### Heat Transfer

1. A grey body is one whose absorptivity

1. varies with temperature
2. varies with the wave length of incident ray
3. varies with both
4. does not vary with temperature and wave length of the incident ray
5. there is no such criterion.

does not vary with temperature and wave length of the incident ray

2. Depending on the radiating properties, body will be transparent when

1. p = 0, x = 0 and a = 1
2. p=l,x = 0,anda = 0
3. p = 0, T= l,anda = 0
4. X = 0, a + p = 1
5. a = 0,x + p= 1.

p = 0, T= l,anda = 0

3. If the temperature of a solid surface changes form 27°C to 627°C, then its emissive power changes in the ratio of

1. 3
2. 6
3. 9
4. 27
5. 81

81

4. Planck's law holds good for

1. black bodies
2. polished bodies
3. all coloured bodies
4. all of the above
5. none of the above.

black bodies

5. 40% of incident radiant energy on the surface of a thermally transparent body is reflected back. If the transmissivity of the body be 0.15, then the emissivity of surface is

1. 0.45
2. 0.55
3. 0.4
4. 0.75
5. 0.60.

0.45

6. A grey body is one whose absorptivity

1. varies with temperature
2. varies with wavelength of the incident ray
3. is equal to its emissivity
4. does not vary with temperature and. wavelength of the incident ray
5. none of the above.

is equal to its emissivity

7. A non-dimensional number generally associated with natural convection heat transfer is

1. Grashoff number
2. Nusselt number
3. Weber number
4. Prandtl number
5. Reynold number.

Grashoff number

8. A perfect black body is one which

1. is black in colour
2. reflects all heat
3. transmits all heat radiations
4. abslprbs heat radiations of all wave lengths falling on it
5. fully opaque.

abslprbs heat radiations of all wave lengths falling on it

9. A steam pipe is to be insulated by two insulating materials put over each other. For best results

1. better insulation should be put over pipe and better one over it
2. inferior insulation should be put over pipe and better one over it
3. both may be put in any order
4. whether to put inferior OIL over pipe or the better one would depend on steam temperature
5. unpredictable.

better insulation should be put over pipe and better one over it

10. Absorptivity of a body will be equal to its emissivity

1. at all temperatures
2. at one particular temperature
3. when system is under thermal equi-librium
4. at critical temperature
5. for a polished body.