This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
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 12°C such that the center of the outer flat surface is at 3°C. The watermelon is initially at 25°C 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 ∞ =12˚C, T i =25˚C, 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 α = 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 = = = = = = π π , 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...
View
Full
Document
 Spring '08
 BENARD
 Heat, Heat Transfer, TI, AISI, Detroit Red Wings, base plate

Click to edit the document details