The Influences of Property Variations on Natural Convection from Vertical Surfaces

1981 ◽  
Vol 103 (4) ◽  
pp. 609-612 ◽  
Author(s):  
A. M. Clausing ◽  
S. N. Kempka
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Ambient Temperature ◽  
Radiative Heat ◽  
Heated Surface ◽  
Convection Heat Transfer ◽  
Absolute Temperature ◽  
Turbulent Regime ◽  
Transfer Rates ◽  
Rayleigh Numbers

The objective of this paper is to show the influences of property variations in natural convection. Heat transfer from a vertical isothermal, heated surface to gaseous nitrogen is experimentally investigated. The ambient temperature, T∞, is varied in order to cover a large range of the Rayleigh number and also to enable the generation of large values of this parameter. The range 80 K < T∞ < 320 K results in Rayleigh numbers between 107 and 2 × 1010 for the 0.28 m model. By using a cryogenic environment, large ratios of the absolute temperature of the wall to the ambient temperature, Tw/T∞, are generated without the results being masked by radiative heat transfer. The range 1 < Tw/T∞ < 2.6 is investigated. Variable properties cause dramatic increases in heat transfer rates in the turbulent regime, and virtually no influence is seen in the laminar regime. The results obtained correlate extremely well with the addition of a single parameter Tw/T∞.

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.

Natural Convection in Horizontal Thin-Walled Honeycomb Panels

1973 ◽  
Vol 95 (4) ◽  
pp. 439-444 ◽  
Author(s):  
K. G. T. Hollands
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Fluid Layer ◽  
Convection Heat Transfer ◽  
Correlation Equation ◽  
Wave Number ◽  
Equivalent Wave ◽  
Side Walls ◽  
The Stability ◽  
Rayleigh Numbers

This paper presents an experimental study of the stability of and natural convection heat transfer through a horizontal fluid layer heated from below and constrained internally by a honeycomb. Examination of the types of boundary conditions exacted on the fluid at the cell side-walls has shown that there are three limiting cases: (1) perfectly conducting side-walls; (2) perfectly adiabatic side-walls; and (3) side-walls having zero thickness. Experiments described in this paper approach the latter category. The fluid used is air and the honeycomb used is square-celled. Measured critical Rayleigh numbers are found to be intermediate between those applying to cases (1) and (2), and consistent with an “equivalent wave number” of approximately 0.95 times that for case (1). The measured natural convective heat transfer after instability is found to be significantly less than that predicted by the Malkus-Veronis power integral technique. However, it is found to approach asymptotically the heat transfer which would take place through a similar fluid layer unconstrained by a honeycomb. A general correlation equation for the heat transfer is given.

Three Dimensional Numerical Simulation of the Natural Convection in an Annulus With an Internal Slotted Cylinder

2011 ◽  
Author(s):  
Mo Yang ◽  
Jin Wang ◽  
Kun Zhang ◽  
Ling Li ◽  
Yuwen Zhang
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Numerical Solutions ◽  
Three Dimensional ◽  
Convection Heat Transfer ◽  
Natural Convection Heat Transfer ◽  
Dimensional Model ◽  
Heat Transfer Characteristics ◽  
Flow And Heat Transfer ◽  
Rayleigh Numbers

Detailed numerical analysis is presented for three-dimensional natural convection heat transfer in annulus with an internal concentric slotted cylinder. The internal slotted cylinder and the outer annulus are maintained at uniform but different temperatures. Governing equations are discretized using control volume technique based on staggered grid formulation and solved using SIMPLE algorithm with QUICK scheme. Flow and heat transfer characteristics are investigated for a Rayleigh number range of 10 to 106 while Prandtl number (Pr) is taken to be 0.7. The results indicate, at Rayleigh numbers below 105, the system shows two dimensional flow and heat transfer characteristics. On the other hand, the flow and heat transfer shows three dimensional characteristics while for Rayleigh numbers greater than 5×105. Comparison with experimental results indicated that the numerical solutions by three dimensional model can obtain more accuracy than the numerical solutions by two dimensional model. Besides, Numerical results show that the average equivalent conductivity coefficient of natural convection heat transfer of this problem can be enhanced by as much as 30% while relative slot width is more than 0.1.

Laminar Natural Convection Heat Transfer From a Horizontal Circular Cylinder to Liquid Metals

1991 ◽  
Vol 113 (1) ◽  
pp. 91-96 ◽  
Author(s):  
K. Sugiyama ◽  
Y. Ma ◽  
R. Ishiguro
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Circular Cylinder ◽  
Liquid Metals ◽  
Convection Heat Transfer ◽  
Boussinesq Approximation ◽  
Large Temperature ◽  
Transfer Rates ◽  
Laminar Natural Convection ◽  
Horizontal Circular Cylinder

The objective of the present study is to clarify the heat transfer characteristics of natural convection around a horizontal circular cylinder immersed in liquid metals. Experimental work concerning liquid metals sometimes involves such a degree of error that it is impossible to understand the observed characteristics in a measurement. Numerical analysis is a powerful means to overcome this experimental disadvantage. In the present paper we first show that the Boussinesq approximation is more applicable to liquid metals than to ordinary fluids and that the present analysis gives accurate heat transfer rates, even for a cylinder with a relatively large temperature difference (>100 K) between the heat transfer surface and fluid. It is found from a comparison of the present results with previous work that the correlation equations that have already been proposed predict values lower than the present ones.

Natural Convection Heat Transfer From a Cylinder With High Conductivity Permeable Fins

2003 ◽  
Vol 125 (2) ◽  
pp. 282-288 ◽  
Author(s):  
Bassam A/K Abu-Hijleh
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Convection Heat Transfer ◽  
High Heat ◽  
Horizontal Cylinder ◽  
High Conductivity ◽  
Transfer Rates ◽  
High Heat Transfer ◽  
Wide Range

The problem of laminar natural convection from a horizontal cylinder with multiple equally spaced high conductivity permeable fins on its outer surface was investigated numerically. The effect of several combinations of number of fins and fin height on the average Nusselt number was studied over a wide range of Rayleigh number. Permeable fins provided much higher heat transfer rates compared to the more traditional solid fins for a similar cylinder configuration. The ratio between the permeable to solid Nusselt numbers increased with Rayleigh number, number of fins, and fin height. This ratio was as high as 8.4 at Rayleigh number of 106, non-dimensional fin height of 2.0, and with 11 equally spaced fins. The use of permeable fins is very advantageous when high heat transfer rates are needed such as in today’s high power density electronic components.

Numerical Investigation of Variable Property Effects on Laminar Natural Convection of Gases Between Two Horizontal Isothermal Concentric Cylinders

1986 ◽  
Vol 108 (4) ◽  
pp. 783-789 ◽  
Author(s):  
D. N. Mahony ◽  
R. Kumar ◽  
E. H. Bishop
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Boussinesq Approximation ◽  
Variable Properties ◽  
Transfer Rates ◽  
Concentric Cylinders ◽  
Weighted Mean Temperature ◽  
Laminar Natural Convection ◽  
Rayleigh Numbers ◽  
Gap Width

A numerical finite difference investigation has been conducted to determine the effects of variable properties on the laminar natural convection of gases between horizontal isothermal concentric cylinders. Velocity profiles, temperature profiles, and heat transfer rates have been computed for diameter ratios of 1.5, 2.28, 2.6, and 5.0 and Rayleigh numbers based on gap width up to 1.8 × 105. The temperature difference ratio θo was varied from 0.2 to 3.0, and the range of validity of the Boussinesq approximation was determined to be θo = 0.2. A volume-weighted mean temperature was shown to be the most effective reference temperature to reduce the heat transfer data for each diameter ratio to a single curve of the form keq = C RaLn, for 0.2 ≤ θo ≤ 3.0 and RaL = 2.0 × 105.

Natural Convection From Horizontal Helicoidal Pipes

2000 ◽  
Author(s):  
Hamed Al-Ahmadi ◽  
Ahmad Fakheri
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Characteristic Length ◽  
Convection Heat Transfer ◽  
Natural Convection Heat Transfer ◽  
Convection Heat ◽  
Coil Diameter ◽  
Horizontal Cylinders ◽  
High Degree ◽  
Rayleigh Numbers

Abstract Natural convection heat transfer from a horizontal helicoidal pipe is experimentally investigated for different coil-pitches. A modified characteristic length incorporating the tube diameter, the coil diameter, and the coil spacing, is proposed as the relevant scale for defining Nusselt and Rayleigh numbers. Using the proposed characteristic length, it is shown that the Nusselt number for horizontal helicoidal pipes can be determined using the available Nusselt versus Rayleigh number correlation for straight horizontal cylinders with high degree of accuracy over the range of the experimental data.

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