Unformatted text preview: when they come out of the oven.
Furnace, 700°C 4–30C How can we use the transient temperature charts
when the surface temperature of the geometry is specified instead of the temperature of the surrounding medium and the
convection heat transfer coefficient?
4–31C A body at an initial temperature of Ti is brought into a
medium at a constant temperature of T . How can you determine the maximum possible amount of heat transfer between
the body and the surrounding medium?
4–32C The Biot number during a heat transfer process between a sphere and its surroundings is determined to be 0.02. 3 cm Brass plate
25°C FIGURE P4–36 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 255 255
CHAPTER 4 4–37 Reconsider Problem 4–36. Using EES (or other)
software, investigate the effects of the temperature of the oven and the heating time on the final surface temperature of the plates. Let the oven temperature vary from
500°C to 900°C and the time from 2 min to 30 min. Plot the
surface temperature as the functions of the oven temperature
and the time, and discuss the results.
4–38 A long 35cmdiameter cylindrical shaft made of stainless steel 304 (k 14.9 W/m · °C,
7900 kg/m3, Cp 477
6
2
J/kg · °C, and
3.95 10 m /s) comes out of an oven at
a uniform temperature of 400°C. The shaft is then allowed to
cool slowly in a chamber at 150°C with an average convection
heat transfer coefficient of h 60 W/m2 · °C. Determine the
temperature at the center of the shaft 20 min after the start of
the cooling process. Also, determine the heat transfer per unit
length of the shaft during this time period.
Answers: 390°C, 16,015 kJ/m 4–39 Reconsider Problem 4–38. Using EES (or other)
software, investigate the effect of the cooling
time on the final center temperature of the shaft and the amount
of heat transfer. Let the time vary from 5 min to 60 min. Plot
the center temperature and the heat transfer as a function of the
time, and discuss the results.
4–40E Long cylindrical AISI stainless steel rods (k 7.74
Btu/h · ft · °F and
0.135 ft2/h) of 4in. diameter are heattreated by drawing them at a velocity of 10 ft/min through
a 30ftlong oven maintained at 1700°F. The heat transfer
coefficient in the oven is 20 Btu/h · ft2 · °F. If the rods enter
the oven at 85°F, determine their centerline temperature when
they leave. 500°C in a fireplace with a heat transfer coefficient of 13.6
W/m2 · °C on the surface. If the ignition temperature of the
wood is 420°C, determine how long it will be before the log
ignites.
4–43 In Betty Crocker ’s Cookbook, it is stated that it takes 2 h
45 min to roast a 3.2kg rib initially at 4.5°C “rare” in an oven
maintained at 163°C. It is recommended that a meat thermometer be used to monitor the cooking, and the rib is considered rare done when the thermometer inserted into the center of
the thickest part of the meat registers 60°C. The rib can be
treated as a homogeneous spherical object with the properties
1200 kg/m3, Cp 4.1 kJ/kg · °C, k 0.45 W/m · °C, and
0.91 10 7 m2/s. Determine (a) the heat transfer coefficient at the surface of the rib, (b) the temperature of the outer
surface of the rib when it is done, and (c) the amount of heat
transferred to the rib. (d) Using the values obtained, predict
how long it will take to roast this rib to “medium” level, which
occurs when the innermost temperature of the rib reaches
71°C. Compare your result to the listed value of 3 h 20 min.
If the roast rib is to be set on the counter for about 15 min
before it is sliced, it is recommended that the rib be taken
out of the oven when the thermometer registers about 4°C
below the indicated value because the rib will continue cooking even after it is taken out of the oven. Do you agree with this
recommendation?
Answers: (a) 156.9 W/m2 · °C, (b) 159.5°C, (c) 1629 kJ, (d) 3.0 h
Oven, 163°C Oven
1700°F
10 ft/min 30 ft
Stainless steel
85°F FIGURE P4–40E
4–41 In a meat processing plant, 2cmthick steaks (k 0.45
W/m · °C and
0.91 10 7 m2/s) that are initially at 25°C
are to be cooled by passing them through a refrigeration room
at 11°C. The heat transfer coefficient on both sides of the
steaks is 9 W/m2 · °C. If both surfaces of the steaks are to be
cooled to 2°C, determine how long the steaks should be kept in
the refrigeration room.
4–42 A long cylindrical wood log (k 0.17 W/m · °C and
1.28 10 7 m2/s) is 10 cm in diameter and is initially at a
uniform temperature of 10°C. It is exposed to hot gases at Rib
Ti = 4.5°C FIGURE P4–43
4–44 Repeat Problem 4–43 for a roast rib that is to be “welldone” instead of “rare.” A rib is considered to be welldone
when its center temperature reaches 77°C, and the roasting in
this case takes about 4 h 15 min.
4–45 For heat transfer purposes, an egg can be considered to
be a 5.5cmdiameter sphere having the properties of water. An
egg that is initially at 8°C is dropped into the boiling water at
100°C. The heat transfer coefficien...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
 Spring '10
 Ghaz

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