Lecture 9 - Lecture 9 Dr. Srinath V. Ekkad Chapter 6 -...

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Lecture 9 Dr. Srinath V. Ekkad Chapter 6 - Convection
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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. δ Why does increase in the flow direction? Manifested by a surface shear stress that provides a drag force, . s τ D F How does vary in the flow direction? Why? s
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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 δ Why does increase in the flow direction? t Manifested by a surface heat flux and a convection heat transfer coefficient h . s q F If is constant, how do and h vary in the flow direction? ( 29 s T T F - s q F
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Local and Average Coefficients Distinction between Local and Average Heat Transfer Coefficients Local Heat Flux and Coefficient : Average Heat Flux and Coefficient for a Uniform Surface Temperature : For a flat plate in parallel flow :
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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:
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Transition (cont.) What may be said about transition if Re L < Re x,c ? If Re L > Re x,c ? Effect of transition on boundary layer thickness and local convection coefficient: Why does transition provide a significant increase in the boundary layer thickness? Why does the convection coefficient decay in the laminar region?
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Lecture 9 - Lecture 9 Dr. Srinath V. Ekkad Chapter 6 -...

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