The Mechanism of Convection The total heat transfer in the convective situation is more than what would flow as a result of conduction alone. --That heat carried by elements of fluid from warmer regions to cooler regions is not a basic heat transfer mechanism. --It can be understood by considering a situation where there is no temperature difference between the fluid the surface (isothermal) Here,no HT takes place,even though fluid motion across layers exists. Convection represents a process of gross movement as opposed to diffusion(conduction). "h"is a gross description of the heat flow between a surface a fluid. (When heat flows from one fluid to another,we use Overall HT Coeff.U) Chapter 6 6
The Mechanism of Convection Chapter 6 6 • The total heat transfer in the convective situation is more than what would flow as a result of conduction alone. -- That heat carried by elements of fluid from warmer regions to cooler regions is not a basic heat transfer mechanism. -- It can be understood by considering a situation where there is no temperature difference between the fluid & the surface (isothermal). Here, no HT takes place, even though fluid motion across layers exists. • Convection represents a process of gross movement as opposed to diffusion (conduction). • “h” is a gross description of the heat flow between a surface & a fluid. ( When heat flows from one fluid to another, we use Overall HT Coeff. U )
Classification OLaminar flow Turbulent flow ONatural and Forced Convection. O Natural convection---Flow is driven by density difference due to temperature gradient in the fluid Forced convection---(External and Internal)Flow is driven by external force Single phase,Two phase,Multi-phase
Classification zLaminar flow & Turbulent flow zNatural and Forced Convection. z Natural convection ---Flow is driven by density difference due to temperature gradient in the fluid z Forced convection---(External and Internal) Flow is driven by external force z Single phase, Two phase, Multi-phase
Boundary Layer Features Boundary Layers:Physical Features Velocity Boundary Layer Free stream 8(x Consequence of viscous effects Velocity boundary -A region characterized by shear layer stresses and velocity gradients. -between the surface and the free u(y=-0.99 stream whose thickness 6 increases in the flow direction.(m) Ou -Manifested by a surface shear stress =0 Us that provides a drag force,Fp. FD=∫t,dA N/m2) N)
Boundary Layer Features Boundary Layers: Physical Features • Velocity Boundary Layer – Consequence of viscous effects – A region characterized by shear stresses and velocity gradients. ( ) 0.99 u y u δ ∞ → = – Manifested by a surface shear stress τs that provides a drag force, FD . FD s y 0 u y τ μ = ∂ = ∂ s D ss A F = ∫ τ dA – between the surface and the free stream whose thickness δ increases in the flow direction. (m) (N/m2) (N)