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BARC 2020 : HT Nuclear Quiz 1

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Question 1

It is desired to increase the heat dissipation rate over the surface of an electronic device of spherical shape of 5 mm radius exposed to convection with h = 20 W/m2k by encasing it in a spherical sheath of conductivity 0.08 W/m-k. For maximum heat flow the diameter of the sheath will be

Question 2

The effect of forced convection can be neglected if

Question 3

A steel ball of 10 mm diameter at 1000 K is required to be cooled to 350 K by immersing it in a water environment at 300 K. The convective heat transfer coefficient is 1000 W/m2‐K. Thermal conductivity of steel is 40 W/m-K. The time constant for the cooling process 𝜏is 16 s. The time required (in s) to reach the final temperature is __________

Question 4

The overall heat transfer coefficient is 500. During the operation the tube surfaces get covered by soot, and the overall heat transfer coefficient is 250. Thus it leads to a factor having a value.

Question 5

A hollow cylinder 10 cm internal diameter and 20 cm outer diameter has an inner surface and outer surface temperatures are 300 and 200. The thermal conductivity of cylinder material is 80 W/m-k. The area of the cylinder per meter length such that cylinder can be transform into an equivalent slab is

Question 6

A triangular duct of unit length with surface temperature given in fig. length of side of triangle are L1 =100 mm, L2 = 120 mm, L3 = 150 mm

Find the ratio of net radiant heat interchange from 1 – 2 surface to the 1 -3 surface.

Question 7

A plane wall of 0.2m thickness and 25 W/m-K thermal conductivity is uniformly  generating heat within the slab. The slab is exposed to a fluid at 92oC & the convection heat transfer coefficient between the wall and the fluid is 500 W/m2-K. If the  uniform volumetric heat generation within the slab is 0.3 MW/m3, the maximum temperature in the wall is

Question 8

A cylindrical pin fin of diameter 5 cm and length 20 cm with negligible heat loss from the tip has an efficiency of 0.5. The effectiveness of this fin is _____.

Question 9

A motor cycle cylinder consist of ten fins, each 150 mm outside diameter and 75 mm inside diameter. The average fin temperature is 500 °C and surrounding air is at 20 °C temperature. The rate of heat dissipation is _______ W from the cylinder fins by convection, when motor cycle is stationery and convection coefficient h = 6 W/m2K.

Question 10

The net radiation exchange between two large parallel plates is to be reduced by 162 times. The number of screens required between these two surfaces are (take emissivity of the screens = 0.04 and that of the surfaces = 0.9)
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