Heat Transfer by Natural Convection Across Vertical and Inclined Air Layers

1982 ◽  
Vol 104 (1) ◽  
pp. 96-102 ◽  
Author(s):  
S. M. ElSherbiny ◽  
G. D. Raithby ◽  
K. G. T. Hollands
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Practical Interest ◽  
Correlation Equations ◽  
Linear Temperature ◽  
Aspect Ratios ◽  
Different Temperatures ◽  
Air Layers ◽  
Rayleigh Numbers

Measurements of the heat transfer by natural convection across vertical and inclined air layers are reported. The air layer is bounded by flat isothermal plates at different temperatures and around the edges by a perfectly conducting boundary (i.e., one that takes on a linear temperature distribution between the two plates). Measurements are reported for six aspect ratios between 5 and 110, covering a portion of the range of practical interest for windows, solar collectors, etc. Rayleigh numbers were in the range 102 to 2 × 107. The present measurements permitted the role of aspect ratio to be clearly defined. In addition, correlation equations are presented which allow the heat trnasfer across a vertical or inclined air layer to be calculated.

Natural Convection Heat Transfer From Horizontal Rectangular Ducts

2006 ◽  
Vol 129 (9) ◽  
pp. 1195-1202 ◽  
Author(s):  
Mohamed E. Ali
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Convection Heat Transfer ◽  
Natural Convection Heat Transfer ◽  
Convection Heat ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Nusselt Numbers ◽  
Aspect Ratios ◽  
Rayleigh Numbers

Experimental investigations have been reported on steady state natural convection from the outer surface of horizontal ducts in air. Five ducts have been used with aspect ratios (Γ=duct height/duct width) of 2, 1, and 0.5. The ducts are heated using internal constant heat flux heating elements. The temperatures along the surface and peripheral directions of the duct wall are measured. Longitudinal (circumference averaged) heat transfer coefficients along the side of each duct are obtained for laminar and transition regimes of natural convection heat transfer. Total overall averaged heat transfer coefficients are also obtained. Longitudinal (circumference averaged) Nusselt numbers are evaluated and correlated using the modified Rayleigh numbers for transition regime using the axial distance as a characteristic length. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers, the aspect ratio, and area ratio for the laminar and transition regimes. The longitudinal or total averaged heat transfer coefficients are observed to decrease in the laminar region and to increase in the transition region. Laminar regimes are obtained only at very small heat fluxes, otherwise, transitions are observed.

Experimental Investigation of Natural Convection in Inclined Rectangular Regions of Differing Aspect Ratios

1976 ◽  
Vol 98 (1) ◽  
pp. 67-71 ◽  
Author(s):  
J. N. Arnold ◽  
I. Catton ◽  
D. K. Edwards
Keyword(s):  
Heat Transfer ◽  
Experimental Investigation ◽  
Natural Convection ◽  
Scaling Law ◽  
Convection Heat Transfer ◽  
Theoretical Work ◽  
Natural Convection Heat Transfer ◽  
Aspect Ratios ◽  
Simple Scaling ◽  
Rayleigh Numbers

An experimental investigation of steady natural convection heat transfer was carried out for finite rectangular regions. The effect of angle of inclination on heat transfer across rectangular regions of several aspect ratios was measured for Rayleigh numbers between 103 and 106. The angle of inclination varied from 0 deg (heated from above) to 180 deg (heated from below) with aspect ratios of one, three, six, and twelve. Comparison is made with past theoretical work, and a simple scaling law is derived which is valid for angles of inclination from 0 to 90 deg (vertical).

scholarly journals Natural Convection in Undivided and Partially Divided Rectangular Enclosures

1981 ◽  
Vol 103 (4) ◽  
pp. 623-629 ◽  
Author(s):  
M. W. Nansteel ◽  
R. Greif
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Temperature Fields ◽  
Transitional Flow ◽  
Two Dimensional ◽  
Injection Flow ◽  
Stabilizing Effect ◽  
Different Temperatures ◽  
Vertical Walls ◽  
Rayleigh Numbers

Heat transfer by natural convection in a two-dimensional rectangular enclosure fitted with partial vertical divisions is investigated experimentally. The horizontal walls of the enclosure are adiabatic while the vertical walls are maintained at different temperatures. The experiments are carried out with water, Pr ≃ 3.5, for Rayleigh numbers in the range, 2.3 × 1010 ⩽ RaL ⩽ 1.1 × 1011, and an aspect ratio, A = H/L = 1/2. The effect of the partial vertical divisions on the fluid flow and temperature fields is investigated by dye-injection flow visualization and by thermocouple probes, respectively. The effect of the partitions on the heat transfer across the enclosure is also studied and correlations for the Nusselt number as a function of RaL and partition length are generated for both conducting and non-conducting partition materials. Partial divisions are found to have a significant effect on the heat transfer; especially when the divisions are adiabatic. The results also indicate that the partial divisions may have a stabilizing effect on the laminar-transitional flow on the heated vertical walls of the enclosure.

Convective Heat Transfer in a Rectangular Porous Cavity—Effect of Aspect Ratio on Flow Structure and Heat Transfer

1984 ◽  
Vol 106 (1) ◽  
pp. 158-165 ◽  
Author(s):  
V. Prasad ◽  
F. A. Kulacki
Keyword(s):  
Heat Transfer ◽  
Rayleigh Number ◽  
Aspect Ratio ◽  
Aspect Ratios ◽  
Family Of Curves ◽  
Different Temperatures ◽  
Vertical Walls ◽  
Rayleigh Numbers ◽  
Cavity Effect ◽  
Cavity Width

Two-dimensional steady natural convection in a porous rectangular cavity bounded by isothermal vertical walls at different temperatures and adiabatic horizontal walls has been studied numerically for aspect ratios less than unity and Rayleigh numbers up to 104. Results indicate the presence of multicellular flow. Also, the average Nusselt number based on cavity width is observed to be a maximum in a restricted range of aspect ratio, depending on the Rayleigh number. Effects of aspect ratio are summarized by a family of curves for constant Rayleigh number, based on cavity height, for aspect ratios from 0.05 to 100. For a cavity with fixed height, the heat transfer rate always increases as the aspect ratio is increased, except when the flow exhibits boundary layers on the vertical walls. Criteria in terms of aspect ratio and Rayleigh number have been established for the existence of different flow regimes.

Steady-State Natural Convection in Empty and Partitioned Enclosures at High Rayleigh Numbers

1990 ◽  
Vol 112 (3) ◽  
pp. 640-647 ◽  
Author(s):  
D. A. Olson ◽  
L. R. Glicksman ◽  
H. M. Ferm
Keyword(s):  
Heat Transfer ◽  
Steady State ◽  
Natural Convection ◽  
Radiation Heat Transfer ◽  
Full Scale ◽  
Scale Model ◽  
Small Scale ◽  
Different Temperatures ◽  
Rayleigh Numbers ◽  
Vertical Partition

Steady-state natural convection, which occurs in building enclosures (Rayleigh numbers of 1010), was studied experimentally in a full-scale room and in a 1:5.5 small-scale physical model containing R114 gas. The model was geometrically similar, had the same Rayleigh number, and had the same dimensionless end wall temperatures as the full-scale room. Configurations were tested with the enclosure empty, with a vertical partition extending from the floor to midheight, and with the vertical partition raised slightly off the floor. For isothermal opposing end walls at different temperatures, excellent agreement was found between the full-scale room and the scale model in flow patterns, velocity levels, temperature distributions, and heat transfer, even though the radiation heat transfer was not scaled between the two models.

The Effect of Thermal Wall Properties on Natural Convection in Inclined Rectangular Cells

1982 ◽  
Vol 104 (1) ◽  
pp. 111-117 ◽  
Author(s):  
B. A. Meyer ◽  
J. W. Mitchell ◽  
M. M. El-Wakil
Keyword(s):  
Heat Transfer ◽  
Cell Wall ◽  
Natural Convection ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Conductive Heat ◽  
Numerical Work ◽  
Aspect Ratios ◽  
Convection Problem ◽  
Natural Convection Problem

The effects of cell wall thickness and thermal conductivity on natural convective heat transfer within inclined rectangular cells was studied. The cell walls are thin, and the hot and cold surfaces are isothermal. The two-dimensional natural convection problem was solved using finite difference techniques. The parameters studied were cell aspect ratios (A) of 0.5 and 1, Rayleigh numbers (Ra) up to 105, a Prandtl number (Pr) of 0.72 and a tilt angle (φ) of 60 deg. These parameters are of interest in solar collectors. The numerical results are substantiated by experimental results. It was found that convection coefficients for cells with adiabatic walls are substantially higher than those for cells with conducting walls. Correlations are given for estimating the convective heat transfer across the cell and the conductive heat transfer across the cell wall. These correlations are compared with available experimental and numerical work of other authors.

Oscillatory Natural Convection of a Liquid Metal in Circular Cylinders

1994 ◽  
Vol 116 (3) ◽  
pp. 627-632 ◽  
Author(s):  
Y. Kamotani ◽  
F.-B. Weng ◽  
S. Ostrach ◽  
J. Platt
Keyword(s):  
Experimental Study ◽  
Natural Convection ◽  
Liquid Metal ◽  
Oscillatory Flow ◽  
Circular Cylinders ◽  
Flow Structures ◽  
Supercritical Flow ◽  
Aspect Ratios ◽  
Oscillation Frequencies ◽  
Rayleigh Numbers

An experimental study is made of natural convection oscillations in gallium melts enclosed by right circular cylinders with differentially heated end walls. Cases heated from below are examined for angles of inclination (φ) ranging from 0 deg (vertical) to 75 deg with aspect ratios Ar (height/diameter) of 2, 3, and 4. Temperature measurements are made along the circumference of the cylinder to detect the oscillations, from which the oscillatory flow structures are inferred. The critical Rayleigh numbers and oscillation frequencies are determined. For Ar=3 and φ = 0 deg, 30 deg the supercritical flow structures are discussed in detail.

Laminar Natural Convection Heat Transfer in a Differentially Heated Square Cavity Due to a Thin Fin on the Hot Wall

2003 ◽  
Vol 125 (4) ◽  
pp. 624-634 ◽  
Author(s):  
Xundan Shi ◽  
J. M. Khodadadi
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Computational Study ◽  
Convection Heat Transfer ◽  
Square Cavity ◽  
Left Wall ◽  
Flow Modification ◽  
Laminar Natural Convection ◽  
Rayleigh Numbers ◽  
Blockage Effect

A finite-volume-based computational study of steady laminar natural convection (using Boussinesq approximation) within a differentially heated square cavity due to the presence of a single thin fin is presented. Attachment of highly conductive thin fins with lengths equal to 20, 35 and 50 percent of the side, positioned at 7 locations on the hot left wall were examined for Ra=104,105,106, and 107 and Pr=0.707 (total of 84 cases). Placing a fin on the hot left wall generally alters the clockwise rotating vortex that is established due to buoyancy-induced convection. Two competing mechanisms that are responsible for flow and thermal modifications are identified. One is due to the blockage effect of the fin, whereas the other is due to extra heating of the fluid that is accommodated by the fin. The degree of flow modification due to blockage is enhanced by increasing the length of the fin. Under certain conditions, smaller vortices are formed between the fin and the top insulated wall. Viewing the minimum value of the stream function field as a measure of the strength of flow modification, it is shown that for high Rayleigh numbers the flow field is enhanced regardless of the fin’s length and position. This suggests that the extra heating mechanism outweighs the blockage effect for high Rayleigh numbers. By introducing a fin, the heat transfer capacity on the anchoring wall is always degraded, however heat transfer on the cold wall without the fin can be promoted for high Rayleigh numbers and with the fins placed closer to the insulated walls. A correlation among the mean Nu, Ra, fin’s length and its position is proposed.

scholarly journals Effects of Anisotropy on Natural Convection in a Vertical Porous Cavity Filled with a Heat Generation

2020 ◽  
pp. 12-27
Author(s):  
Degan Gerard ◽  
Sokpoli Amavi Ernest ◽  
Akowanou Djidjoho Christian ◽  
Vodounnou Edmond Claude
Keyword(s):  
Heat Transfer ◽  
Porous Medium ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Temperature Fields ◽  
Poisson Equations ◽  
Gravitational Vector ◽  
Side Walls ◽  
Rayleigh Numbers ◽  
Vertical Cavity

This research was devoted to the analytical study of heat transfer by natural convection in a vertical cavity, confining a porous medium, and containing a heat source. The porous medium is hydrodynamically anisotropic in permeability whose axes of permeability tensor are obliquely oriented relative to the gravitational vector and saturated with a Newtonian fluid. The side walls are cooled to the temperature  and the horizontal walls are kept adiabatic. An analytical solution to this problem is found for low Rayleigh numbers by writing the solutions of mathematical model in polynomial form of degree n of the Rayleigh number. Poisson equations obtained are solved by the modified Galerkin method. The results are presented in term of streamlines and isotherms. The distribution of the streamlines and the temperature fields are greatly influenced by the permeability anisotropy parameters and the thermal conductivity. The heat transfer decreases considerably when the Rayleigh number increases.

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