Natural Convection Research Papers - Academia.edu (original) (raw)

2025, Quality Institute of Technology Press (QIT Press)

The study investigates the formation and breakdown of coherent structures in transitional wall-bounded flows using Large Eddy Simulation (LES) with Dynamic Subgrid Scale (SGS) modeling. Transitional wall-bounded flows are characterized by complex interactions between laminar and turbulent structures, making them critical for understanding flow instabilities and transition mechanisms. LES provides a high-fidelity approach to resolving large-scale turbulence structures while modeling smaller scales with SGS techniques. This paper presents a detailed numerical analysis using LES to capture the formation and breakdown of coherent structures in transitional boundary layers and internal flows. The dynamic SGS model adapts to local flow characteristics, improving accuracy and physical realism. Results reveal key insights into the mechanisms driving coherent structure breakdown and the role of small-scale turbulence in the transition process. Statistical and spectral analyses are conducted to quantify turbulent energy transfer and identify dominant coherent modes. The study provides new perspectives on improving turbulence models and enhancing predictive capabilities for transitional flows.

2025, Al-Rafidain Engineering Journal (AREJ)

This research aims to investigate the performance solar water collector by varing five parameters; which are mass flowrate, inclination angle, total solar radiation, pipe size, and number of glass covers. The test rig was established to collect the data for the whole months of September and October and use it as a focal point for analysis of the solar water heating system's performance. The dynamic behavior simulated and optimizated with MATLAB software for the practical data to investigate the performance of the flat plate solar collector. The novelity in this study is the first time the authors use the whole practical data instead of using an average to approximate the theoretical dynamic investigation of the flat plate solar collector. The achievements are as follows: The increase in collector efficiency was from 62.17% to 71.26% when the collector pipe spacing was reduced from 186 mm to 86 mm; the increase in efficiency was approximately 2% as the collector pipeline diameter grew from 1 mm to 50 mm; the optimum efficiency was achieved with triple glazing and was about 0.83%; the increase in mass flowrate from; 1 to 5 liters per minute, would improve the efficiency of the system from 64% to 83%. Moreover, the best tilt angle for the flat plate solar collector was 30°.Also, heat loss coefficient rises by around 50% when wind speed is increased from 1 m/s to 5 m/s. Thus, the use of dynamic investigation with actual data will assist the researcher in improving the performance of the solar water flat plate collector.

2025, International Journal of Numerical Methods for Heat & Fluid Flow

Purpose The purpose of this study is to indicate the effect of mounting heat generating porous matrix in a close cavity on the Brownian term of CuO-water nanofluid and its impact on improving the Nusselt number. Design/methodology/approach Because of the presence of heat source in porous matrix, couple of energy equations is solved for porous matrix and nanofluid separately. Thermal conductivity and viscosity of nanofluid were assumed to be consisting of a static component and a Brownian component that were functions of volume fraction of the nanofluid and temperature. To explain the effect of the Brownian term on the flow and heat fields, different parameters such as heat conduction ratio, interstitial heat transfer coefficient, Rayleigh number, concentration of nanoparticles and porous material porosity were investigated and compared to those of the non-Brownian solution. Findings The Brownian term caused the cooling of porous matrix because of rising thermal conductivity. Mountin...

2025, Advances in Space Research

It is demonstrated in floating-zone configurations utilizing silicone oil and nitrate salts that mechanically induced vibration effectively minimizes detrimental, gravity independent, thermocapillary flow. The processing parameters leading to crystal improvement and aspects of the on-going modeling effort are discussed. Plans for applying the crystal growth technique to commercially relevant materials, e.g., silicon, as well as the value of processing in a microgravity environment are presented.

2025, Modern Physics Letters B

This study examined the influence of geometric parameters on free convective heat transfer in a zigzag-walled cavity filled with a hybrid nano-fluid composed of magnesium oxide (MgO) and single-walled carbon nanotubes (SWCNT) suspended in water. Utilizing validated multiphysics software grounded in the Galerkin finite element method (GFEM), we systematically analyzed how variations in Rayleigh number (Ra) from 10^4 to 10^6 , nanofluid volume fraction (') ranging from 0.01 to 0.04, and the number of wall undulations (from 1 to 8) affect thermal performance. The findings revealed that both the Rayleigh number and the number of wall undulations had a significant positive impact on the average Nusselt number (Nu).

2025, Computer Modeling in Engineering & Sciences

Maximizing the efficiency of thermal engineering equipment involves minimizing entropy generation, which arises from irreversible processes. This study examines thermal transport and entropy generation in viscous flow over a radially stretching disk, incorporating the effects of magnetohydrodynamics (MHD), viscous dissipation, Joule heating, and radiation. Similarity transformations are used to obtain dimensionless nonlinear ordinary differential equations (ODEs) from the governing coupled partial differential equations (PDEs). The converted equations are then solved by using the BVP4C solver in MATLAB. To validate the findings, the results are compared with previously published studies under fixed parameter conditions, demonstrating strong agreement. Various key parameters are analyzed graphically to assess their impact on velocity and temperature distributions. Additionally, Bejan number and entropy generation variations are presented for different physical parameters. The injection parameter (S < 0) increases the heat transfer rate, while the suction parameter (S > 0) reduces it, exhibiting similar effects on fluid velocity. The magnetic parameter (M) effectively decreases entropy generation within the range of approximately 0 ≤ η ≤ 0.6. Beyond this interval, its influence diminishes as entropy generation values converge, with similar trends observed for the Bejan number. Furthermore, increased thermal radiation intensity is identified as a critical factor in enhancing entropy generation and the Bejan number.

2025, Oceanography

Lake in a bottle-A laboratory demonstration of the unusual stability properties of freshwater.

2025, Oceanography

Lake in a bottle-A laboratory demonstration of the unusual stability properties of freshwater.

2025

A diffusion cell has been used to measure the effective diffusion coefficients of proteins. The method is applied to lysozyme and BSA at different pH and ionic strength. A parameter optimization technique is used to estimate the diffusion coefficients directly from experimental data. The importance and influence of pH and ionic strength on the diffusive properties in an agarose gel have been demonstrated. A comparison with other methods showed that there is good agreement. The diffusion cell is an accurate and easy-to-use method for the measurement of protein diffusion coefficients, in spite of the long time required for a protein diffusion experiment. For smaller proteins this time can be shortened considerably by the present parameter optimization technique.

2025

In the aftermath of the Chernobyl nuclear accident and following more than ten years of research and development, the Real time On-line DecisiOn Support system (RODOS) offers a wide range of alternatives to dealing in an effective and efficient fashion with an accidental release of radioactivity to the environment in Europe. The system is intended to provide assistance to decision-makers throughout the various phases of the accident, covering the plume phase (prediction of the atmospheric dispersion), the post-plume phase (assessing the impact of the contamination on the food chain through both aquatic and terrestrial pathways), and the evaluation of the effectiveness of countermeasures issued to protect the public and the environment against the hazards of ionizing radiation. In the framework of the European Commission's Nuclear Fission Programme, a research project on data assimilation has the task of introducing a practical improvement in the predictive capabilities of the RO...

2025, Microgravity - Science and Technology

It is well known that temperature and concentration gradients at an immiscible interface can result in fluid flow due to surfacetension-induced convection and such systems have been the subject of numerous investigations in microgravity [2]. We propose that an analogous situation can result when a sharp concentration gradient is imposed between two miscible fluids. Such miscible interfacial phenomena can only be studied in the absence of buoyancy-driven convection. Thus it can only be studied in weightlessness, i.e., "microgravity". We are interested in knowing how important could be such transient phenomena to materials processing in weightlessness. We propose that temperature and/or concentration gradients in miscible polymeric systems, such as could occur in the photopolymerization of dodecyl acrylate/poly (dodecyl acrylate), will cause macroscopically observable fluid flow in the absence of buoyancy-driven convection. Many types of problems could involve sharp temperature and concentration gradients in N. Bessonov et al: Numerical simulations of convection induced by Korteweg stresses in miscible polymer-monomer systems

2025, Mathematics and Computers in Simulation

In this study, coupled non-linear partial differential equations governing the natural convection from an isothermal wall of a trapezoidal porous enclosure have been solved numerically by finite element method (FEM) in conjunction with GMRES, a Krylov subspace based solver. In view of the enormous amount of computation, a parallel numerical algorithm for incomplete LU-conjugate gradient (ILU-CG) solver on eight-noded ANUPAM cluster under MIMD paradigm based on ANULIB message passing library has been developed. Parallel computations have been carried out for various values of flow and geometric parameters both under Darcian and non-Darcian assumptions on the porous model. Cumulative heat fluxes and Nusselt number (Nu) associated with convection process are presented through computer generated plots.

2025, Journal of Chemical & Engineering Data

The diffusion coefficient of diiodomethane in air has been measured using a natural convection method. The diffusion coefficient at 25 OC and 760 mmHg was found to be (6.0 f 0.5) X lo-' m2/s.

2025, MDPI

This study numerically investigates unsteady natural convection (NC) heat transfer (HT) and entropy generation (E gen) in trapezoidal cavities filled with two thermally stratified fluids. Both air-filled and water-filled configurations are analyzed to evaluate and compare their thermal performance under varying conditions. The cavities are characterized by a heated base, thermally stratified sloped walls, and a cooled top wall. The governing equations are numerically solved using the finite volume (FV) approach. The study considers a Prandtl number (Pr) of 0.71 for air and 7.01 for water, Rayleigh numbers (Ra) ranging from 10 3 to 5 × 10 7 , and an aspect ratio (AR) of 0.5. Flow behavior is examined through various parameters, including temperature time series (TTS), average Nusselt number (Nu), average entropy generation (E avg), average Bejan number (Be avg), and ecological coefficient of performance (ECOP). Three bifurcations are identified during the transition from steady to chaotic flow for both fluids. The first is a pitchfork bifurcation, occurring between Ra = 10 5 and 2 × 10 5 for air, and between Ra = 9 × 10 4 and 10 5 for water. The second, a Hopf bifurcation, is observed between Ra = 4.7 × 10 5 and 4.8 × 10 5 for air, and between Ra = 10 5 and 2 × 10 5 for water. The third bifurcation marks the onset of chaotic flow, occurring between Ra = 3 × 10 7 and 4 × 10 7 for air, and between Ra = 4 × 10 5 and 5 × 10 5 for water. At Ra = 10 6 , the average HT in the air-filled cavity is 85.35% higher than in the water-filled cavity, while E avg is 94.54% greater in the air-filled cavity compared to water-filled cavity. At Ra = 10 6 , the thermal performance of the cavity filled with water is 4.96% better than that of the air-filled cavity. These findings provide valuable insights for optimizing thermal systems using trapezoidal cavities and varying working fluids.

2025

The effect of pressure work on free convection about a truncated cone is considered. Non-Similarity solutions for boundary layer equations are found to exist.. The governing boundary layer equations are reduced to non similarity boundary layer equations and solved numerically by using Mathematica technique. Comparisons with previously published work on special cases of the problem are performed and the results are found to be in excellent agreement. The solutions are presented in terms of local skin friction, local Nusselt number, velocity and temperature profiles for values of Prandtl number and pressure work parameter.

2025

The flow patterns and the heat transfer rate of a rotating-Boussinesq fluid sphere are presented. The convective flow is induced by (i) a gravity field acting axially downwards (ii) a homogeneous internal heat source in the fluid, and (iii) a uniform low temperature at the wall of the sphere. We show the influence of the Taylor number on the heat transfer rate for different Rayleigh numbers. The natural convection analysis is carried out from the subcritical steady state (pure conduction) regime to the supercritical non-steady state regime. The mesh based Spectral Element method (SEM) has been used to solve the fluid equations in a Cartesian coordinate system in a rotating reference frame. The fluid sphere is discretized by using non-regular hexahedra with straight sides macro elements. Using this approach the singularity that appears at the poles of the sphere, when the governing equations are formulated in the spherical coordinate system, is avoided. The flow patterns and the heat...

2025

𝑫 𝒂,𝒃 is the mass diffusivity, 𝒘 𝒂 𝟎 is the initial mass fraction of component "a" and 𝑫 𝑻 is the thermodiffusion coefficient.

2025, Proc. Of Waste …

A team led by BNFL, Inc. was awarded the contract to remediate and immobilize the Hanford radioactive tank waste in support of the Hanford River Protection Program (RPP). BNFL, Inc. is teamed with BNFL Engineering, LTD., Bechtel National, GTS Duratek, and Science Application International Corporation to develop and design integrated facilities for pretreatment and vitrification in support of the RPP mission. This facility will pretreat and immobilize approximately 0.375 MT/day of high level waste (HLW, producing 1.5 MT/day of HLW glass) and approximately 4.5 MT/day of low activity waste (LAW, producing 30 MT/day of LAW glass). During the initial phase of the project (FY98 -FY00), the technology will be validated and optimized to the point that it can be used as the basis for final design, construction, and operation of a vitrification facility in Hanford, Washington.

2025, Applied and Computational Mechanics

In this paper, laminar natural convection of copper/water nanofluid in an open-ended L-shaped cavity is investigated by Lattice Boltzmann Model (LBM). The results are compared by previous studies, that are in good agreement. Influences of Rayleigh number (Ra=〖10〗^3,〖10〗^4,〖10〗^5,〖10〗^6), cavity aspect ratio (AR=0.2, 0.4, 0.6) and volume concentration of Cu nanoparticles (0≤φ≤0.1) on the momentum, thermal ﬁelds and heat transfer in the enclosure are studied. Also, the effect of changing the boundary conditions, on the heat transfer rate has been investigated. It is observed that maximum heat transfer enhancement by adding the nanoparticles for Ra=〖10〗^6 with AR=0.4 (32.76%) occurs. Results illustrate that increasing the cavity aspect ratio decreases heat transfer rate for Ra=〖10〗^3 and Ra=〖10〗^4. The least and most heat transfer rate for Ra=〖10〗^5 occurs in enclosures by aspect ratios of 0.2 and 0.4 respectively, while it was observed at Ra=〖10〗^6 for minimum and maximum rate of heat...

2025, International Journal of Hydrogen Energy

In this paper a study of the thermal performance of a commercial alkaline water electrolyzer (HySTAT from Hydrogenics) designed for a rated hydrogen production of 1 N m 3 H 2 /h at an overall power consumption of 4.90 kW h/N m 3 H 2 is presented. The thermal behaviour of the electrolyzer has been analyzed under different operating conditions with an IR camera and several thermocouples placed on the external surface of the main electrolyzer components. It has been found that the power dissipated as heat can be reduced by 50-67% replacing the commercial electric power supply unit provided together with the electrolyzer by an electronic converter capable of supplying the electrolyzer with a truly constant DC current. A lumped capacitance method has been adopted to mathematically describe the thermal performance of the electrolyzer, resulting in a thermal capacitance of 174 kJ C À1 . The effect of the AC/DC converter characteristics on the power dissipated as heat has been considered. Heat losses to the ambient were governed by natural convection and have been modeled through an overall heat transfer coefficient that has been found to be 4.3 W m À2 C À1 . The model has been implemented using ANSYS Ò V10.0 software code, reasonably describing the performance of the electrolyzer. A significant portion of the energy dissipated as heat allows the electrolyzer operating at temperatures suitable to reduce the cell overvoltages.

2025, Journal of Power Sources

In this study, direct borohydride fuel cells (DBFCs) potentialities are evaluated. These emerging systems make it possible to reach maximum powers of about 200 mW cm -2 at room temperature and ambient air (natural convection) with high concentrated borohydride solutions. On the other hand, a part of borohydride hydrolyses during cell operating which leads to hydrogen formation and fuel loss: the practical capacity represents about only 18% of the theoretical one. In order to improve fuel efficiency, thiourea is tested as an inhibitor of the catalytic hydrolysis associated with BH 4 electro-oxidation on Pt. The practical capacity is drastically improved: it represents about 64% of the theoretical one. Against, electrochemical performances (I-E curves) are affected by the presence of thiourea. Last, both playing with platinum anodic mass loading and the nature of catalyst, a good compromise has been found in terms of both specific capacity and power.

2025, Solar Energy

It is important to enhance the availability of SACs either in cloudy weather or after sunset for space heating and agricultural drying process. In this context, this study aims to determine experimentally thermal performance of cherry pits as sensible heat storage material under natural convection conditions. In the first collector (Type I), powdered cherry pits (32 kg) were used as heat storage material and cherry pits (24 kg) were used in the second (Type II). The third collector (Type III) with a flat absorber plate is for comparison. Although the maximum thermal efficiencies were found 26.54% and 27.03% for Type I and Type II respectively, average thermal efficiency of Type I is higher than Type II. According to the test results, the inlet-outlet temperatures were equalized 46 min for the Type III, 200 min for Type II, and 270 min for Type I collector after sunset. As a result, it can be said that the use of cherry pits and powder as sensible heat storage material is promising.

2025, Solar Energy Materials and Solar Cells

Phase change material (PCM) is at the head of materials commonly used in latent heat storage. However, usually the main problem is that the heat conductivity of PCMs is low. Therefore, a novel solar air collector (SAC) combined the PCM-Rubitherm RT54HC with aluminum honeycomb was manufactured and investigated the effect of using honeycomb core on the collector thermal performance regarding heat storage with PCM under natural convection conditions. In the experiments, the heat storage material was tested in two forms with 26 kg capacity, PCM with honeycomb core in the first collector (type I) and only PCM in the second collector (type II), and also the third collector (type III) with a flat absorber plate for comparison. The first collector reached the PCM melting point about 50 min ago. Also, when we consider the peak point, an apparent 10 °C advantage is achieved in sensible heat storage in favor of type I. It is observed that the honeycomb core used as a heat transfer enhancer is effective in shortening the charge-discharge times, also a noticeable difference in sensible heat storage during the noon. The production of useful energy in Type I lasted 469 min and 539 min longer than type III, respectively. The thermal efficiencies under daylight were calculated as 10.1%, 10.9% and 13.6%, respectively. As a result, it can be said that the use of honeycomb core as heat conductivity enhancer material is very functional, in particular during the discharge period.

2025

This study explores the Fluid-Structure Interaction (FSI) and heat transfer dynamics in a wavy L-shaped enclosure with a flexible baffle simulated using COMSOL Multiphysics. The analysis focuses on the maximum von Mises stress, streamlines, isotherms, mean Nusselt number, and scaled temperature under varying elasticity modulus (1 GPa ≤ E ≤ 50 GPa), Rayleigh number (10 3 ≤ Ra ≤ 10 6), baffle length (0.05 m ≤ B ≤ 0.25 m), and Prandtl number (0.71 ≤ Pr ≤ 13.2). The results demonstrate that the presence of a flexible baffle within the wavy L-shaped enclosure significantly alters fluid dynamics and heat transfer characteristics. The baffle acts as an obstruction, disrupting natural convection currents, yet its flexibility enables it to conform to fluid motion, thereby reducing drag while maintaining an extended surface area crucial for efficient heat transfer. Comparative analyses reveal that the inclusion of the baffle enhances heat transfer rates by approximately 74.9 % and 82.5 % for Rayleigh numbers of 10 5 and 10 6 , respectively, by optimizing flow dynamics and improving heat exchange efficiency. Furthermore, increasing the Rayleigh and Prandtl numbers enhances heat transfer efficiency, with mean Nusselt numbers increasing by up to 60 % for higher parameter values. Longer baffle lengths improve heat transfer but result in increased mechanical stress, with the maximum von Mises stress rising by up to 45 % when B = 0.25 m. These findings underscore the importance of optimizing material properties and geometric configurations to balance thermal performance and structural integrity in fluid-structure interaction applications.

2025

This paper presents a computational analysis of two-diffusive convection in 3D pyramid solar still using conductive fins. The study explores heat and mass transmission inside the solar still, focusing on the influences of buoyancy ratio, Rayleigh number and fin conductivity. Using COMSOL Multiphysics software, a three-dimensional diffusive double convection model is developed. The analysis reveals the significant impact of fin conductivity on isotherms and iso-concentration, emphasizing enhanced convective heat transfer with increasing fin conductivity. The findings provide valuable insights for improving solar still systems through material and buoyancy adjustments. The study underscores the critical role of buoyancy in shaping both convective heat and mass transfer characteristics, with optimal performance observed at negligible buoyancy and high fin conductivity.

2025

Hybrid photovoltaic thermal PVT solar collectors are a good option to produce electrical power and thermal energy simultaneously from solar radiation, where the transfer fluid circulating in this system works as a coolant, and at once, it gains additional heat to produce heat. This study is based on a numerical investigation, and aims to improve the performances of a hybrid photovoltaic thermal (PVT)collectorby using porous media, and taking into account the air buoyancy force effect (i.e., natural convection effect).For this purpose, the impacts of panel tilt angle (α), Richardson number (Ri), thickness of the porous layer (Ep) and Darcy number (Da) on the PVT system performances including its velocity contours, thermal field, thermal and electrical efficiencies are examined. The obtained results showed that by taking into account the buoyancy force, an enhancement in both electrical and thermal efficiencies is observed which can reach up 9.88% and 10% respectively if the PVT is er...

2025, International Journal of Heat and Mass Transfer

Heat transfer over a sub-millimeter spheroidal solid is of interest in many engineering processes. One important mechanism of heat transfer in the above processes is natural convection which leads to heat transfer rates many times larger than that of pure conduction. Despite the huge literature devoted to natural convection heat transfer rates over spheres (and to a smaller extent over spheroids) there is not a generally accepted correlation especially for small Rayleigh numbers. Existing correlations for external geometries predict a progressively increasing contribution of natural convection to heat transfer with respect to gravity (starting from zero gravity). To test the validity of these correlations, experiments are performed for the estimation of heat transfer rates at low gravity. Heat pulses are given to a miniature thermistor with a nearly spheroidal shape immersed in a liquid and its thermal response is registered during heating in parabolic flights. The contribution of natural convection to heat transfer is undoubtedly estimated from runs in which acceleration varies from 0 to 1.8 g. Surprisingly enough, the experiments showed that the Rayleigh number must take a minimum value before non-negligible effect of natural convection on heat transfer appears (existence of a threshold Rayleigh number). In the absence of natural convection (below Ra thr ) the experimental thermal response curves can be successfully described by approximating solutions of the transient heat conduction equation for the spheroidal geometry of the thermistor. Apparently, additional research is needed regarding the natural convection around submillimeter objects for small Rayleigh numbers.

2025, Mechanics Research Communications

2025, International Journal for Research in Applied Science and Engineering Technology

2025, WSEAS TRANSACTIONS ON HEAT AND MASS TRANSFER

The numerical study on the improvement of the cooling of a microprocessor by the use of Nanofluids has been made. Natural convection is analyzed in a box fence with a temperature source encountered at its lower border and loaded with an Ethylene Glycol-Copper nanoparticle. This article explores the influences of relevant aspects such as thermal Rayleigh number, solid volume fraction, and enclosure dimensions on the thermal efficacy of the box fence, which are enhanced with an enlargement in thermal Rayleigh number and solid volume fraction. The results also illustrate that the change of the warmth transfer rate concerning the box dimensions of the enclosure is unlike at inferior and elevated thermal Rayleigh numbers. A simile is offered between the upshots got and the literature. Results were presented in terms of heat transfer rate depending on thermal Rayleigh number (Rat = 10^3 , and 10^6 ), nanoparticle solid volume fraction (0 ≤ φ < 5%), and box dimensions. The results show ...

2025, Research article

Improper drying of hot pepper after harvest is a prominent cause of post-harvest losses. In Ethiopia, hot pepper is mostly dried using the sun drying method before being processed into a powder. However, it's physicochemical qualities and application of other drying techniques were not well studied. Therefore, this study was aimed to evaluate the physicochemical properties of two hot pepper varieties dried under sun and oven drying methods. The experiment was arranged using factorial design and conducted by drying two hot pepper varieties under sun at a maximum temperature of 28.5 °C for 96 hours and oven drying at a temperature of 80 °C for 7 hours. The determined physicochemical properties were moisture content, pH, total soluble solids (TSS), oleoresin content, total carotenoids, browning index, extractable and surface color. The result showed that the highest moisture content (11.33%) was recorded for sun dried Marako fana hot pepper whereas the lowest moisture content (3.17%) was noted for Gababa hot-pepper dried under oven drying method. The highest pH (5.92), bulk density (0.52 g/cm 3), extractable color (228.62) and surface color (22.94) were noted for sun-dried Marako fana. In addition, the highest total soluble solids (3.83 o Brix) and yellowness (29.02) were recorded for oven dried Marako fana hot pepper. However, the highest oleoresin (9.94%) and redness (a* value) (20.79) were noted for oven dried Gababa hot pepper. The present finding also revealed that Statistically, non-significant (p˃0.05) differences were observed for luminosity (L* value) and carotenoids of dried hot pepper under sun and oven drying methods. Some physicochemical properties of hot pepper powder were improved by sun and oven drying methods, and further study is needed to optimize these drying methods for the production of hot pepper powder.

2025, Applied Thermal Engineering

Four wall solar chimneys have been constructed and put at each wall and orientation of a small-scale test room so as to be used for the evaluation and measurement of their thermal behavior and the certification of their efficiency. At this stage, research focuses on the study of the buoyancy-driven flow field and heat transfer inside them. A numerical investigation of the thermo-fluid phenomena that take place inside the wall solar chimneys is performed and the governing elliptic equations are solved in a twodimensional domain using a control volume method. The flow is turbulent and six different turbulence models have been tested to this study. As the realizable k-model is likely to provide superior performance for flows boundary layers under strong adverse pressure gradients, it has been selected to be used in the simulations. This is also ACCEPTED MANUSCRIPT 2 confirmed by comparing with the experimental results. Predicted velocity and temperature profiles are presented for di erent locations, near the inlet, at different heights and near the outlet of the channel and they are as expected by theory. Important parameters such as average Nusselt number are also compared and calculated at several grid resolutions. The developed model is general and it can be easily customised to describe various solar chimney's conditions, aspect ratios, etc. The results from the application of the model will support the effective set-up of the next configurations of the system.

2025, Journal of Semiconductors

This paper provides an analytical approach to determine the optimum pitch by utilizing a thermal resistance network, under the assumption of constant luminous efficiency. This work allows an LED array design which is mounted on a printed circuit board (PCB) attached with a heat sink subject to the natural convection cooling. Being validated by finite element (FE) models, the current approach can be shown as an effective method for the determination of optimal component spacing in an LED array assembly for SSL.

2025

Flow measurements in indoor office environment BALA VARMA DATLA, SRIKAR KALIGOTLA, MARK GLAUSER, Syracuse University -Experiments are being conducted to study the nature of flow near the human breathing zones. PIV measurements are being made around two thermal manikins and a table placed in an office space provided with displacement ventilation. The velocity profiles at various positions near the breathing zone and at various instants of the breathing cycle are being studied to understand how the breathing influences the air flow and mixing near the breathing zone, in a flow that is primarily driven by natural convection. These results can help us understand the transport of freely suspended particulate matter in indoor environments into the breathing zone and be used for validating computer models for indoor environments.

2025, Solar Energy Materials and Solar Cells

2025, The European Physical Journal B

We study thermal convection in a colloidal glass of Laponite in formation. Low concentration preparation are submitted to destabilizing vertical temperature gradient, and present a gradual transition from a turbulent convective state to a steady conductive state as their viscosity increases. The time spent under convection is found to depend strongly on sample concentration, decreasing exponentially with mass fraction of colloidal particles. Moreover, at fixed concentration, it also depends slightly on the pattern selected by the Rayleigh Bénard instability: more rolls maintain the convection state longer. This behavior can be interpreted with recent theoretical approaches of soft glassy material rheology.

2025

The present numerical study deals with the laminar natural convection within a square cavity filled with a Newtonian incompressible fluid. The vertical walls are isotherms whereas the horizontal ones are adiabatic except of a portion at the center of the inferior wall which is maintained at a temperature greater than that of the vertical walls. The governing equations are solved using the finite volume method and the SIMPLER algorithm. The study focuses on the effect of both the intensity of buoyancy forces and the length of the thermal source on hydrodynamic and thermal properties of the flow. The results show that heat transfer is enhanced when both the Rayleigh number and the thermal source’s length increase. 23 ème Congrès Français de Mécanique Lille, 28 Août au 1 er Septembre 2017 Mots clefs : Convection naturelle, cavité carrée, source chauffante, méthode des volumes finis.

2025, Theoretical Foundations of Chemical Engineering

Abstract⎯This paper reports a two-dimensional numerical investigation of mixed convection inside a liddriven square enclosure, completely filled with a non-Newtonian fluid obeying the Bingham model, having two rectangular adiabatic partitions mounted in different dispositions. Due to the problem' complexity, the latter is solved using finite volume method when the SIMPLER algorithm is adopted for the pressure-velocity coupling. In order to investigate the yield stress effects on flow field and heat transport, we maintain the Richardson number (Gr/Re 2 ) as 0.01, 1 and 10, respectively, which generates a good simulation of forced, mixed and natural convection dominated flow. The Prandtl and Grashof numbers are fixed at 50 and 10 4 , respectively, while the Bingham number covers the range from 0 to 30. The validity of the computing code was ascertained by comparing our results with the numerical ones already available in the literature and that for both cases: Newtonian and non-Newtonian fluid. The phenomenon is analyzed through the streamlines, the isotherms and the Nusselt numbers with special attention to the partitions' arrangement and its size. It is found that all parameters related to the geometrical dimensions of the partitions play a crucial role on the temperature distribution, flow field and heat transfer enhancement. For all values of the Bingham number, the mean Nusselt number is found as an increasing function of the decrease Richardson number. Thus, the heat transfer improves.

2025, arXiv: Fluid Dynamics

In this paper, global sensitivity analysis (GSA) and uncertainty quantification (UQ) have been applied to the problem of natural convection (NC) in a porous square cavity. This problem is widely used to provide physical insights into the processes of fluid flow and heat transfer in porous media. It introduces however several parameters whose values are usually uncertain. We herein explore the effect of the imperfect knowledge of the system parameters and their variability on the model quantities of interest (QoIs) characterizing the NC mechanisms. To this end, we use GSA in conjunction with the polynomial chaos expansion (PCE) methodology. In particular, GSA is performed using Sobol’ sensitivity indices. Moreover, the probability distributions of the QoIs assessing the flow and heat transfer are obtained by performing UQ using PCE as a surrogate of the original computational model. The results demonstrate that the temperature distribution is mainly controlled by the longitudinal the...

2025, International Journal of Heat and Mass Transfer

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2025, Solar Energy Advances

The system proposed in this article is a collector made up of hemispherical concentrators. The prototype consists of four reflecting bowls covered with 8 mm glass in which 42 L of sunflower oil circulates. The oil being the absorber itself is heated by the hot spots resulting from the concentration of the spherical reflectors. So the proposed collector stands against usual collectors by canceling the black plate absorber and by assuming that the hot spots heating process is a better way of heating a fluid in natural convection while keeping the permanent positioning advantage. The aim of the first series of tests was to confirm that the hot spots could better heat the oil, which was assessed by the oil excess temperature with the ambient reaching 32 • C for a maximum irradiation value of 970 W m-2. The second series of tests was carried out on the same collector heating a 60 L water tank in indirect heat transfer mode, resulting in an average daily efficiency of 86.30%. The annual daily effective heat gain was estimated at 6.21 MJ for a receptive surface area of 0.39 m 2 and an average irradiation value of 506 W m-2. 73.02% of this heat gain was allocated to water. In conclusion, direct heating of a liquid by hot-spots is effective. The proposed collector, while operating entirely in passive mode and retaining the convenience of flat plate collectors, has shown competing performances with them and can also be approved as an ICS after key actions against its drawbacks.

2025

A convergent solar chimney requires that the velocity of air flow at the top of the chimney is many times that at the base of the chimney. The multiple is the ratio of the area of the base of the chimney to the area of the top of the chimney. Available kinetic energy for a turbine sited at the top of the chimney is multiplied by the square of this ratio. It should thus be possible to design a convergent solar chimney of modest height but with high efficiency. Calculations have been carried out on models of height 3, 5, 10 and 20m using high area ratios. In this way an efficiency of about 50% could be achievable.

2025, Journal of Theoretical and Applied Mechanics

W pracy wyprowadzono stowarzyszone równania cał kowe dla równania H elmholtza i pokazano ich zastosowania do rozwią zywani a zagadnień odwrotnych przewód* nictwa cieplnego. Przez zagadnienia odwrotne rozumie się przy tym zagadnienia wyznaczania strumienia ciepł a lub temperatury na brzegu i wewną tr z pewnego obszaru D a E"\ m = 1,2, 3, na podstawie tzw. wewnę trznyc h odpowiedzi temperaturowych lub strumieniowych, . Z tego typu problemami moż n a się spotkać w wielu dziedzinach techniki; doczekał y się one także dosyć bogatej literatury (por. [8, 9, 10]). W niniejszej pracy omówiono trzy typy jednowamiarowych zagadnień odwrotnych, a mianowicie zagadnienia dotyczą c e warstwy pł askiej, kuli i warstwy kulistej oraz walca i warstwy walcowej. D o rozwią zywani a tych zagadnień zastosowano równania cał kowe wyprowadzone w pracy [1], Równania te w wymienionych przypadkach moż n a był o rozwią za ć w postaci zamknię tej , co pozwolił o otrzymać wzory rekurencyjne okreś lają c e pewne ukł ady funkcji, które stanowią przybliż on e rozwią zani a tych zagadnień. Analiza tych wzorów doprowadził a do interesują cyc h wniosków dotyczą cyc h wpł ywu bł ę d u danych na wyniki obliczeń. Wnioski te znalazł y swoje potwierdzenie w przykł adach liczbowych. gdzie x eQ\ 8Q*, t e T, zaś n(x*) jest normalną zewnę trzn ą do 3Q*. N iech funkcja ® 0 (x), x eQ, opisuje począ tkow y rozkł ad temperatury w obszarze Q, tzn.

2025, Journal of Scientific Research

The aim of this study is to present an exact analysis the effect of heat generation parameter and Dufour number on the magneto hydrodynamic (MHD) free convection flow of an electrically conducting incompressible viscous fluid over an inclined plate embedded in a porous medium. The impulsively started plate with variable temperature and mass diffusion is considered. The dimensionless momentum equation coupled with the energy and mass diffusion equations are analytically solved using the closed analytical method. Expressions for velocity, temperature and concentration fields are obtained. They satisfy all imposed initial and boundary conditions and can be reduced, as special cases, to some known solutions from the literature. Expressions for skin friction, Nusselt number and Sherwood number are also obtained. Finally, the effects of parameters on velocity, temperature and concentration profiles and skin friction coefficient are graphically displayed.

2025, International Communications in Heat and Mass Transfer

A three-dimensional computer model is used to calculate the convective flow in a 400 W high-pressure Hg discharge lamp. The model gives the expected axisymmetric flow when the are tube axis is vertical, and more complex threedimensional... more

2025, Bulletin of Materials Science

Epitaxial layers of silicon are grown on single crystal Si-substrate from a solution of silicon in indium using conventional graphite slider boat technique. The important problems of natural convection due to lower density of silicon compared to indium, poor wetting of substrate due to high angle of contact of indium solution on silicon substrate resulting in poor nucleation, melt removal from the growth substrate and saturation wafer associated with LPE in this technique are practically eliminated using sandwich method with simple modifications of the boat and the method of growth. Some experimental studies on the effect of different surface preparations of growth substrate are also reported. Growth results are shown and discussed. Further, improvization of slider boat to facilitate better study of growth parameters is suggested in the line of modification already carried out.

2025

Recentemente foram estudados aspectos importantes do tratamento numérico, incluindo comparações com a solução exata, de uma equação de advecção com termo fonte [3]. Nossa análise mostra que a simulação convencional para a solução T (x, t) que é feita evoluindo em t precisa ser melhorada, devendo considerar também uma evolução em x devido à condição inicial T (0, t) = 1 para t ≥ 0. Isto impõe uma adequação da condição de estabilidade e convergência de Courant, Friedrichs e Lewys (CFL), vide [1]. Assim, considere o problema de advecção com termo fonte :

2025

This work investigates mixed convection numerically using a heated square cylinder at the middle of a vented cavity filled with fluid-saturated porous media. External cold fluid enters into the cavity through the opening located either at the bottom wall or at the bottom and side walls, whereas the hot plume is vented through an opening at the top wall. All the walls of the cavity are insulated. The governing equations are solved by in-house code. Flow through porous medium has been modeled using Brinkman-Forchheimer-Darcy Model (BFDM). The influence of flow parameters (Reynolds number Re = 10-500, Richardson number Ri = 0.1-100) and porous-medium parameters (Darcy number Da = 10-7-10-3 , porosity e = 0.1-1) and inlet openings are investigated systematically. It is observed that the heat transfer increases as Ri and Re increase.

2025

Conjugate forced convection in air and heat conduction with solidification of water content in food freezing is predicted. Continuity, linear momentum and energy equations are solved to predict the fluid dynamics and heat transfer around the food. Solidification of water is calculated by using a temperature dependent apparent specific heat. The method of finite volumes, developed in generalized body fitted coordinates is used along the SIMPLER algorithm to solve the discretized equations. Time dependent results for velocity and temperature distributions are presented for two cases: a plate shaped and a piece of a cross section of salmon meat being cooled in air.

2025, International Journal of Heat and Technology

Numerical prediction of heat transfer by natural convection of a Herschel-Bulkley non-Newtonian fluid inside a square cavity has been computationally analyzed. Unsteady 2D fluid mechanics and heat transfer were described in terms of the non-linear coupled continuity, momentum and heat equations. These equations were solved by the control volume finite element method (CVFEM) with Gauss-Seidel/System Over-Relaxation coupling algorithm. The effect of the Ra, Pr, Bn and the rheological behavior index (n) on the non-Newtonian fluid thermal and momentum behavior were studied. The non-Newtonian fluid flow was described by the rheological model of Herschel-Bulkley. Results for the streamlines and isotherms along the enclosure walls are presented. It was found that the effect of the Pr and Bn is more important when the Ra is lower (10 3 ). In addition, the behavior index had a significant effect on the CPU time for the different studied cases.