Heat transfer course
Topics to prepare for the next seminar on chapter 12-13:
1. When treating heat transfer via radiation the following terms are used. Explain each one of them:
Ø Gray body
Ø Emissivity (different types)
Ø View factor
Ø Radiosity
Ø Irradiation
Ø Monochromatic
Ø Spectral intensity
Ø Spectral and total emissive power
2. Which are the basic laws of radiation heat transfer and what are the assumptions they are based on?
3. For heat transfer by radiation, Kirchhoff´s identity states that e = a, i.e. that the emissivity is equal to the absorptivity. State under what circumstances this applies and show that e ¹ a leads to unreasonable results under these conditions.
4. Draw radiation networks for the following cases and indicate how each resistance is defined:
Ø Two surfaces which see each other and nothing else
Ø Three surfaces which see each other and nothing else
Ø Two plane or convex surfaces enclosed by a third surface which is non-conducting but re-radiating (insulated).
5. One square plate with an insulated back side and another plate, with the temperature 727°C, are placed in a large surrounding room with the wall temperatures 27°C. The plates are arranged perpendicular to each other and with one edge (almost) in contact (no conduction assumed between surfaces). The length of each side is 0.5 m. The emissivity of the heated plate is e=0.6. The insulated plate is in thermal equilibrium with the other surface and the surrounding room. What is the temperature of the insulated plate and how large are the heat flows between the three surfaces?
6. To measure the effective sky temperature at night the following experiment is performed: A quadratic surface with the side 0.1 m is placed horizontally on the ground and the temperature difference between the surface and the ambient air is measured. What is the effective sky temperature if the air temperature is 20°C, the surface temperature is 15°C and the air velocity is 3 m/s? The emissivity of the surface is assumed to be 0.9, the plate is insulated on the bottom side and the plate is placed on a large open area so that it sees only the sky.
7. A circular ice rink is 20 m in diameter is to be temporarily enclosed in a hemispherical dome of the same diameter. The ice is maintained at 270 K and on a particular day the inner surface of the dome is measured to be 290 K. Estimate the radiant heat transfer from the dome to the rink if both surfaces can be taken as black. What would be the difference if the ice is assumed to have an emissivity of 0.85?