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.

scholarly journals Transient Natural Convection in Porous Square Cavity Heated and Cooled on Adjacent Walls

2012 ◽  
Vol 2012 ◽  
pp. 1-10 ◽  
Author(s):  
M. S. Selamat ◽  
I. Hashim ◽  
M. K. Hasan
Keyword(s):  
Steady State ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Vertical Wall ◽  
Transient State ◽  
Governing Equations ◽  
Square Cavity ◽  
Transient Natural Convection ◽  
Temperature Distributions ◽  
Rayleigh Numbers

Transient natural convection in a square cavity filled with a porous medium is studied numerically. The cavity is assumed heated from one vertical wall and cooled at the top, while the other walls are kept adiabatic. The governing equations are solved numerically by a finite difference method. The effects of Rayleigh number on the initial transient state up to the steady state are investigated for Rayleigh number ranging from 10 to2×102. The evolutions of flow patterns and temperature distributions were presented for Rayleigh numbers,Ra=102and103. It is observed that the time taken to reach the steady state is longer for low Rayleigh number and shorter for high Rayleigh number.

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].

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.

Natural Convection Inside a Bidisperse Porous Medium Enclosure

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
Arunn Narasimhan ◽  
B. V. K. Reddy
Keyword(s):  
Heat Transfer ◽  
Porous Medium ◽  
Natural Convection ◽  
Volume Fraction ◽  
Solid Phase ◽  
Darcy Number ◽  
The Core ◽  
Side Walls ◽  
Porous Blocks ◽  
Rayleigh Numbers

Bidisperse porous medium (BDPM) consists of a macroporous medium whose solid phase is replaced with a microporous medium. This study investigates using numerical simulations, steady natural convection inside a square BDPM enclosure made from uniformly spaced, disconnected square porous blocks that form the microporous medium. The side walls are subjected to differential heating, while the top and bottom ones are kept adiabatic. The bidispersion effect is generated by varying the number of blocks (N2), macropore volume fraction (ϕE), and internal Darcy number (DaI) for several enclosure Rayleigh numbers (Ra). Their effect on the BDPM heat transfer (Nu) is investigated. When Ra is fixed, the Nu increases with an increase in both DaI and DaE. At low Ra values, Nu is strongly affected by both DaI and ϕE. When N2 is fixed, at high Ra values, the porous blocks in the core region have negligible effect on the Nu. A correlation is proposed to evaluate the heat transfer from the BDPM enclosure, Nu, as a function of Raϕ, DaE, DaI, and N2. It predicts the numerical results of Nu within ±15% and ±9% in two successive ranges of modified Rayleigh number, RaϕDaE.

Transient Natural Convection Between Two Zones in an Insulated Enclosure

1988 ◽  
Vol 110 (1) ◽  
pp. 116-125 ◽  
Author(s):  
P. A. Litsek ◽  
A. Bejan
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Numerical Experiments ◽  
Thermal Stratification ◽  
Convection Flow ◽  
Natural Convection Flow ◽  
Flow And Heat Transfer ◽  
Vertical Opening ◽  
The Right ◽  
Rayleigh Numbers

The natural convection flow and heat transfer between two enclosures that communicate through a vertical opening is studied by considering the evolution of an enclosed fluid in which the left half is originally at a different temperature than the right half. Numerical experiments show that at sufficiently high Rayleigh numbers the ensuing flow is oscillatory. This and other features are anticipated on the basis of scale analysis. The time scales of the oscillation, the establishment of thermal stratification, and eventual thermal equilibrium are determined and tested numerically. At sufficiently high Rayleigh numbers the heat transfer between the communicating zones is by convection, in accordance with the constant-Stanton-number trend pointed out by Jones and Otis (1986). The range covered by the numerical experiments is 102 < Ra < 107, 0.71 < Pr < 100, and 0.25 < H/L < 1.

scholarly journals Lattice Boltzmann simulation of turbulent natural convection in tall enclosures

2015 ◽  
Vol 19 (1) ◽  
pp. 155-166 ◽  
Author(s):  
Hasan Sajjadi ◽  
Reza Kefayati
Keyword(s):  
Nusselt Number ◽  
Natural Convection ◽  
Lattice Boltzmann ◽  
Average Nusselt Number ◽  
Large Eddy Simulations ◽  
Turbulent Natural Convection ◽  
Aspect Ratios ◽  
Lattice Boltzmann Simulation ◽  
Large Eddy ◽  
Rayleigh Numbers

In this paper Lattice Boltzmann simulation of turbulent natural convection with large-eddy simulations (LES) in tall enclosures which is filled by air with Pr=0.71 has been studied. Calculations were performed for high Rayleigh numbers (Ra=107-109) and aspect ratios change between 0.5 to 2 (0.5<AR<2). The present results are validated by finds of an experimental research at Ra=1.58x109. Effects of the aspect ratios in different Rayleigh numbers are displayed on streamlines, isotherm counters, vertical velocity and temperature at the middle of the cavity, local Nusselt number and average Nusselt number. The average Nusselt number increases with the augmentation of Rayleigh numbers. The increment of the aspect ratio causes heat transfer to decline in different Rayleigh numbers.

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 New Method for Calculating the Heating of the Conductor

Energies ◽  
2019 ◽  
Vol 12 (14) ◽  
pp. 2769
Author(s):  
Voršič ◽  
Maruša ◽  
Pihler
Keyword(s):  
Heat Transfer ◽  
Tensile Strength ◽  
Steady State ◽  
Natural Convection ◽  
Simple Model ◽  
Solar Radiation ◽  
Balance Of Power ◽  
Stable Operation ◽  
The Core ◽  
Steel Core

The paper describes the core heating of ACSR (aluminum conductor steel—reinforced) conductor in stable operation under different environmental conditions. The calculations are greatly simplified in a steady state—we can calculate on a balance of power instead of a balance of energies. At a known surface, the temperature of the conductor due to solar radiation, natural convection, and joules heating as well as the temperature of the steel core were calculated, which is relevant for the tensile strength of the rope. Measurements of the surface of the conductor and the core rejected a simple model of heat transfer—it is also necessary to take into account empty air spaces between the wires of a rope. On the basis of measurements, a new model has given satisfactory compliance with the measured values.

A Numerical Study of Natural Convection in Partially Open Enclosures With a Conducting Side-Wall

2004 ◽  
Vol 126 (1) ◽  
pp. 76-83 ◽  
Author(s):  
G. Desrayaud ◽  
G. Lauriat
Keyword(s):  
Natural Convection ◽  
Numerical Study ◽  
Finite Thickness ◽  
Vertical Wall ◽  
Side Wall ◽  
Practical Applications ◽  
Hot Air ◽  
Aspect Ratios ◽  
Vertical Walls ◽  
Rayleigh Numbers

A numerical study of natural convection generated by a cold vertical wall of an enclosure with two openings on the opposite wall of finite thickness is presented. The enclosure is connected to an infinite reservoir filled with hot air. A two-dimensional laminar flow is assumed both within the enclosure and along the side of the bounding wall immersed into the reservoir. The effects of the size of the openings, spacing between the vertical walls and thermal resistance of the bounding wall are investigated. Numerical results are discussed for aspect ratios of the enclosure and Rayleigh numbers relevant to practical applications.

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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