Introduction to Convection: Flow and Thermal Considerations Chapter Six
Chapter 1 Review: Newton’s Cooling Law - Definition of convective heat transfer coefficient, h
Boundary Layer Features Boundary Layers:• Velocity Boundary Layer What is the boundary layer? What is the shear stress at wall?
Boundary Layer Features (cont.) • Thermal Boundary Layer ()0.99stsTTyTTδ∞−→=What is the thermal boundary layer?
Velocity profiles in Laminar and Turbulent Flows
Local and Average Coefficients Local and Average Heat Transfer Coefficients • Local Heat Flux and Coefficient: ()∞ʹ′ʹ′=−ssqh TT• Average Heat Flux and Coefficient for a Uniform Surface Temperature: ()ssqhATT∞=−• For a flat plate in parallel flow: 1LohhdxL=∫
Transition Transition from Laminar to Turbulent Flow (Chapter 7) • Transition criterion for a flat plate in parallel flow: ,critical Reynolds numbercx cu xReρμ∞≡→location at which transition to turbulence beginscx→105<Rex,c<3 ×106
Does heat transfer increase or decrease with x?
Effect of transition on boundary layer thickness and local convection coefficient: Why does transition increase the boundary layer thickness? Why? Why?
Boundary Layer Equations How to Characterize the Boundary Layer Flows? Velocity Boundary Layer: 2222,uupdpxdxxy∞∂∂∂<<≈∂∂∂Thermal Boundary Layer: 2222TTxy∂∂<<∂∂
Example 6.5 Objectives: Determine h, the convective heat transfer coefficient on the outer wall of the blade. Given: Local heat flux = 95 kW/m2. Ts= 800oC through internal cooling.