1. TEM – Lecture 5
Radiative Heat Flow

2. Radiation Radiation intensity increases with temperature!
Every surface above zero K Radiates!

4. Radiation: Emission and Absorption
Stefan Boltzmann Law (Black Body)
Radiation from a Real Body:
Absorption:
Grey Surface

5. Emissivity Coefficients

6. Surface radiate and absorb simultaneously
The incident flux of energy depends on the source.
If the energy absorbed is higher then the energy radiated, the temperature increases and the radiated flux increases up to a value where both fluxes balance (in the absence of other heat transfer processes).
What about solar energy?

7. Factors affecting solar radiation
Varies daily. Is a function of the hour of the day,
Varies with the latitude,
Varies seasonally (distance to the sun and inclination of the rotation axis)
Depends on the atmospheric particles (dust, clouds).

8. Factors affecting solar radiation at the ground level
Of the order of 400W/m2
@our latitude

9. Solar radiation is uniform over the surface.
Varies because the Earth distance to the Sun varies along the year (Earth Translation).
The inclination of Earth Rotation axis changes seasonally and thus I0 changes as well.

10. In Polar Zones total daily radiation is higher than at lower latitudes because the day is longer too.

11. In Polar Zones total daily radiation perpendicular to the solar rays is higher than at lower latitudes because the day is longer too.
In the Artic it is maximum or zero, because part of the year is dark during the whole day and part of the year there is light during the whole day.

12. Night Radiation (1/2)
Would be zero, without atmosphere. Assuming the sky as a surface, its temperature would be zero:
But there is an atmosphere. The radiation hitting the earth surface depends mostly on:
Altitude,
Cloud cover,
Atmospheric dust (pollution).

13. Night Radiation (2/2) Sky Temperature:
Desert, clean sky: Tsky= -50 C
Cloudy Equatorial Regions: Tsky= 20 C
Typical Sky Temperature: Tsky= 0 C
Sky temperature can be measured using an “Infrared Thermometer”, but the value will depend on the orientation of the thermometer.

14. Empirical formulae

15. Examples

16. Grey Surface surrounded by a closed surface

17. Can one produce Ice in a desert without a freezer?
Yes:
without other forms of heat transfer, the stable temperature of water with a free surface would be the sky temperature…
One needs only a clear atmosphere, no wind and adiabatic container walls.

19. Hot pipes are usually on stainless steel because this material has low emissivity.