MEGI+412+Study+Notes+Ch7.pdf - 7.1 Chapter 7 External flow...

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7. 1 Chapter 7: External flow We confine our attention to problems of low speed, forced convection, with no phase change. 7.1 The empirical method Re Pr with Re Pr m n L L L m n L L Nu C hL Nu k k k h Nu C L L = =     = =         This is an empirical correlation derived from experimental measurements. The specific values of , , C m n vary with the nature of the surface geometry and the type of flow. Fluid properties vary with temperature across the boundary layer and this variation can influence the heat transfer rate. This influence may be handled by evaluating all properties at a mean boundary layer temperature T f termed the film temperature given by 2 + = T T T s f . An alternative method is to evaluate all properties at T and to multiply the right-hand side of the L Nu equation by an additional parameter to account for the property variations: ( ) Pr Pr r s or ( ) r s μ μ . 7.2 Flat plate in parallel flow Despite its simplicity, parallel flow over a flat plate occurs in numerous engineering applications. Laminar boundary layer development begins at the leading edge and transition to turbulence may occur at a downstream location for which a critical Reynolds number is achieved. 7.2.1 Laminar flow ( ) 5 Re 5 10 < The assumption is made that the flow is steady, incompressible and laminar with constant fluid properties and negligible viscous dissipation. The correlated local Nusselt number is of the form
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7. 2 0.6 Pr Pr Re 332 . 0 3 1 2 1 = x x x k x h Nu . The average Nusselt number is given by 1 2 1 3 0.664Re Pr Pr 0.6 L L Nu = .
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