High Rayleigh Number Laminar Natural Convection in an Asymmetrically Heated Vertical Channel

1989 ◽  
Vol 111 (3) ◽  
pp. 649-656 ◽  
Author(s):  
B. W. Webb ◽  
D. P. Hill
Keyword(s):  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Vertical Channel ◽  
Local Heat Transfer ◽  
Heated Wall ◽  
Uniform Heat Flux ◽  
Parallel Plates ◽  
Heating Rates ◽  
Wide Range

Experiments have been performed to determine local heat transfer data for the natural convective flow of air between vertical parallel plates heated asymmetrically. A uniform heat flux was imposed along one heated wall, with the opposing wall of the channel being thermally insulated. Local temperature data along both walls were collected for a wide range of heating rates and channel wall spacings corresponding to the high modified Rayleigh number natural convection regime. Laminar flow prevailed in all experiments. Correlations are presented for the local Nusselt number as a function of local Grashof number along the channel. The dependence of both average Nusselt number and the maximum heated wall temperature on the modified Rayleigh number is also explored. Results are compared to previous analytical and experimental work with good agreement.

scholarly journals Enhancement of natural convection heat transfer in a U-shaped cavity filled with Al2O3-water nanofluid

2012 ◽  
Vol 16 (5) ◽  
pp. 1317-1323 ◽  
Author(s):  
Ching-Chang Cho ◽  
Her-Terng Yau ◽  
Cha’o-Kuang Chen
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Volume Fraction ◽  
Convection Heat Transfer ◽  
Heated Wall ◽  
Natural Convection Heat Transfer ◽  
Convection Heat ◽  
The Mean

This paper investigates the natural convection heat transfer enhancement of Al2O3-water nanofluid in a U-shaped cavity. In performing the analysis, the governing equations are modeled using the Boussinesq approximation and are solved numerically using the finite-volume numerical method. The study examines the effects of the nanoparticle volume fraction, the Rayleigh number and the geometry parameters on the mean Nusselt number. The results show that for all values of the Rayleigh number, the mean Nusselt number increases as the volume fraction of nanoparticles increases. In addition, it is shown that for a given length of the heated wall, extending the length of the cooled wall can improve the heat transfer performance.

scholarly journals NATURAL CONVECTION IN HIGH ASPECT RATIO THREE-DIMENSIONAL ENCLOSURES WITH UNIFORM HEAT FLUX ON THE HEATED WALL

2004 ◽  
Vol 3 (2) ◽  
pp. 100
Author(s):  
T. Dias Jr. ◽  
L. F. Milanez
Keyword(s):  
Heat Flux ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Aspect Ratio ◽  
Prandtl Number ◽  
High Aspect Ratio ◽  
Three Dimensional ◽  
Uniform Heat Flux ◽  
Uniform Heat

In this work, the laminar natural convection in high aspect ratio three-dimensional enclosures has been numerically studied. The enclosures studied here were heated with uniform heat flux on a vertical wall and cooled at constant temperature on the opposite wall. The remaining walls were considered adiabatic. Fluid properties were assumed constant except for the density change with temperature on the buoyancy term. The governing equations were solved using the finite volumes method and the dimensionless form of these equations has the Prandtl number and the modified Rayleigh number as parameters. The influences of the Rayleigh number and of the cavity aspect ratio on the Nusselt number, for a Prandtl number of 0.7, were analyzed. Results were obtained for values of the modified Rayleigh number up to 106 and for aspect ratios ranging from 1 to 20. The results were compared with two-dimensional results available in the literature and the variation of the average Nusselt number with the parameters studied were discussed.

Thermal Drag and Critical Heat Flux for Natural Convection of Air in Vertical Parallel Plates

1993 ◽  
Vol 115 (1) ◽  
pp. 124-129 ◽  
Author(s):  
Zeng-Yuan Guo ◽  
Xiao-Bo Wu
Keyword(s):  
Heat Flux ◽  
Natural Convection ◽  
Critical Heat Flux ◽  
Numerical Solutions ◽  
Vertical Channel ◽  
Wall Heat Flux ◽  
Uniform Heat Flux ◽  
Parallel Plates ◽  
Constant Property ◽  
Flux Boundary Condition

Variable property effects on vertical channel natural convection in air are studied systematically. Numerical solutions of the governing equations show that both the mass flow rate and heat transfer in the channel are not only lower than the constant property results, but also show a nonmonotonic variation with increasing wall temperature or wall heat flux. This phenomenon, which seemingly conflicts with the conventional knowledge, has also been identified by experiments. For a vertical channel with a uniform heat flux boundary condition, the wall may experience a sharp rise in temperature up to damage of the channel if the wall heat flux is greater than the critical heat flux. This implies that the crisis phenomenon (or burnout) may occur in channel natural convection in gas as well as in the boiling process.

scholarly journals NATURAL CONVECTION IN HIGH ASPECT RATIO THREE-DIMENSIONAL ENCLOSURES WITH UNIFORM HEAT FLUX ON THE HEATED WALL

2004 ◽  
Vol 3 (2) ◽  
Author(s):  
T. Dias Jr. ◽  
L. F. Milanez
Keyword(s):  
Heat Flux ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Aspect Ratio ◽  
Prandtl Number ◽  
High Aspect Ratio ◽  
Three Dimensional ◽  
Uniform Heat Flux ◽  
Uniform Heat

In this work, the laminar natural convection in high aspect ratio three-dimensional enclosures has been numerically studied. The enclosures studied here were heated with uniform heat flux on a vertical wall and cooled at constant temperature on the opposite wall. The remaining walls were considered adiabatic. Fluid properties were assumed constant except for the density change with temperature on the buoyancy term. The governing equations were solved using the finite volumes method and the dimensionless form of these equations has the Prandtl number and the modified Rayleigh number as parameters. The influences of the Rayleigh number and of the cavity aspect ratio on the Nusselt number, for a Prandtl number of 0.7, were analyzed. Results were obtained for values of the modified Rayleigh number up to 106 and for aspect ratios ranging from 1 to 20. The results were compared with two-dimensional results available in the literature and the variation of the average Nusselt number with the parameters studied were discussed.

scholarly journals NATURAL CONVECTION IN RECTANGULAR CAVITIES WITH DIFFERENT ASPECT RATIOS

2011 ◽  
Vol 10 (1-2) ◽  
pp. 44
Author(s):  
R. M. Nogueira ◽  
M. A. Martins ◽  
F. Ampessan
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Flow Behavior ◽  
Heated Wall ◽  
Flow Profile ◽  
Height Ratio ◽  
Dimensionless Variable ◽  
Aspect Ratios ◽  
Wide Range

Natural convection in closed cavities has been extensively studied in recent decades. This spontaneous method of heat transfer has a wide range of applications in engineering. In the present work, natural convection was numerically analyzed in a rectangular cavity heated on one of the sides and cooled on the opposite side. Temperatures of the heated wall and of the cooled wall were assumed to be constant. The objective of these studies was to determine the effects of the aspect ratio and the Rayleigh number on flow behavior and heat transfer in the cavity. In the simulations, the Rayleigh number drastically influenced the flow profile and heat transfer inside de cavity, as well as the thickness of the thermal boundary layer. It was also verified that the Nusselt number is strongly dependent on the L/D (Length/Height) ratio, and that this dimensionless variable increases with the increase of the W/L. The simulation of natural convection problems in the CFD Studio satisfactorily described the studied situations.

Study on Natural Convection in Horizontal Parallel Plate Channels Heated on Upper Wall and Partially Filled With Metal Foam

2020 ◽  
Author(s):  
Bernardo Buonomo ◽  
Vincenzo Fardella ◽  
Oronzio Manca ◽  
Sergio Nardini ◽  
Salvatore Pragliola
Keyword(s):  
Porous Medium ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Average Nusselt Number ◽  
Metal Foam ◽  
Open Cavity ◽  
Temperature Fields ◽  
Local Thermal Equilibrium ◽  
Uniform Heat Flux

Abstract In this work, a numerical investigation on two-dimensional steady state natural convection in a horizontal channel partially filled with a porous medium and heated at uniform heat flux from above is carried out. The lower plate is adiabatic. The porous medium is modeled using the Brinkman–Forchheimer-extended Darcy model and the local thermal equilibrium (LTE) hypothesis is assumed. The structure of the porous medium is homogenous and isotropic, the thermophysical properties of the air and the porous medium are temperature independent and the fluid flow is laminar and incompressible. The aluminum foam has 10, 20 and 40 pore per inches (PPI) and its porosity ranges from 0.90 and 0.95. Rayleigh number values are examined, from 6.0 × 104 and 1.2 × 107. Results are presented in terms of velocity and temperature fields, temperature and velocity profiles at different significant sections are shown, to obtain a description of the natural convection inside the open-ended cavity. Finally, Average Nusselt number values are evaluated. The horizontal open cavity partially filled with metal foam presents improved heat transfer behavior for higher Rayleigh numbers. The enhancement depends on the porosity and pore density. The average Nusselt number for the partially filled open cavity is the double of the configuration without the foam, clear configuration, for the highest considered Rayleigh number.

Natural Convection Heat Transfer and Entropy Generation From a Horizontal Cylinder With Baffles

2000 ◽  
Vol 122 (4) ◽  
pp. 679-692 ◽  
Author(s):  
B. A/K Abu-Hijleh
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Entropy Generation ◽  
Convection Heat Transfer ◽  
Horizontal Cylinder ◽  
Natural Convection Heat Transfer ◽  
Convection Heat ◽  
Wide Range

The problem of laminar natural convection heat transfer from a horizontal cylinder with multiple, equally spaced, low conductivity baffles on its outer surface was investigated numerically. The effect of several combinations of number of baffles and baffle height on the average Nusselt number was studied over a wide range of Rayleigh numbers. The computed velocity and temperature fields were also used to calculate the local and global entropy generation for different cylinder diameters. The results showed that there was an optimal combination of a number of baffles and baffle height for minimum Nusselt number for a given value of the Rayleigh number. Short baffles slightly increased the Nusselt number at small values of the Rayleigh number. The global entropy generation increased monotonically with increasing Rayleigh number and decreased with increasing cylinder diameter, baffle height, and number of baffles. [S0022-1481(00)01203-2]

On the Low Rayleigh Number Asymptote for Natural Convection Through an Isothermal, Parallel-Plate Channel

1991 ◽  
Vol 113 (4) ◽  
pp. 899-905 ◽  
Author(s):  
L. Martin ◽  
G. D. Raithby ◽  
M. M. Yovanovich
Keyword(s):  
Heat Transfer ◽  
Asymptotic Behavior ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Boundary Conditions ◽  
Parallel Plate ◽  
Parallel Plates ◽  
Numerical Studies ◽  
Plate Channel

The problem of natural convection through a channel formed by isothermal, parallel plates forms a cornerstone of our understanding of a class of natural convection flows. Following the pioneering study of Elenbaas, it is widely accepted that there is a fully developed re´gime, at low Rayleigh number, in which the Nusselt number becomes directly proportional to the Rayleigh number. This paper gives a detailed analysis of heat transfer in this re´gime. It is concluded that the previous numerical studies, which appeared to confirm this asymptote, used inappropriate boundary conditions, and that the asymptotic behavior should, in fact, not be expected except under very special conditions.

Transient Natural Convection Slip Flow in a Vertical Microchannel Heated at Uniform Heat Flux

2008 ◽  
Author(s):  
Bernardo Buonomo ◽  
Oronzio Manca
Keyword(s):  
Heat Flux ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Mass Flow Rate ◽  
Knudsen Number ◽  
Flow Rate ◽  
Wall Temperature ◽  
Uniform Heat Flux ◽  
Vertical Microchannel

Miniaturization of devices has received a rapid expansion in the last years and a great attention of research activities is given to microflow due to its new applications of microfluidic systems and components. In the present paper a transient investigation on natural convection in parallel-plate vertical microchannels is carried out numerically. The vertical microchannel is considered asymmetrically or symmetrically heated at uniform heat flux. The first-order model for slip velocity and jump temperature is assumed in microscale conditions. The analysis is performed in laminar boundary layer assumption for different values for different values of Knudsen number, Rayleigh number and the ratio of wall heat flux in order to evaluate their effects on wall temperatures, mass flow rate and Nusselt number. Wall temperature overshoots are detected for the different conditions. These values increase increasing the Knudsen number, Kn, at high Rayleigh number, Ra, whereas for lower Ra the lowest wall temperature are obtained for Kn = 0.05. Mass flow rate increases increasing Kn whereas Nusselt number decreases increasing Kn.

Experimental Analysis of Natural Convection and Flow Visualization in an Asymmetrically Heated Open Vertical Channel

2019 ◽  
Vol 11 (5) ◽  
Author(s):  
Yassine Cherif ◽  
Emilio Sassine ◽  
Laurent Zalewski ◽  
Kaies Souidi ◽  
Stephane Lassue
Keyword(s):  
Natural Convection ◽  
Rayleigh Number ◽  
Aspect Ratio ◽  
Thermal Flux ◽  
Vertical Channel ◽  
Velocity Profiles ◽  
Heated Wall ◽  
Image Velocimetry ◽  
Channel Aspect Ratio ◽  
Thermal Phenomena

An experimental device was designed to perform the thermal and dynamic study of natural convection airflow in an open vertical channel. The two side walls of the vertical channel are made of Plexiglas allowing the visualization of the flow via the particle image velocimetry (PIV) method. For the two other vertical walls, one is heated at a constant temperature, and the other is insulated with a 9-cm thick polystyrene insulation. The dynamic characterization of convection is carried out by nonintrusive measurements (PIV), and thermal phenomena are analyzed using nonintrusive heat flux instrumentation (simultaneous temperature and velocity measurements have been carried out across the channel at different elevations). Moreover, this study deals with the influence of the Rayleigh number on the measured vertical velocity profiles as well as the thermal flux densities recorded along the heated wall. To do this, different values of the modified Rayleigh numbers were considered in the interval with the channel aspect ratio of A = 5 and A = 12.5. The obtained Nusselt number values have been compared successfully with those of the literature. The impacts of the Rayleigh number and the aspect ratio on the velocity profiles and the convective and radiative heat transfer have been examined.

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