Q. Engineering problems of practical interest are involved with heat exchange between two or more surfaces, and this exchange is strongly dependent upon

(i) Radiative properties

(ii) Temperature levels

(iii) Surface geometrics

Identify the correct statements

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Q. The fraction of the radiative energy that is diffused from one surface element and strikes the other surface directly with no intervening reflections is called

(i) Radiation shape factor

(ii) Geometrical factor

(iii) Configuration factor

Choose the correct answer

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Q. The interchange factor is also known as

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Q. For the same type of shapes, the value of radiation shape factor will be higher when

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Q. A thin shield of emissivity E3 on both sides is placed between two infinite parallel plates of emissivities E1 and E2 and temperatures T1 and T2. If E1 = E2 = E3, then the fraction radiant energy transfer without shield takes the value

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Q. The grey body shape factor for radiant heat exchange between a small body (emissivity = 0.4) in a large enclosure (emissivity = 0.5) is

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Q. Two long parallel surfaces, each of emissivity 0.7 are at different temperatures and accordingly have radiation exchange between them. It is desired to reduce 75% of this radiant heat transfer by inserting thin parallel shields of equal emissivity 0.7 on both sides. What should be the number of shields?

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Q. An enclosure consists of four surfaces 1, 2, 3 and 4. The view factors for radiation heat transfers are

F 11 = 0.1

F 12 = 0.4

F 13 = 0.25

The surface areas A 1 and A 2 are 4 m² and 2 m². The view factor F 41 is

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Q. The value of shape factor depends on how many factors?

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Q. A large plane, perfectly insulated on one face and maintained at a fixed temperature T 1 on the bare face, has an emissivity of 0.84 and loses 250 W/m² when exposed to surroundings at nearly 0 K. The radiant heat loss from another plane of the same size is 125 W/m² when bare face having emissivity 0.42 and is maintained at temperature T 2 is exposed to the same surroundings. Subsequently these two planes are brought together so that the parallel bare faces lie only 1 cm apart and the heat supply to each is so regulated that their respective temperatures T 1 and T 2 remains unchanged. Determine he net heat flux between the planes

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Q. Interchange factor for body 1 completely enclosed by body 2 (body 1 is large) is given by

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Q. A thermos flask has a double walled bottle and the space between the walls is evacuated so as to reduce the heat flow. The bottle surfaces are silver plated and the emissivity of each surface is 0.025. If the contents of the bottle are at 375 K, find the rate of heat loss from the thermos bottle to the ambient air at 300 K

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Q. A 250 mm by 250 mm ingot casting, 1.5 m high and at 1225 K temperature, is stripped from its mold. The casting is made to stand on end on the floor of a large foundry whose wall, floor and roof can be assumed to be at 300 K temperature. Make calculation for the rate of radiant heat interchange between the casting and the room. The casting material has an emissivity of 0.85

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Q. Interchange factor for infinitely long concentric cylinders is given by

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Q. What is the geometric factor for infinitely long concentric cylinders?

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Q. What is the geometric factor for concentric spheres?

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Q. The net heat interchange between non-black bodies at temperature T 1 and T 2 is given by

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Q. A thermos flask has a double walled bottle and the space between the walls is evacuated so as to reduce the heat flow. The bottle surfaces are silver plated and the emissivity of each surface is 0.025. If the contents of the bottle are at 375 K and temperature of ambient air is 300 K. What thickness of cork (k = 0.03 W/m degree) would be required if the same insulating effect is to be achieved by the use of cork?

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Q. The total radiant energy leaving a surface per unit time per unit surface area is represented by

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Q. Determine the radiant heat flux between two closely spaced, black parallel plates radiating only to each other if their temperatures are 850 K and 425 K. The plates have an area of 4 m²

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