Linear stability of two-dimensional natural convective flow in horizontal cylindrical annuli is performed for three-dimensional perturbations and radius ratios in the range 1.2 ≤ R ≤ 3. Flow transitions for moderate to large gap annuli which have not been reported before are thoroughly investigated. Then stability diagrams are obtained for finite and for infinite length annuli. The leading disturbances and thresholds values are found to agree well with experimental data and three-dimensional numerical solutions previously published.
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