上海交通大学：《对流换热 Convection Heat Transfer》教学资源_分类视图_Chapter 6 Principles of Convection

Convection Heat Transfer Ch 6 Fundamental Concepts Ch 7 庄xternal flow .Heat Ch 8 Internal flow transfer Ch 9 Free convection coefficient Sh 10 Convection with phase change Ch 11 Heat exchanger

Convection Heat Transfer • Ch 6 Fundamental Concepts • Ch 7 External flow • Ch 8 Internal flow • Ch 9 Free convection • Ch 10 Convection with phase change • Ch 11 Heat exchanger •Heat transfer coefficient

Chapter 6 Principles of Convection >Boundary Layers >Laminar and Turbulent Flow Chapter 6 2

Chapter 6 2 Chapter 6 Principles of Convection ¾Boundary Layers ¾Laminar and Turbulent Flow

Introduction to Convection Convection heat transfer (HT)denotes energy transfer between a surface a fluid moving over the surface. The dominant contribution is due to the bulk(or gross)motion of fluid particles. General idea of convective HT problems: (1)simple convection problems between a surface a fluid (liquid gas) (2)between a liquid and a gas across the interface (with direct contact) (3)between two immiscible liquids across the interface. Chapter 6 3

Introduction to Convection Chapter 6 3 • Convection heat transfer (HT) denotes energy transfer between a surface & a fluid moving over the surface. • The dominant contribution is due to the bulk (or gross) motion of fluid particles. General idea of convective HT problems: (1) simple convection problems between a surface & a fluid (liquid / gas) (2) between a liquid and a gas across the interface (with direct contact) (3) between two immiscible liquids across the interface

The Mechanism of Convection Convection is not a basic heat transfer mechanism: If the fluid were perfectly at rest,heat is transferred into the fluid by molecular conduction. .Temperature gradients transfer of heat are in a direction normal to the surface. uly)a A solid surface and a fluid exchanging heat with it. Chapter 6 4

Chapter 6 4 The Mechanism of Convection Convection is not a basic heat transfer mechanism: • If the fluid were perfectly at rest, heat is transferred into the fluid by molecular conduction. • Temperature gradients & transfer of heat are in a direction normal to the surface. A solid surface and a fluid exchanging heat with it

The Mechanism of Convection T(y) .The first layer of fluid adheres to the surface because of viscous friction (no-slip condition at the wall.ie,v,velocity of surface itself) In subsequent layers,the velocity increases progressively to Uo In the first layer (no slip layer),heat transfer by conduction. other layers,also conduction. Velocity gradient exists,the fluid moving from warmer to colder part carries the stored heat (i.e,enthalpy). Chapter 6 5

Chapter 6 5 The Mechanism of Convection •The first layer of fluid adheres to the surface because of viscous friction (no-slip condition at the wall. ie, vs = velocity of surface itself) • In subsequent layers, the velocity increases progressively to U∞ • In the first layer (no slip layer), heat transfer by conduction. other layers, also conduction. • Velocity gradient exists, the fluid moving from warmer to colder part carries the stored heat (i.e, enthalpy)

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)

Boundary Layer Features(cont.) Thermal Boundary Layer -A consequence of heat transfer Free stream一δ，lx) Thermal -A region of the flow characterized boundary layer by temperature gradients and heat fluxes. T,-T(=0.99 -Between the surface and T、-T the free stream whose thickness ncreases in the flow direction.(m) Manifested by a surface heat flux a and a convection heat transfer (Wim) -k OT18yy-0 coefficient h. (W/m2●K) T、-T

Boundary Layer Features (cont.) – A region of the flow characterized by temperature gradients and heat fluxes. – Manifested by a surface heat flux and a convection heat transfer coefficient h. Thermal Boundary Layer – A consequence of heat transfer – Between the surface and the free stream whose thickness δt increases in the flow direction. s q ′′ ( ) 0.99 s t s T Ty T T δ ∞ − → = − s f y 0 T q k y = ∂ ′′ = − ∂ 0 / f y s kT y h T T = ∞ −∂ ∂ ≡ − (m) (W/m2) (W/m2 •K )

Concentration Boundary Layer -A consequence of mass transfer Mixture of A+B Free stream一&.) Concentration -A region of the flow characterized boundary layer by concentration gradients and molar/mass fluxes. -A region between the surface and 6。→ C4s-CA=0.99 the free stream whose thickness Oc (m) C4.s-C4.0 increases in the flow direction. CA Manifested by a surface molar flux N”=-D (m/s) Nand a convection heat transfer coefficientC (kmol/s°m2) hm= C4.s-C4

Concentration Boundary Layer – A consequence of mass transfer – A region of the flow characterized by concentration gradients and molar/mass fluxes. – A region between the surface and the free stream whose thickness δ c increases in the flow direction. – Manifested by a surface molar flux and a convection heat transfer coefficient h m NA . ′′ , , , 0.99 As A c As A C C C C δ ∞ − → = − 0 A A AB y C N D y = ∂ ′′ = − ∂ 0 , , / AB A y m As A DC y h C C = ∞ −∂ ∂ = − (m) (kmol/s • m 2 ) (m/s )