Lecture 10

# Lecture 10 - Lecture 10 Convection(External Flat Plate...

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Lecture 10 Convection (External) Flat Plate Dr.Srinath V.Ekkad

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Physical Features Physical Features As with all external flows, the boundary layers develop freely without constraint. Boundary layer conditions may be entirely laminar, laminar and turbulent, or entirely turbulent. To determine the conditions, compute and compare with the critical Reynolds number for transition to turbulence, , laminar flow t Re Re hroughout L x c < , , transition to turbulent flow Re Re at / Re R e / L x c c x c L x L
Velocity Boundary Layer τ s = C f ( ρ u 2 / 2 ) = Local surface shear stress ( N/m 2 ) C f = Local friction coefficient = Function of x * and Re L τ s = C f ( ρ u 2 / 2 ) = Average surface shear stress ( N/m 2 ) C f = Average friction coefficient = Function of Re L Thermal Boundary Layer q s ” = h (T s - T ) = Local surface heat flux (W/m 2 ) h = Local heat transfer coefficient( W/m 2 K) Nu x = h x / k f = Local Nusselt # = Function of x * , Re L , Pr q s = A s h (T s - T ) = Total heat transfer rate (W) h = Average heat transfer coefficient( W/m 2 K) Nu L = h L / k f = Average Nusselt # = Function of Re L and Pr Property Evaluation The best temperature to evaluate properties is the average fluid temperature, T T T f s = + 2 Most correlations are based on T f . A few correlations use the ratio af propeties evaluated at the freestream and surface temperatures.

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Physical Features (cont.) , Re x c Value of depends on free stream turbulence and surface roughness. Nominally, 5 , 5 10 Re . x c x x If boundary layer is tripped at the leading edge and the flow is turbulent throughout. , Re 0 x c = Surface thermal conditions are commonly idealized as being of uniform temperature or uniform heat flux . s
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Lecture 10 - Lecture 10 Convection(External Flat Plate...

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