Lecture Objectives Review Heat transfer Convection Conduction Radiation

Lecture Objectives: • Review - Heat transfer – Convection – Conduction – Radiation Analysis of a practical problem

Example Problem –radiant barrier in attic

Example Problem –heat transfer in window construction

Radiation

Radiation wavelength

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

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

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

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

View factor relations A 2 A 3 A 1 F 11=0, F 12=1/2 F 22=0, F 12=F 21 F 31=1/3, F 13=1/3 A 1=A 2=A 3

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