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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Q. What is the value of grey body factor for concentric cylinders?

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Q. The net rate at which the radiation leaves the surface is given by

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Q. A ring (E = 0.85) of 8 cm inner and 16 cm outer diameter is placed in a horizontal plane. A small element (E = 0.7) of 1 cm² is placed concentrically 8 cm vertically below the center of the ring. The temperature of the ring is 800 K and that of small area is 400 K. Find the radiant heat gain by the small ring

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Q. Two opposed, parallel, infinite planes are maintained at 420 K and 480 K. Calculate the net heat flux between these planes if one has an emissivity of 0.8 and other an emissivity of 0.7

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Q. The total radiant energy incident upon a surface per unit time per unit area is known as

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Q. Which one of the following is true for opaque non-black surface?

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Q. A radiation shield should

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Q. Radiation shield are used between the emitting surfaces such that

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Q. Which of the following can be used as a radiating shield?

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Q. Determine the net radiant heat exchange per m 2 area for two infinite parallel plates held at temperature of 800 K and 500 K. Take emissivity as 0.6 for the hot plate and 0.4 for the cold plate

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Q. Two parallel square plates, each 4 m² area, are large compared to a gap of 5 mm separating them. One plate has a temperature of 800 K and surface emissivity of 0.6, while the other has a temperature of 300 K and surface emissivity of 0.9. Find the net energy exchange by radiations between the plates

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