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UBC Theses and Dissertations

Combined free and forced convection from horizontal plates Classen, Lutz


A theoretical analysis and experimental results are presented for .free convection and combined free and forced convection from a heated horizontal surface. The principal objective was to investigate a laminar boundary layer flow which had been shown, theoretically only, to form above a heated surface. This boundary layer flow is fundamentally different from flows above inclined or vertical surfaces since the driving force or buoyancy force acts perpendicular to the primary boundary layer motion. The flows analyzed are those for which the system of partial differential equations describing the flow can be reduced to simultaneous total differential equations. The method involves the introduction of similarity parameters and then the numerical integration of the resulting simplified system of total differential equations. These solutions are restricted, for 2-dimensional flow, to a semi-infinite surface, and for axially-symmetrical flow, to an infinite disc. In conjunction with the former only free convection is considered while for the latter combined convection is considered as well. The flow was examined experimentally with a semi-focusing colour schlieren system. From the photographs it may be concluded that the semi-infinite surface analysis would correctly portray a physical flow. The flow, though, remains laminar for a short distance only and then breaks down into an unstable cellular pattern. The axially-symmetrical analysis, although it yielded analytically a valid boundary layer solution, appears to have no physical parallel above a disc of finite radius.

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