BF-Ch6 - Introduction to Convection Flow and Thermal Considerations Chapter Six and Appendix D Sections 6.1 through 6.8 and D.1 through D.3 Boundary

BF-Ch6 - Introduction to Convection Flow and Thermal...

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Introduction to Convection: Flow and Thermal Considerations Chapter Six and Appendix D Sections 6.1 through 6.8 and D.1 through D.3
Boundary Layer Features Boundary Layers: Physical Features Velocity Boundary Layer A consequence of viscous effects associated with relative motion between a fluid and a surface. A region of the flow characterized by shear stresses and velocity gradients. A region between the surface and the free stream whose thickness increases in the flow direction. d ( ) 0.99 u y u d = Why does increase in the flow direction? d Manifested by a surface shear stress that provides a drag force, . s t D F 0 s y u y t m = = s D s s A F dA t = How does vary in the flow direction? Why? s t
Boundary Layer Features (cont.) Thermal Boundary Layer A consequence of heat transfer between the surface and fluid. A region of the flow characterized by temperature gradients and heat fluxes. A region between the surface and the free stream whose thickness increases in the flow direction. t d Why does increase in the flow direction? t d Manifested by a surface heat flux and a convection heat transfer coefficient h . s q ᆬᆬ ( ) 0.99 s t s T T y T T d - = - 0 s f y T q k y = ᆬᆬ =- 0 / f y s k T y h T T = - - If is constant, how do and h vary in the flow direction? ( ) s T T - s q ᆬᆬ
Local and Average Coefficients Distinction between Local and Average Heat Transfer Coefficients Local Heat Flux and Coefficient: ( ) s s q h T T ᆬᆬ = - Average Heat Flux and Coefficient for a Uniform Surface Temperature: ( ) s s q hA T T = - s s A q q dA ᆬᆬ = ( ) s s s A T T hdA = - 1 s s A s h hdA A = For a flat plate in parallel flow: 1 L o h hdx L =
Transition Boundary Layer Transition How would you characterize conditions in the laminar region of boundary layer development? In the turbulent region? What conditions are associated with transition from laminar to turbulent flow? Why is the Reynolds number an appropriate parameter for quantifying transition from laminar to turbulent flow? Transition criterion for a flat plate in parallel flow: , Re critical Reynolds number c x c u x r m location at which transition to turbulence begins c x 5 6 , ~ ~ 10 Re 3 x 10 x c < <
Transition (cont.) What may be said about transition if ReL < Rex,c? If ReL > Rex,c?

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