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Free Convection: Cylinders, Spheres, and Enclosures 1
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The Long Horizontal Cylinder Boundary Layer Development and Variation of the Local Nusselt Number for a Heated Cylinder: The Average Nusselt Number: How do conditions change for a cooled cylinder? 2
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Spheres The Average Nusselt Number: In the limit as how may conditions be characterized? Ra D 10 11 Pr 0,7 3
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Enclosures Rectangular Cavities Characterized by opposing walls of different temperatures, with the remaining walls well insulated. Horizontal Cavity Vertical Cavity 4
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Horizontal Cavities Heating from Below – Fluid layer is thermally stable. – Thermal instability yields a regular convection pattern in the form of roll cells. – Buoyancy drive flow is turbulent 5
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Heating from Above – Fluid layer is unconditionally stable. Vertical Cavities – A primary cellular flow is established, as the core becomes progressively more quiescent, and secondary (corner) cells develop with increasing 6
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Inclined Cavities Relevant to flat plate solar collectors. Heat transfer depends on the magnitude of relative to a critical angle, whose value depends on H/L (Table 9.4). Heat transfer also depends on the magnitude of relative to a critical Rayleigh number of Heat transfer correlations Eqs. (9.54) – (9.57). 7
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Annular Cavities Concentric Cylinders Critical Rayleigh Number: 8 K of a stationary fluid to transfer the same amount of heat as the moving fluid
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Concentric Spheres Critical Rayleigh Number: 9
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