elasticity, diffusion, and the thermal effects under the TPL framework. This study provides a comprehensive understanding of wave propagation in advanced materials with complex thermoelastic and piezothermoelastic properties.

This paper analyses the transition temperature and duration of the cooling regimes (film boiling, transition boiling, nucleate boiling, and convection) during quenching by immersion of specimens covering a range of Biot numbers $��\; ��\left(�\; �\mathrm{Bi}�\; �\right)��$ from 0.70 to 1.32. Test data were acquired under an industrial environment, thus capturing the in situ conditions, information that is rarely available in the literature. The experimentally acquired cooling curves were processed to determine characteristic points, in the form of time, temperature, and cooling rate, representing the transition between cooling regimes. These points were precisely identified as per the peaks of the cooling curve derivatives with respect to time. The results show that transition temperatures tend to decrease with the $��\; ��\mathrm{Bi}��$ number following a linear behavior. Correlation models between the duration of each regime and the $��\; ��\mathrm{Bi}��$ number were developed following the nonlinear behavior of the regimes. The duration of boiling phases and convection evolve inversely; the former prevails as the $��\; ��\mathrm{Bi}��$ number decreases, while the latter drives the cooling time for larger $��\; ��\mathrm{Bi}��$ numbers. The models provide reference points for time and temperature that constitute the basis for a more comprehensive thermal modeling of immersion quenching.

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.

Retinal laser therapies assume a critical function within the field of ophthalmology, since the human eye is exposed to a multitude of ocular disorders, and diabetics and individuals with prolonged hypertension levels are at significant risk of developing retinal disease. Accurate modeling of the thermal behavior of ocular tissues is critical to optimizing treatment efficacy while minimizing adverse effects. In this paper, a novel finite element model is studied for the analysis of the thermal effects of two types of laser therapies on the retina. Two common laser wavelengths used in ophthalmology are simulated: 532 nm for pan-retinal photocoagulation and 810 nm for transpupillary thermotherapy. The effects of optical properties, such as absorption, scattering, and anisotropy index are incorporated. Also included are melanin granules in the retinal pigment epithelium, vascular networks in the neural retina and choroid, and fluid dynamics in the posterior segment of the eye. Some of the key limitations of previous approaches are overcome by the model. In addition, a comparative analysis is carried out between two intensity profiles: Top-Hat and Gaussian beams. The results show that Gaussian and Top-Hat laser beam profiles significantly affect thermal distributions, underscoring the critical role of beam shape and laser parameters in optimizing therapeutic outcomes. Sensitivity analysis showed that changes in perfusion of $��\; ��\pm �\; �50�\; �\%��$ shifted the peak temperature by less than 0.2 $��\; ��°�\; �C��$. Therefore, while perfusion helps to dissipate heat, it remains secondary to the beam profile and vitreous convection.

The Organic Rankine Cycle (ORC) has gained widespread adoption in recent decades due to its efficiency in converting medium-temperature heat sources into power. This study simulates and assesses ORC performance using AspenPlus (V10), focusing on subcritical conditions to achieve near-critical pressure to generate the highest net power output. This study examines the performance of pure and zeotropic mixtures of R1234ze(E), R1234yf (environmentally friendly hydrofluoroolefins), and R134a (a widely used hydroflurocarbon) in an ORC system. This study also addresses integrating the Subcritical Organic Rankine Cycle (SORC) system with a simulated and validated refinery boiler using actual operating data. The boiler was adjusted by modifying the combustion chamber pressure to generate medium-temperature flue gas output as a heat source in the SORC system. This integration demonstrates that the flue gas can be effectively utilized to increase the net power output of the system while simultaneously reducing the flue gas output temperature. Among the evaluated pure and binary zeotropic mixtures, the ternary mixture R1234ze(E)/R134a/R1234yf (0.7/0.2/0.1) achieved the highest network output of 1470.42 kW, surpassing all other assessed configurations. The binary mixture R1234ze(E)/R134a (0.8/0.2) followed closely, with a net power output of 1468.18 kW. The superior performance of zeotropic mixtures stems from their enhanced heat recovery capability, as evidenced by a 73.3%–76.6% reduction in flue gas outlet temperature. The zeotropic mixtures require a greater heat recovery demand due to lower flue gas output temperatures, leading to increased evaporator and condenser sizes. In turn, this raises the Specific Purchased Equipment Cost, impacting key economic indicators, such as Net Earning, Return on Investment (ROI), and Payback Period. While zeotropic mixtures significantly enhance heat recovery and net power output, they also increase capital costs. Understanding these implications is crucial for assessing ORC systems in industrial waste heat recovery applications, sustainability, and cost-effectiveness.

Natural convection heat transfer from a closed vertical cylindrical vessel with a shroud has been investigated experimentally. The experimental study has been performed for the large-diameter cylindrical vessel having a diameter of 0.8 m. The steam at atmospheric pressure has been injected into the vertical cylindrical vessel. The shroud around the vessel has been used to enhance the heat transfer performance. Experiments were conducted at different steam injection rates at atmospheric pressure and saturated steam conditions. The vessel outer wall temperatures and the air temperatures in the gap between the vessel and shroud at different vertical locations were measured during experiments using thermocouples. The heat transfer coefficients, Nusselt numbers, and Rayleigh numbers at different vertical locations have been calculated. The Rayleigh number varies from 4.48 × 10⁷ to 6.03 × 10⁹ based on the locations from the bottom to the top section of the closed vertical cylindrical vessel with ellipsoidal head. In the steady state, similar natural convection behavior of air between the vessel and shroud has been observed for different steam injection rates, as the outer wall temperature of the vessel at various vertical locations reaches a temperature close to that of saturated steam temperature, irrespective of the steam injection rate. The heat transfer coefficient was observed to decrease slightly from the bottom to the top vertical location on the vessel. The average heat transfer coefficient was found to be ≈ 7.98 ± 1.09 W/m² K. The variation in air temperature, vessel temperature, Nusselt number, and Rayleigh number along the vertical locations of the vessel has been discussed. The Rayleigh number was found to increase significantly along the height of the vessel. A correlation for the Nusselt number as a function of the Rayleigh number has been developed for the closed vertical cylindrical vessel with ellipsoidal head using experimental values. The comparison between developed correlation and well-known published correlations has been performed. The values predicted by the published correlations were found to underpredict the results, possibly due to differences in shape and geometry.

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.