Which of the following is generally considered as opaque surface towards radiations ?

Related Mcqs:

1. The sum of reflectivity and absorptivity for an opaque body is equal to____________________?

   A. 0.5
   B. 1
   C. 0
   D. 2

2. Electromagnetic radiations propagate in vacuum with a velocity of _______________ metre/second?

   A. 3 × 105
   B. 3 × 108
   C. 3 × 1010
   D. 3 × 1012

3. Fouling factor for a heat exchanger is given by (where, U1 = heat transfer co-efficient of dirty surface U2 = heat transfer co-efficient of clean surface) ?

   A. U1 – U2
   B. 1/U1 – 1/U2
   C. 1/U2 – 1/U1
   D. U2 – U1

4. The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.°C- 1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m-2.°C-1. The rate of heat loss from the outer surface in W.m-2is__________________?

   A. 165.4
   B. 167.5
   C. 172.5
   D. 175

5. The interchange factor for radiation heat transfer from surface ‘x’ to surface ‘y’ in case of an infinite parallel planes with emissivities εx & εy is given by_______________?

   A. εx + εy
   B. εx . εy
   C. 1/εx + 1/εy
   D. (εx + εy)/( εx + εy – εx . εy)

6. Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be_________________?

   A. 0.35
   B. 0.45
   C. 0.55
   D. 0.85

7. The most conducive surface for dropwise condensation to occur is the _____________ surface?

   A. Coated
   B. Oily
   C. Glazed & polished
   D. Smooth

8. Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin efficiency is defined as the ratio of heat transferred across the fin surface to the theoretical heat transfer across an equal area held at the________________?

   A. Surrounding temperature
   B. Average temperature of the fin
   C. Temperature of the fin end
   D. Constant temperature equal to that of the base

9. The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by__________________?

   A. Painting the surface black
   B. Painting the surface white (with aluminium paint)
   C. Giving the surface a mirror finish
   D. Roughening the surface

10. The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness) _________________?

    A. ∝ t
    B. ∝ 1/t
    C. ∝ √t
    D. Independent of ‘t’