t-Δt
t
t+Δt
Q
dA n
温度梯度是一个点的概念。
温度梯度是一个向量。 方向垂直于该点所在等温面，以温度增加的方向为正
Basic law of conduction
Fourier’s law (傅立叶定律)
dq = −k ∂T dA ∂n
(10-1)
negative --- gradient is opposite that of
Heat transfer by conduction Equal temperature surface 等温面
在同一时刻，温度场中所有温度相同的点组成的面。
★不同温度的等温面不相交
等温
t1
面
t2
Q
t1>t2
temperature gradient 温度梯度
r grad t
=
lim
Δt
=
∂t
Δn→0 Δn ∂n
Chapter10. Heat transfer by conduction Basic conception
¾Steady-state conduction 稳态导热
( ) Ttimhtee=,tceamnfpbexera,thtyuer,fezuanrcetiionn∂∂dθtoepf=epn0odsietniotnof,
对流--- 流体内部质点发生相对位移的热量传递过程
Newton’s law of cooling
q A
=
h(ts
−
t
f
)
h---heat transfer coefficient
ts--- surface temperature
tf----fluid temperature (ts-tf)---temperature driving force
Thermal conductivity
constant k
----a physical property of the substance. independent of the temperature gradient. It from Experiment like the Newtonian Viscosity µ,.
Conduction 热传导
Temperature gradient exists in continuous substance, heat can flow by any observable
motion of matter.
Fourier’s law (傅立叶定律)
dq = −k dt dA dx
G2，t1 T2
G1，T1
t2 A
zDouble-tube heat exchanger
套管式换热器
2 shell-and-tube heat exchanger 列管式换热器
• Single-pass shell-and-tube condenser
double-pass shell-and-tube condenser
thermalconductivity k can be write:
k =a+bT
(10-6)
★Linear function ★ k solid > k liquid > k gases
There are two types of convection :
znatural convection 自然对流
由于流体内温度不同造成的浮升力 引起的流动
zforce convection 强制对流
流体受外力作用而引起的流动。
Radiation 热辐射
Radiation is the transfer of energy through space by electromagnetic waves.
(1) heat up or cooling , (2) heat exchange (3) heat preservation, reduce heat loss
Nature of heat flow
The mechanisms by the heat flows are three:
Conduction ----热传导 Convection ----对流 Radiation --热辐射
the heat flow (from hot to cold) ,.
dq = −k ∂T dA ∂n
(10-1)
--- heat transfer area导热面积，m2
x ---distance,m
k ── thermal conductivity导热系数， W/(m·℃)或W/(m·K)
In gases, ---conduction occurs by the random motion of molecules
z In metals,.
-- conduction results from the motion of free electrons
Convection 对流
convective flow of heat --- a particle of fluid crosses a specific surface, such as the boundary of a control volume, it carries with quantity of enthalpy.
物体因热的原因发出辐射能的过程称为热辐射 能量转移、能量形式的转化
Fourth Power Law 四次方定律
Wb = σT 4
σ=stefan-boltzmann constant
T=absolute temperature
3.Typical heat-exchange equipment
1.Double-tube heat exchanger 套管式换热器
Section 3 Heat Transfer
summarize
1.Heat Transfer and its application
2.Nature of heat flow (three type)
3.Typical heat-exchange equipment
• 1.Heat Transfer and its application • The purpose of heat transfer involve: