HeatTransfer-I-Section-10

These include the concepts of surface radiosity and

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Unformatted text preview: ansmissivity € € € *We can also write these on a spectral basis as well. 19 Radia%on Heat Transfer Reflec%vity 20 Radia%on Heat Transfer Transmissivity Solar Irradia%on 21 Example 2 •  Assume solar irradia%on is propor%onal to a blackbody with a temperature of 5800 K, is incident upon a quartz glass window with a transmissivity of 0.93 over the bandwidth 0.2 µm – 4 µm and approximately 0, elsewhere in the spectrum. What is the total transmissivity of the glass. 22 Radia%on Heat Transfer •  Kirchoff’s Law and Grey Surfaces –  Kirchoff’s Law in radia%on states either α = ε, or αλ = ελ, for diffuse surfaces in thermal equilibrium. –  It can be shown that it holds in general for any condi%on, since α and ε only depend on the surface condi%on and its temperature. –  It is also valid under condi%ons illustrated below, i.e. αλ and ελ independent of λ over the spectral regions of irradia2on and emission, providing we have diffuse surfaces and diffuse irradia2on. 23 Radia%on Heat Transfer •  Radia2on Exchange: –  We now consider essen%al concepts for diffuse radiant exchange between surfaces. –  These include the concepts of surface Radiosity and View Factor. –  We will present the basic exchange equa%ons for enclosures with both black surfaces and grey surfaces. –  In general we assume surfaces are at fixed uniform temperatures, but we may also specify heat fluxes. –  We will also limit our analysis to two and three surface systems. 24 Radia%on Heat Transfer •  Surface Radiosity: is the combined effect of reflected incident radia%on (ρG) and surface radia%on emission (E). In other words the rate at which radia%on leaves a surface: J = E + ρG € 25 Radia%on Heat Transfer •  View Factors: 1 Fij = Ai 1 F ji = Aj ȹ cosθ i cosθ j ȹ dqi → j = J i ȹ ȹdAi dA j πR 2 ȹ Ⱥ ȹ cosθ j cosθ i ȹ dq j → i = J j ȹ ȹdA j dAi πR 2 ȹ Ⱥ € qi → j Fij = Ai J i ȹ cosθ i cos θ j dAi dA j ȹ ∫∫ ȹ ȹ πR 2 ȹ Ⱥ ȹ cosθ j c...
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This document was uploaded on 02/14/2014 for the course ENGR 6901a at Memorial University.

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