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• 3D ice storage spheres with different metal foam methods and proportions are designed. • Solidification performance of PCMs augmented by partially metal foam is studied. • The competitive mechanism of natural convection and heat conduction is analyzed. • Considering the economic benefit, the optimal filling radius 6/13 is obtained. The advancement of ice-ball thermal energy storage systems is limited by the poor thermal conductivity of phase change materials(PCM). This paper presents a numerical investigation into enhancing heat transfer in ice balls by partially filling them with metal foam. Dynamic temperature changes, solid phase fraction, and cold storage capacity are analyzed for various filling radius ratios (2/13, 4/13, 6/13, 8/13, and 13/13). We quantitatively assess the specific impact of metal foam filling on heat transfer by calculating changes in the comprehensive thermal conductivity coefficient. Our findings reveal that the comprehensive thermal conductivity coefficient increases nonlinearly with the growing metal foam filling radius ratio, indicating that full filling may not the most optimal configuration. Furthermore, the energy storage capacity per unit time, per unit weight and per unit cost of the ice ball filled with metal foam under different radius ratios was evaluated by comprehensive evaluation criteria, and the optimal filling radius ratio was determined to be 6/13. Contrary to prior findings, this research highlights the efficacy of partial filling strategies, offering valuable insights for optimizing ice ball performance in thermal energy storage applications. [ABSTRACT FROM AUTHOR]

The behavior of non-Newtonian power-law nanofluids under free convection heat transfer conditions in a cooled square enclosure equipped with a heated fin is investigated numerically. In particular, the impact of nanofluids, composed of water and Al₂O₃ , TiO₂ , and Cu nanoparticles, on heat transfer enhancement is examined. The aim of this research is also to analyze the influence of different parameters, including the Rayleigh number (Ra = 104− 106), nanoparticle volume fraction (φ = 0% − 20%), non-Newtonian power-law indexes (n = 0.6 − 1.4), and fin dimensions (Ar = 0.3, 0.5, and 0.7). Streamlines and isotherms are used to depict flow and related heat transfer characteristics. Results indicate that thermal performance improves with increasing Rayleigh number, regardless of the nanoparticle type or nanofluid rheological behavior. This suggests that the buoyancy force has a significant impact on heat transfer, particularly near the heat source. The Nusselt number is more sensitive to variations in Cu nanoparticle volume fractions compared to Al₂O₃ and TiO₂. Moreover, the average Nusselt numbers for power-law nanofluids with n < 1 (n > 1) are greater (smaller) than for Newtonian fluids due to the decrease (increase) in viscosity with increasing (decreasing) shear rate, at the same values of Rayleigh number Ra owing to the amplification (attenuation) of the convective transfer. Notably, the most substantial enhancement is observed with Cu–water shear-thinning nanofluid, where the Nusselt number increases by 136% when changing from Newtonian to shear thinning behavior and by 154.9% when adding 16% nanoparticle volume fraction. Moreover, an even larger increase of 57% in the average Nusselt number is obtained on increasing the fin length from 0.3 to 0.7. Graphic Abstract [ABSTRACT FROM AUTHOR]

This paper reports the numerical study of natural convection in a square enclosure filled with CNT-water nanofluid and exposed to a uniform external magnetic field. Heating is ensured by twothin fins. Using the control volume method, the effects of the fins position, their length and spacing as well as the solid volume fraction, the Rayleigh number and the Hartmann number on the thermal performance of the cavity were examined. The results obtained show that the heat transfer rate increases with the Rayleigh number, solid volume fraction and fins length; but decreases with Hartmann numbers. A comparison is also carried out between the results obtained from the Maxwell and Xue models. The results prove that the mean Nusselt number is higher based on the Xue model. [ABSTRACT FROM AUTHOR]

This work is devoted to the study of the influence of height and the number of heaters on the nature of the flow and isotherms in a three-dimensional cube filled with Al2O3/H2O nanofluid with a volume fraction of 1% at Pr = 7.06586. Numerical simulation was performed on the base of the numerical finite volume method and the coupled scheme. It simulated the natural convection with nanofluid. Verification of the mathematical model was performed by comparing the results with the results of other authors. Having obtained satisfactory results, numerical results of U and V velocities, temperature contours and calculations of average Nusselt numbers for six cases of the main problems were obtained. [ABSTRACT FROM AUTHOR]

Buoyancy-driven flow and heat transfer characteristics of the solid/hollow cylinders with concave shapes were numerically analyzed in a turbulent regime. The effects of Ra , L / D , and D th / D on heat dissipation have been studied in the range of 1010 to 1012, 2 to 10, and 0.2 to 0.9, respectively. It has been found that the Nu ¯ increases with Ra for solid/hollow cylinders. For a hollow cylinder, Nu ¯ in is always higher than Nu ¯ out except at D th / D = 0.2. The Nu ¯ in of a hollow cylinder increases up to D th / D = 0.6, irrespective of the Ra , and L / D ratios thereafter reduced drastically has been observed. Whereas, at a low D th / D , and high Ra (i.e., Ra ≥ 1011), Nu ¯ out of a hollow cylinder significantly decreases with D th / D . However, it shows a marginal decrement at a high length-to-diameter ratio. For solid and hollow cylinders, the thermo-fluid dynamics have been delineated through the temperature contours, streamline patterns, and vortex generation at the inner as well as outer wall of the hollow cylinder. This numerical study also proposes a correlation for the Nu ¯ of the solid/hollow cylinders to have an accuracy ± 6 % of relevant parameters. [ABSTRACT FROM AUTHOR]

AbstractThis investigation aims to scrutinize the flow and heat transfer characteristics of a hybrid nanofluid composed of Ag-MgO within an eccentric annular space, where the inner circular wall is held at a constant elevated temperature and the outer circular wall is uniformly cooled. Natural convection in such eccentric annular enclosures, with thermal radiation and the effects of heat generation/absorption taken into consideration, plays a vital role in various engineering applications, including solar collectors, nuclear reactors, thermal storage systems, and heat exchangers. The resolution of the fundamental governing equations was executed employing the Gauss-Seidel approach, with the inclusion of a sub-relaxation process, all within the framework of the finite volume method. The impact of the Rayleigh number, radiation number, volume fraction as well as heat generation/absorption coefficient on thermal transfer, flow, and temperature distributions were investigated. It was observed that for low Rayleigh numbers, radiation influences enhance flow stability and improve the conductive heat transfer mode. Conversely, at high Rayleigh numbers, radiation intensifies convective heat exchange. The growth of the Rayleigh number diminishes the impact of heat generation and reinforces the radiation heat transfer process, while heat absorption exhibits the opposite effect. From Nr = 1 onwards, the average Nusselt number values are no longer influenced by an increase in nanoparticle volumetric fraction, and they are solely linked to the radiation number. [ABSTRACT FROM AUTHOR]

This study focuses on how the porous medium affects the plume generated due to line heat source when an aligned magnetic field is present. For this study, the momentum equation of the flow model is modified for porous medium by including the porosity term. A mathematical model is developed as coupled partial differential equations in order to study the flow problem. Later, a numerical solution is found for the system of coupled partial differential equations that are transmuted in to ordinary differential equations. For this purpose, the numerical characteristics of the problem are derived employing a shooting approach in combination with the built-in MATLAB tool bvp4c. The graphical illustrations of missing and specified boundary conditions demonstrate the impacts of porosity parameter Ω, magnetic force parameter S, Prandtl number Pr and magnetic Prandtl number γ accompanied by a discussion of their corresponding physical implications. The novelty of this developed problem is proclaimed with justification by its emphasis on the principal characteristics of heat and fluid flow affected predominantly by the presence of a porous medium. The thorough examination of the porosity parameter Ω for missing conditions depicts that the temperature and velocity profiles enhance while current density drops for the increasing values of the porosity parameter Ω. Whereas, for specified conditions, the skin friction and magnetic flux enhance but heat transfer rate declines with increment in Ω. [ABSTRACT FROM AUTHOR]

The work numerically investigated laminar natural convection heat transfer from the single sphere with a constant heat flux surface in air over the wide range of Grashof number ( 10 ≤ G r ≤ 10 7 ). The more efficient and precise numerical method based on Bejan et al. was employed here, the accuracy of which has been confirmed through validation against a single sphere case. The heat transfer characteristics were systemically analyzed in terms of isothermal contours and streamlines around the sphere, dimensionless temperature and velocity profiles. Additionally, local Nusselt number as well as local pressure and friction drag coefficients were studied with different Grashof number. In comparison to the sphere with uniform heat flux surface, the heat transfer from the isothermal sphere was found to be enhanced attributable to a more robust buoyancy force and a steeper temperature gradient. Moreover, the average Nusselt number for the sphere with a constant heat flux between 60.4 and 98.6% of the isothermal sphere's value, this range being contingent upon the specific Grashof number. What's more, the proposed correlation addresses a notable void in the predictive understanding of heat transfer from the sphere with uniform heat flux, which is scenario prevalent in various engineering applications, particularly for the cooling of electrical and nuclear systems, and offer values for academic research. [ABSTRACT FROM AUTHOR]

Abstract\nHIGHLIGHTSThe single-phase approach, in which nanofluids are treated as pure fluids assuming that the solid and liquid phases are in local thermal and hydrodynamic equilibrium, has been used with good results to simulate forced convection flows. Conversely, its extension to natural convection has revealed to be more problematic, as the obtained numerical results are substantially different from the experimental data, mainly due to the effects of the slip motion occurring between the suspended nanoparticles and the base liquid, whose consequent nonuniform distribution of the solid phase concentration can significantly affect both heat and momentum transfer. In the present article, a two-phase model based on a double-diffusive approach is proposed to study the natural convection flows of water-based metal oxide nanofluids inside cylindrical and rectangular enclosures heated and cooled at their opposite sides with the scope to reproduce a number of experimental data-sets available in the literature. Given the assumption of local thermal equilibrium between nanoparticles and host liquid, and considering the Brownian and thermophoretic diffusion as responsible for causing significant relative velocity between the solid and liquid phases, the governing equations are solved by a control-volume numerical method implemented using the open-source platform OpenFOAM (Open Field Operation and Manipulation). It has been found that the nanoparticle diffusion gives the major contribution to the decrease of the heat transfer rate red usually observed experimentally in natural convection flows of nanofluids inside differentially-heated enclosures. This means that a two-phase model, in conjunction with proper correlations for the evaluation of the nanofluid effective physical properties, must be necessarily enforced to obtain reliable results for many engineering applications.A two-phase model has been tested and validated reproducing different sets of experimental data available in the literature for natural convection flows of nanofluids inside differentially-heated enclosures.A description of the nanoparticle diffusion is given with the scope to point out the two-phase behavior of nanofluids.Optimal values of both the nanoparticle volume fraction and average nanofluid temperature are found to be imposed to enhance the heat transfer rate.A two-phase model has been tested and validated reproducing different sets of experimental data available in the literature for natural convection flows of nanofluids inside differentially-heated enclosures.A description of the nanoparticle diffusion is given with the scope to point out the two-phase behavior of nanofluids.Optimal values of both the nanoparticle volume fraction and average nanofluid temperature are found to be imposed to enhance the heat transfer rate. [ABSTRACT FROM AUTHOR]

The integrity of a pressure vessel (PRV) is affected by the decay heat of the molten pool in the lower head, so it is very important to study the natural convection heat transmission in the lower head. Scholars from all over the world have carried out a lot of experimental studies and calculations to determine the convection mechanism and heat transmission characteristics of the molten pool. Most of them were based on the empirical formula of convective heat transmission on the wall of a hemispherical molten pool based on two-dimensional slice experiments and simulation calculations. In this study, FLUENT 2021R1 software was used to simulate the three-dimensional convective heat transmission process of the hemispherical molten pool, and the temperature, velocity and wall heat transmission coefficients of the flow field were analyzed. The results were in good agreement with experimental data from UCLA (University of California, Los Angeles). Through research on the heat transmission of the lower head, the results showed that in the region where the wall angle was θ between approximately 72 and 90 degrees, heat transmission coefficients had larger fluctuations, and a more reasonable empirical relation was proposed. Comparing between the simulation results of CFD and the two-dimensional empirical formula, it was found that the latter one was smaller under the same θ angle condition. Finally, the convection phenomena of different external temperatures were simulated, and the main factors affecting the flow field by temperature were analyzed. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate fluid‐structure interaction (FSI) within conjugate natural convection. An oscillating fin, featuring a heat source placed at different locations, is examined using the Arbitrary Lagrangian–Eulerian formulation. The Galerkin finite element method is utilized to solve nonlinear dimensionless equations. Verification of grid independence is conducted and the model undergoes validation. Simulation outcomes for three fin positions (left, center, and right) and three heat source locations (bottom, middle, and top) are presented, illustrating streamlines, isotherms, and the average Nusselt number. The governing equations and boundary conditions are addressed using the finite element method. Temperature profiles in four scenarios are analyzed, along with horizontal velocities at different levels (0, D/2, D from the bottom wall). Dimensionless time (10−5 ≤ t ≤ 3), Ra = 106, Kr = 10, E = 1011 are used as parameters. The impact of the heat source position on vibratory motion is evaluated through Nusselt number variation, affecting heat exchange in different cases. The results show that heat transfer is minimal for a source location at the center of the fin (c). These findings also offer insights into FSI applications in economics and industry, guiding practical design considerations. Additionally, coupling the vibratory motion of the heat source with the flexible oscillating fin at the same frequency enhances understanding of the system. [ABSTRACT FROM AUTHOR]

Natural convection in enclosures is a phenomenon significantly affected by the geometry of the physical domain, however, comparing different formats is challenging because features such as the characteristic length varies with the geometry. This study proposes examining natural convection in 3D convex regular polyhedra, also known as Platonic solids, using the number of faces as a parameter. Differentially heated enclosures were analyzed to investigate the effect of shape on fluid flow and energy distribution. By comparing the different solids, it was found that the dodecahedron and the icosahedron exhibit very similar behaviors and tend to be the more efficient shapes for improving convection heat transfer. In contrast, the tetrahedron is much less efficient due to its structure, which does not favor the presence of a single convective cell. In general, increasing the number of faces, which leads to a more spherical shape and eliminates sharp corners, tends to enhance fluid recirculation and increase the average Nusselt number. For example, the icosahedron showed a 12% increase in the Nusselt number compared to the commonly studied hexahedron for Ra = 106. On the other hand, the tetrahedron demonstrated a 50% decrease in the Nusselt number relative to the cube, making it the least efficient shape among those evaluated. [ABSTRACT FROM AUTHOR]

Alumina nanoparticles have been accumulating attention in recent years because of their unique physical characteristics and broad range of engineering and industrial applications. Scientists are paying close attention to analyzing the flow and heat transfer features of alumina-based nanofluids filled within a cavity due to their wide range of applications, such as heat exchangers, fuel cells, room ventilation, and cooling electronic systems. Motivated by the applications, the current study numerically analyzes the natural convective flow and heat transfer features of A l 2 O 3 - H 2 O and A l 2 O 3 / C 2 H 6 O 2 - H 2 O nanofluids in a rectangular porous cavity under the presence of thermal radiation and heat generation/absorption by using the non-Fourier heat flux model. The system of equations is numerically solved by utilizing the Marker and Cell technique. The comparative fluid flow and heat transfer features are performed by using different shapes of A l 2 O 3 nanoparticles (spherical, brick, cylindrical, platelet, and blade) dispersed in two different kinds of base fluids ( H 2 O and C 2 H 6 O 2 - H 2 O ). The flow domain is filled with an isotropic porous medium. Several pertinent parameters are considered in this investigation, such as the Darcy number, Rayleigh number, heat source/sink, radiation parameter, and nanoparticle volume fractions, which are analyzed in terms of streamlines, isotherms, and local and average Nusselt numbers. According to the findings, the average Nusselt number increases by augmenting the nanoparticle volume concentration and the heat source/sink parameters, while it decreases by improving the radiation influence. Thermal performance within the cavity is affected by varying radiation influences. By increasing the spherical-shaped A l 2 O 3 nanoparticle concentration from 0 to 5% in H 2 O and C 2 H 6 O 2 - H 2 O base fluids, the average heat transfer rate is augmented by 27.37% and 27.62%, respectively. The blade-shaped nanoparticles produce a better heat transfer rate than other-shaped nanoparticles. Finally, it was concluded that A l 2 O 3 - H 2 O nanofluid delivers better fluid flow and temperature distribution than A l 2 O 3 / C 2 H 6 O 2 - H 2 O nanofluid. [ABSTRACT FROM AUTHOR]

This work focuses on the study of natural convection in a flat porous microchannel with asymmetric heating. The novelty of the work lies in the fact that for the first time the method of discrete symmetries was used to analyze the complete system of Navier–Stokes and energy equations in a two-dimensional approximation. Analytical solutions for velocity and temperature profiles have been derived based on symmetry analysis, taking into account boundary conditions such as slip and temperature jump at the channel walls. The effect of Grashof, Knudsen, Darcy, and Prandtl numbers on the flow characteristics in the microchannel and heat transfer coefficients was elucidated. At high Grashof numbers, an ascending flow near the hot wall and a descending flow near the cold wall arise. Increasing the Knudsen number leads to an increase in the velocity, temperature jump at the walls and a decrease in heat transfer coefficients. As the Darcy number increases, velocities amplify in both ascending and descending flows. The temperature jump at the hot wall grows up, while it remains unchanged at the cold wall. In the same time, the heat transfer coefficient at the hot wall decreases. [ABSTRACT FROM AUTHOR]

The phenomenon of natural convection is the subject of significant research interest due to its widespread occurrence in both natural and industrial contexts. This study focuses on investigating natural convection phenomena within triangular enclosures, specifically emphasizing a valley-shaped configuration. Our research comprehensively analyses unsteady, non-dimensional time-varying convection resulting from natural fluid flow within a valley-shaped cavity, where the inclined walls serve as hot surfaces and the top wall functions as a cold surface. We explore unsteady natural convection flows in this cavity, utilizing air as the operating fluid, considering a range of Rayleigh numbers from Ra = 100 to 108. Additionally, various non-dimensional times τ, spanning from 0 to 5000, are examined, with a fixed Prandtl number (Pr = 0.71) and aspect ratio (A = 0.5). Employing a two-dimensional framework for numerical analysis, our study focuses on identifying unstable flow mechanisms characterized by different non-dimensional times, including symmetric, asymmetric, and unsteady flow patterns. The numerical results reveal that natural convection flows remain steady in the symmetric state for Rayleigh values ranging from 100 to 7 × 103. Asymmetric flow occurs when the Ra surpasses 7 × 103. Under the asymmetric condition, flow arrives in an unsteady stage before stabilizing at the fully formed stage for 7 × 103 < Ra < 107. This study demonstrates that periodic unsteady flows shift into chaotic situations during the transitional stage before transferring to periodic behavior in the developed stage, but the chaotic flow remains predominant in the unsteady regime with larger Rayleigh numbers. Furthermore, we present an analysis of heat transfer within the cavity, discussing and quantifying its dependence on the Rayleigh number. [ABSTRACT FROM AUTHOR]

The heat transfer by natural convection of a nanofluid, which is ethylene glycol−Al2O3 has been analyzed in an open cavity numerically using the multiple-relaxation-time - lattice Boltzmann method by the graphics processing unit high-performance parallel computing. The right side of the cavity is open, and different boundary conditions have been applied to all the walls. Besides, one adiabatic fin has been installed on each side of the enclosure’s top and bottom sides. Here, the Prandtl number is fixed at 16.6, and the Rayleigh number changes from 104−106 with the nanoparticle volume fraction from 0%−5% has been used for numerical simulations. Besides, in this work, the power-law index is an important parameter as well, and 0.7, 0.8, 1, 1.2, and 1.4 are the values of this parameter. Results are presented concerning both the average and local Nusselt numbers in the form of streamlines, isotherms, temperature distributions, velocity distributions, heat transfer rate, and entropy production. It is observed when increases, average Nusselt number increases 607.94%, and for this reason, the overall heat transfer rate rises because of buoyancy force. In addition, the average Nusselt number falls by 83.28% when the power-law index rises; as a result, the total heat transfer rate falls because fluid viscosity increases with the power-law index. It is also observed that for shear-thickening fluids, the temperature gradient is higher. On the contrary, the temperature started decreasing with the increase of the power-law index. Additionally, the local Nusselt number value rises as power-law index falls. Moreover, the heat transfer rate increases by 7.08% when volume fraction increases. The intensity of buoyancy force reduces with the increase of volume fraction. Besides, the overall entropy generation rises when the Rayleigh number and the volume fraction increase, but it decreases when the power-law index increases. So, when the Rayleigh number is 106, the power-law index is 0.7, and the volume fraction is 0.00 then the entropy generation is the highest. This current research has many applications for example heat exchangers, electronic cooling equipment, solar heating systems, aerospace applications, medical devices, and entropy generation-related systems.

This study conducts a comprehensive numerical investigation into enhancing thermal transfer within square enclosures filled with water-based oxide nanoparticle suspensions, subjected to central sinusoidal heating. Further flow configuration, influenced by an inclined magnetic field, is designed with a focus on enhancing thermal efficiency for engineering applications. Key innovations include the application of sinusoidal heating elements to enhance thermal performance significantly. Computational analysis supported by finite element analysis, quantifies the impact of these parameters on flow dynamics and thermal transmission, presenting a substantial advance in the understanding of nanofluid-filled enclosure thermal management. The study reveals that the undulation of the heating element plays a crucial role in the heat transfer rate, with improvements observed as undulation increases. The introduction of magnetic fields further controls flow distribution and buoyancy effects, as demonstrated by our findings that an increase in the Rayleigh number correlates with enhanced convection, dominating the cavity's thermal dynamics. Additionally, the report outlines the conditions under which the Nusselt number increases, indicating enhanced thermal performance. These insights are pivotal for designing optimized heat transfer systems and energy-efficient applications, setting a new benchmark for thermal management strategies in practical engineering contexts. [ABSTRACT FROM AUTHOR]

The Particle Image Velocity method was used to investigate the velocity fields when the liquid layer height changed from 0.6 to 5.1 mm. The loss of stability of the velocity field during local heating is realized at the Bond number (Bo) 0.01–0.02, which is 10–20 times lower than the critical value of Bo(cr) compared to heat exchange at a uniform wall temperature. The Marangoni flow depends both on the height and the layer diameter. The critical height of the layer 2–2.5 mm leads to a sharp change in the flow pattern. [ABSTRACT FROM AUTHOR]

In order to improve convective flow, this study primarily used techniques including curved surfaces and a strong magnetic force. Cold temperatures and even flow are experienced by both the circular outer wall and the sinusoidal inner wall. FHD can have a bigger effect when combined with iron oxide in the base fluid. The electric current-carrying wire was positioned close to the interior wall in order to produce Kelvin force. A novel modeling approach was chosen to ascertain the number of scalars throughout the entire domain, and the process was validated using earlier numerical work. Gravity forces, ferrofluid concentrations, and the Kelvin force serve as the main pillars of current research. Conduction features increase and the Nu rises by 12.44% when nanopowders are used. Due to gravity and magnetic forces, the nanofluid can move more easily through the container. At Ra = 1 e 3 and 1e4, respectively, Nu increases by 93.04% and 35.43%; with an increase in MnF. Additionally, Nu rises with Ra at Mn F = 0 and Mn F = 4 0 0 0 by 43.55% and 0.71%, respectively. [ABSTRACT FROM AUTHOR]

Natural convection is a heat transfer mechanism driven by temperature or density differences, leading to fluid motion without external influence. It occurs in various natural and engineering phenomena, influencing heat transfer, climate, and fluid mixing in industrial processes. This work aims to use the Updated Lagrangian Particle Hydrodynamics (ULPH) theory to address natural convection problems. The Navier-Stokes equation is discretized using second-order nonlocal differential operators, allowing a direct solution of the Laplace operator for temperature in the energy equation. Various numerical simulations, including cases such as natural convection in square cavities and two concentric cylinders, were conducted to validate the reliability of the model. The results demonstrate that the proposed model exhibits excellent accuracy and performance, providing a promising and effective numerical approach for natural convection problems. [ABSTRACT FROM AUTHOR]

Natural convection in a square porous cavity with a partial magnetic field is investigated in this work. The magnetic field enters a part of the left wall horizontally. The horizontal walls of the cavity are thermally insulated. The wave vertical wall on the right side is at a low temperature, while the left wall is at a high temperature. The Brinkman-Forchheimer-extended Darcy equation of motion is utilized in the construction of the fluid flow model for the porous media. The Finite Element Method (FEM) was used to solve the problem's governing equations, and the current study was validated by comparing it to earlier research. On streamlines, isotherms, and Nusselt numbers, changes in the partial magnetic field length, Hartmann number, Rayleigh number, Darcy number, and number of wall waves have been examined. This paper will show that the magnetic field negatively impacts heat transmission. This suggests that the magnetic field can control heat transfer and fluid movement. Additionally, it was shown that heat transfer improved when the number of wall waves increased. [ABSTRACT FROM AUTHOR]

Hybrid nanofluids have shown promise as advanced novel heat transfer fluids by effective utilization of properties of its sub-constituents. This study explores the synergistic effect of silver-doped titania nanoparticles in ethylene glycol, focusing on thermal conductivity and viscosity improvements. A two-step synthesis method is employed to create the hybrid nanofluids, followed by comprehensive measurements of their thermal properties. Results demonstrate an impressive 18.9% increase in effective thermal conductivity compared to the base fluid. Furthermore, the viscosity of the hybrid nanofluids shows a concentration-dependent increase and a temperature-dependent decrease. Buoyancy-driven convective heat transfer experiments are conducted to evaluate the heat transfer performance of the hybrid nanofluids. A cubic cavity filled with the nanofluids allows for the assessment of Nusselt number and heat transfer coefficient values. Significant enhancements are observed compared to the base fluid, with maximum increases of approximately 16.8% for the Nusselt number and 26.87% for the heat transfer coefficient. However, these enhancements reach their peak at an optimal concentration before diminishing. Correlations are developed to predict the Nusselt number for natural convection within the cavity based on the experimental data. Additionally, an artificial neural network (ANN) model is employed for prediction and is found to outperform the correlations in accurately estimating the Nusselt number. These findings contribute to the understanding and advancement of hybrid nanofluids for efficient heat transfer applications. [ABSTRACT FROM AUTHOR]

The objective of this study is to create a simulation of a cavity containing high-heat rack server computing equipment. The aim is to explore various numbers of openings (two and four apertures) and rack layouts (shelf spacing of 30 and 60 mm and shelf height spacing of 35 and 17 mm) in order to minimize indoor temperature and achieve optimal heat dissipation. The numerical results are evaluated against the experimental data through the utilization of the least squares approach to determine unknown physical quantities. Next, a turbulence model that is appropriate is chosen using root mean square error analysis. The zero-equation model was selected for scenarios involving four ventilation openings, whereas the RNG k-ε model was good for scenarios involving two openings. Then, the resulting temperature and flow fields are assessed thereafter. Results revealed that expanding the distance between two racks has a minimal impact on the temperature of the rack surface and the convection coefficients. Thus, this research suggested using a shelf arrangement with a 30 mm shelf spacing to mitigate the occurrence of localized eddy currents at the upper part of the cavity, potentially diminishing the efficiency of ventilation. The presence of openings at the bottom of the cavity led to a 42% improvement in convection heat transfer coefficients, compared to cases without such apertures. Hence, it was recommended to incorporate apertures at the lower part of the cavity to facilitate the intake of cold air. Furthermore, reducing the shelf height spacing resulted in an increase in temperature of around 2 K on the surface of the rack. Nevertheless, it was deemed suitable for optimizing space utilization. [ABSTRACT FROM AUTHOR]

This numerical investigation explores the enhanced control of the 3D natural convection (NC) within a cubic cavity filled with graphene–water nanofluids, utilizing a bottom-center-located tree-shaped obstacle and a horizontal magnetic field (MF). The analysis includes the effects of the Rayleigh number (Ra), the solid volume fraction of graphene ( φ ), the Hartmann number (Ha), and the fins' length (W). The results show complex flow patterns and thermal behavior within the cavity, indicating the interactive effects of nanofluid properties, the tree-shaped obstacle, and magnetic field effects. The MHD effects reduce the convection, while the addition of graphene improves the thermal conductivity of the fluid, which enhances the heat transfer observed with increasing Rayleigh numbers. The increase in the fins' length on the heat transfer efficiency is found to be slightly negative, which is attributed to the complex interplay between the enhanced heat transfer surface area and fluid flow disruption. This study presents an original combination of non-destructive methods (magnetic field) and a destructive method (tree-shaped obstacle) for the control of the fluid flow and heat transfer characteristics in a 3D cavity filled with graphene–water nanofluids. In addition, it provides valuable information for optimizing heat transfer control strategies, with applications in electronic cooling, renewable energy systems, and advanced thermal management solutions. The application of a magnetic field was found to reduce the maximum velocity and total entropy generation by about 82% and 76%, respectively. The addition of graphene nanoparticles was found to reduce the maximum velocity by about 5.5% without the magnetic field and to increase it by 1.12% for Ha = 100. Varying the obstacles' length from W = 0.2 to W = 0.8 led to a reduction in velocity by about 23.6%. [ABSTRACT FROM AUTHOR]

The concept behind this article is an extensive analysis of the most recent findings about free convection thermal flow in an enclosure or cavity, as well as the methods employed to enhance heat transfer (HT). There are different heat transfer enhancers such as (a) nanofluids, (b) porous media, (c) large particle suspensions, (d) phase-change devices, (e) flexible seals, (g) fins and microfins, and (h) ultra high thermal conductivity composite materials. The majority of heat transfer enhancement techniques that use porous media, fins, and nanofluids are reviewed in the literature. In this review, both experiments and theoretical studies on different cavities are highlighted through investigating the subsequent parameters: impacts of the configuration of the cavities, effect of the type of nanoparticles, significance of cavities in nanofluids, effects of the cavities' tilt angle, wave amplitude, the magnetic field's (MF) influence in cavities, and cooler and heater impacts. Through a review of the literature, it was found that the use of nanofluid in all forms of cavities increases its thermal performance. The most used cavity is square, which has many applications. The results obtained from previous studies can be used to improve the design and heat refinement of the geometry applied in diverse energy systems. Additionally, as the Hartmann number rises, there is a rising tendency observed in the average Nusselt number. Furthermore, as the concentration of nanofluid particles and Rayleigh numbers rise, so does the rate of heat transfer. The heat transfer within the cavity is improved by the hybrid nanoparticles. The amount of the augmentation would, however, decrease as the concentration of nanoparticles increased. The lowest volume fraction of nanoparticles, Ø = 0.2 vol.%, showed the greatest increase. Also, the inner wave affects the flow pattern when the wave amplitude rises but the flow strength stays constant. The most important findings showed that the heterogeneous porous medium transfers heat more effectively than the homogeneous porous medium in different enclosures. [ABSTRACT FROM AUTHOR]

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In this study, we introduced modifications to a prior existing enthalpic lattice Boltzmann method (LBM) tailored for simulating the conjugate heat transfer phenomena in non-homogeneous media with time-dependent thermal properties. Our approach is based upon the incorporation of the remaining terms of a conservative energy equation, excluding only the terms regarding flow compressibility and viscous dissipation, thereby accounting for the local and transient variations in the thermophysical properties. The solutions of verification tests, comprising assessments of both transient and steady-state solutions, validated the accuracy of the proposed model, further bolstering its reliability for analyzing heat transfer processes. The modified model was then used to perform an analysis on structured cavities under free convection, revealing compelling insights, particularly regarding transient regimes, demonstrating that the structured cavities exhibit a beneficial impact on enhancing the heat transfer processes, hence providing insights for potential design enhancements in heat exchangers. These results demonstrate the potential of our modified enthalpic LBM approach for simulating complex heat transfer phenomena in non-homogeneous media and structured geometries, offering valuable results for heat exchanger engineering and optimization. [ABSTRACT FROM AUTHOR]

This paper introduces an innovative analytical model to compute flame velocities and temperatures within a premix burner in a domestic gas oven. This model significantly streamlines the heat transfer simulation process by simplifying the modeling of the thermo-chemical energy release during combustion, effectively reducing complexity and computation time. Accelerated solutions are essential at the initial design stages when comparing the effect of the oven parameter variation on the overall performance. The validation of the proposed analytical model involved experimental assessments of the temperature of the false bottom plate in a natural gas oven. The resulting data were then compared against CFD simulations performed utilizing the proposed model. The results revealed a marginal discrepancy of 4% between the experimental measurements and the outcomes generated by the model. Simulations were executed under differing conditions, encompassing scenarios with and without radiation effects. This exploration identified natural convection as the predominant heat transfer mechanism, with heat radiation contributing only modestly to the heating of the false bottom plate. Among its advantages, the proposed model offers a notable reduction in the numerical complexity of the modeling of the combustion process. Furthermore, its straightforward integration into numerical simulations involving premixed flames underscores its practical utility and versatility in evaluating design performance at the early stages of the design. Highly accurate models can be left for the final oven configuration validation. [ABSTRACT FROM AUTHOR]

Natural convection is numerically studied in a triangular cavity whose inclined walls that is isothermal at temperature TC, while its base is thermally insulated. The cavity contains a hot isothermal cylindrical heat source TH of diameter D. In this study, we used the nanofluid (water + TiO2). The nanoparticle volume fraction is varied within the range 0.01 ≤ ϕ ≤ 0.05, and the Rayleigh number is set between 10³ and 106. The main objective of this study is to explore the impact of nanoparticle concentration, Rayleigh number (Ra), and heat source position (h) on the enhancement of convective thermal transfer. The simulation results show that thermal exchange improves with increasing Ra, heat source diameter, and nanoparticle volume fraction (ϕ). [ABSTRACT FROM AUTHOR]

This study is devoted to an analysis of natural convection and the emergence of delamination in an incompressible fluid encapsulated in a closed region heated from the side. Weak, medium and intensive modes of stationary laminar thermal and thermo-concentration convection are considered. It is shown that nonlinear flow features can radically change the flow structure and characteristics of heat and mass transfer. Moreover, the temperature and concentration segregation in the center of the square region display a non-monotonic dependence on the Grashof number (flow intensity). The formation of a nonstationary periodic structure of thermal convection in boundary layers and in the core of a convective flow in the closed region is also examined. Details of the formation of countercurrents inside the region with the direction opposite to the main convective flow are given. Finally, the influence of vertical and horizontal vibrations on oscillatory convection is analyzed in detail. [ABSTRACT FROM AUTHOR]

This study investigates fluid flow and natural convection heat transfer in a parallelogram-shaped cavity with a porous medium under non-uniform temperature condition. The bottom wall is heated with a non-uniform temperature, the side walls are cooled, and the upper wall is adiabatic. The governing equations, along with corresponding boundary conditions, are formulated in dimensionless stream function and vorticity using the extended Darcy model for the porous medium. The equations are solved by employing the finite difference method. The study presents outcomes for isotherms, streamlines, and Nusselt numbers across range of parameters, including the Rayleigh number (10 4 ≤ R a ≤ 10 8), Darcy number (10 − 1 ≤ D a ≤ 10 − 6), Prandtl numbers (P r = 0.71 , 1 , 7.1), fluid porosity (ϵ = 0.8) , and inclination angles (30 ∘ ≤ ϕ ≤ 90 ∘), about flow and heat transfer characteristics. The results indicate that an increase in Ra , Pr , and ϕ , along with a decrease in Da , leads to an enhancement in the strength of vortexes. It is observed that for constant R a ⁎ = R a D a , Rate of heat transfer increases with an increase in Ra and a decrease in Da. The maximum value observed is 10.15. Also rate of Heat transfer increases with the increase in angle and maximum rate of heat transfer is observed at ϕ = 90 ∘. [ABSTRACT FROM AUTHOR]

The phenomenon of natural convection in rectangular cavities of different aspect ratios is considered. The water is considered a base fluid associated with copper nanoparticles. The flow is induced only due to buoyancy force that arises by heating the right side of the cavity. The left side is set cold while the other walls are assumed to be at zero flux temperature. The governing equations of the present communication are simulated by the finite difference method. This study explores the impacts of Rayleigh number (Ra), Prandtl number (Pr), nanoparticles volume fraction (ϕ), and aspect ratio (A). Different combinations of these parameters are investigated. Compared to other parameters, Rayleigh number (10 4 < Ra < 1 0 8 ), aspect ratio (1 ≤ A ≤ 3), and volume fraction of nanoparticles (0. 0 1 ≤ ϕ ≤ 0. 1), A is found more effective on the flow field and isotherms. Regression curves are determined for the mean Nusselt number (Nuavg) as a function of Ra for different cases. It is found that Nuavg more precisely fits the exponential function. Also, it is found that Nuavg decreases as the values of A increase. But, ϕ , shows the opposite behavior. It is noticed that when the A of the cavity grows, so does the mean heat transfer Nu. With rising Ra, the local heat transfer NuL decreases and the heat transfer rises. In the case of the square cavity, the regression coefficient for Nuavg is found to be 0.3673 and 0.2514 for an exponential function. [ABSTRACT FROM AUTHOR]

The transition of the steady fluid flow in a differently heated cavity with an adiabatic wavy fin installed on its hot wall to the unsteady one under the action of this fin was simulated numerically depending on the geometry of the fin. It was established that the thermal behavior of the fluid in the cavity is greatly dependent on the relative undulation of the fin, and the heat transfer in the cavity increases with increase in the fin-developed heat exchange surface of the cavity. [ABSTRACT FROM AUTHOR]

This study explores the numerical investigation of magnetohydrodynamic free convection flow within a rectangular cavity featuring a triangular obstacle near a corner heater. The upper horizontal wall is maintained at a cool temperature, while the right bottom corner is heated, and the remaining walls are kept adiabatic. Solving the governing nonlinear differential equations for general flow problems, along with boundary conditions, is accomplished using the Galerkin weighted residual finite element method. The simulation spans a broad range of parameters, including Rayleigh number Ra(102 = Ra = 104), Hartmann number Ha(0 = Ha = 100), and Prandtl number Pr (0.70 = Pr = 7.00). Results are presented in terms of stream functions, temperature profiles, and Nusselt Numbers. At low Rayleigh numbers (Ra = 103), the isotherms exhibit near-parallel alignment with the upper portion of the triangular obstacle, while higher Ra values lead to more distorted isotherms. An increase in Rayleigh number corresponds to a significant enhancement in flow circulation and heat transfer. Isotherms distributions remain consistent with increasing Hartmann numbers, but higher Hartmann numbers influence streamlines. Validation of the numerical model against previously published results demonstrates good agreement. [ABSTRACT FROM AUTHOR]

Analyzing fluid dynamics and heat transfer holds significant importance in the design and enhancement of engineering systems. The current investigation utilizes the finite element method to explore natural convection and heat transfer intricacies within a novel cavity containing an inner circular cylinder under steady and laminar flow conditions. The principal aim of this study is to assess the impact of Rayleigh number (Ra), Bejan number (Be), and the presence of adiabatic, hot, and cold cylinders on heat transfer, entropy generation, and fluid flow. The range of Ra considered in this investigation spans from 103 to 106, while the Prandtl number for the air is fixed at 0.71. The findings illustrate that the presence of a cylinder leads to higher Be as Ra increase, compared to scenarios where no cylinder is present. This observation suggests that buoyancy forces dominate in the absence of a cylinder, resulting in significantly enhanced convective heat transfer efficiency. However, the presence of a heated cylinder within the tooth‐shaped cavity exerts a substantial influence on the overall thermal performance of the system. Notably, the average Nusselt Number (Nu) experiences a remarkable increase of 41.97% under the influence of a heated cylinder, when compared to situations where a cold cylinder is present. This elevated average Nu signifies improved heat transfer characteristics, ultimately resulting in an overall improvement in the thermal system's efficiency. [ABSTRACT FROM AUTHOR]

Natural convection heat transfer from periodically ribbed vertical surfaces is targeted for upscaling, incorporating the analysis of thermal conduction through the microscale ribs. Asymptotic homogenization theory is employed, considering the steady conjugate heat transfer problem, to formulate second-order accurate effective conditions for velocity and temperature at a fictitious plane surface, beyond which the macroscale behavior of the flow is computed. This allows to avoid the numerically expensive resolution of fields within and through the microstructured corrugations. For the streamwise velocity component, the no-slip boundary condition is corrected at first order (in terms of a small parameter ϵ , ratio of microscopic to macroscopic length scales) by the classical Navier-slip condition plus a buoyancy term, while the gradient of the normal stress appears at second order together with a temperature-gradient term. The temperature at the virtual boundary deviates from the uniform value at the baseplate; the thermal slip is described via a first-order temperature-gradient term with a coefficient depending on rib geometry and thermal conductivity. Different case studies are conducted on the case of transverse square ribs, varying the density of the pattern and the rib-to-fluid thermal conductivity ratio, to provide extensive validation of the model against full feature-resolving simulations. Beyond the validation phase, a better understanding of the effects of different parameters on the heat transfer performance is pursued. The presence of ribs is found to decrease the overall heat transfer rate from the surface, and this deterioration is only partially alleviated by raising the thermal conductivity of the ribs. Increasing the number of conducting ribs on the hot surface has a complex, non-monotonic effect on the heat transfer rate, unlike the case of adiabatic ribs where the average Nusselt number decays monotonically. The performance of low-thermal-conductivity elements (e.g. wooden ribs) may considerably differ from that of perfectly adiabatic ones. [ABSTRACT FROM AUTHOR]

This paper proposes a new correlation to evaluate the heat transfer coefficient between a vertical wall containing a phase change material (PCM) and air in a square enclosure. This correlation was determined in order to simulate the transient process during PCM discharge and its effect on the heat transfer inside the cavity without using complex CFD models. A 2D CFD model based on the resolution of Navier–Stokes and energy equations inside the air and the PCM was previously validated. It was used to generate numerical data in order to build the proposed heat transfer correlation. The new correlation is Nu = 0.186 · Ra 0.28 θ 0.271 Ste 0.022 valid for: 10 5 ≤ Ra ≤ 4.310 7 , 0.05 ≤ θ ≤ 1 and 0.05 ≤ ste ≤ 0.6 . The accuracy of the proposed correlation versus the correlations established without phase change is analysed through a simplified model considering only the PCM layer and replacing the air cavity by a flux condition with an appropriate heat transfer coefficient. The relative error being lower than 1%, the new correlation shows a better agreement with the CFD results than existing correlations. [ABSTRACT FROM AUTHOR]

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The installation of successional heating devices in industrial buildings will result in thermal corridors. To improve the thermal environment in and around these corridors, buoyancy-driven ventilation is commonly utilized to dissipate heat, which is based on the natural convection design for buildings. However, the flow and heat exchange patterns of natural convection related to thermal corridors have not been clearly clarified, and no relevant correlations have been established to quantify them. The conducted numerical study aimed to analyze the flow and heat transfer characteristics of natural convection within thermal corridors in industrial buildings. Experimental data were utilized to validate a computational fluid dynamics (CFD) model developed for this purpose. The study considered the influence of various parameters on the results obtained. In the side corridor, the prevalence of reverse flow dominates much of the channel, while in the middle corridor, reverse flow near the bottom corner is observed. The ambient air temperature significantly impacts the temperature distribution in both corridors. Increasing the ambient air temperature at the inlet from 22 to 28 °C results in a substantial temperature rise within the corridor, by approximately 6–7 °C. When the outlet size is constant and the inlet size drops by 30%, the air temperature in the corridor increases by 3 °C. Finally, correlations were established based on the simulation data to predict the surface-averaged N u ¯ of the heated wall and the induced mass flow rate, m ˙ , of the natural convection. The correlations have relative errors of less than 16% when compared to the simulation data. [ABSTRACT FROM AUTHOR]

The thermal radiation and Fourier flux have drawn the attention of scientists and experts due to their extensive applications in the fields of medicine, manufacturing modern airplanes, water distillation, more efficient electronic devices, more efficient batteries, radiating treatment, and the textile industry. Through careful consideration of this, the mathematical investigation of natural convection and Fourier flux on the flow of energy and diffusion transmission of a body of cone and cylinder revolution, located in a saturated medium, has been described for the dual situations (flow over a cone and a cylinder). After then, the Runge–Kutta technique was used to explain the leading mechanism. Possessions of governing physical measures of local Nusselt and Sherwood numbers as well as velocity, thermal and diffusion figures are provided with support of tables and diagrams. It is motivating to declare that mass transfer rate is advanced in cone kind of revolution matched to cylinder kind of revolution with rising N R , N t , Q S . Also, the heat transmission rate is upper in cylinder revolution equated to cone revolution. [ABSTRACT FROM AUTHOR]

The finned-tube sodium-to-air heat exchanger plays a crucial role in ensuring the safety of prototype Gen IV sodium-cooled fast reactors. It is designed to actively operate with power, but it is also important to consider its passive performance to ensure availability during power-out conditions. This study focused on addressing this limitation through a natural convection experiment using a dedicated facility, and the obtained results were compared with the safety analysis code MARS-LMR. The overall temperature difference was observed to be within an acceptable range of less than 5 %. While the code calculations aligned well with the experimental results at higher temperatures, they underestimated the experimental results at lower temperatures. These findings can serve as a basis for other codes employing different heat transfer correlations and for future reactor licensing purposes. [ABSTRACT FROM AUTHOR]

Enhancing heat transfer for both natural and forced convection is a common issue for any heat transfer process. Experimental studies have been carried out for six different geometrical shapes of solid bars for natural convection and forced convection with four different air velocities while keeping the same perimeter and length of the solid bars, which means the lateral surface area of the bars is the same. Results reveal that both the natural and forced convective heat transfer characteristics are greatly influenced by the geometrical shape in terms of Nusselt number (Nu), heat transfer coefficient (h), and heat transfer rate (q). In addition, isosceles and cylindrical shape geometry contribute to the lowest and highest heat transfer, respectively. As well, it is obtained from the results that convective heat transfer characteristics are directly related to the cross-sectional area, even if the perimeters are the same. Moreover, among the different geometrical shapes, the isosceles and hexagonal shapes take the shortest and longest duration to attain the steady-state condition in the conductive heat transfer process. The convective heat transfer characteristics are well-validated, with available results for both natural and forced convection heat transfer. [ABSTRACT FROM AUTHOR]

This study examines the heat and mass transfer aspects of the natural convective flow of a nanofluid along a vertical flat surface, incorporating electrified nanoparticles and electric Reynolds number. While conventional nanofluid models like Buongiorno's model overlook the nanoparticle electrification and electric Reynolds number mechanisms, this study addresses the nanoparticle electrification and electric Reynolds number mechanisms by justifying its relevance, particularly when tribo-electrification results from Brownian motion. This incorporation of the electric Reynold number and nanoparticle electrification mechanism is a unique aspect of this investigation. Using the similarity method and nondimensionalization, the governing partial differential equations of the flow are transformed into a set of locally similar equations. MATLAB's bvp4c solver is employed to solve this set of equations, along with the boundary conditions. The obtained results are validated by comparison with those from previously published works. Graphical representations are provided for the numerical outcomes of non-dimensional velocity, concentration and temperature concerning the nanoparticle electrification parameter and electric Reynolds number. The combined effects of the nanoparticle electrification parameter and the electric Reynolds number on non-dimensional heat and mass transfer coefficients are examined in tabular form. Furthermore, the impact of the nanoparticle electrification parameter on both heat and mass transfer for varying values of the Brownian motion parameter is explored graphically. The primary finding of this investigation indicates that the electrification mechanism of nanoparticles quickens the transfer of heat and mass from a flat surface to nanofluid, suggesting promising prospects for utilization in cooling systems and biomedical applications. [ABSTRACT FROM AUTHOR]

This paper represents an endeavor to review the advancements achieved in the topic of coupled radiation-free convection (CRFC) across various investigated models. Through detailed bibliometric analysis, the paper not only underscores the importance of CRFC publications but also highlights their frequency, signaling an increasing interest and significant contributions in this domain. The literature review is systematically structured, categorizing studies based on the types of cavities/enclosures with or without internal heating sources, thereby providing a comprehensive and structured overview of this field. It then delves into the crux of CRFC, giving the governing equations, assumptions, parameters, and variables, establishing a robust knowledge. Transitioning seamlessly to application, the paper explores simulated models, boundary conditions, and insightful result discussions. Notably, the proven influence of surface thermal radiation in enhancing free convection heat transfer throughout the reviewed studies underscores CRFC's transformative potential, paving the path for innovative advancements in thermal engineering systems.

Effective cooling mechanisms are considered advancements in thermal control systems for diverse engineering purposes. Considering this fact, this investigation explores the analysis of natural convection and entropy generation by employing a decagonal enclosure using Galerkin Residual Finite Element method. In this study, influence of Rayleigh number (103 ≤ Ra ≤ 106), Bejan number (Be), Prandtl number (0.71 ≤ Pr ≤ 7), and isothermal boundary condition are considered. The findings reveal that both Ra and Pr significantly impact the characteristics of convection and flow patterns within the system. In cases with and without the circular object, higher values of Pr and Ra lead to a more pronounced cooling effect on the upper heated surface compared to the lower surface. Also, it enhances the overall thermal efficiency of the system by 112.05 % in comparison to the absence of a cylinder enclosure. These findings shed light on thermal control system advancements and contribute to field progress.