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ISRO Mechanical Engineering Heat Transfer Questions, Answers and Explanation

ISRO Mechanical Engineering Heat Transfer Questions, Answers and Explanation

Conduction

2017.2.62. The distribution of a temperature T, at a particular instant , across a large concrete wall, 1m thick, which is heated from one side is given by the equation T = 150-80x+16x2, where the distance x is in meters and T is in degree Celsius. Thermal conductivity of wall is 0.6W/mK, area of cross section is 10 m2. The heat accumulated in unit time at this instant in the concrete wall is
a) 768J/sec
b) 384J/sec
c) 96J/sec
d) 192J/sec

Answer
d) 192J/sec

Explanation
Heat accumulated = Heat conducted away - Heat conducted in
Q = kA*[dT/dx] at x = 1 minus at at x = 0
= 0.6*10*(-80+32x) at x = 1 minus at x = 0
=6*32 = 192J/sec

General

2017.2.29. Which of the following is FALSE?
a) Heat flow by conduction is directly proportional to the area and temperature gradient
b) Heat flow by convection is directly proportional to the area and temperature difference
c) Heat flow by radiation is directly proportional to the cube of absolute temperature of the radiating body
d) Emissivity of perfect black body is unity


Answer
c)

Explanation
Heat flow by radiation is directly proportional to the fourth power of absolute temperature of the radiating body.

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