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Unformatted text preview: t at the surface of the egg is
estimated to be 800 W/m2 · °C. If the egg is considered cooked
when its center temperature reaches 60°C, determine how long
the egg should be kept in the boiling water. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 256 256
HEAT TRANSFER 4–46 Repeat Problem 4–45 for a location at 1610m elevation such as Denver, Colorado, where the boiling temperature
of water is 94.4°C.
4–47 The author and his 6yearold son have conducted the
following experiment to determine the thermal conductivity of
a hot dog. They first boiled water in a large pan and measured
the temperature of the boiling water to be 94°C, which is not
surprising, since they live at an elevation of about 1650 m in
Reno, Nevada. They then took a hot dog that is 12.5 cm long
and 2.2 cm in diameter and inserted a thermocouple into the
midpoint of the hot dog and another thermocouple just under
the skin. They waited until both thermocouples read 20°C,
which is the ambient temperature. They then dropped the hot
dog into boiling water and observed the changes in both temperatures. Exactly 2 min after the hot dog was dropped into the
boiling water, they recorded the center and the surface temperatures to be 59°C and 88°C, respectively. The density of the hot
dog can be taken to be 980 kg/m3, which is slightly less than
the density of water, since the hot dog was observed to be floating in water while being almost completely immersed. The
specific heat of a hot dog can be taken to be 3900 J/kg · °C,
which is slightly less than that of water, since a hot dog is
mostly water. Using transient temperature charts, determine
(a) the thermal diffusivity of the hot dog, (b) the thermal conductivity of the hot dog, and (c) the convection heat transfer
coefficient.
Answers: (a) 2.02
(c) 467 W/m2 · °C. 10 Boiling water
94°C 7 m2/s, (b) 0.771 W/m · °C, Refrigerator
5°F Chicken
Ti = 72°F FIGURE P4–49E
minimum. The chicken can be treated as a homogeneous spherical object having the properties
74.9 lbm/ft3, Cp 0.98
Btu/lbm · °F, k 0.26 Btu/h · ft · °F, and
0.0035 ft2/h.
4–50 A person puts a few apples into the freezer at 15°C to
cool them quickly for guests who are about to arrive. Initially,
the apples are at a uniform temperature of 20°C, and the heat
transfer coefficient on the surfaces is 8 W/m2 · °C. Treating the
apples as 9cmdiameter spheres and taking their properties to
be
840 kg/m3, Cp 3.81 kJ/kg · °C, k 0.418 W/m · °C,
and
1.3
10 7 m2/s, determine the center and surface
temperatures of the apples in 1 h. Also, determine the amount
of heat transfer from each apple.
4–51 Tsurface HOT DOG
Tcenter FIGURE P4–47
4–48 Using the data and the answers given in Problem 4–47,
determine the center and the surface temperatures of the hot
dog 4 min after the start of the cooking. Also determine the
amount of heat transferred to the hot dog.
4–49E In a chicken processing plant, whole chickens averaging 5 lb each and initially at 72°F are to be cooled in the racks
of a large refrigerator that is maintained at 5°F. The entire
chicken is to be cooled below 45°F, but the temperature of the
chicken is not to drop below 35°F at any point during refrigeration. The convection heat transfer coefficient and thus the
rate of heat transfer from the chicken can be controlled by
varying the speed of a circulating fan inside. Determine the
heat transfer coefficient that will enable us to meet both temperature constraints while keeping the refrigeration time to a Reconsider Problem 4–50. Using EES (or other)
software, investigate the effect of the initial temperature of the apples on the final center and surface temperatures and the amount of heat transfer. Let the initial
temperature vary from 2°C to 30°C. Plot the center temperature, the surface temperature, and the amount of heat transfer
as a function of the initial temperature, and discuss the results.
4–52 Citrus fruits are very susceptible to cold weather, and
extended exposure to subfreezing temperatures can destroy
them. Consider an 8cmdiameter orange that is initially at
Ambient air
–15°C Orange
Ti = 15°C FIGURE P4–52 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 257 257
CHAPTER 4 15°C. A cold front moves in one night, and the ambient temperature suddenly drops to 6°C, with a heat transfer coefficient of 15 W/m2 · °C. Using the properties of water for the
orange and assuming the ambient conditions to remain constant for 4 h before the cold front moves out, determine if any
part of the orange will freeze that night.
4–53 An 8cmdiameter potato (
1100 kg/m3, Cp 3900
J/kg · °C, k 0.6 W/m · °C, and
1.4 10 7 m2/s) that is
initially at a uniform temperature of 25°C is baked in an oven
at 170°C until a temperature sensor inserted to the center of the
potato indicates a reading of 70°C. The potato is then taken out
of the oven and wrapped in thick towels so that almost no heat
is lost from the baked potato. Assuming the heat transfer coefficient in the oven to be 25 W/m2 · °C, det...
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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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