Introduction to Convection:Flow and Thermal ConsiderationsChapter Six and Appendix DSections 6.1 through 6.8 and D.1 through D.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.d()0.99u yudﾥﾥ=–Why does increase in the flow direction? d–Manifested by a surface shearstress that provides a drag force, .stDF0syuytm=ﾥ=ﾥsDssAFdAt=ﾥ–How does vary in the flowdirection? Why?st
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.td–Why does increase in theflow direction?td–Manifested by a surface heatflux and a convection heattransfer coefficient h.sqﾥﾥ()0.99stsTTyTTdﾥ-ﾥ=-0sfyTqky=ﾥﾥﾥ=-ﾥ0/fyskTyhTT=ﾥ-ﾥﾥﾥ-–If is constant, how do and hvary in the flow direction? ()sTTﾥ-sqﾥﾥ
Local and Average CoefficientsDistinction between Local andAverage Heat Transfer Coefficients•Local Heat Flux and Coefficient:()ssqh TTﾥﾥﾥ=-•Average Heat Flux and Coefficient for a Uniform Surface Temperature:()ssqhATTﾥ=-ssAqq dAﾥﾥ=ﾥ()sssATThdAﾥ=-ﾥ1ssAshhdAA=ﾥ•For a flat plate in parallel flow:1LohhdxL=ﾥ
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 criterionfor a flat plate in parallel flow:,Recritical Reynolds numbercx cu xrmﾥﾥﾥlocation at which transition to turbulence beginscxﾥ56,~~10Re3 x 10x c<<
Transition (cont.)What may be said about transition if ReL < Rex,c? If ReL > Rex,c?