Lecture Objectives: Review - Heat transfer Convection Conduction

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Presentation transcript:

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 <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

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

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 F11=0, F12=1/2 F22=0, F12=F21 F31=1/3, F13=1/3 A1=A2=A3 A1

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