Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

104 | Thermodynamics

convection heat transfer coefficient and surface emissivity on

the heat transfer rate from the ball. Let the heat transfer coef-

ficient vary from 5 to 30 W/m^2 · °C. Plot the rate of heat

transfer against the convection heat transfer coefficient for

the surface emissivities of 0.1, 0.5, 0.8, and 1, and discuss the

results.

2–102 Hot air at 80°C is blown over a 2-m 4-m flat sur-

face at 30°C. If the convection heat transfer coefficient is 55

W/m^2 · °C, determine the rate of heat transfer from the air to

the plate, in kW.

2–103 A 1000-W iron is left on the ironing board with its

base exposed to the air at 20°C. The convection heat transfer

coefficient between the base surface and the surrounding air

is 35 W/m^2 · °C. If the base has an emissivity of 0.6 and a

surface area of 0.02 m^2 , determine the temperature of the

base of the iron.

2–104 A thin metal plate is insulated on the back and

exposed to solar radiation on the front surface. The exposed

surface of the plate has an absorptivity of 0.6 for solar radia-

tion. If solar radiation is incident on the plate at a rate of

20 °C5°C

Brick

wall

30 cm

FIGURE P2–95 1000-W

iron

Air

20 °C

FIGURE P2–103

700 W/m^2

α = 0.6

25 °C

FIGURE P2–104

2–95 The inner and outer surfaces of a 5-m 6-m brick wall
of thickness 30 cm and thermal conductivity 0.69 W/m · °C are
maintained at temperatures of 20°C and 5°C, respectively.
Determine the rate of heat transfer through the wall, in W.

2–96 The inner and outer surfaces of a 0.5-cm-thick 2-m 
2-m window glass in winter are 10°C and 3°C, respectively.
If the thermal conductivity of the glass is 0.78 W/m · °C,
determine the amount of heat loss, in kJ, through the glass
over a period of 5 h. What would your answer be if the glass
were 1-cm thick?

2–97 Reconsider Problem 2–96. Using EES (or other)
software, investigate the effect of glass thickness
on heat loss for the specified glass surface temperatures. Let
the glass thickness vary from 0.2 to 2 cm. Plot the heat loss
versus the glass thickness, and discuss the results.

2–98 An aluminum pan whose thermal conductivity is
237 W/m · °C has a flat bottom whose diameter is 20 cm
and thickness 0.4 cm. Heat is transferred steadily to boiling
water in the pan through its bottom at a rate of 500 W. If
the inner surface of the bottom of the pan is 105°C, deter-
mine the temperature of the outer surface of the bottom of
the pan.

2–99 For heat transfer purposes, a standing man can be
modeled as a 30-cm diameter, 170-cm long vertical cylinder
with both the top and bottom surfaces insulated and with the
side surface at an average temperature of 34°C. For a convec-
tion heat transfer coefficient of 15 W/m^2 · °C, determine the
rate of heat loss from this man by convection in an environ-
ment at 20°C. Answer:336 W

2–100 A 5-cm-diameter spherical ball whose surface is
maintained at a temperature of 70°C is suspended in the mid-
dle of a room at 20°C. If the convection heat transfer coeffi-
cient is 15 W/m^2 · C and the emissivity of the surface is 0.8,
determine the total rate of heat transfer from the ball.

2–101 Reconsider Problem 2–100. Using EES (or
other) software, investigate the effect of the