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This paper introduces an innovative approach for modelling unsteady incompressible natural convection flow over an inclined oscillating plate with an inclined magnetic effect that employs the Atangana-Baleanu time-fractional derivative (having a non-singular and non-local kernel) and the Mittag-Leffler function. The fractional model, which includes Fourier and Fick's equations, investigates memory effects and is solved using the Laplace transform. The Mittag-Leffler function captures power-law relaxation dynamics, which improves our understanding of thermal and fluid behaviour. Graphical examination shows the influence of fractional and physically involved parameters, leading to the conclusion that concentration, temperature, and velocity profiles initially grow and then decrease asymptotically with time. Moreover, the study emphasizes the impact of effective Prandtl and Schmidt numbers on temperature, concentration, and velocity levels in the fluid. [ABSTRACT FROM AUTHOR]

This computational study looks at the dynamics of free convection in a differentially heated square chamber occupied by ionized propane and saturated permeable partition. The modified Navier–Stokes equations and temperature equations are used to describe fluid and porous domain flow and heat transport phenomena. To solve these equations, the Galerkin finite element technique is used. Parametric adjustments are made for various heat source and sink lengths, Rayleigh number, Hartman number, and Darcy number. The results are quantitatively described in terms of the mean Nusselt number along the heated wall as the Rayleigh number increases (104 ≤ Ra ≤ 106). Streamlines and isotherms are used to visualize qualitative insights on these parametric changes. A comparative study is also performed without the porous media. Conclusive data demonstrate that using a permeable partition over a solid partition improves the mean Nusselt value by 26.28% at Ra = 104 and to a highest value of 56.5% at Ra = 106. Notably, this work presents a sensitivity analysis based on response surface methodology, demonstrating subtle connections between magnetohydrodynamics, Rayleigh number, and porous material effects. These findings provide important perspectives for improving thermal control and energy efficiency in advanced magnetohydrodynamic systems. [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]

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]

In this paper, the entropy generation of natural convection Jeffrey fluid flow through an inclined porous channel is investigated by taking into account the impact of angled magnetic field, Soret parameter, thermal radiation, and Hall current. On the basis of simplified assumptions, the governing equations are transformed into dimensionless equations by using suitable transformations, and the spectral quasi-linearization method (SQLM) is used for the numerical solution. Graphs are utilized to address the effects of new thermophysical parameters. The calculations reveal that increasing the Soret parameter, inclination angle, Hall parameter, magnetic parameter, and Jeffrey fluid parameter induces an increase in entropy generation. Conversely, it decreases as the radiation parameter and channel angle of inclination increase. [ABSTRACT FROM AUTHOR]

The multitube design in the shell-and-tube type latent heat thermal energy storage (LHTES) system has received intensive attention due to its promising benefits in enhancing heat storage efficiency. In this paper, single and multi-tube shell LHTES systems were experimentally investigated. First, this study experimentally compared the thermal characteristics between a multiple-tube heat exchanger (MTHX) and a single-tube heat exchanger (STHX). The STHX's geometrical parameters coincided with a virtual cylindrical domain in the MTHX, being similar to the single-tube model formulated by simplifying the numerical solution to investigate the MTHX. The experimental data was then used to validate the simplified numerical model commonly used in the literature that converted the multi-tube problem to a single-tube model by formulating a virtual cylindrical domain for each tube in the MTHX system. The results showed that there was a noticeable difference in the thermal characteristics between the actual STHX and the virtual cylindrical STHX domain in the MTHX system. The comparison indicated that the simplified numerical model could not accurately reflect the thermal performance of the MTHX system. An experimental study or three-dimensional numerical modelling was required for the thermal analysis of the multi-tube problems. Second, the effect of tube number in the MTHX was experimentally investigated. It was found that an increase in tube number boosted both charging and discharging rates without inhibiting the natural convection. The five-tube configuration decreased the total charging and discharging duration by 50% compared to the two-tube one. Finally, the effect of heat transfer fluid (HTF) operating parameters on the system performance was evaluated on the five-tube MTHX system. The results revealed that the adoption of higher HTF temperature considerably improved the charging performance. The charging time decreased by up to 41% with the HTF temperature increasing from 70 to 80 °C. Meanwhile, a variation in the HTF flow rate from 5 to 20 L/min showed a more pronounced influence on charging than on discharging due to the different dominant heat transfer mechanisms. [ABSTRACT FROM AUTHOR]

The investigation of entropic variations in the thermal transport mechanism produced by buoyantly driven temperature gradients has attracted significant attention because of excellent physical significance. Therefore, the prime consent to manipulate the current investigation is to explore the impact of change in the aspect ratio of the trapezoidal cavity in the optimization of the entropy phenomenon. After attaining motivation from its practical essence different entropies including thermal, viscous, and local are estimated. Additionally, global quantities such as average Bejan and Nusselt numbers calculated along with total entropy are measured against flow concerning parameters (aspect ratio (AR), Rayleigh number (Ra) and irreversibility ratio (ϕ )). Numerical experiments are performed by implementing a finite element approach using open-source software renowned as COMSOL Multiphysics. Before the accomplishment of the outcomes, confirmation of the numerical technique is assured by establishing grid sensitivity testing. Comparison of results between present and previous studies is also demonstrated. A wide range of involved sundry parameters varying from 10 - 4 ≤ ϕ ≤ 10 - 2 , 10 2 ≤ Ra ≤ 10 5 and 0.25 ≤ AR ≤ 0.75 are accounted. It is concluded that by escalating the aspect ratio from 0.50 to 0.75, the magnitude of the local entropy enhances from 3370 to 3424. It is revealed that the highest value of viscous entropy that is, 45, is achieved at Ra = 105 and by keeping the aspect ratio of enclosure equal to 0.75, whereas, the thermal entropy approaches 2 for the same situation of parameters. The magnitude of the average Bejan number reaches unity at AR = 0.5 and Ra = 105, whereas for low and high aspect ratios it depicts a magnitude less than 1 for the same Rayleigh number. [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]

In this manuscript, an attempt is made to investigate the natural convection process in hybrid nanofluid (Al2O3–Cu/water) contained in an isosceles triangular porous conduit with heated side/congruent walls by considering the performance of various thermal conditions. The bottom side of the enclosure is kept isothermally cooled, whereas uniformly and non-uniformly thermal boundary conditions are applied on congruent/side walls. Thermophysical properties of water and nanoparticles are used to model the problem with the help of continuity, momentum, and energy equations. Appropriate combination of variables is used to make the transport equations dimensionless. The obtained partial differential equations (PDEs) have been solved numerically by using Galerkin-based finite element method (GFEM). Trends of Darcy number Da (10 - 5 ≤ Da ≤ 10 - 3) , Rayleigh number Ra (10 3 ≤ Ra ≤ 10 6) , and solid volume fractions ϕ 1 , ϕ 2 (0.00 ≤ ϕ 1 , ϕ 2 ≤ 0.06) on streamlines, local Nusselt number, and isotherms have been presented in pictorial form. Bifurcation analysis is also described to make the contribution more versatile. Finally, results obtained from present simulation are matched for limited version (i.e., ϕ 1 = ϕ 2 = 0.0 ) in order to ensure the accuracy of the GFEM and modeling procedure of the mathematical model. [ABSTRACT FROM AUTHOR]

This study examines the simultaneous impact of temperature-dependent viscosity and Robin boundary conditions on velocity, focusing on analyzing the stability of buoyant parallel flow in a differentially heated vertical porous layer. The neutral stability condition and the instability thresholds are determined numerically for various values of governing parameters. The onset of instability of the base flow is accurately analyzed by introducing a non-negative parameter that measures the extent of departure of boundaries from impermeable to permeable. It is established that the base flow becomes unstable when this parameter exceeds a threshold value, which significantly depends on the variable viscosity parameter. This work demonstrates a clear bridge between impermeable and permeable boundaries in the context of a variable viscosity fluid. [ABSTRACT FROM AUTHOR]

The requirement for heat sinks to better reject excess thermal energy is ever increasing due to the recent improvements in output power capacity in the Information and Communications Technology (ICT) industry. Current ICT thermal management strategies rely on single phase heat transfer techniques which have attained their upper limit. The present work aims to demonstrate that two-phase thermal system strategies can decrease heat sink size. A comparison of the heat dissipation capacity of a natural convection heat sink with and without the thermal transport mechanism of vaporization are measured and discussed. A discussion relating to the mathematical analysis of the heat transfer mechanisms leads to quantified results showing the efficiency gains of a two phase micro-porous heat sink. It is shown that the presence of evaporation from the holes on the front surface of the radiator makes it possible to reduce its size by 37.6% compared to a radiator in which heat removal is carried out only by natural convection. [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]

The allocation of energy through buoyancy, along with the generation of irreversibility, has been examined in a porous, air-filled, quadrantal-shaped enclosure at different inclinations. The contour has a continuous heat source at the lowest wall, while the left and curved walls remain at colder temperature. The results are presented using vortex patterns and non-dimensional temperature distributions, Nusselt number along the hot leading lowermost wall, and the generation of entropy inside the cavity. Adjusting the Rayleigh number (Ra), Darcy number (Da), and inclination angle (φ ) to values between 103 and 106, 10−4 to 10−1, and 15–60°, respectively, allows for the explanations to be prepared. The analysis has revealed that altering these parameters causes noteworthy alterations in the flow dynamics and entropy generation. The prime design for thermal applications is initiated to be a contour with an inclination of 30° attributable to restrained magnitudes of average heat allocation and less establishment of irreversibility. [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]

The creation of heat is inextricably tied to the functioning of vehicle headlights, which presents a substantial problem for dependable and long-term operations. The concomitant rise in heat generation has grown to be a significant problem as the luminous effectiveness of light-emitting diode (LED) headlights continues to improve. Heat sinks have been commonly employed to remove surplus heat from the LED chips and other parts of the lighting system to solve this problem. This research investigates the impact of different heat sink fin architectures on the thermal performance of LED headlights by subjecting numerical models to varying LED powers ranging from 6 to 12 W across four fin models. Outcomes reveal that the diverse thermal characteristics resulting from variations in the number of fins in the heat sink lead to enhanced thermal dissipation of up to 73.6%. Furthermore, these heat sinks are subjected to vibrational testing under the ISO 16750 standard, assessing vibrational characteristics in three distinct scenarios: Passenger Car Engine, Passenger Car Sprung Masses, and Passenger Car Unsprung Masses. Through analysis, it reveals that the vibrational characteristics of the heat sinks align with established standards, reinforcing their reliability and suitability for automotive applications. [ABSTRACT FROM AUTHOR]

The application of a new sedimentation similarity parameter, containing an electrochemical number, is considered. Generalization of the results of experimental studies in various media under conditions of natural convection was carried out. New dimensionless equations of heat exchange have been derived. A new method has been developed for calculating the emission of heat during local formation of deposits on the heat exchange surface. [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 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]

For combined sensible-latent heat storage system (CSLHS) (termed as the hybrid configuration), macro encapsulation can effectively solve the leakage problem of PCMs. However, due to the poor thermal conductivity of PCMs, the charging performance of the hybrid configuration slightly increases compared to the solid structure (with only sensible materials). Meanwhile, the natural convection in the PCMs zone could improve the charging performance. So, how to improve natural convection intensity is a key issue for the CSLHS by macro encapsulating. It is found that adding fins can significantly enhance natural convection and accelerate the melting of PCM. In this paper, we proposed the hybrid configuration with fins built-in by macro encapsulation, and analyzed its charging performance with different fin structural parameters in the PCM zone by CFD simulation. In the case, the sensible heat storage material is high-temperature concrete and the PCM is a low-melting-point mixed molten salt. We analyzed the effects of fin number, fin length and fin thickness on the charging performance of the hybrid configuration respectively. From the result, the charging performance increases with the fin number, but the increase rate gradually decreases. When the fin number is 6, the charging performance increases by 20.18% compared to the situation without fin. The charging performance increases gradually with the fin length. Compared with the hybrid configuration without fin, for each 10 mm increase in fin length, its charging performances increase by 4.09%, 5.26%, 7.02%, 8.77%, 11.70%, and 15.79%, respectively. Different from number and length of fins, the effect of thickness on the charging performance is very small. When the fin thickness increased from 1 mm to 4 mm, the charging performance only increased by 2.3%. It indicates that the main reason for the improving the charging performance is to increase the natural convection intensity by dividing the PCM zone through fins. These results show that the charging performance of the CSLHS with macro encapsulation can be improved by optimizing fin structural parameters. [ABSTRACT FROM AUTHOR]

In the present study, natural convection heat transfer is investigated in a porous cavity filled with Cu/water nanofluid and equipped with horizontal fins. Optimization and sensitivity analysis of the fin's geometry, porous medium and nanofluid properties to maximize heat transfer rate is the aim of this work. To achieve this purpose, a design space is created by input parameters which include length, number of fins, distance between fins, porosity, Darcy number and volumetric fraction of the nanoparticles. Several tools have been used to implement optimization methods including the Taguchi method (TM) for design points generation, sensitivity analysis of design variables by using signal-to-noise ratio (SNR) and analysis of variance (ANOVA), response surface method (RSM) for interpolation and regression by using nonparametric regression, and genetic algorithm (GA) for finding optimum design point. The double multi-relaxation time lattice Boltzmann method (MRT-LBM) is used to analyze and simulate the flow field and heat transfer in each design point. The results show that the optimal configuration leads to an average Nusselt number of 5.56. This optimal configuration is at the length of fins L/2, the number of fins 2, the distance between fins L/12, porosity 0.8, Darcy number 0.1, and the volumetric fraction of the nanoparticles 0.02. By using the SNR results, the Darcy number and the number of fins have the most and the least effect in maximizing the average Nusselt number, respectively. The ANOVA results and global sensitivity analysis (GSA) findings further validated this conclusion. [ABSTRACT FROM AUTHOR]

A deep collocation method (DCM) is proposed for studying the natural convection phenomenon in the porous media (NCPM). The buoyancy-driven convection analysis inside the porous media is a complex process governed by dynamical conservation laws. Furthermore, these conservation laws involve complex nonlinear governing equations, which are required special computational techniques in well-known numerical methods like finite element method (FEM), finite difference method (FDM), finite volume method (FVM), and others. Such numerical schemes often face computational limitations like mesh generation, dimensionality limitations, increased computation errors for complex domains, and challenges in modeling physics. This research employs an unsupervised deep learning (DL) approach to address and resolve the typical computational challenges encountered in traditional numerical methods when dealing with natural convection in complex porous enclosures. In contrast to mesh-based numerical methods, the computational procedure in the DL approach involves domain and boundary discretization, followed by the random sampling of collocation points throughout the entire physical domain and its boundaries. Furthermore, a loss function is defined based on the governing differential equations and boundary conditions, which are minimized at the collocation points to achieve the desired solution. A combination of gradient-based optimizers is deployed to obtain a better set of parameter values using the backpropagation algorithm. The entire setup of the feedforward neural network is trained to approximate the solution with acceptable accuracy. The study explores four configurations of porous enclosures for a nonlinear mathematical model of natural convection in porous media, with various combinations of Neumann and Dirichlet boundary conditions. Additionally, the results from the mesh-based FEM are chosen as reference data to validate the consistency and accuracy of the DCM results. In all cases, the DCM results exhibit excellent agreement with the FEM results. [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]

In this work, vegetation effects on the characteristics of radiation-induced natural convection (isotherms, circulation patterns, exchange flow rate) in sloping water bodies are investigated numerically. The water body consists of (a) a sloping vegetated region (with a bottom slope equal to 0.1) and (b) a deep region with a horizontal bottom. The vegetation of porosity 0.85 (typical of aquatic plants found in lakes) has a length equal to the length of the sloping region. It can block (totally or partially) the radiation and as a result a non-uniform (differential) heating is developed along the free surface of the water body. The Volume-Averaged Navier–Stokes equations together with the Volume-Averaged Energy equation are solved numerically in the vegetated region. The radiation-induced natural convection in a water body with only a sloping region (with no vegetation) is also considered for validation purposes since numerical and scaling analysis results are available in literature. The results indicate significant vegetation effects on the thermal and flow patterns especially for vegetation which blocks completely surface heating. [ABSTRACT FROM AUTHOR]

This article investigates natural convection with double-diffusive properties numerically in a vertical bi-layered square enclosure. The cavity has two parts: one part is an isotropic and homogeneous porous along the wall, and an adjacent part is an aqueous fluid. Adiabatic, impermeable horizontal walls and constant and uniform temperatures and concentrations on other walls are maintained. To solve the governing equations, the finite element method (FEM) employed and predicted results shows the impact of typical elements of convection on double diffusion, namely the porosity thickness, cavity rotation angle, and thermal conductivity ratio. Different Darcy and Rayleigh numbers effects on heat transfer conditions were investigated, and the Nusselt number in the border of two layers was obtained. The expected results, presented as temperature field (isothermal lines) and velocity behavior in X and Y directions, show the different effects of the aforementioned parameters on double diffusion convective heat transfer. Also results show that with the increase in the thickness of the porous layer, the Nusselt number decreases, but at a thickness higher than 0.8, we will see an increase in the Nusselt number. Increasing the thermal conductivity ratio in values less than one leads to a decrease in the average Nusselt number, and by increasing that parameter from 1 to 10, the Nusselt values increase. A higher rotational angle of the cavity reduces the thermosolutal convective heat transfer, and increasing the Rayleigh and Darcy numbers, increases Nusselt. These results confirm that the findings obtained from the Finite Element Method (FEM), which is the main idea of this research, are in good agreement with previous studies that have been done with other numerical methods. [ABSTRACT FROM AUTHOR]

This research investigates the influence of various shape pores of porous media (circular, hexagonal, and square) on nanofluids' natural convection heat transfer. The experimental setup comprises a cube-shaped heat transfer chamber with two opposing hot and cold walls, while the remaining walls are adiabatic. The nanofluids used include Al2O3-water, and TiO2-water, each at two volumetric concentrations of 0.5% and 1%. The heat transfer performance was evaluated at three inclination chamber angles (0, 30, and 60°) toward the hot wall. The results reveal that the highest values of heat transfer coefficient and Nusselt number for all nanofluids and porous media occur at a chamber angle of 30°, while the lowest values occur at an angle of 0°. The highest values of heat transfer coefficient, Nusselt number, Nu* (illustrating the combined effect of porous medium and nanofluids on the Nusselt number of distilled water) are observed for the porous medium with square porosity, while the lowest values of these parameters are associated with the porous medium with circular porosity. Subsequently, all other cases encompassing the influence of angles and volumetric concentration percentages of nanofluids were meticulously investigated and analyzed. [ABSTRACT FROM AUTHOR]

The design of thermal conductivity enhancers (TCE) is quite critical to overcoming the disadvantage of the poor thermal conductivity of phase change materials (PCM). The main contribution of this study is firstly to discuss how to actively enhance natural convection of the melted PCM in cellular structure by the fin formed in the structure under the condition of the same metal mass, apart from simultaneously improving heat conduction, which can boost the heat transfer performance. Also, a tailored hybrid fin-lattice structure (HFS) as TCE is designed and fabricated by additive manufacturing (AM). A two-equation numerical method is applied to study the heat transfer of the PCM, and its feasibility is validated with the experimental data. The numerical results indicate that enhanced natural convection and improved heat conduction can be obtained simultaneously when a well-designed fin is embedded into a lattice structure. The enhanced natural convection results in the improved melting rate and the decreased wall temperature; e.g., the complete melting time and the wall temperature are reduced by 11.6% and 19.7%, respectively, because of the fin for metal aluminum. Moreover, the parameters of HFS including the porosity, pore density, and fin dimension have a great impact on the heat transfer. The enhancement effect of the fin for HFS on the melting rate of the PCM increases as the thermal conductivity of the base material decreases. For example, when the fin is introduced into the lattice structure, the complete melting time is reduced by 24.1% for metal titanium. In summary, this study enables us to obtain a good understanding of the mechanism of the heat transfer and provides necessary experimental data for the structural design of HFS fabricated by AM. [ABSTRACT FROM AUTHOR]

Novel designs have been proposed for the phase change material (PCM) heat sink of concentrated photovoltaic (CPV) cells to enhance both convective and conductive heat transfer mechanisms. Trapezoid (with two different thickness ratios) and zigzag geometry designs are suggested for the CPV-heat sink. To enhance the performance, two improving treatments have also been presented for the proposed trapezoid phase change material heat sink, (i) using a baffle and (ii) using two small triangle fins. A set of energy, exergy, economics, and environmental analyses has been established by computational fluid dynamics simulation to compare the performance of the proposed heat sinks with the conventional foamed and finned rectangular heat sinks. The results reveal that a trapezoid heat sink with two small triangle fins reduces the average concentrated photovoltaic temperature by 22 °C, the complete melting time of the phase change material by 1800s (50% reduction), and increases the absorbed heat by 3.13 kJ (7.43% increment), the exergy efficiency by 2.22% (23.9% increment), the generated electricity by 10.55%, and CO2 emission reduction by 10.75% compared to the concentrated photovoltaic with the simple rectangular heat sink. These results of the trapezoid heat sink are very similar to the rectangular heat sink with four long fins but with almost 74.3% lower levelized copper cost. [ABSTRACT FROM AUTHOR]

Radon exhaled from building materials infiltrates the indoor atmosphere and is transported into the indoor space by buoyancy-driven airflow. This paper investigated the dynamic coupling of radon concentration in the building wall area and indoor space. An indoor radon migration model under natural convection caused by temperature gradient was established. The radon exhalation rate, average Nusselt number, and average Sherwood number at the building wall and indoor space interface were quantified. The mechanism of radon migration from building materials into the indoor atmosphere was elucidated. Results show that natural convection influences the flow of indoor air and the radon concentration distribution, which increases with the increase of temperature gradient. [ABSTRACT FROM AUTHOR]

In this work, diurnal heating, by solar radiation, and cooling in a sloping water body with vegetated shallow regions are investigated numerically. The sloping water body consists of a vegetated region with a bottom slope equal to 0.1 and a deep region with a horizontal bottom. The Volume-Averaged Navier-Stokes equations together with the Volume-Averaged Energy equation are solved numerically in the vegetated region. The latter has porosity equal to 0.85 (typical of aquatic plants found in lakes) and a length equal to the total length of the sloping region. At the top free surface, a time-dependent thermal forcing is applied, which is reduced in the vegetated region. Shallow water is considered with a maximum water depth being less than the penetration depth of the solar radiation. The non-vegetated sloping water body is also considered for validation purposes. The results (isotherms, streamlines, exchange flow rate), after ten full thermal forcing cycles, indicate significant vegetation effects on the daytime and night circulation and the exchange flow rates. [ABSTRACT FROM AUTHOR]

The main objective of the present analysis is to characterize the transient buoyancy-motivated free convection turbulent flow and heat transfer characteristic features of an incompressible viscous fluid past a vertical cylinder with low-Reynolds-number (LRN) k–ε turbulence model in a two-dimensional coordinate system numerically. The Reynolds averaged Navier–Stokes equations (RANS) such as continuity, momentum, and energy are considered in terms of cylindrical coordinate system. The extra stress tensors obtained from the RANS model are closed using the eddy diffusive model. The local value of turbulent kinematic viscosity ( ν t ) is determined by utilizing the kinetic energy (k) and dissipation rate (ϵ) equations. The resulting system of partial differential equations (PDEs) with high nonlinearity, governing the turbulent boundary layer flow are solved using the implicit Crank–Nicolson technique. The discretized set of dimensionless tridiagonal algebraic equations are simplified by utilizing Thomas algorithm. Also, the simulated results are expressed in terms of graphs to analyse the average velocity, temperature, kinetic energy, dissipation rate, and also average momentum and heat transfer rates for the varying values of turbulent Prandtl ( Pr t ), Grashof (G r t) and Reynolds ( Re t ) numbers. It is noted that the average velocity, kinetic energy, dissipation rate of kinetic energy fields suppressed, and temperature field enhanced with increasing Re t . Also, the rising turbulent Prandtl parameter decreased the average velocity, temperature, turbulent kinetic energy, and dissipation rate profiles. Further, the increasing turbulent Grashof number decreased the kinetic energy and dissipation rate profiles. Further, the obtained results from the present turbulent investigation are compared with the existing results and observed an excellent agreement. [ABSTRACT FROM AUTHOR]

The aim of this study is to better understand the behavior of the nanofluid in a specific configuration, aiding in the creation of new models and designs for heat transfer systems, by investigating the MHD natural convection in an annular partially porous metal space between two vertical concentric cylinders, which is saturated by (Cu–water) nanofluid. The inside cylinder undergoes a regular heat flux, whereas the outer cylinder maintains a uniform temperature. The upper and lower walls are impermeable and insulated. In the upward direction, an exterior magnetic field with constant intensity is used. The nonlinear coupled conservation equations with specified boundary conditions in the vorticity-stream function form are solved using the finites differences method in conjunction with the successive over relaxation method. The numerical results obtained are presented to show the impact of a variety of control parameters depicted in Darcy number 10 - 5 ≤ Da ≤ 10 - 1 , Rayleigh number 10 4 ≤ Ra ≤ 10 6 , Hartmann number 0 ≤ Ha ≤ 100 , heater size, the porous layer thickness 0.25 ≤ Xp ≤ 0.75 , and nanoparticle concentration 0.01 ≤ ϕ ≤ 0.05 . From this study, the increase in the Ra number from 10 4 to 10 6 causes a thermal energy transmission improvement of 50%. Furthermore, a rise in the Da number from Da = 10 - 5 to Da = 10 - 1 enhances the thermal energy transport by approximately 30 % , while it reduces by 4.8% when we increase the Hartmann number from 0 to 100. Also, the rise in nanoparticle concentration leads to an enhancement of the average Nusselt number, while the heat transfer rate is reduced by extending the heater size. The numerical results also show a significant improvement in the thermal energy transport in active walls by using an optimum thickness layer of stainless steel porous medium, according to the Da number. Furthermore, this study demonstrates that there is a critical value of porosity for a given nanoparticle concentration and porous layer thickness for better heat transfer enhancement. [ABSTRACT FROM AUTHOR]

Nowadays, the application of renewable energies such as solar energy in the building sector has increased notably considering the adverse impacts of climate change on human life; hence many studies have focused on the application of photovoltaic panels in buildings. In the current study, a 3D computational fluid dynamics (CFD) model has been developed to evaluate the performance of a newly designed building-integrated photovoltaic (BIPV) system. Given the negative influence of overheating on the lifespan and performance of PV panels, their passive air cooling has been studied. Further, the potential of rooftop-mounted solar panels in passive ventilation of buildings by generating natural convective currents has been explored. The developed CFD model takes into consideration the effects of radiation, conduction, and buoyancy-driven natural convective currents generated by solar PV panels which are heated due to the exposure to solar radiation heat flux. The results suggest that applying a high surface emissivity for the part of the roof beneath the PV panels intensifies the natural convective currents which in turn provides better cooling for PV panels with higher cooling effects at higher solar heat fluxes. Up to a 34% increase in the convective mass flow rate and a 3 K decrease in the mean temperature of the panels were attained by modifying the emissivity of roof surface. Such a 3 K decrease in the operating temperature of the PV panels can enhance their efficiency and lifespan by about 1.56% and 21 %, respectively. Based on the operating conditions and system characteristics, the BIPV system yielded an air change rate (ACH) in the range of 3–13 which was considered to be highly prevalent in providing the required passive ventilation for a wide range of applications. It was also observed that the flow dynamics inside the building were affected by both the amount of solar heat load incident on the solar panels and the emissivity of the roof surface behind the panels. [ABSTRACT FROM AUTHOR]

In recent years, heat storage system combining sensible and latent heat materials has received more and more attentions. In this paper, we proposed the hybrid configuration with a macro-encapsulation, and analyzed its charging performance with different influencing factors by CFD simulation. In the case, the sensible heat storage materials are magnesia brick or HT concrete and the phase change materials (PCMs) are mixed molten salts. Firstly, we analyzed the heat transfer characteristics of the hybrid configuration in charging process. Then, we analyzed the effect of heating power on charging performance. The maximum temperature of the heating surface shall not exceed 500°C as the constraint condition, the heat storage capacity increases at first and then decreases with the heating power. Then, we compared the charging performance of different solid structure and the hybrid configurations. Whether magnesia brick or HT concrete, the charging performance of the solid structure is better than that of the hybrid configuration, because the thermal conductivity of the molten salt is significantly lower than that of the two sensible heat storage materials. Then, we compared the charging performance of different molten salts. The hybrid configuration with lower melting point molten salt has better performance because of more intensity natural convection. Finally, we analyzed the charging performance of the hybrid configuration used the composite phase change material (CPCM) with high thermal conductivity and specific heat. From the result, the charging performance increases by 22.5% compared with the solid structure. These results indicate that the hybrid configuration with the macro-encapsulation method is a potential form of thermal energy storage, but it needs to be further optimized. [ABSTRACT FROM AUTHOR]

This study numerically simulates optimal thermal convection and entropy production minimization in a steady magnetohydrodynamic micropolar hybrid nanofluid (Ag–Al2O3/H2O) flow through an annulus region bounded between an elliptical cylinder (inner) and a square cylinder (outer) with partially insulated and partially heated walls. Four different configurations with heater locations were considered when the heater was placed continuously or discretely at different locations along the annulus' boundaries. The governing equations were derived from the Navier–Stokes equations along with the angular momentum equation of microrotation. The flow domain, bounded between two dissimilar geometrical surfaces, was transformed into a computational domain. The modeled set of equations was solved using an in-house developed MATLAB program code based on the finite difference method with the succesive over relaxation, succesive under relaxation, and Gauss–Seidel iteration techniques. The impacts of the Hartmann number, Rayleigh number, volume fraction of nanoparticles, and vortex viscosity parameter on the streamlines, isotherms, average Nusselt number, average Bejan number, and entropy generation number were computed to analyze the flow dynamics and convective heat transfer mechanism. The average Nusselt number increased linearly with ϕhnf, higher for the inner elliptic cylinder. As K0 rises, heat convection declines. In contrast to Cases II and I, Cases III and IV exhibit a greater attenuation of heat convection at the inner elliptical cylinder. The Nuo declines rapidly for Cases I and III compared to Cases II and IV. Increasing the concentration of nanoparticles improved heat convection. Both Nui and Nuo increase linearly with ϕhnf, but the rise is more pronounced from the inner elliptical cylinder than the outer square cylinder. It is found that Cases II and III have the highest and lowest Beavg values, while Cases III and I have the highest and lowest Ns values. Beavg diminishes and the total entropy generation Ns increases with Ha, Ra, and ϕhnf, while reverse effect of vortex viscosity Ko. [ABSTRACT FROM AUTHOR]

In the present work, the effect of natural convection caused by immiscible intermediate fluid on melting process in a horizontal shell-and-tube latent heat storage is examined numerically. The intermediate fluid is placed between the section filled with phase change material and the storage outer hot wall (as heat source). Water is considered as the intermediate fluid to transfer heat from the hot wall to PCM (n-octadecane) by the buoyancy impact of heated water. Water and the liquid phase of the n-octadecane are immiscible. Numerical analysis is performed employing the enthalpy-porosity method with the assistance of a commercial CFD code, ANSYS FLUENT 18.2, based on the finite volume method. The present work consists of two parts. In the first part, a simple 2D latent heat storage with various volume proportions of water as intermediate fluid, including; 25, 50, 75, and 100%, is considered. In the second part, the effect of the number and arrangement of heat transfer fluid channels is evaluated and analyzed. In all investigated models, the volume of the storage is constant and the amount of the PCM varies. The solid PCM was subcooled to 1 ℃, and the temperature of the surface of the storage was maintained at 329 K. The pertinent dimensionless parameters, including; Prandtl, Rayleigh, and Stefan numbers, are 59.5, 6.3 × 108, and 0.27, respectively. The results depicted that the maximum saved energy inside the proposed latent heat storage belongs to the case with 50% of storage volume capacity filled by water. Moreover, the case with four heat transfer fluid tubes indicated the highest absorbed energy, and among them, the four tubes with diamond pattern arrangement stored the highest amount of energy. [ABSTRACT FROM AUTHOR]

Investigations were performed of heat transfer to a forced upward flow of mercury in a tube inserted into a heated channel with a rectangular cross-section under the effect of a transverse magnetic field. The outer channel is filled with mercury and connected to a natural circulation loop. Liquid metal heat transfer is simulated in a cell of the cooling system of the channel-type liquid metal blanket for a Tokamak fusion reactor. Experimental data on temperature fields and heat-transfer performance in the inner tube and the outer channel were obtained in the mercury magnetohydrodynamic test rig using microthermocouple probes. Three different cases of natural circulation loop operation are examined: (I) the loop is off, convective flow can occur only in the space between the tube and the channel wall; (II) the loop is open and operates under adiabatic conditions; (III) the loop is open, water cooling is on. The results of measurement in the inner tube demonstrate that heat transfer in the tube-in-channel system is enhanced compared to the heat transfer in a separate tube both with and without a magnetic field. Under the experimental conditions, natural convection is induced by the buoyancy and electromagnetic forces in the gap between the tube and the channel wall. The configuration and structure of the flow in the gap change drastically in a transverse magnetic field, and the heat-transfer rate depends on the operating conditions in the natural circulation loop. Convection reduces temperature nonuniformities in the gap, and the heat transfer in the investigated "tube-in-channel" enhances greater when the natural circulation loop is activated and, especially, when it is additionally cooled. Low-frequency high-amplitude fluctuations induced by the instability of the natural convection and magnetohydrodynamic flows are observed in the gap. [ABSTRACT FROM AUTHOR]

In this study, the efficiency of air velocity on solar panels during cooling was studied based on temperature and solar radiation in the environment where the panels are located. When the panels cool down, the temperature of the rear panel decreases and, accordingly, the idle voltage of the panels increases. Currently, the most significant losses in panels are associated with an increase in the temperature of the panels, depending on solar radiation and outdoor temperature. The article presents mathematical modeling of turbulent natural air convection in a heated photovoltaic solar panel. The considered problem, despite its relative simplicity, contains all the main elements characteristic of currents near the wall caused by buoyancy forces. A significant disadvantage of the algebraic Reynolds-Averaged Navier—Stokes (RANS) turbulence models for solving this problem is that for them it is necessary to set the transition point from the laminar to turbulent mode from the experiment. Therefore, the work uses the modern Spalart—Allmares (SA) turbulence model, which has a high rating in the NASA database. In order to verify the model, the obtained results are compared with known experimental data. It is shown that the SA model describes the turbulence zone well. The paper shows that an additional force arises as a result of the temperature gradient, which plays an important role in describing turbulent natural convection. The results show good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

In this study, the unknown boundary heat fluxes in a square enclosure containing a cylinder were estimated by an inverse technique. A series of computations was conducted for the two-dimensional, steady-state, and buoyancy-driven heat transfer in a square section containing a cylinder with variable heat fluxes and at a Rayleigh number (Ra) of 106 and Prandtl number (Pr) of 0.7. The generated datasets were used to construct a physics-based neural network, which acted as a proxy model for natural convection to reduce the computational time for inverse estimation. The trained network was embedded in a genetic algorithm and Bayesian framework to estimate the boundary conditions of the heat fluxes from synthetic experimental temperatures. The results indicated that the genetic algorithm accurately predicted the heat flux, but the estimation failed with increasing measurement error/noise. The solutions of the genetic algorithm were then used as informative priors for the Bayesian framework, which outperformed the genetic algorithm at estimating unknown boundary heat fluxes with measurement noise. The estimated heat fluxes were then used as input for the direct problem and investigated the thermal and flow characteristics in an enclosure containing a cylinder. [ABSTRACT FROM AUTHOR]

In this research paper, the results of the numerical investigations on combined surface radiation and natural convection heat transfer in a square enclosure with partition are presented. The vertical walls of the square enclosure are differentially heated, with the left wall at a higher temperature and the right wall at a lower temperature. The partition is placed parallel to these isothermal vertical walls while the horizontal walls are adiabatic. The simulation is done for a Rayleigh Number ranging between 103 and 106, Prandtl number of 0.7 (air). In this research work, the numerical simulation is carried out using the software ANSYS FLUENT 2022R1 in which the governing equations of fluid flow and heat transfer with surface radiation are solved using finite volume-based solver and coupled algorithm. Discrete Ordinate (DO) is used as the radiation model. The thickness of the partitioned is fixed and is equal to one twentieth of the length of the square enclosure. An analysis from the obtained streamlines, temperature contours and Nusselt number values shows the significant influence of surface radiation on natural convection heat transfer. It has been observed that increasing the length of partition reduces the Nusselt number, when the height of partition is varied from h = L to L/4, the value of Nusselt number increases to 7.2% for Ra 103 and 30.3% for Ra 106. It is also established that increasing the distance of partition from hot wall increases the Nusselt number. Likewise, an increase in emissivity leads to an increase in total Nusselt number which can be noted that when emissivity is changed from 0.25 to 1, the increase in total Nusselt number is found to be 24.9% and 97.1% respectively when compared with pure convection for Ra = 103. For Rayleigh number 106 the increase in Nusselt number is 32.6% and 170.9% sequentially when compared with pure convection. [ABSTRACT FROM AUTHOR]

In this work, Al-4.5wt.%Cu was selected as the research object, and a phase field-lattice Boltzmann method (PF-LBM) model based on compute unified device architecture (CUDA) was established to solve the problem of low serial computing efficiency of a traditional CPU and achieve significant acceleration. This model was used to explore the evolution of dendrite growth under natural convection. Through the study of the tip velocities, it is found that the growth of the dendrite arms at the bottom is inhibited while the growth of the dendrite arms at the top is promoted by natural convection. In addition, research on the inclined dendrite under natural convection was conducted. It is observed that there is a deviation between the actual growth direction and the preferred angle of the inclined dendrite. With the increase of the preferred angle of the seed, the difference between the actual growth direction and the initial preferred angle of the inclined dendrite shows a trend of increasing at first and then decreasing. In the simulation area, the relative deflection directions of the primary dendrite arms in the top right corner and the bottom left corner of the same dendrite are almost counterclockwise, while the relative deflection directions of the other two primary dendrite arms are clockwise. [ABSTRACT FROM AUTHOR]

A way of harvesting water from the air that avoids the discontinuity of the adsorption/desorption cycles is theoretically analyzed. A rectangular prism-shaped adsorbent bed is immersed in low-humidity air, at an angle to the horizontal and subject to a temperature gradient between two opposite and open faces. The other four faces of the prism remain isolated. Water is adsorbed on the adsorbent colder face, causing a density gradient in the surrounding air, parallel to the surface, that results in a self-sustained continuous air flow. On the opposite face, a self-sustained continuous air flow parallel to the surface also arises, but this time due to a temperature gradient in the air surrounding the hot bed face. In addition, its higher temperature causes the desorption of water from the adsorbent. This overall water exchange produces the enrichment of water content in one of the air streams that is crucial to produce water harvesting. The performance of Al-Fumarate, MOF-303, SAPO-34 and Zeolite 13X is tested, unveiling the key factors that increase flow rate and water concentration at the enriched phase. It has been found that the diffusive mass transport at the air-solid interphase is the bottleneck of water harvesting in continuous flow conditions. Therefore, if high concentration of water is desired, it is necessary to use porous materials with very high diffusitivities. These findings provide the foundations for the design of continuous water harvesting devices. Article Highlights: The performance of four molecular sieves with enhanced water adsorption properties is tested for continuous harvesting. A theoretical model is done by coupling natural air convection with non-fickean water transport inside molecular sieves. High diffusion coefficient of water in porous media plays a key role for efficient continuous water harvesting. [ABSTRACT FROM AUTHOR]

In this study, the local thermal non-equilibrium model (LTNE) and the parabolic boundary layer governing differential equations were used to investigating the effect of the flow and solid matrix parameters on the Nusselt number for free convection over a horizontal cylinder embedded in a saturated infinite packed bed. The Forchheimer–Brinkman-extended Darcy and the local thermal non-equilibrium scheme were solved by the Keller box numerical method. Using the variables of the boundary layer, the physical environment is transferred to a rectangular computational domain; then with the new variables, one order of the derivatives of the equations is reduced and the three-diagonal matrix of the coefficients is calculated. The impacts of the porosity, thermal conductivity ratio, Rayleigh number, the ratio of cylinder diameter to spherical particle diameter and Biot number parameters on the local and average Nusselt number have been studied. The obtained results showed that, the increasing Rayleigh number and porosity increase the mean Nusselt number of two phases. In such a way that increasing the porosity from 0.2 to 0.85 can increase the Nusselt of fluid by 8 times. Also, increasing the ratio of cylinder diameter to spherical particle diameter from 20 to 100 causes the mean fluid Nusselt to decrease by 64%. In addition, increasing the ratio of solid to fluid conductivity and Biot number reduces the average Nusselt of the fluid. Except for the Rayleigh number, the changes of other fluid and solid parameters did not have a significant effect on the amount of solid Nusselt. [ABSTRACT FROM AUTHOR]

Magnetohydrodynamic (MHD) natural convection of non-Newtonian ferrofluid and entropy generation in a square enclosure containing a wavy cylinder was investigated. The inner wavy cylinder was assumed to be heated and the outer square enclosure to be cold. The ferrofluid's rheology was presented by the power-law model, while density fluctuations owing to thermal expansion were described using the Boussinesq approximation. Numerical calculations had been performed using dimensionless parameters such as Hartmann number, power-law index, Rayleigh number, wave number, and volume fraction. Results are discussed in terms of isotherms, velocity field, average Nusselt number, and entropy generation, taking into account the variations in physically significant parameters. Results indicate that thermal convection dominates the isotherms of shear-thinning fluids, while conduction is more prominent in shear-thickening fluids. The power-law index (n) greatly influences the streamlines and isotherms. The non-Newtonian ferrofluid's average Nusselt number ( Nu ¯ ) rises as the Hartmann number is reduced and the Rayleigh number (Ra) is increased. In this simulation, the maximum value of Nu ¯ is found to be 8.38 because of the addition of ferroparticles. Additionally, the irreversibility caused by fluid friction, heat transfer, and magnetic field for the shear-thinning (n < 1), Newtonian (n = 1), and shear-thickening (n > 1) cases can be minimized by using the ideal parametric combination. [ABSTRACT FROM AUTHOR]

In this work, the pressure decay method was implemented to measure the diffusion coefficient of carbon dioxide in light oil at different temperatures and pressures. The mechanism of molecular diffusion of gas inside the liquid was studied by evaluating the kinetics of gas absorption. Assuming one-way diffusion from the gas phase to the liquid, mathematical modeling was developed based on Fick's second law, and a mathematical relation was obtained for predicting the gas phase pressure versus time. Finally, the molecular diffusion coefficient was calculated using two methods including graphical and trial-and-error methods. Results showed that for carbon dioxide-crude oil system, the numerical values of the molecular diffusion coefficient were in the order of 10–8 m2/s which were greater than the values expected for the gas diffusion in liquids. It was also found that the effect of temperature on the diffusion coefficient was not in accordance with the behavior of diffusion mechanism, i.e., by increasing temperature, the effective diffusion coefficient decreased. It was concluded that in addition to the molecular diffusion, there should be a stronger mass transfer mechanism such as natural convection within the liquid phase whose origin is the concentration gradient within the liquid column. Therefore, the value of diffusion coefficient obtained in this work was assigned as effective diffusion coefficient (Deff). The solubility decreases with increasing temperature, and it causes the concentration gradient to be smaller. Therefore, natural convection is expected to be weaker at higher temperatures which leads to lower effective diffusion coefficient. [ABSTRACT FROM AUTHOR]

Efficient heat management is a serious challenge of integrating and miniaturizing high-power electronic devices. Fin heat sink based on phase change material (PCM) is an efficient passive cooling technology for electronic devices with alternative high-power density. It is developed to cool down electronic parts in transient applications. The efficiency of PCM-based heat sink is acceptable only below the melting point so that afterward, the temperature of electronic parts increases notably. The present study, in addition to using an aluminum heat sink with PCM for passive thermal management, forced airflow (2.5 ms−1 and 4.5 ms−1) was employed to examine the effects on the system performance. In addition, two PCM materials (n-Eicosane and paraffin) with two different layouts were used at power levels 10 W (8.3 kW m−2) and 15W (12.5 kW m−2). The results indicated that with paraffin in the middle and airflow velocity of 2.5 ms−1 and 4.5 ms−1 the maximum temperature of the heat sink decreased by 45% and 58%, respectively, compared to the design without forced convection. The layout of PCM in multi-PCM design had a notable impact on the performance in forced convection condition in particular. The increment rate at 100 mW mode and airflow of 4.5 ms−1 with paraffin in the middle was 150% higher than that of the alternate layout. [ABSTRACT FROM AUTHOR]

Turbulent natural convection (Ra = 1.58·109) in a confined 3D square cavity with two differentially heated side walls are simulated numerically using the in-house EDF code (Code_Saturne) based on the unstructured finite volume solver. The objective of the present work is to investigate the performance of the low-Reynolds-number models known by their good suitability for the near-wall treatment. The low-Reynolds-number models, shear stress transport (SST) k–ω model, φ–f model which is a developed version of the original υ ¯ 2 f model, and the LES (large-eddy simulation) technique are used, and the results of their using are compared with the experimental benchmark data. The numerical results show quantitative and qualitative agreements. In general, the SST k–ω model gives good predictions for the temperature profiles, and the φ –f model is more accurate for the velocity profile prediction. This is mainly due to the good resolution of the turbulence properties in the near-wall region and to the ability to mimic the physical flow features in this type of geometries. [ABSTRACT FROM AUTHOR]

The present work examines experimentally the natural convection heat transfer within a three-dimensional novel I-shaped enclosure with wavy-walled and inner circular pipe. The left layer is filled by Al2O3–water nanofluid while the right layer is filled with nanofluid/porous medium. The nanofluid thermophysical properties had been measured experimentally. The thermal conductivity had been measured utilizing hot-wire approach, viscometer is used to measure the nanofluid viscosity, and densitometer is used to measure the nanofluid density. It noted that there is a good agreement between the measured nanofluid thermophysical properties and the calculated properties based upon the theoretical models within nanofluid concentrations [0–0.06]. K-type thermocouples had been installed on the left and right walls as well as along the center of the midsection in order to measure the temperature experimentally. Two cores of magnetic field had been installed with magnetic intensity of 20 mT. It had been proved that there is a slight increase in the temperature with the existence of the magnetic field. For example, at 20 mT, the temperature increases into 29.1 °C while the temperature equals to 28.8 °C at the absence of the magnetic field. Additionally, it had been illustrated that increasing the hot side wavy-walled temperature leads to an increase in the temperature difference which increases the temperature level in the nanofluid–porous region and the nanofluid region. [ABSTRACT FROM AUTHOR]

The stability of natural convection in an internally heated vertical porous layer confined between two impermeable boundaries which are kept at different constant temperatures is investigated. The momentum transfer is modeled by adopting Darcy's law including time-dependent velocity term contribution. The conduction stream function and temperature fields are significantly altered due to internal heating, and the linear instability is analyzed through a study of normal mode perturbations on the base flow. The neutral stability curves and the critical Darcy–Rayleigh number for the onset of instability are evaluated by solving the stability eigenvalue problem numerically. It has been established that a uniform volumetric heat source and the Prandtl–Darcy number reinforce together in initiating the instability of the base flow under certain conditions despite their isolation presence evidences stability for all infinitesimal perturbations. Although the internal heat source strength is to hasten the onset of instability, and the Prandtl–Darcy number is found to induct both destabilizing and stabilizing impact on the base flow. [ABSTRACT FROM AUTHOR]

Cavities are often applied in the thermal management of electronic products, to improve the performance of cavities, a new sinusoidal cavity was developed instead of square cavity, some metal foams were partially filled in inner surface of hot and cold wall instead of smooth surface, Fe3O4–H2O nanofluids were chosen as the heat-transfer medium instead of water. Natural convection of Fe3O4–H2O nanofluids in partially filled metal foam sinusoidal cavities is investigated. Some main variables, including various nanoparticle mass fractions (wt% = 0.0%, 0.1%, 0.3%, 0.5%), heating power (Q = 10 W, 15 W, 20 W, 25 W), and pore density (PPI = 20, 30, 40), were considered. Compared with deionized water, with the increase of nanofluid concentration from wt% = 0.1% to wt% = 0.5%, the Nusselt number firstly increases significantly and then decreases gradually. Results showed that when Q = 25 W, PPI = 40, wt% = 0.5%, the Nusselt number is relatively large and the heat transfer effect is the better, which gives certain advices for the structure design and selection of working condition of electronic components. Article Highlights: A new partially filled metal foam sinusoidal cavity is developed. Effects of pore density and nanoparticle concentration are studied. A working condition of PPI = 40, Q = 25 W and wt% = 0.5% is the best one for cooling of electronic components. With the increase of nanoparticle concentration, Nusselt number firstly rises and then reduces. [ABSTRACT FROM AUTHOR]