178 pág.

ch09

DisciplinaTransferência de Calor2.833 materiais18.634 seguidores
Pré-visualização38 páginas
```264 K= <
From Eq. (1), the convection coefficient is then
( )( )
( )( ) ( )
4 4
elec,1 1 s sur 2
meas 2
s
P A / 2 T T 9.70 W 4.60 W
h 8.49 W / m K
A / 2 T T 0.01 60 m K
\u3b5 \u3c3
\u221e
\u2212 \u2212
\u2212
= = = \u22c5
\u2212 × \u22c5
<
With RaL = 2.58 × 10
5
, Eq. 9.30 yields
( )1/ 41/ 4 5 2Lk 0.0278 W / m Kh 0.054 Ra 0.54 2.58 10 8.46 W / m KL 0.04m \u22c5= = × = \u22c5 <
Again, agreement between the two values of h is well within the experimental uncertainty of the
measurements.
COMMENTS: Because the semi-circular disks are at the same temperature, the characteristic length
corresponds to that of the circular disk, L = D/4.
PROBLEM 9.40
KNOWN: Horizontal, circular grill of 0.2m diameter with emissivity 0.9 is maintained at a uniform
surface temperature of 130°C when ambient air and surroundings are at 24°C.
FIND: Electrical power required to maintain grill at prescribed surface temperature.
SCHEMATIC:
ASSUMPTIONS: (1) Room air is quiescent, (2) Surroundings are large compared to grill surface.
PROPERTIES: Table A-4, Air (Tf = (T¥ + Ts)/2 = (24 + 130)°C/2 = 350K, 1 atm): n = 20.92 ´ 10
-
6
m
2
/s, k = 0.030 W/m×K, a = 29.9 ´ 10-6 m2/s, b = 1/Tf.
ANALYSIS: The heat loss from the grill is due to free convection with the ambient air and to
( ) ( )4 4q A h T T T T .s s s suresé ù= - + -¥ê úë û (1)
Calculate RaL from Eq. 9.25,
( ) 3Ra g T T L /L s cb na= - ¥
where for a horizontal disc from Eq. 9.29, Lc = As/P = (pD
2
/4)/pD = D/4. Substituting numerical
values, find
( ) ( ) ( )329.8m/s 1/350K 130 24 K 0.25m/4 6Ra 1.158 10 .L 6 2 6 220.92 10 m / s 29.9 10 m / s
-
= = ´- -´ ´ ´
Since the grill is an upper surface heated, Eq. 9.30 is the appropriate correlation,
( )1 / 41/4 6Nu h L / k 0.54Ra 0.54 1.158 10 17.72L cL L= = = ´ =
( ) 2h Nu k / L 17.72 0.030W/m K / 0.25m/4 8.50W/m K.L cL= = ´ × = × (2)
Substituting from Eq. (2) for h into Eq. (1), the heat loss or required electrical power, qelec, is
( ) ( ) ( ) ( )( )W W8 42 4 4q 0.25m 8.50 130 24 K 0.9 5.67 10 130 273 24 273 K2 2 44 m K m K
p -= - + ´ ´ + - +
× ×
é ù
ê ú
ë û
q 44.2W 46.0W 90.2W.= + = <
COMMENTS: Note that for this situation, free convection and radiation modes are of equal
importance. If the grill were highly polished such that e » 0.1, the required power would be reduced by
nearly 50%.
PROBLEM 9.41
KNOWN: Plate dimensions and maximum allowable temperature. Freestream temperature.
FIND: Maximum allowable power dissipation.
SCHEMATIC:
ASSUMPTIONS: (1) Steady-state conditions, (2) Constant properties, (3) Negligible heat loss
from sides and bottom, (4) Isothermal plate.
PROPERTIES: Table A-4, Air (Tf = 325K, 1 atm): n = 18.4 ´ 10
-6 m2/s, k = 0.028 W/m×K, a
= 26.2 ´ 10-6 m2/s.
ANALYSIS: The power dissipated by convection is
( )P q hA T T .elec s s= = - ¥
With ( ) ( )2L A / P 1.2m / 4 1.2m 0.3ms= = =
( ) ( ) ( ) ( )
( ) ( )
1 33 2g T T L 9.8m/s 325K 50K 0.3msRaL 6 2 6 218.4 10 m / s 26.2 10 m / s
b
na
-- ¥= =
- -´ ´
7Ra 8.44 10 .L = ´
With the upper surface heated, Eq. 9.31 yields
hL 1 / 3Nu 0.15Ra 65.8L Lk
= = =
0.028W/m K 2h 65.8 6.14W/m K
0.3m
×= = ×
and the power dissipated is
( ) ( )22q 6.14W/m K 1.2m 50K= ×
P q 442W.elec = = <
COMMENTS: This result corresponds to an average surface heat flux of 442 W/1.44 m
2
= 307
W/m
2
= 0.03 W/cm
2
, which is extremely small. Heat dissipation by free convection in this manner is
a poor option compared to the heat flux with forced convection (u¥ = 15 m/s) of 0.15 W/cm
2.
PROBLEM 9.42
KNOWN: Material properties, inner surface temperature and dimensions of roof of refrigerated
truck compartment. Solar irradiation and ambient temperature.
FIND: Outer surface temperature of roof and rate of heat transfer to compartment.
SCHEMATIC:
ASSUMPTIONS: (1) Negligible irradiation from the sky, (2) Ts,o > T\u221e (hot surface facing upward)
and RaL > 10
7
, (3) Constant properties.
PROPERTIES: Table A-4, air (p = 1 atm, Tf \u2248 310K): \u3bd = 16.9 × 10-6 m2/s, k = 0.0270 W/m\u22c5K, Pr
= 0.706, \u3b1 = \u3bd/Pr = 23.9 × 10-6 m2/s, \u3b2 = 0.00323 K-1.
ANALYSIS: From an energy balance for the outer surface,
s,o s,i
S S conv cond
tot
T T
G q E q
R
\u3b1
\u2212
\u2032\u2032 \u2032\u2032
\u2212 \u2212 = =
\u2032\u2032
( ) s,o s,i4S S s,o s,o
p i
T T
G h T T T
2R R
\u3b1 \u3b5\u3c3
\u221e
\u2212
\u2212 \u2212 \u2212 =
\u2032\u2032 \u2032\u2032+
where ( ) ( )5 2 2p 1 p i 2 iR t / k 2.78 10 m K / W and R t / k 1.923 m K / W.\u2212\u2032\u2032 \u2032\u2032= = × \u22c5 = = \u22c5 For a hot surface
facing upward and ( ) 3 7L s,oRa g T T L / 10 , h\u3b2 \u3b1\u3bd\u221e= \u2212 > is obtained from Eq. 9.31. Hence, with
cancellation of L,
( )
1/ 32 1
1/ 31/ 3
L s,o12 4 2
k 9.8 m / s 0.00323 K
h 0.15 Ra 0.15 0.0270 W / m K T T
L 16.9 23.9 10 m / s
\u2212
\u221e
\u2212
×
= = × \u22c5 \u2212
× ×
\uf8eb \uf8f6\uf8ec \uf8f7\uf8ec \uf8f7\uf8ed \uf8f8
( )4 / 32 4 / 3 s,o1.73 W / m K T 305 K= \u22c5 \u2212
Hence,
( ) ( ) ( )
s,o2 2 4 / 3 8 2 4 44 / 3
s,o s,o 5 2
T 263K
0.5 750 W / m K 1.73 W / m K T 305 0.5 5.67 10 W / m K T
5.56 10 1.923 m K / W
\u2212
\u2212
\u2212
\u22c5 \u2212 \u22c5 \u2212 \u2212 × × \u22c5 =
× + \u22c5
Solving, we obtain s,oT 318.3K 45.3 C= = ° <
Hence, the heat load is ( ) ( ) ( )t cond 2
45.3 10 C
q W L q 3.5m 10m 1007 W
1.923m K / W
+ °
\u2032\u2032= \u22c5 = × =
\u22c5
<
COMMENTS: (1) The thermal resistance of the aluminum panels is negligible compared to that of
the insulation. (2) The value of the convection coefficient is ( )1/ 3 2s,oh 1.73 T T 4.10 W / m K.\u221e= \u2212 = \u22c5
PROBLEM 9.43
KNOWN: Inner surface temperature and composition of a furnace roof. Emissivity of outer surface and
temperature of surroundings.
FIND: (a) Heat loss through roof with no insulation, (b) Heat loss with insulation and inner surface
temperature of insulation, and (c) Thickness of fire clay brick which would reduce the insulation
temperature, Tins,i, to 1350 K.
SCHEMATIC:
ASSUMPTIONS: (1) Steady-state conditions, (2) One-dimensional conduction through the composite
wall, (3) Negligible contact resistance, (4) Constant properties.
PROPERTIES: Table A-4, Air (Tf \u2248 400 K, 1 atm): k = 0.0338 W/m\u22c5K, \u3bd = 26.4 × 10-6 m2/s, \u3b1 = 38.3
× 10-6 m2/s, Pr = 0.69, \u3b2 = (400 K)-1 = 0.0025 K-1; Table A-1, Steel 1010 (600 K): k = 48.8 W/m\u22c5K;
Table A-3 Alumina-Silica blanket (64 kg/m3, 750 K): k = 0.125 W/m\u22c5K; Table A-3, Fire clay brick (1478
K): k = 1.8 W/m\u22c5K.
ANALYSIS: (a) Without the insulation, the thermal circuit is
Performing an energy balance at the outer surface, it follows that
cond conv radq q q= + ( ) ( )s,i s,o 4 4s,o s,o sur
1 1 3 3
T T
hA T T A T T
L k A L k A
\u3b5\u3c3
\u221e
\u2212
= \u2212 + \u2212
+
(1,2)
where the radiation term is evaluated from Eq. 1.7. The characteristic length associated with free
convection from the roof is, from Eq. 9.29 2sL A P 16m 16 m 1m= = = . From Eq. 9.25, with an
assumed value for the film temperature, Tf = 400 K,
( ) ( )( )( ) ( )3123 s,os,o 7L s,o6 2 6 29.8m s 0.0025K T T l mg T T LRa 2.42 10 T T26.4 10 m s 38.3 10 m s
\u3b2
\u3bd\u3b1
\u2212
\u221e
\u221e
\u221e
\u2212 \u2212
\u2212
\u2212
= = = × \u2212
× × ×
Hence, from Eq. 9.31
( ) ( )1/ 3 1/ 31/ 3 7L s,ok 0.0338 W m Kh 0.15Ra 0.15 2.42 10 T TL 1 m \u221e\u22c5= = × \u2212 = \u2212 \u22c5\u221e1 1 3 2.47 , /T T W m Ks o\ufffd \ufffd .(3)
Continued...
PROBLEM 9.43 (Cont.)
The energy balance can now be written
( )
( ) ( )
4 /3s,o
s,o
1700 T K
1.47 T 298K
0.08m 1.8 W m K 0.005m 48.8W m K
\u2212
= \u2212
\u22c5 + \u22c5
( )4 48 2 4s,o0.3 5.67 10 W m K T 298K\u2212 \uf8ee \uf8f9+ × × \u22c5 \u2212\uf8ef \uf8fa\uf8f0 \uf8fb
and from iteration, find Ts,o \u2248 895 K. Hence,
( ) ( ) ( ){ }4 / 3 4 42 2 8 2 4q 16m 1.47 895 298 W m 0.3 5.67 10 W m K 895K 298 K\u2212= \u2212 + × × \u22c5 \u2212\uf8ee \uf8f9\uf8ef \uf8fa\uf8f0 \uf8fb
{ }2 2 5q 16m 7,389 10,780 W m 2.91 10 W= + = × . <
(b) With the insulation, an additional conduction resistance is provided and the energy balance at the
outer surface becomes
( ) ( )s,i s,o 4 4s,o s,o sur
1 1 2 2 3 3
T T
hA T T A T T
L k A L k A L k A
\u3b5\u3c3```