unsteady state conduction
DESCRIPTION
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Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
Outline Basic concepts Application The Biot number Lumped parameter method
Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
Basic concepts Steady-state conduction: temperature do not change with time; equilibrium condition unsteady-state conduction: temperature change with time
Q: Is the concept of thermal resistance suitable for the analysis of unsteady-state conduction?
Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
temperature profile of unsteady-state conduction
t 1
t 0 0
1
2
34
non-regular regime
non-regular regime
regular regimeregular regime
Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
Q: Is the heat flow rate constant in the process of unsteady-state conduction?Q: Is the heat flow rate constant in the process of unsteady-state conduction?
Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
Application
the transient heating or cooling process
which takes place in the interim period
before equilibrium is established
Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
The Biot number
hrh 1tf
h
tf
h
x
t
0
tf
h
x
t
0
r
h
hrr
Bih
1
Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
h
hrr
Bih
1
Bi hrr 0Bihrr
the influence of Bi on temperature distributionthe influence of Bi on temperature distribution
Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
t
iB iB0 0Bi
12
2
3
12 12
01 01
0 0 00tt 0tt 0tt
Li XuemeiLi Xuemei
CH3 Unsteady-state Conduction
Lumped parameter method• the resistance to heat transfer by
conduction were small compared with the convection resistance at the surface
• the internal resistance of the body is negligible in comparison with the external resistance
Li XuemeiLi Xuemei
Exercise
Li XuemeiLi Xuemei
Thank you!