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17-78
Review Problems
17-119 Two persons are wearing different clothes made of different materials with different surface areas. The fractions of
heat lost from each person‟s body by perspiration are to be determined.
Assumptions 1 Heat transfer is steady. 2 Heat transfer is one-dimensional. 3 Thermal conductivities are constant. 4 Heat
transfer by radiation is accounted for in the heat transfer coefficient. 5 The human body is assumed to be cylindrical in shape
17-81
17-122 Steam is flowing inside a steel pipe. The thickness of the insulation needed to reduce the heat loss by 95 percent and
the thickness of the insulation needed to reduce outer surface temperature to 40C are to be determined.
Assumptions 1 Heat transfer is steady since there is no indication of any change with time. 2 Heat transfer is one-
dimensional since there is thermal symmetry about the centerline and no variation in the axial direction. 3 Thermal
conductivities are constant. 4 The thermal contact resistance at the interface is negligible.
17-82
17-123 A spherical vessel is used to store a fluid. The thermal resistances, the rate of heat transfer, and the temperature
difference across the insulation layer are to be determined.
Assumptions 1 Steady operating conditions exist. 2 Heat transfer is one-dimensional.
17-83
17-124 One wall of a refrigerated warehouse is made of three layers. The rates of heat transfer across the warehouse without
and with the metal bolts, and the percent change in the rate of heat transfer across the wall due to metal bolts are to be
determined.
Assumptions 1 Steady operating conditions exist. 2 Heat transfer coefficients are constant.
Properties The thermal conductivities are given to be kAl = 200 W/mK, kfiberglass = 0.038 W/mK, kgypsum = 0.48 W/mK, and
17-87
L, m
T(x), °C
Part (a)
Part (b)
Part (c)
Part (d)
0
350
350
350
350
0.005
318
326
328
325
0.010
290
305
308
304
0.015
264
288
292
285
0.020
241
272
279
270
0.025
220
260
268
256
0.030
201
250
259
246
0.035
184
242
253
237
0.040
169
237
250
231
0.045
155
233
249
227
0.050
142
232
250
224
The temperature variations for parts (a) to (d) are plotted in the following figure:
x, m
0.00 0.01 0.02 0.03 0.04 0.05
T, °C
100
150
200
250
300
350
Infinitely long fin
Adiabatic fin tip
Fin with tip temperature of 250 °C
Convection from the fin tip
Discussion The differences in the temperature variations show that applying the proper boundary condition is very important
in order to perform the analysis correctly.
17-89
17-129 Aluminum pin fins of parabolic profile with blunt tips are attached to a plane surrface. The heat transfer rate from a
single fin and the increase in the heat transfer as a result of attaching fins are to be determined.
Assumptions 1 Heat conduction is steady and one-dimensional. 2 Thermal properties are constant. 3 Heat transfer by
radiation is negligible.
)m 004.0)(C W/m230(
kD
24
2/3
2
2
4
2/3
2
2
4
fin
m 10099.2
11
m 004.0
m 025.0
16
)m 025.0(96
)m 004.0(
1116
96
D
L
L
D
A
3/)3497.0(4
3
)3/4(
3
1
1
I
mLI
9738.0
fin
W1.610
C )25200)(m 10099.2)(C W/m45)(9738.0()( 242
finfinfin TThAQb
Heat transfer from 100 fins is
W161 W)610.1)(100(
totalfin, Q
17-92
