Exam2_S09 - ME 315 Exam 2 8:00 -9:00 PM Tuesday, March 10,...

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ME 315 Exam 2 8:00 -9:00 PM Tuesday, March 10, 2009 This is a closed-book, closed-notes examination. There is a formula sheet at the back. You must turn off all communications devices before starting this exam, and leave them off for the entire exam. Please write legibly and show all work for your own benefit. Please show your final answers in the boxes provided. State all assumptions. Please arrange all your sheets in the correct order. Make sure they are all included. Name: ____________________________________ Last First CIRCLE YOUR DIVISION Div. 1 (9:30 am) Div. 2 (12:30 pm) Prof. Murthy Prof. Choi Problem Score 1 (20 Points) 2 (40 Points) 3 (40 Points) Total (100 Points)
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2 1. Consider the following two objects with the same geometry but different sizes exposed to nitrogen gas. The two objects are at 500 K, while the N 2 gas has a temperature of 300 K. All properties may be evaluated at 400 K: For nitrogen, you are given thermal conductivity, k = 26.0 ± 10 -3 W/mK; density, ± = 1.0 kg/m 3 ; specific heat, c p = 1000 J/kgK; kinematic viscosity, ² = 18.2 ± 10 -6 m 2 /s. With given conditions, the heat flux in object A is found to be q s1 ’’ = 2,000 W/m 2 . What is the average convective heat transfer coefficient (h 2 ) and heat flux in q s2 ’’ in object B? h 2 = W/m 2 K q s2 ’’ = W/m 2
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3 Solution Nu = f ( Re , Pr ) = hL c k ; Re = ± UL c μ Pr = ² ³ 1 = 2 ´ Pr 1 = Pr 2 20 µ L c 1 = L c 2 , U 1 = 20 µ U 2 ´ Re 1 = 2 Nu 1 = Nu 2 ´ h 1 L c 1 k = h 2 L c 2 k h 2 = h 1 L c 1 L c 2 = h 1 L c 1 20 µ L c 1 = h 1 20 q 1 " = h 1 µ ( T s ,1 · T ¸ ) = h 1 µ (500 · 300) = 2000 Wm 2 ´ h 1 = 10 2 · K h 2 = h 1 20 = 0.5 2 ² K q 2 " = h 2 ± ( T s ,2 ² T ³ ) = (0.5 2 ² K ) ± (500 ² 300) K = 100 2 q 2 " = 100 2
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4 2. A spherical metal ball of a radius, r o = 0.1 m is initially at T i = 400 K. At t = 0, the ball is
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This note was uploaded on 02/09/2012 for the course ME 352 taught by Professor Staff during the Fall '08 term at Purdue University-West Lafayette.

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Exam2_S09 - ME 315 Exam 2 8:00 -9:00 PM Tuesday, March 10,...

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