HeatTransfer-I-Section-10

Although radiaon emiced by a blackbody is a funcon of

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: problems we assume that surfaces behave in a diffuse manner. 7 Radia%on Heat Transfer •  Blackbody Radia%on: we now consider the special case of a blackbody. A blackbody is defined as: –  A body which absorbs all incident radia%on regardless of wavelength and direc%on •  For a prescribed temperature and wavelength, no surface can emit more energy than a black body. •  Although radia%on emiCed by a blackbody is a func%on of temperature and wavelength, it is independent of direc%on, i.e. a diffuse emiCer. 8 Radia%on Heat Transfer •  The emissive power of a blackbody is defined using Planck’s spectral distribu%on: C1 E λ ,b ( λ, T ) = 5 λ [exp(C2 / λT ) − 1] € 2 C1 = 2πhc o = 3.742 × 10 8 W ⋅ µm / m 2 and C2 = hc o / kB = 1.439 × 10 4 µm⋅ K •  h = 6.26 x 10 ­34 Js and kB = 1.381 x 10 ­23 J/K are the Planck and Boltzmann constants, € respec%vely. co is the speed of light in a vacuum. 9 Radia%on Heat Transfer •  Spectral Emissive Power 10 Radia%on Heat Transfer •  General Observa%ons: –  Emission varies con%nuously with wavelength –  At any wavelength, as T increases, the emissive power increases –  The peak emission is shieed to the lee (shorter wavelengths) as T increases. •  Wien’s Displacement Law –  Defines the locus of maximum emission λmax T = 2898 µm⋅ K •  Note that temperature is in absolute [K] and wavelength in µm. Don’t Forget!!!!! € 11 Radia%on Heat Transfer •  Stefan ­Boltzmann Law –  If we integrate the spectral distribu%on over the en%re bandwidth, we obtain the total emissive power of a blackbody [W/m2]: ∞ Eb = ∫ 0 C1dλ = σT 4 λ5 [exp(C2 / λT ) − 1] •  The constant σ = 5.67 x 10 ­8 W/m2K4 is the € Stefan ­Boltzmann constant. •  The above integra%on is...
View Full Document

This document was uploaded on 02/14/2014 for the course ENGR 6901a at Memorial University.

Ask a homework question - tutors are online