The distance between the ends is 40 cm determine the

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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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