On the Existence of an Oscillatory Approach to Steady Natural Convection in Cavities

1984 ◽  
Vol 106 (1) ◽  
pp. 104-108 ◽  
Author(s):  
J. C. Patterson
Keyword(s):  
Steady State ◽  
Natural Convection ◽  
Aspect Ratio ◽  
Transient Flow ◽  
Wave Activity ◽  
Scaling Analysis ◽  
Transient Phase ◽  
Low Aspect Ratio ◽  
Approach To Steady State ◽  
Rayleigh Numbers

The recent paper by Patterson and Imberger [1], which analysed the transient phase of natural convection in low aspect ratio cavities, gave a criterion for the presence of an oscillatory approach to steady state in such flows. However, the experiments of Yewell et al. [3] displayed no evidence of this oscillatory approach, even though the criterion was apparently satisfied. In this paper, it is shown that since the flow regimes are described by a series of critical Rayleigh numbers, the ordering of which changes with changing aspect ratio, a criterion for the existence of transient internal waves, valid for all orderings, may be established. Further, the results of [1] are extended by means of flow descriptions valid for two additional orderings. Consequently, it is shown that the experiments addressed a transient flow regime in which internal wave activity would not be expected and that the experimental results are in support of the scaling analysis of [1].

Unsteady natural convection in a cavity with internal heating and cooling

1984 ◽  
Vol 140 ◽  
pp. 135-151 ◽  
Author(s):  
John C. Patterson
Keyword(s):  
Steady State ◽  
Natural Convection ◽  
Numerical Solutions ◽  
Transient Flow ◽  
Scaling Analysis ◽  
Steady State Flow ◽  
Internal Heating ◽  
Sources And Sinks ◽  
Heating And Cooling ◽  
Unsteady Natural Convection

The problem of transient natural convection in a cavity of aspect ratio A < 1 driven by internal buoyancy sources and sinks distributed linearly in the horizontal and uniformly in the vertical is considered. Scaling analysis is used to show that a number of possible transient flow regions are possible, collapsing ultimately onto one of conductive, transitional, or convective steady-state flow regimes. A number of numerical solutions are obtained, and their relationships to the scaling analysis are discussed.

Transient Natural Convection Experiments in Shallow Enclosures

1982 ◽  
Vol 104 (3) ◽  
pp. 533-538 ◽  
Author(s):  
R. Yewell ◽  
D. Poulikakos ◽  
A. Bejan
Keyword(s):  
Steady State ◽  
Natural Convection ◽  
Theoretical Prediction ◽  
Thermal Stratification ◽  
Wave Motion ◽  
The Core ◽  
Transient Natural Convection ◽  
Aspect Ratios ◽  
Approach To Steady State ◽  
Rayleigh Numbers

This paper reports experimental observations on transient natural convection in enclosures at high Rayleigh numbers (1.28×109, 1.49×109) and low aspect ratios (0.0625, 0.112). The phenomenon consists of the establishment of thin intrusion layers along the horizontal adiabatic surfaces; in time, the intrusion layers exchange heat with the isothermal core of the cavity, leading to the thermal stratification of the core. The approach to steady state is gradual, contrary to the theoretical prediction of Brunt-Vaisala wave motion (Patterson and Imberger [6]). The measured durations of the observed transients agree very well with theoretical estimates.

Natural convection in an infinite porous medium with a concentrated heat source

1978 ◽  
Vol 89 (1) ◽  
pp. 97-107 ◽  
Author(s):  
Adrian Bejan
Keyword(s):  
Porous Medium ◽  
Steady State ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Heat Source ◽  
Flow Pattern ◽  
Transient Flow ◽  
Steady State Solution ◽  
Upward Flow ◽  
Rayleigh Numbers

The phenomenon of buoyancy-induced convection in an infinite porous medium with a concentrated heat source is studied analytically. The transient and steady-state temperature distribution and flow pattern around the source are determined using a perturbation analysis in the Rayleigh number based on the heat generation rate at the source. The first-order transient solution derived in the paper is valid for Rayleigh numbers less than 10. The transient flow pattern consists of an expanding vortex ring situated in the horizontal plane containing the source. The steady-state solution, valid for Rayleigh numbers of the order of 20 or less, reveals an upward flow pattern which becomes very intense near the source. The upward flow extends throughout the medium. Both solutions show that as the Rayleigh number increases the region situated above the source is effectively heated by natural convection in addition to direct heat conduction from the source.

scholarly journals Steady-State Approach to Low-Aspect-Ratio Reversed-Field Pinch Nuclear Fusion Reactor

2005 ◽  
Vol 81 (11) ◽  
pp. 932-943 ◽  
Author(s):  
Shoichi SHIINA ◽  
Yasuyuki YAGI ◽  
Hisaya SUGIMOTO ◽  
Hisao ASHIDA ◽  
Yoichi HIRANO ◽  
...  
Keyword(s):  
Steady State ◽  
Aspect Ratio ◽  
Fusion Reactor ◽  
Nuclear Fusion ◽  
Reversed Field Pinch ◽  
Nuclear Fusion Reactor ◽  
Low Aspect Ratio ◽  
Reversed Field

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.

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.

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.

Transient, Laminar, Combined Free and Forced Convection in a Duct

1962 ◽  
Vol 84 (2) ◽  
pp. 141-148 ◽  
Author(s):  
S. L. Zeiberg ◽  
W. K. Mueller
Keyword(s):  
Steady State ◽  
Forced Convection ◽  
Wall Temperature ◽  
Axial Direction ◽  
Transient Heating ◽  
Fully Developed Flow ◽  
Fluid Properties ◽  
Free And Forced Convection ◽  
Approach To Steady State ◽  
Rayleigh Numbers

Transient, laminar, combined free and forced convection in a duct is analyzed under the assumptions of constant fluid properties, and fully developed flow. The transient heating is taken to be a result of wall temperature variations; the wall temperatures vary linearly with the axial co-ordinate of the duct (this is shown to be the only permissible axial dependence, other than no wall temperature variation in the axial direction). Numerical results show that for certain combinations of the Prandtl and Rayleigh numbers, an oscillatory approach to steady state exists. This phenomenon can induce a large reduction of the Nusselt number (compared to steady state) during the transient period.

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