lecture 24 - Thermal and Fluids Engineering I Lecture 24...

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Thermal and Fluids Engineering I Lecture 24 Page 1 Lecture 24 – Natural Convection In natural convection, flow is driven by buoyancy forces. For example, consider a hot vertical flat plate exposed to a cold fluid on one side. Both thermal and velocity boundary layers develop on the plate, although they generally have different thicknesses. The velocity boundary layer is unusual in that velocity is zero both at the plate and far from the plate. The heat transfer coefficient for natural convection varies along the plate, being higher at the leading edge, where the boundary layers are thinner. In natural convection, there is no obvious velocity to use in determining the heat transfer coefficient. Instead we utilize the variation of density with temperature in the form
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Thermal and Fluids Engineering I Lecture 24 Page 2 p T ρ ⎛⎞ ⎜⎟ ⎝⎠ We normalize this derivative by defining volume expansivity – the rate of change in density with temperature per unit density 1 p T β =−
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This note was uploaded on 04/08/2008 for the course ENGR 2250 taught by Professor Borca-tasciuc during the Spring '08 term at Rensselaer Polytechnic Institute.

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lecture 24 - Thermal and Fluids Engineering I Lecture 24...

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