Fundamentals of Heat and Mass Transfer, 6^th edition

Presented ：杜文静 E-mail: wjdu@sdu.edu.cn Telephone: 88399000-2511

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Chapter 6 Introduction to convection

Convection: energy transfer between a surface and a fluid moving over the surface Objectives Understand the physical mechanism of convection; Develop means to perform convection transfer calculation

Contents: Basic contents on convection heat transfer and convection mass transfer physical origin, dimensionless parameters, analogies

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Chapter 6 Introduction to convection

Convection: energy transfer between a surface and a fluid moving over the surface ( 表面和流过表面的流体之间的能量传递 ) the bulk fluid motion (advection) the random motion of fluid molecules (conduction or diffusion)

The determination of convection transfer coefficient and related heat transfer enhancement method are the central problems in the study of convection issue.

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• Chapter 7 External flow

• Chapter 8 Internal flow

• Chapter 9 Free convection

• Chapter 10 Boiling and condensation

• Chapter 11 Heat exchangers

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6.1 the convection boundary layer

Boundary layers: velocity, thermal , concentration,

• 速度边界层，温度边界层，浓度边界层

• Friction coefficient （摩擦系数）

• Convection heat transfer coefficient （对流换热系数）

• Convection mass transfer coefficient （对流传质系数）

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6.1 the convection boundary layer

The velocity boundary layer

• Fluid velocity=0 at the surface 无滑移边界条件

• Velocity grow Velocity u

• Shear stress ( 切应力 ), parallel to fluid velocity

Velocity boundary layer thickness u = .099 u

In boundary layer ( a thin layer): velocity gradients and shear stresses are large Region outside the boundary layer: velocity gradients and shear stresses are negligible

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6.1 the convection boundary layer

Velocity boundary layer: developed whenever there is fluid flow over a surface

For a Newtonian fluid ，牛顿流体

Cf: the local friction coefficient,

dynamic viscosity ，动力粘度 kg/m S

当地摩擦系数，局部摩擦系数

to determine the friction drag ， a dimensionless parameter 摩擦力

u ： velocity along the x direction m/s

u Cf y

x

s surface shear stress ，表面切应力 N/m2

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6.1 the thermal boundary layer

Developed when the flow free stream and the surface temperatures differ t Thermal boundary layer thickness when ( Ts - T ) /( Ts - T ) = .099

T x h y

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6.1 the concentration boundary layer

• Example: air past the surface of a pool of water evaporation 蒸发

• • a binary mixture of chemical species A and B, Sublimation 升华

CA: molar concentration , kmol/m³

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6.1 the concentration boundary layer

• Concentration boundary layer: concentration gradients exist( 浓度梯度 ) - c :thickness is defined as ( C A , s C A ) = .099 - ( C A , s C A , )

Molar flux of species A , NA’’ (kmol / s m2) ， 组分 A 的摩尔流密度 Binary diffusion coefficient, 二元扩散系数 DAB

'' C
A
= -
N D
A AB

y
'' C
N = - D A
A , s AB y = 0
y

Fick ‘s law Diffusion mass transfer, 斐克定律

At the surface y=0, diffusion mass transfer

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