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Unformatted text preview: ME 410 Spring 2008 Name Exam 2 Exam is open notes, open book. Problems have indicated point value. Problem 1 (30 pts) Often a watermelon is cut in half and put into the freezer to cool it quickly. If it is left in the freezer to long, a frozen layer will appear on top. To avoid this, determine how long to keep one half of a 30 cm diameter spherical watermelon in a freezer at 12C such that the center of the outer flat surface is at 3C. The watermelon is initially at 25C and the freezer has a convection coefficient of 30 W/m 2 K. Assume the watermelon to have the properties of water. h = 30 W/m 2 C, T =12C, T i =25C, r = .15m Properties of H 2 O taken at T average ~ 285 K Cp = 4189 J/kgK, k = 0.590 W/mK, v = 0.001 m 3 /kg = 1.41E7 m 2 /s Check LCM r r r A V L k r h k hL Bi s c c 3 2 3 4 2 1 1 . 08475 . 5 3 2 2 3 = = = = = = , not valid Fourier number is unknown, looking to cool something quickly, so assume short time and model with semiinfinite. Also, unclear how to map to 1 st term geometry which requires symmetry. Use semiinfinite for convective condition at the surface...
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 Spring '08
 BENARD
 Heat Transfer

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