Introduction to Convection:Flow and Thermal ConsiderationsChapter Six and Appendix ESections 6.1 through 6.8 and E.1 through E.3
Boundary Layer FeaturesBoundary Layers: Physical Features•Velocity Boundary Layer –A consequence of viscous effectsassociated with relative motionbetween a fluid and a surface.–A region of the flow characterized byshear stresses and velocity gradients.–A region between the surfaceand the free stream whosethickness increases in the flow direction.0.99u yu–Why does increase in the flow direction? –Manifested by a surface shearstressthat provides a drag force, .sDF0syuysDssAFdA–How does vary in the flowdirection? Why?s
Boundary Layer Features (cont.)•Thermal Boundary Layer–A consequence of heat transfer between the surface and fluid.–A region of the flow characterizedby temperature gradients and heatfluxes.–A region between the surface andthe free stream whose thickness increases in the flow direction.t–Why does increase in theflow direction?t–Manifested by a surface heatfluxand a convection heattransfer coefficient h.sq0.99stsTTyTT0sfyTqky 0/fyskTyhTT–If is constant, how do and hvary in the flow direction? sTTsq
Local and Average CoefficientsDistinction betweenLocalandAverage Heat Transfer Coefficients•Local Heat Flux and Coefficient:ssqh TT•Average Heat Flux and Coefficient for a Uniform Surface Temperature:ssqhATTssAqq dAsssATThdA1ssAshhdAA•For a flat plate in parallel flow:1LohhdxL
TransitionBoundary Layer Transition•How would you characterize conditions in the laminar regionof boundary layerdevelopment?In the turbulent region?•What conditions are associated with transitionfrom laminar to turbulent flow?•Why is the Reynolds number an appropriate parameter for quantifying transitionfrom laminar to turbulent flow?
Transition (cont.)What may be said about transition if ReL< Rex,c?If ReL> Rex,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? Why does it increase significantly with transition to turbulence, despite the increase in the boundary layerthickness? Why does the convection coefficient decay in the turbulent region??
Boundary Layer EquationsThe Boundary Layer Equations•Consider concurrent velocity and thermal boundary layer development for steady,two-dimensional, incompressible flowwith constant fluid propertiesand negligible body forces.