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Unformatted text preview: os θ i dA j dAi ȹ
∫∫ ȹ ȹ
πR 2
ȹ Ⱥ Fij Ai = F ji A j €
€ € Fij is the view factor which deﬁnes the amount of radia%on leaving surface i which is intercepted by surface j. 26 Radia%on Heat Transfer • View Factors can be calculated from the view factor integral or computed numerically. • In most cases we use a catalog of view factor formulas or graphical representa%ons of the results. • Only in advanced analysis do we calculate the view factors using this formula. • We also u%lize view factor algebra to help deduce view factors for most systems. • In most cases once a few view factors are known, we can solve for the rest using simple analysis. 27 Radia%on Heat Transfer • View Factor Algebra: – Two important rela%onships we use in determining view factors is the reciprocity property and the summa%on rule. Ai Fij = A j F ji Reciprocity N ∑F ij =1 Summation j=1 – In general if we have N surfaces in an enclosure, we require N2 view factors. As a minimum N(N
1)/2 view factors are €
required. The remaining factors can be deduced using the above formulas. – If a surface “sees itself”, i.e. concave, then Fii >0. Otherwise for ﬂat or convex surfaces Fii = 0. 28 Radia%on Heat Transfer • View Factors: 29 Radia%on Heat Transfer • View Factors: 30 Radia%on Heat Transfer • View Factors: 31 Radia%on Heat Transfer • View Factors: 32 Example 3 • A simple conical enclosure is the result of two parallel disks of diameter 10 cm and 25 cm which are connected with a third surface to form an enclosed system. The dista...
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This document was uploaded on 02/14/2014 for the course ENGR 6901a at Memorial University.
 Fall '12
 DrMuzychka
 Heat Transfer

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