scholarly journals Thermally Optimum Spacing between Inner Plates in Natural Convection Flows in Cavities by Numerical Investigation

Processes ◽  
2020 ◽  
Vol 8 (5) ◽  
pp. 554 ◽  
Author(s):  
Blas Zamora
Keyword(s):  
Natural Convection ◽  
Aspect Ratio ◽  
Numerical Investigation ◽  
Thermophysical Properties ◽  
Electronic Equipment ◽  
Passive Devices ◽  
Wide Range ◽  
Optimum Spacing ◽  
Rayleigh Numbers ◽  
Wall Spacing

Buoyancy-driven airflow that included two isothermal inner plates established in a vented cavity is investigated numerically. The thermally optimum wall-to-wall spacing of the immersed channel, as well as its dependence with respect to the relevant governing parameters, are determined. Results are presented as a function of the aspect ratio b/H for a wide range of Rayleigh numbers RaH. A logarithmic correlation for the optimum (b/H)opt as a function of RaH is presented. In addition, since the outlined configuration might be subject to intense heating conditions, the influence of considering variable thermophysical properties is also included in the analysis. In fact, an appreciable influence of the variation of properties on (b/H)opt is also detected for a representative value of RaH = 109. Obtained results can be directly applied to the optimization of electronic equipment cooling, or even to thermal passive devices in buildings.

Natural Convection of Cu-Gallium Nanofluid in Enclosures

2011 ◽  
Vol 133 (12) ◽  
Author(s):  
Cong Qi ◽  
Yurong He ◽  
Yanwei Hu ◽  
Juancheng Yang ◽  
Fengchen Li ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Nusselt Number ◽  
Natural Convection ◽  
Aspect Ratio ◽  
Convection Heat Transfer ◽  
Low Velocity ◽  
Grashof Number ◽  
Aspect Ratios ◽  
Wide Range ◽  
Temperature Dependent Properties

In this work, the natural convection heat transfer of Cu-gallium nanofluid in a differentially heated enclosure is investigated. A single-phase model is employed with constant or temperature-dependent properties of the fluid. The results are shown over a wide range of Grashof numbers, volume fractions of nanoparticles, and aspect ratios. The Nusselt number is demonstrated to be sensitive to the aspect ratio. It is found that the Nusselt number is more sensitive to thermal conductivity than viscosity at a low velocity (especially for a low aspect ratio and a low Grashof number), however, it is more sensitive to the viscosity than the thermal conductivity at a high velocity (high aspect ratio and high Grashof number). In addition, the evolution of velocity vectors, isotherms, and Nusselt number for a small aspect ratio is investigated.

Effect of 3D Diffusion Over Vertical Thin Rectangular Geometries in Natural Convection Heat Transfer

2006 ◽  
Author(s):  
Mustafa Gursoy ◽  
Mehmet Arik ◽  
Tunc Icoz ◽  
Michael Yovanovich ◽  
Theodorian Borca-Tasciuc
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Fluid Motion ◽  
Heat Removal ◽  
Air Entrainment ◽  
Published Data ◽  
Diffusion Term ◽  
Nusselt Numbers ◽  
Wide Range ◽  
Rayleigh Numbers

Natural convection over vertical plates is a very well known problem in heat transfer. There are many available correlations to predict Nusselt numbers for a wide range of Rayleigh numbers. These benchmark studies on natural convection for vertical plates were conducted on rather large surfaces leading to Rayleigh numbers in the range of 0.1 to 109. In natural convection the sole driving force of fluid motion is the change in fluid density, when the diffusive limit is small compared to convective heat transfer. However, conduction to air, as well as air entrainment from sides also contributes to the heat removal from heater surfaces. An experimental study has been carried out with small and large heaters compared to published data for 2×103<Ra<4×107. Square surfaces of 12.5 and 25.4 mm, and rectangular heaters of sizes 25.4×101.6 and 25.4×203.2 mm were tested for a range of heat inputs such that the surface temperatures are controlled between 30 °C and 80 °C. It is found that published correlations underpredict the Nusselt numbers as much as 20%. It is observed that widely known correlations underpredict the experimental values since the 3D conduction and side air drifts on heat transfer are not accounted for in these correlations. However, the cuboid model which includes the 3D diffusion term showed much better agreement with the experimental results.

The phase diffusion and mean drift equations for convection at finite Rayleigh numbers in large containers

1990 ◽  
Vol 220 ◽  
pp. 187-252 ◽  
Author(s):  
Alan C. Newell ◽  
Thierry Passot ◽  
Mohammad Souli
Keyword(s):  
Aspect Ratio ◽  
Prandtl Number ◽  
Solvability Condition ◽  
Boussinesq Equations ◽  
Phase Diffusion ◽  
Drift Field ◽  
Wide Range ◽  
The Mean ◽  
Circular Target ◽  
Rayleigh Numbers

We derive the phase diffusion and mean drift equations for the Oberbeck–Boussinesq equations in large-aspect-ratio containers. We are able to recover all the long-wave instability boundaries (Eckhaus, zigzag, skew-varicose) of straight parallel rolls found previously by Busse and his colleagues. Moreover, the development of the skew-varicose instability can be followed and it becomes clear how the mean drift field conspires to enhance the necking of phase contours necessary for the production of dislocation pairs. We can calculate the wavenumber selected by curved patterns and find very close agreement with the dominant wavenumbers observed by Heutmaker & Gollub at Prandtl number 2.5, and by Steinberg, Ahlers & Cannell at Prandtl number 6.1. We find a new instability, the focus instability, which causes circular target patterns to destabilize and which, at sufficiently large Rayleigh numbers, may play a major role in the onset of time dependence. Further, we predict the values of the Rayleigh number at which the time-dependent but spatially ordered patterns will become spatially disordered. The key difficulty in obtaining these equations is the fact that the phase diffusion equation appears as a solvability condition at order ε (the inverse aspect ratio) whereas the mean drift equation is the solvability condition at order ε2. Therefore, we had to use extremely robust inversion methods to solve the singular equations at order ε and the techniques we use should prove to be invaluable in a wide range of similar situations. Finally, we discuss the introduction of the amplitude as an active order parameter near pattern defects, such as dislocations and foci.

Natural Convection Liquid Cooling of a Substrate-Mounted Protrusion in a Square Enclosure: A Parametric Study

1992 ◽  
Vol 114 (2) ◽  
pp. 401-409 ◽  
Author(s):  
S. B. Sathe ◽  
Y. Joshi
Keyword(s):  
Natural Convection ◽  
Boundary Conditions ◽  
Flow Behavior ◽  
Air Cooling ◽  
Square Enclosure ◽  
Simple Boundary ◽  
Wide Range ◽  
Solid Region ◽  
Rayleigh Numbers ◽  
Thermal Conductivities

The coupled conduction and natural convection transport from a substrate-mounted heat generating protrusion in a liquid-filled square enclosure is numerically examined. The governing steady two-dimensional equations are solved using a finite-difference method for a wide range of Rayleigh numbers, protrusion thermal conductivities and widths, substrate heights, and enclosure boundary conditions. The results presented apply to liquids with 10≤Pr≤1000. It was established that in many situations it may be inappropriate to specify simple boundary conditions on the solid surface and decouple the conduction within the substrate or the protrusion. Higher Rayleigh numbers, protrusion thermal conductivities, and widths enhanced cooling. A variation in the substrate height did not affect the maximum protrusion temperature; however, the flow behavior was considerably altered. An empirical correlation for the maximum protrusion temperature was developed for a wide range of parametric values. The enclosure thermal boundary conditions changed the heat transfer in the solid region to only a small extent. Immersion cooling in common dielectric liquids was shown to be advantageous over air cooling only if the thermal conductivity of the protrusion was larger than that of the liquid.

scholarly journals NUMERICAL ANALYSIS OF THE NATURAL CONVECTION IN HORIZONTAL ANNULI AT LOW AND MODERATE Ra

2006 ◽  
Vol 5 (2) ◽  
pp. 58
Author(s):  
E. L. M. Padilla ◽  
R. Campregher ◽  
A. Silveira-Neto
Keyword(s):  
Natural Convection ◽  
Three Dimensional ◽  
Staggered Grid ◽  
Governing Equations ◽  
Concentric Cylinders ◽  
Wide Range ◽  
Horizontal Annuli ◽  
Rayleigh Numbers ◽  
The Heat Transfer Coefficient ◽  
Good Agreement

The natural convection at low and moderate Rayleigh numbers (Ra) incylindrical horizontal annuli with imposed temperatures in both surfaces isnumerically studied. This flow inside concentric cylinders classic configuration has a wide range of practical and technological applications, which justifies its growing studies efforts. In this work, the governing equations are discretized by the volume finite technique over a staggered grid, with second-order accuracy in space and time. The flow pattern is presented by several Rayleigh numbers, with an analysis of the heat transfer coefficient and flow properties. Furthermore, a three-dimensional field is shown at a moderate Ra number. The results showed a good agreement with the experimental data.

Transient Natural Convection in Enclosures at High Rayleigh Number

1991 ◽  
Vol 113 (3) ◽  
pp. 635-642 ◽  
Author(s):  
D. A. Olson ◽  
L. R. Glicksman
Keyword(s):  
Natural Convection ◽  
Rayleigh Number ◽  
Aspect Ratio ◽  
Transient Behavior ◽  
Full Scale ◽  
Scale Model ◽  
Mixed Gas ◽  
Transient Natural Convection ◽  
Physical Scale ◽  
Rayleigh Numbers

Transient natural convection at Rayleigh numbers of 1010 was studied experimentally in two enclosures of aspect ratio 1/3, one a 1:5.5 physical scale model containing the dense refrigerant gas R114, and the second a full-scale room containing air. In one type of transient the vertical endwall temperature was suddenly changed, while in a second type of transient the isothermal, well-mixed gas was suddenly exposed to hot and cold vertical endwalls. The experiments indicated that the dominant time constant was a convective one. Comparisons between the scale model and full scale show that R114 gas can simulate the transient behavior of air-filled enclosures.

scholarly journals A Dimensionless Model for Transient Turbulent Natural Convection in Isochoric Vertical Thermal Energy Storage Tubes

2018 ◽  
Vol 10 (3) ◽  
Author(s):  
Reza Baghaei Lakeh ◽  
Richard E. Wirz ◽  
Pirouz Kavehpour ◽  
Adrienne S. Lavine
Keyword(s):  
Natural Convection ◽  
Energy Storage ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Convection Heat Transfer ◽  
Thermal Storage ◽  
Fourier Number ◽  
Turbulent Natural Convection ◽  
Wide Range ◽  
Rayleigh Numbers

In this study, turbulent natural convection heat transfer during the charge cycle of an isochoric vertically oriented thermal energy storage (TES) tube is studied computationally and analytically. The storage fluids considered in this study (supercritical CO2 and liquid toluene) cover a wide range of Rayleigh numbers. The volume of the storage tube is constant and the thermal storage happens in an isochoric process. A computational model was utilized to study turbulent natural convection during the charge cycle. The computational results were further utilized to develop a conceptual and dimensionless model that views the thermal storage process as a hot boundary layer that rises along the tube wall and falls in the center to replace the cold fluid in the core. The dimensionless model predicts that the dimensionless mean temperature of the storage fluid and average Nusselt number of natural convection are functions of L/D ratio, Rayleigh number, and Fourier number that are combined to form a buoyancy-Fourier number.

Numerical Experiments in Turbulent Natural Convection Using Two-Equation Eddy-Viscosity Models

2008 ◽  
Vol 130 (7) ◽  
Author(s):  
X. Albets-Chico ◽  
A. Oliva ◽  
C. D. Pérez-Segarra
Keyword(s):  
Natural Convection ◽  
Aspect Ratio ◽  
Numerical Experiments ◽  
Eddy Viscosity ◽  
Computational Effort ◽  
Simulation Data ◽  
Turbulent Natural Convection ◽  
Viscosity Models ◽  
Buoyancy Driven Flows ◽  
Rayleigh Numbers

This work is focused on the simulation and prediction of turbulent natural convection flows by means of two-equation eddy-viscosity models. In order to show the generality, precision, and numerical issues related to these models under natural convection, three different buoyancy-driven cavities have been simulated: a tall cavity with a 30:1 aspect ratio, a cavity with a 5:1 aspect ratio, and, finally, a 4:1 aspect ratio cavity. All cases are solved under moderate and∕or transitional Rayleigh numbers (2.43×1010, 5×1010, and 1×1010, respectively) and all simulations are compared to experimental and∕or direct numerical simulation data available in literature. These different situations allow to check the applicability of two-equation eddy-viscosity models in buoyancy-driven flows, giving criteria on computational effort∕precision and their physical behavior.

Nu-Ra correlations for natural convection at high Ra numbers in air-filled tilted hemispherical cavities with dome oriented upwards. Disk submitted to constant heat flux

2015 ◽  
Vol 25 (3) ◽  
pp. 504-512 ◽  
Author(s):  
Abderrahmane Baïri ◽  
Juan Mario García de María ◽  
Nacim Alilat ◽  
Najib Laraqi ◽  
Jean-Gabriel Bauzin
Keyword(s):  
Heat Flux ◽  
Natural Convection ◽  
Solar Energy ◽  
Constant Heat Flux ◽  
Constant Heat ◽  
Content Type ◽  
Turbulent Natural Convection ◽  
Wide Range ◽  
Building Safety ◽  
Rayleigh Numbers

Purpose – The purpose of this paper is to propose correlations between Nusselt and Rayleigh numbers for the case of inclined and closed air-filled hemispherical cavities. The disk of such cavities is subjected to a constant heat flux. The study covers a wide range of Rayleigh numbers from 5×107 to 2.55×1012. Design/methodology/approach – Correlations are obtained from numerical approach validated by experimental measurements on some configurations, valid for several angles of inclination of the cavity between 0° (horizontal disk) and 90° (vertical disk) in steps of 15°. Findings – The statistical analysis of a large number of calculations leads to reliable results covering laminar, transitional and turbulent natural convection heat transfer zones. Practical implications – The proposed correlations provide solutions for applications in several fields of engineering such as solar energy, aerospace, building, safety and security. Originality/value – The new relations proposed are the first published for high Rayleigh numbers for this type of geometry. They supplement the knowledge of natural convection in hemispherical inclined cavities and constitute a useful tool for application in various engineering areas as solar energy (thermal collector, still, pyranometer, albedometer, pyrgeometer), aerospace (embarked electronics), building, safety and security (controlling and recording sensors).

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