The results of nil available experimental investigations into the characteristics of free convective flow of air between horizontal isothermal concentric cylinders are reviewed and several discrepancies are pointed out. An experimental study is described which was directed at resolving these discrepancies and categorizing the several flow patterns which have been observed.Using six different cylinder sets and varying both the annulus pressure and temperature difference between the cylinder surfaces, a range of Grashof numbers (based on annulus width) from 300 to 3.4 X /0 6 was achieved.The resulting air flow patterns were made visible with the use of tobacco smoke and are documented by written descriptions, photographs, motion pictures, and quantitative data. One steady and three unsteady flow patterns were observed and comparison with the results of other investigators is presented.A chart is presented which allows prediction of the type of unsteady flow that will occur for a wide range of cylinder combinations and annulus operating conditions..1 comparison with cylinders in forced crossflow is used to satisfactorily predict the onset of one of the unsteady flow patterns. Also, the flow patterns observed experimentally are compared lo those predicted by an available analytical solution.

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The results of a finite-difference solution for natural convection within horizontal cylindrical annuli for Prandtl numbers near 0.7 (air) are presented.The ranges of Rayleigh number and inverse relative gap width over which such a solution yields valid results are investigated.It is shown that this solution, though formulated for steady flow, can be used to obtain an indication of the Rayleigh number at which transition from ft steady to an unsteady flow will occur for a wide range of inverse relative gap widths (2.8-12.5 ). A recent experimental investigation has shown that steady secondary cellular flows occur immediately prior to transition for this range of inverse relative gap widths, and the Rayleigh number at which these secondary flows occur is accurately predicted by the numerical solution, thereby yielding a good indication of the Rayleigh number at which transition to an unsteady flow occurs. Flow patterns predicted by the numerical solution near transition are compared with photographs of the flow, and excellent qualitative agreement is noted. It is thus shown that this numerical technique gives, at least qualitatively, valid residts for all Rayleigh numbers at which the flow is steady for the range of inverse relative gap widths under consideration.Heretofore unavailable data on temperature profiles, radial and angidar velocity components, and local Nusselt numbers for Rayleigh numbers near the transition value are presented.It is found that the foregoing parameters (velocity, temperature, and Nusselt number) are little affected by the appearance of the secondary flow for the smaller inverse relative gap widths considered, whereas for the larger inverse relative gap widths a pronounced increase in magnitude of temperatures and velocity components is noted. The overall or mean Nusselt number is not affected by the appearance of these secondary flows for any of the inverse relative gap widths considered.Transactions of the ASME Copyright © 1971 by ASME Downloaded From: http://heattransfer.asmedigitalcollection.asme.org/ on 06/08/2015 Terms of Use: http://asme.org/terms

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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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