Problem 12-2 - Example 12-2 A plate of size 1 ft by 2 ft is...

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Example 12-2 A plate of size 1 ft by 2 ft is maintained at 135°F. One side is insulated and the other exchanges heat by natural convection with air at 65°F. Calculate h if the plate is held horizontally with the hot side up and also if the plate is held with the hot side down. Solution: Use the correlations in Table 12-3 on p. 603. ( )( ) () 12 1 ft 21 2 3 Aa r e a L P perimeter == = = + 23 2 sf gT T L Gr βρ µ = Use the film temperature for all properties o 135 65 100 F 22 film TT T + + = 3 lbm density(air, 100, 14.7) 0.0709 ft TP ρ = = lbm viscosity(air, 100) 0.04615 ft h T =
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() 32 o3 2 3 22 ft 1 1 lbm 32.17 135 65 F ft 0.0709 s 100 460 R 3 ft lbm 1 h 0.04615 ft h 3600 s Gr ⎛⎞ ⎜⎟ + ⎝⎠ = 6 4.55 10 Gr Pr Prandtl(air, 100) 0.726 T == = ( ) 66 Pr 4.55 10 0.726 3.31 10 Ra Gr × = × If the hot side is up 1/4 0.54 Nu Ra = 0.54 hL Ra k = Btu conducivity(air, 100) 0.0153 ft h R kT = 6 Btu 0.0153 ft h R 0.54 0.54 3.31 10 1 ft
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Problem 12-2 - Example 12-2 A plate of size 1 ft by 2 ft is...

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