To enable cooking a wider range of foods in microwave ovens, thin, metallic packaging materials have been developed that will readily absorb microwave energy. As the packaging material is heated by the microwaves, conduction simultaneously occurs from the hot packaging material to the cold food. Consider the spherical piece of frozen ground beef of Problem 5.33 that is now wrapped in the thin microwave-absorbing packaging material. Determine the time needed for the beef that is immediately adjacent to the packaging material to reach T = 0 ° C if 50 % of the oven power ( P = 1 kW total ) is absorbed in the packaging material.
To enable cooking a wider range of foods in microwave ovens, thin, metallic packaging materials have been developed that will readily absorb microwave energy. As the packaging material is heated by the microwaves, conduction simultaneously occurs from the hot packaging material to the cold food. Consider the spherical piece of frozen ground beef of Problem 5.33 that is now wrapped in the thin microwave-absorbing packaging material. Determine the time needed for the beef that is immediately adjacent to the packaging material to reach T = 0 ° C if 50 % of the oven power ( P = 1 kW total ) is absorbed in the packaging material.
Solution Summary: The author explains the time required for the beef to adjust the packaging material to reach 0°C.
To enable cooking a wider range of foods in microwave ovens, thin, metallic packaging materials have been developed that will readily absorb microwave energy. As the packaging material is heated by the microwaves, conduction simultaneously occurs from the hot packaging material to the cold food. Consider the spherical piece of frozen ground beef of Problem 5.33 that is now wrapped in the thin microwave-absorbing packaging material. Determine the time needed for the beef that is immediately adjacent to the packaging material to reach
T
=
0
°
C
if
50
%
of the oven power
(
P
=
1
kW
total
)
is absorbed in the packaging material.
A cylinder 6”in diameter and 18”long is suspended horizontally in a largeroom. The air and wall surfaces of the room are at a temperature of 60 °Fwhile the surface temperature of the cylinder is 440 °F. Compute (a) thesurface coefficient due to free convection, (b) the heat transferred by freeconvection (neglecting the end areas), (c) the surface coefficient due toradiation if the surface emissivity is 0.75, and (d) the total heattransferred by free convection and radiation (neglecting end areas)
Air at 4 °C is being carried within a metal cylindrical pipe to astorage room at a bioprocessing facility, where a heat-sensitive protein product must be stored until it is transferred to another facility. The outer diameter of the pipe is 400 mm, and the thickness of the pipe is 2 mm. The pipe is installed within a larger room where the room temperature is kept at 20°C. There is a layer of insulating material around the metal pipe. Thermal conductivity of the metal pipe is 60 W/m*K, whereas the thermal conductivity of the insulating material is 0.04 W/m*K. Heat gain by the air being transported occurs at a steady rate of 40 W. A while later, it becomes necessary to reduce the temperature of the flowing air down to 0 °C for another protein product. By how much (in percentages) should the insulation material thickness should increase in order to maintain this air temperature within the pipe? Assume the inner surface temperature of the metal pipe is equal to the air temperature in the…
Arrays of parallel equally spaced vertical thin plates are connected to the surface of a natural convection air heater. The plates are maintained at a fixed temperature of 75°C by embedding electrical heaters. The plates are 305-mm high and 305-mm long. The whole set-up is in an area where the air temperature is 20°C. The total width of the array cannot exceed 150-mm. What is the plate spacing in millimeters that maximizes heat transfer from the array?
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