文档介绍：第四节对流传热(2)
Xiamen University Department of Chemical and
Biochemical Engineering
(一)流体在管内作强制对流
1. 流体在光滑圆形直管内作强制湍流(Re > 10 4)
(1)低粘度(<2倍常温水的粘度)流体
Nu = Prn
n
k æ diub ρö æ cp µ ö
或 h = ç ÷ ç ÷
di è µ ø è k ø
流体被加热时,n=
n
流体被冷却时,n=
Xiamen University Department of Chemical and
Biochemical Engineering
1
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(2)高粘度流体
1/3
Nu = Re Pr ϕW
1/3
k ædiubρö æcpµö æ µ ö
或 h= ç ÷ ç ÷ ç ÷
di è µ ø è k ø è µW ø
式中,µW 为壁温下的粘度
被加热时,ϕW =
因壁温较难确定,故对液体:
被冷却时,ϕW =
对气体:无论加热或冷却,
均取ϕW =
Xiamen University Department of Chemical and
Biochemical Engineering
例1 p350例5-7
例2 水在一定流量下通过某套管换热器的内管,
温度可从20℃升到80℃,并测得对流传热系
数为1000W /( m 2 × ° C ) ,试求同样体积流量
的苯通过该换热器内管的对流传热系数为多
少?已知两种情况下流动型态皆为湍流,苯
进口与出口的平均温度为60℃。
解: 水的定性温度=(20+80)/2=50℃
由附录查得50℃水的物性为:
­3
ρ=1000kg / m3 µ = ´10 Pa × s
Xiamen University Department of Chemical and
Biochemical Engineering
2
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3
cp = ´10 J /(kg ×°C) k = /(m ×°C)
60℃苯的物性为:
ρ¢ = 840kg / m3 µ¢ = ´10­3 Pa × s
3
c¢p =´10 J /(kg ×°C) k¢ = /(m×°C)
据题意,两种情况的对流传热系数均可用下式计算:
n
k æ diub ρö æ c p µ ö
h = ç ÷ ç ÷
di è µ ø è k ø
ρ c p k ub
=
µ di
Xiamen Unive