第五章对流换热分析

wton’s law of cooling:

−=W/m 2

dx

dt

q λ−=

Contents

第一节对流换热概述

Analysis on Convection

第二节对流换热微分方程组

The Convection Heat Transfer Equations

第三节边界层换热微分方程组

Convection Differential Equations of Boundary Layer 第四节边界层换热积分方程(自学)

第五节动量传递和热量传递的类比(自学)

第六节相似理论基础

Basis of similarity theory

Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion.

(流体与固体壁直接接触时所发生的热量传递过程，称为对流换热)

The faster the fluid motion, the greater the convection heat transfer.

We will study how to calculate the convection heat-transfer coefficient h in Chapter 5 and Chapter 6.

5-1 Analysis on Convection(对流换热概述) Convection transfer problem

Heat exchangers Tubes in steam

boiler (蒸汽锅炉的管束)

Tube-shell heat exchanger (管壳式换热器)Condenser of

refrigerator Tubes with

fins (翅片管束)

图5-1几种常见的换热设备示意图

Factors influencing convection heat transfer ：

Flow causes of fluid (流体流动的起因)、flow states ( 流动状态)、properties of fluid (流体物性)、change of phase of fluid (流体物相变化)、geometry parameters (壁面的几何参数),and so on 。

1.Causes and States of Flow (流动的起因和状态)

(1) Causes of flow(流动起因)

Forced convection(受迫对流):

if the fluid is forced to flow over the surface by external means such as a fan ,pump,or the wind .

natural

forced h h >Natural (or free) convection(自然对流):

if the fluid motion is caused by buoyancy (浮力) forces that is induced(引起) by density difference due to the variation of the temperature in the fluid.

Examples ：air free convection ：h=5~25W/（m 2.K ）

air forced convection ：h=10~100W/（m 2.K ）

(2) flow states ( 流动状态)

Whatever the causes of flow are ,the flow states are consist of laminar flow (层流)and turbulent flow (紊流)

Laminar flow ——characterized by smooth streamlines (流线) and highly ordered motion due to the fluid flowing in laminae(层) or layers(整个流场呈一簇互相平行的流线)

Turbulent flow ——characterized by velocity fluctuation (波动) and highly disordered motion （

流体质点做复杂无规则的运动）

l

t h h >

2.The thermal properties of fluid (流体热物性)

Properties:

density(密度) ρ[kg/m 3], specific heat(比热) c [J/（kg. ℃）]thermal conductivity(导热系数) λ[ W/(m. ℃)],dynamic viscosity(动力粘度) μ[kg/（s.m)]kinematic viscosity(运动粘度) ν（m 2/s ），volume expansion coefficient (体积膨胀系数) α(1/K),

ρμ=v p p T T ⎟

⎠⎞

⎜⎝⎛∂∂−=⎟⎠⎞⎜⎝⎛∂∂=ρρυυα11自然对流换热增强

↑⇒α)

( 多能量单位体积流体能携带更、↑↑⇒h c ρ)

( 热对流有碍流体流动、不利于↓↑⇒h μ↑↑⇒h λ)(间导热热阻小流体内部和流体与壁面where Ideal gas

α=1/T