VF3-1
VF3-1
VF3-1
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Värmeledningsekvationen (Heat<br />
conduction equation),<br />
isotropt (isotropic) material<br />
∂t<br />
∂ ⎛ ∂t<br />
ρc<br />
= ⎜λ<br />
∂τ<br />
∂x<br />
⎝ ∂x<br />
∂ ⎛ ∂t<br />
⎜<br />
∂z ⎝<br />
λ ∂z<br />
⎞<br />
⎟ +<br />
⎠<br />
⎞<br />
⎟ +<br />
⎠<br />
∂ ⎛ ∂t<br />
⎜λ<br />
∂y<br />
⎝ ∂y<br />
Q′<br />
⎞<br />
⎟ +<br />
⎠<br />
( 1−18)<br />
Om λ = konstant, if λ constant ⇒ homogent<br />
(homogeneous) material<br />
∂t<br />
∂τ<br />
=<br />
2<br />
⎧ ∂ t<br />
a⎨<br />
2<br />
⎩<br />
∂<br />
x<br />
+<br />
∂<br />
2<br />
∂<br />
y<br />
t<br />
2<br />
+<br />
2<br />
∂ t ⎫<br />
+<br />
2 ⎬<br />
∂<br />
z<br />
⎭<br />
Q′<br />
ρ<br />
c<br />
(1 −19)<br />
a<br />
λ ⎧termisk diffusivitet⎫<br />
= ⎨ ⎬<br />
ρ c<br />
⎩ thermal diffusivity<br />
⎭
Enkel Plan Vägg, Simple Plane<br />
Wall, Kap., Ch. 3<br />
t 1 b<br />
t 1 > t<br />
t 2<br />
2<br />
x<br />
Q & ∂<br />
stationär värmeledning ⇒ = 0<br />
∂τ<br />
steady heat conduction:<br />
∂ ∂<br />
endimensionellt:<br />
= ≡ 0<br />
∂yy<br />
∂zz<br />
one dimensional case:<br />
ingen intern värmegenerering: Q′ = 0<br />
no internal heat generation:<br />
λ = konstant ober av t och x: λ constant independent of<br />
t and x:<br />
2<br />
(1 - 19) ⇒<br />
d t<br />
= 0<br />
2<br />
dx<br />
lösning, solution: t = c 1 x + c 2<br />
RV, BC: x = 0 t = t 1 ; x = b t = t 2<br />
⇒ linjär temperaturfördelning
Enkel Plan Vägg,forts, Simple plane<br />
wall<br />
t t<br />
t<br />
2 −<br />
= 1 + x (3 − 3)<br />
b<br />
t<br />
1<br />
Värmeflödet, The heat flow:<br />
dt<br />
Q&<br />
= −λA<br />
dx<br />
Q<br />
1<br />
Q&<br />
= λA<br />
(t1 − t 2 ) (3 − 4)<br />
b<br />
Alternativ formulering, Alternate formulation<br />
b<br />
t<br />
{<br />
1− t2<br />
= ⋅<br />
{<br />
Q&<br />
λ A<br />
drivkraft { flöde<br />
" spänning " resi−<br />
" ström "<br />
potential stan s current<br />
resis tan ce
Sammansatt plan vägg-<br />
Composite Wall<br />
t 1<br />
λ1<br />
λ 3<br />
λ 2<br />
t 4<br />
λ<br />
b 1 b 2 b 3<br />
λ 3<br />
Q &<br />
”Seriekoppling - Serial circuit”<br />
Q<br />
t<br />
1<br />
∑<br />
t 1− t4 = ( termisk res, thermal res)<br />
⋅Q&<br />
− t<br />
4<br />
=<br />
⎛<br />
⎜<br />
⎝<br />
b1<br />
b<br />
+<br />
2<br />
+<br />
λ A λ A<br />
1<br />
2<br />
b3<br />
λ A<br />
⇒<br />
t<br />
t<br />
Q&<br />
1 −<br />
=<br />
4<br />
(3 − 6)<br />
b1<br />
b b<br />
+<br />
2<br />
+<br />
3<br />
λ1A<br />
λ2A<br />
λ3A<br />
Konvektion, Convection<br />
Q&<br />
= α A( t −t ) ⇒ Konvektiv resis tan s :<br />
f<br />
3<br />
⎞<br />
⎟⋅<br />
⎠<br />
Q&<br />
1<br />
Convective resis tan ce<br />
α A<br />
w
Sammansatt Vägg- Composite Wall,<br />
forts, continuation<br />
t f1<br />
α 1<br />
λ 1 λ 2<br />
λ 3<br />
α 2<br />
t f2<br />
Q&<br />
=<br />
1<br />
α A<br />
1<br />
+<br />
b 1 b 2 b 3<br />
)<br />
t<br />
f 1<br />
3<br />
∑<br />
i=<br />
l<br />
− t<br />
i<br />
f 2<br />
bi<br />
+<br />
λ A<br />
1<br />
α A<br />
2<br />
(3 − 7
Cirkulärt rör eller skikt-Circular<br />
tube or layer (shell)<br />
• Innerradie, inner radius r i ;Ytterradie, outer radius r o ; Innerytans<br />
temperatur, inner temperature t i ; Ytterytans temperatur, outer<br />
temperature t o<br />
r o<br />
radiell temperaturfördelning<br />
t i<br />
> t o<br />
r i<br />
t i<br />
r<br />
dr<br />
r i<br />
radial temperature distribution<br />
r o<br />
t o<br />
t i<br />
> t o<br />
)<br />
&<br />
t<br />
−t<br />
i o<br />
Q= −1<br />
1<br />
⎛<br />
r<br />
ln<br />
i ⎞<br />
⎜ ⎟<br />
2πλL<br />
⎝ro<br />
⎠<br />
(3−10
Cirkulärt rör eller skikt sammansatt av olika<br />
material, Composite circular wall (shell)<br />
radiell temperaturfördelning<br />
t fi > t fo<br />
r 1<br />
λ 1<br />
λ 2<br />
t fi<br />
r 2<br />
r 3<br />
t 1<br />
t 2<br />
t 3<br />
t fo<br />
radial a temperature e distribution<br />
t fi > t fo<br />
0<br />
Q&<br />
=<br />
tf<br />
−t<br />
i fo<br />
1 1 r 1 r 1<br />
+ ln + ln +<br />
2 πrL<br />
π<br />
r<br />
L<br />
α<br />
2<br />
3<br />
1L<br />
α<br />
i<br />
2<br />
π<br />
L<br />
λ<br />
1<br />
r<br />
1<br />
2<br />
π<br />
L<br />
λ<br />
2<br />
r<br />
2<br />
2<br />
3<br />
0