Heat Transfer Mock Test 6

1. Find the shape factor F ₁₂ for the arrangement shown in the figure. The areas A ₁ and A ₂ are perpendicular but do not share the common edge

2. A conical cavity of base diameter 15 cm and height 20 cm has inside surface temperature 650 K. If emissivity of each surface is 0.85, determine the net radiative heat transfer from the cavity

3. 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

4. The nature of the fluid flow is governed by the following parameters
(i) Mean flow velocity
(ii) Density of fluid
(iii) Dynamic viscosity of the fluid
Identify the correct statements

5. Two radiating surface A ₁ = 6 m² and A ₂ = 4 m² have shape factor F ₁₂ = 0.1. Then the shape factor F ₂₁ will be

6. 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

7. A large plane, perfectly insulated on one face and maintained at a fixed temperature T ₁ on the bare face, has an emissivity of 0.84 and loses 250 W/m² when exposed to surroundings at nearly 0 K. The radiant heat loss from another plane of the same size is 125 W/m² when bare face having emissivity 0.42 and is maintained at temperature T ₂ is exposed to the same surroundings. Subsequently these two planes are brought together so that the parallel bare faces lie only 1 cm apart and the heat supply to each is so regulated that their respective temperatures T ₁ and T ₂ remains unchanged. Determine he net heat flux between the planes

8. Two large parallel planes with emissivity 0.4 are maintained at different temperatures and exchange heat only by radiation. What percentage change in net radiative heat transfer would occur if two equally large radiation shields with surface emissivity 0.04 are introduced in parallel to the plates?

9. The furnace of a boiler is laid from fire clay brick with outside lagging from the plate steel, the distance between the two is quite small compared with the size of the furnace. The brick setting is at an average temperature of 365 K whilst the steel lagging is at 290 K. Calculate the radiant heat flux. Assume the following emissivity values
For brick = 0.85
For steel = 0.65

10. 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