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4-172 In a production facility, large plates made of stainless steel (k = 15 W/mºC, = 3.9110-6 m2/s) of 40 cm thickness are
taken out of an oven at a uniform temperature of 750ºC. The plates are placed in a water bath that is kept at a constant
temperature of 20ºC with a heat transfer coefficient of 600 W/m2ºC. The time it takes for the surface temperature of the
plates to drop to 120ºC is
(a) 0.6 h (b) 0.8 h (c) 1.4 h (d) 2.6 h (e) 3.2 h
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4-175 Consider a 7.6-cm-long and 3-cm-diameter cylindrical lamb meat chunk (ρ = 1030 kg/m3, cp = 3.49 kJ/kgºC, k = 0.456
W/mºC,
= 1.310-7 m2/s). Such a meat chunk initially at 2ºC is dropped into boiling water at 95ºC with a heat transfer
coefficient of 1200 W/m2ºC. The time it takes for the center temperature of the meat chunk to rise to 75ºC is
(a) 136 min (b) 21.2 min (c) 13.6 min (d) 11.0 min (e) 8.5 min
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4-177 A small chicken (k = 0.45 W/mK, = 0.1510-6 m2/s) can be approximated as an 11.25–cm-diameter solid sphere. The
chicken is initially at a uniform temperature of 8ºC and is to be cooked in an oven maintained at 220ºC with a heat transfer
coefficient of 80 W/m2K. With this idealization, the temperature at the center of the chicken after a 90-min period is
(a) 25ºC (b) 61ºC (c) 89ºC (d) 122ºC (e) 168ºC
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4-178 A potato may be approximated as a 5.7–cm-diameter solid sphere with the properties ρ = 910 kg/m3, cp = 4.25 kJ/kgK,
k = 0.68 W/mK, and
= 1.7610-7 m2/s. Twelve such potatoes initially at 25ºC are to be cooked by placing them in an oven
maintained at 250ºC with a heat transfer coefficient of 95 W/m2K. The amount of heat transfer to the potatoes during a 30–
min period is
(a) 77 kJ (b) 483 kJ (c) 927 kJ (d) 970 kJ (e) 1012 kJ
the number of potatoes”
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4-179 When water, as in a pond or lake, is heated by warm air above it, it remains stable, does not move, and forms a warm
layer of water on top of a cold layer. Consider a deep lake (k = 0.6 W/mK, cp = 4.179 kJ/kgK) that is initially at a uniform
temperature of 2oC and has its surface temperature suddenly increased to 20oC by a spring weather front. The temperature of
the water 1 m below the surface 400 hours after this change is
(a) 2.1oC (b) 4.2oC (c) 6.3oC (d) 8.4oC (e) 10.2oC
4-180 A large chunk of tissue at 35C with a thermal diffusivity of 110-7 m2/s is dropped into iced water. The water is well-
stirred so that the surface temperature of the tissue drops to 0C at time zero and remains at 0C at all times. The temperature
of the tissue after 4 minutes at a depth of 1 cm is
(a) 5C (b) 30C (c) 25C (d) 20C (e) 10C
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