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Homework7_Solution

# Homework7_Solution - Cr Solve problem 3-40 29 29 29 29 29...

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BAEN-366 SPRING/09 Homework # 7 Read Chapter 3 Pages 116 – 126 Answer the following questions 1) What is contact resistance ? It’s the thermal resistance resulting from the contact of two surfaces due to imperfections or to the roughness of each material. 2) What is the overall heat transfer coefficient? It’s the inverse of the sum of the resistances divided by the area. A R U thermal * 1 = 3) What is the physical basis for existence of a critical insulation radius? How the thermal conductivity and the convection coefficient affect its value? h k r Cr = the critical insulation radius is directly proportional to k and inversely proportional to h. Below critical values q’ increases with the increase of r Cr , and above critical radius q’ decreases with the increase of r

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Unformatted text preview: Cr . Solve problem 3-40 ( 29 ( 29 ( 29 [ ] ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 [ ] 4 4 4 2 8 2 3 3 4 4 3 2 3 1 2 298 323 10 68 . 5 2 . 298 323 6 2 15 . 18 . ln 40 15 . 18 . ln 323 873 2 , , , 2 ln ln , , 2-+-= +--+ ∞-= + --K m W x K m W r mK W m r mK W m m K Tsur o Ts o T o Ts h r k r r k r r o Ts i Ts o ins st π εσ Solving for m r 468 . 3 = and m W q 01 . 541 ' = Solve problem 3-35 ( 29 ( 29 ( 29 m W q m m K mK W q r r Ts Ts k L q q r 68 . 581 ' 075 . 105 . ln 500 850 089 . 2 ' ln , , 2 ' 1 2 2 1 = -= -= =...
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