Linear stability of natural convection in spherical annuli

1990 ◽  
Vol 221 ◽  
pp. 105-129 ◽  
Author(s):  
David R. Gardner ◽  
Rod W. Douglass ◽  
Steven A. Trogdon
Keyword(s):  
Natural Convection ◽  
Prandtl Number ◽  
Outer Sphere ◽  
Radius Ratio ◽  
Grashof Number ◽  
Concentric Spheres ◽  
Different Temperatures ◽  
Fixed Radius ◽  
Higher Temperature ◽  
Boussinesq Fluid

Natural convection in a Boussinesq fluid filling the narrow gap between two isothermal, concentric spheres at different temperatures depends strongly on radius ratio, Prandtl number, and Grashof number. When the inner sphere has a higher temperature than the outer sphere, and for fixed values of radius ratio and Prandtl number, experiments show the flow to be steady and axisymmetric for sufficiently small Grashof number and quasi-periodic and axisymmetric for Grashof numbers greater than a critical value. It is our hypothesis that the observed transition is a flow bifurcation. This hypothesis is examined by solving an appropriate eigenvalue problem. The critical Grashof number, critical eigenvalues, and corresponding eigenvectors are obtained as functions of the radius ratio, Prandtl number, and longitudinal wavenumber. Critical Grashof numbers range from 1.18 × 104 to 2.63 × 103 as Prandtl number Pr increases from zero to 0.7, for radius ratios of 0.900 and 0.950. A transitional Prandtl number Prt exists such that for Pr < Prt the bifurcation is time-periodic and axisymmetric. For Pr > Prt the bifurcation is steady and non-axisymmetric with wavenumber twoA first approximation to the bifurcated flow is obtained using the critical eigenvectors. For Pr < Prt the bifurcation sets in as a cluster of relatively strong cells with alternating directions of rotation. The cells remain fixed in location, but pulsate with time. The cluster moves toward the top of the annulus as Pr increases toward Prt. An important feature of the non-axisymmetric bifurcation for Pr > Prt is a set of four cells located at each pole of the annulus in which the radial velocity alternates direction in moving from any one cell to an adjacent one. For fixed radius ratio, the average Nusselt number at criticality varies only slightly with Prandtl number.

Laminar natural convection about an isothermally heated sphere at small Grashof number

1968 ◽  
Vol 34 (1) ◽  
pp. 163-176 ◽  
Author(s):  
Francis E. Fendell
Keyword(s):  
Natural Convection ◽  
Three Dimensional ◽  
Perturbation Expansion ◽  
Outer Sphere ◽  
Convective Transport ◽  
Convection Flow ◽  
Constant Density ◽  
Boussinesq Model ◽  
Grashof Number ◽  
Recirculating Flow

The flow induced by gravity about a very small heated isothermal sphere introduced into a fluid in hydrostatic equilibrium is studied. The natural-convection flow is taken to be steady and laminar. The conditions under which the Boussinesq model is a good approximation to the full conservation laws are described. For a concentric finite cold outer sphere with radius, in ratio to the heated sphere radius, roughly less than the Grashof number to the minus one-half power, a recirculating flow occurs; fluid rises near the inner sphere and falls near the outer sphere. For a small heated sphere in an unbounded medium an ordinary perturbation expansion essentially in the Grashof number leads to unbounded velocities far from the sphere; this singularity is the natural-convection analogue of the Whitehead paradox arising in three-dimensional low-Reynolds-number forced-convection flows. Inner-and-outer matched asymptotic expansions reveal the importance of convective transport away from the sphere, although diffusive transport is dominant near the sphere. Approximate solution is given to the nonlinear outer equations, first by seeking a similarity solution (in paraboloidal co-ordinates) for a point heat source valid far from the point source, and then by linearization in the manner of Oseen. The Oseen solution is matched to the inner diffusive solution. Both outer solutions describe a paraboloidal wake above the sphere within which the enthalpy decays slowly relative to the rapid decay outside the wake. The updraft above the sphere is reduced from unbounded growth with distance from the sphere to constant magnitude by restoration of the convective accelerations. Finally, the role of vertical stratification of the ambient density in eventually stagnating updrafts predicted on the basis of a constant-density atmosphere is discussed.

Instability of steady natural convection in a vertical fluid layer

1978 ◽  
Vol 84 (4) ◽  
pp. 743-768 ◽  
Author(s):  
R. F. Bergholz
Keyword(s):  
Natural Convection ◽  
Prandtl Number ◽  
Fluid Layer ◽  
Travelling Wave ◽  
Linear Stability Theory ◽  
Wave Instability ◽  
Theoretical Predictions ◽  
Different Temperatures ◽  
Prandtl Numbers ◽  
Moderate Range

The instability of steady natural convection of a stably stratified fluid between vertical surfaces maintained at different temperatures is analysed. The linear stability theory is employed to obtain the critical Grashof and Rayleigh numbers, for widely varying levels of the stable background stratification, for Prandtl numbers ranging from 0·73 to 1000 and for the limiting case of infinite Prandtl number. The energetics of the critical disturbance modes also are investigated. The numerical results show that, if the value of the Prandtl number is in the low to moderate range, there is a transition from stationary to travelling-wave instability if the stratification exceeds a certain magnitude. However, if the Prandtl number is large, the transition, with increasing stratification, is from travelling-wave to stationary instability. The theoretical predictions are in excellent agreement with the experimental observations of Elder (1965) and of Vest & Arpaci (1969), for stationary instability, and in fair to good agreement with the experimental results of Hart (1971), for travelling-wave instability.

Heat Transfer by Laminar Natural Convection Within Rectangular Enclosures

1970 ◽  
Vol 92 (1) ◽  
pp. 159-167 ◽  
Author(s):  
M. E. Newell ◽  
F. W. Schmidt
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Three Dimensional ◽  
Time Dependent ◽  
Two Dimensional ◽  
Numerical Techniques ◽  
Grashof Number ◽  
Different Temperatures ◽  
Laminar Natural Convection ◽  
Steady State Solutions

Two-dimensional laminar natural convection in air contained in a long horizontal rectangular enclosure with isothermal walls at different temperatures has been investigated using numerical techniques. The time-dependent governing differential equations were solved using a method based on that of Crank and Nicholson. Steady-state solutions were obtained for height to width ratios of 1, 2.5, 10, and 20, and for values of the Grashof number, GrL′, covering the range 4 × 103 to 1.4 × 105. The bounds on the Grashof number for H/L = 20 is 8 × 103 ≤ GrL′ ≤ 4 × 104. The results were correlated with a three-dimensional power law which, yielded H/L=1Nu¯L′=0.0547(GrL′)0.3972.5≤H/L≤20Nu¯L′=0.155(GrL′)0.315(H/L)−0.265 The results compare favorably with available experimental results.

The stability of natural convection in a vertical fluid layer

1976 ◽  
Vol 73 (1) ◽  
pp. 65-75 ◽  
Author(s):  
Jiro Mizushima ◽  
Kanefusa Gotoh
Keyword(s):  
Natural Convection ◽  
Temperature Gradient ◽  
Fluid Layer ◽  
Vertical Direction ◽  
Grashof Number ◽  
Numerical Integrations ◽  
Stable Temperature ◽  
Different Temperatures ◽  
The Stability ◽  
Stationary Disturbances

The stability of natural convection in fluid between two parallel vertical plates is investigated theoretically. The two plates are maintained at different temperatures and a uniform stable temperature gradient β is present in the vertical direction. The Prandtl number of the fluid is fixed at 7.5. An orthonormalization method is used in numerical integrations of the disturbance equations. It is shown how the critical Grashof number varies with β for both stationary and travelling disturbances. It is found that for β ≤ 7.1 × 10−3 the convection is unstable to stationary disturbances and for β ≥ 7.1 × 10−3 it is unstable to travelling disturbances. The critical Grashof number is given by \[ G_c = \left\{\begin{array}{@{}l@{\quad}c@{\quad}l@{}} 500 & {\rm for} & \beta < 1.0\times 10^{-3},\\ 1.3\times 10^6\beta^3 & {\rm for} &\beta > 4.1\times 10^{-2}, \end{array}\right. \] and even for intermediate values of β the variation of Gc is rather simple but not monotonic.

Thermoconvective Motion of Low Prandtl Number Fluids Within a Horizontal Cylindrical Annulus

1977 ◽  
Vol 99 (4) ◽  
pp. 596-602 ◽  
Author(s):  
J. R. Custer ◽  
E. J. Shaughnessy
Keyword(s):  
Heat Flux ◽  
Natural Convection ◽  
Prandtl Number ◽  
Perturbation Expansion ◽  
Constant Heat Flux ◽  
Basic Flow ◽  
Constant Heat ◽  
Grashof Number ◽  
Cylindrical Annulus ◽  
Prandtl Numbers

Steady natural convection in very low Prandtl number fluids is investigated using a double perturbation expansion in powers of the Grashof and Prandtl numbers. The fluid is contained in a horizontal cylindrical annulus, the walls of which either are held at constant temperature or support a constant heat flux. In both cases the evolution of the flow for increasing Grashof number is of interest. It is found that the basic flow pattern consists of one eddy. For both boundary conditions the center of this eddy falls into the lower half of the annulus as the Grashof number increases. Such behavior is directly opposite to experimental results obtained in fluids of higher Prandtl number.

An Analysis of Laminar Forced Convection Between Concentric Spheres

1976 ◽  
Vol 98 (4) ◽  
pp. 601-608 ◽  
Author(s):  
K. N. Astill
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Temperature Distribution ◽  
Prandtl Number ◽  
Stokes Equations ◽  
Initial Conditions ◽  
Radius Ratio ◽  
Flow And Heat Transfer ◽  
Concentric Spheres ◽  
Inner Surface

A numerical solution for predicting the behavior of laminar flow and heat transfer between concentric spheres is developed. Axial symmetry is assumed. The Navier-Stokes equations and energy equation are simplified to parabolic form and solved using finite-difference methods. Hydrodynamic and energy equations are uncoupled, which allows the hydrodynamic problem to be solved independently of the heat-transfer problem. Velocity and temperature are calculated in terms of the two spatial coordinates. Solutions depend on radius ratio of the concentric spheres, Reynolds number of the flow, Prandtl number, initial conditions of temperature and velocity, temperature distribution along the spherical surfaces, and azimuthal position of the start of the flow. The effect on flow and heat transfer of these variables, except surface temperature distribution, is evaluated. While the computer solution is not restricted to isothermal spheres, this is the only case treated. Velocity profiles, pressure distribution, flow losses, and heat-transfer coefficients are determined for a variety of situations. Local and average Nusselt numbers are computed, and a correlation is developed for mean Nusselt number on the inner surface as a function of Reynolds number, Prandtl number, and radius ratio. Flow separation is predicted by the analysis. Separation is a function of Reynolds number, radius ratio, and azimuthal location of the initial state. Separation was observed at the outer surface as well as from the inner surface under some conditions. In cases where separation occurred, the solution was valid only to the point of separation.

Heat Transfer by Natural Convection between Vertically Eccentric Spheres

1973 ◽  
Vol 95 (1) ◽  
pp. 47-52 ◽  
Author(s):  
N. Weber ◽  
R. E. Powe ◽  
E. H. Bishop ◽  
J. A. Scanlan
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Convective Motion ◽  
Outer Sphere ◽  
Correlation Equation ◽  
Mapping Technique ◽  
Transfer Rates ◽  
Concentric Spheres ◽  
Wide Range ◽  
Inner Sphere

Natural convection to a cooled sphere from an enclosed, vertically eccentric, heated sphere is described in this paper. Water and two silicone oils were utilized in conjunction with four different combinations of sphere sizes and six eccentricities for each of these combinations. Both heat-transfer rates and temperature profiles are presented. The effect of a negative eccentricity (inner sphere below center of outer sphere) on the temperature distribution was an enhancement of the convective motion, while a positive eccentricity tended to stabilize the flow field and promote conduction rather than convection. As for concentric spheres, a multicellular flow pattern was postulated to explain the thermal field observed for the largest inner sphere utilized. In all cases the heat-transfer rates were increased by moving the inner sphere to an eccentric position, and the utilization of a conformal-mapping technique to transform the eccentric spheres to concentric spheres enabled the application of existing empirical correlations for concentric spheres to the eccentric-sphere data. It is significant to note that this technique yields a single correlation equation, in terms of only keff/k and a modified Rayleigh number, which is valid for an extremely wide range of diameter ratios, eccentricities, Rayleigh numbers, and Prandtl numbers.

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