上海交通大学：《传热学》课程PPT教学课件（英文版）CHAPTER 6 Introduction to convection

HEAT TRANSFER CHAPTER 6 Introduction to convection 们au Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 1 HEAT TRANSFER CHAPTER 6 Introduction to convection

CH6- INTRODUCTION Where we’ ve been. Basic conduction Heat Transfer inished Fourier’slaw: k kvt Where we’ re going: Begin study of convective heat transfer Newtons law of cooling =h( T-T Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 2 CH6 – INTRODUCTION Where we’ve been …… Basic Conduction Heat Transfer Finished Fourier’s law: Where we’re going: Begin study of convective heat transfer. Newton’s law of cooling: dx dt q  = −k q  = −kt ( ) = −  q  h Ts T

Convective transfer problem A FAMILY CIRCUS 2-6 201 Eh ty Krg mures siN. "I forget Does holding the door open let the cold air in, or the warm out? Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 3 Convective transfer problem

CH6 INTRODUCTION KEY POINTS THIS CHAPTER What are the key variables when analyzing convection heat transfer? Review boundary layer concept and significance Generalidea of relationship between velocity and thermal profiles in a boundary layer. Effect oflaminar versus turbulent flow on heat transfer potential Boundary layer similarity This chapter will be taught in two lectures: the firstincludes text book sections $6. 1 to 6.4 the other includes text book sections 86.5 to 6.10 Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 4 CH6 INTRODUCTION KEY POINTSTHIS CHAPTER • What are the key variables when analyzing convection heat transfer? • Review boundary layer concept and significance • General idea of relationship between velocity and thermal profiles in a boundary layer. • Effect of laminar versus turbulent flow on heat transfer potential • Boundary layer similarity • This chapter will be taught in two lectures: the first includes text book sections §6.1 to 6.4 the other includes text book sections §6.5 to 6.10

Convection overview Consider a flat plate oflength L, in air flow with velocity uoo and temperature Too y uoo, Too 1,7 Local heat flux is where h is the q"=H(T-7) local heat transfercoefficient Total heat transfer rate. ∫ql4=(,-T)hll q=hA(T-T average heat transfer coefficient Determination ofh'will rely on analytical as well as empirical data Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 5 Convection overview • Consider a flat plate of length L, in air flow with velocity u and temperature T • Local heat flux is: where h is the local heat transfer coefficient • Total heat transfer rate: h = average heat transfer coefficient Determination of ‘h’ will rely on analytical as well as empirical data ( ) = −  q  h Ts T    =  = − s As s s A q q dAs (T T ) hdA ( ) q = hAs Ts −T

Convection overview(Contd) Same principal applies to any arbitrary shape, not Just a flat plate Average convection heat transfer coefficient hdA. or, for unit width hdx S So. we need to know how h varies with x. the distance from the leading edge What do you think key parameters that might influence h? Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 6 Convection overview (Cont’d) • Same principal applies to any arbitrary shape, not just a flat plate • Average convection heat transfer coefficient: So, we need to know how h varies with x, the distance from the leading edge…….. What do you think key parameters that might influence h? q  dAs As Ts , or, for unit width:  = As s s hdA A h 1  = L hdx L h 0 1

Key parameters Transfer potential: forced flow or free flow Phase change: boiling and condensation Flow conditions laminar or turbulent flow Geometries: shape, size, position and roughness Properties: density, viscosity, thermal conductivity, specific heat, and so on Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 7 Key parameters • Transfer potential: forced flow or free flow • Phase change: boiling and condensation • Flow conditions: laminar or turbulent flow • Geometries: shape, size, position and roughness. • Properties: density, viscosity, thermal conductivity, specific heat, and so on

E Xample Given Experimental results for measured local heat transfer coefficient h for flow over a flat plate with a rough sur face where: a=coefficient h,(x)=ax 0.3 x= distance from leading edge Find expression for average heat transfer coefficient. and the relation of average heat transfer coefficient to the local coefficient sh Jar 03 dx=jx. dx 0 0.7 0.7x Distance from leading edge Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 8 Example Given • Experimental results for measured local heat transfer coefficient h for flow over a flat plate with a rough surface • where: a = coefficient x = distance from leading edge – Find expression for average heat transfer coefficient, and the relation of average heat transfer coefficient to the local coefficient ( ) −0.3 h x = ax x 0.7 0 0.3 0 0.3 0.7 1 1 x x a h x dx x a a x dx x h x x x x = =  =  − −

The Convection Boundary layers Velocity Boundary Layer Free stream 6(x Velocity boundary a For fluid flow over a flat plate which disturbs the fluid flow Asy→>∞:u= u where u is velocity in X-direction As y>0: u=0(no-slip condition) The boundary layer thickness is defined as the value at which: u(y)=0.99u The boundary layer thickness 8 varies with x Shear Stress Dynamic viscosity 0 Local friction coefficient Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 9 The Convection Boundary Layers Velocity Boundary Layer • For fluid flow over a flat plate, which disturbs the fluid flow: – As y→: where u is velocity in x-direction – As y→0: (no-slip condition) – The boundary layer thickness is defined as the value at which: – The boundary layer thickness  varies with x • Shear Stress • Local friction coefficient =0   = y s y u           =  2 2 u C s f   u = u u = 0 = u u(y) 0.99 Dynamic viscosity

The Convection Boundary layers Thermal boundary laver Free stream (x) Therma T boundary rer A hot or cold plate alters the temperature distribution in the air Asy→>∞:7()=T Asy→>0:T(y) The thermal boundary layer thickness is defined as the value at which 0.99 T-To The thermal boundary layer thickness, st also varies(increases)with x Heat Transfer Su Yongkang School of Mechanical Engineering

Heat Transfer Su Yongkang School of Mechanical Engineering # 10 The Convection Boundary Layers Thermal Boundary Layer • A hot or cold plate alters the temperature distribution in the air – As y→: – As y→0: – The thermal boundary layer thickness is defined as the value at which: – The thermal boundary layer thickness, t also varies (increases) with x 0.99 ( ) = −  − T T T T y s s air Ts T =T T(y) Ts T( y) =