1. Lecture Objectives: Review - Heat transfer Convection Conduction
Radiation
Analysis of a practical problem

2. Example Problem –radiant barrier in attic

3. Example Problem –heat transfer in window construction

4. Radiation

5. Radiation wavelength

6. Short-wave & long-wave radiation
Short-wave – solar radiation
<3mm
Glass is transparent
Does not depend on surface temperature
Long-wave – surface or temperature radiation
>3mm
Glass is not transparent
Depends on surface temperature

7. Radiation emission The total energy emitted by a body,
regardless of the wavelengths, is given by:
Temperature always in K ! - absolute temperatures
– emissivity of surface
– Stefan-Boltzmann constant
A - area

8. Surface properties
absorbed (α), transmitted (t), and reflected (ρ) radiation
Emission ( e ) is same as Absorption ( a ) for gray surfaces
Gray surface: properties do not depend on wavelength
Black surface: e = a = 1
Diffuse surface: emits and reflects in each direction equally

9. View (shape) factors http://www.me.utexas.edu/~howell/
For closed envelope – such as room

10. View factor relations F11=0, F12=1/2 F22=0, F12=F21 F31=1/3, F13=1/3
A1=A2=A3 A1

11. Radiative heat flux between two surfaces
Simplified equation for non-closed envelope
Exact equations for closed envelope
ψi,j - Radiative heat exchange factor