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SAMPLE-Exam -2 - Answer all questions EMA 4125.Exam 2...

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Unformatted text preview: Answer all questions: EMA 4125 .Exam 2 Answer all questions. Time 50 min (1) A plane wall is a composite of two materials A and B. The wall of materials A has thermal conductivity K, = 75/WmK, and thickness L1. = 50 mm. The wall of material B has thickness Lb = 20 mm and kb = 150 W/mK. The inner surface of material A is fixed at 1000 C while the outer surface of material B is cooled by a water stream with T = 30 C . andh= 1000 W/mZK. [30pts] (a) Schematically show the temperature gradients across the composite wall. Assume that the interface temperature between Wall A and B is T2. and Wall B and fluid is T1. Please draw the profile on a reasonable scale {15 pts}. (b) Set up the equation for heat flux (i) across wall A and (ii) Wall B and fluid [15 pts] '2 ”K T 4000 OVA ——(—o—T~“ z .0 D al/g: —Kb(T\ ’T’LD 0'07, Ol’h - b (1'30). (c) Calculate the ratio of the slope change at the interface between Wall A and Wall B. {Bonus 10 points} 8%.“ Shh Ol/A= WE, W m —\<A @“HP ' \‘a “@313: (2) Determine view factors F12 and F21 view factors for the following arrangement. The diameter of the hemisphere is “D”, while the diameter of the disk is D/2. [20 3) You have a computer chip sitting on a thermal metal heat spreader. The temperature at the chip is 100 C, while the temperature of the environment is 25 C. The back end of the heat spreader is cooled by a fan. Show the thermal profiles you would expect across the heat spreader and across the gas if Biot # is (a) 0.1 and (b) 1.0 and (c) 10 .0. Please draw the profiles in a reasonable scale for each condition. [ 20 pts] [00° EDA-:0" (4) State True or False. Please provide a succinct explanation to back-up your answer. [30 pts ] (a) Diamond has a higher thermal conductivity than silicon at room temperature because it has a lower concentration of electrons. mm 5 ’ F DIAMOND ‘ than Wisdom) my” 66% P/W/ (b) The absorptivity and the emissivity of a metal can be increased by polishing the surface so that the surface roughness is reduced considerably. FALSE , DEC/2573f £D c) The emission power of a materials increases 16 times if the temperature of the materials is raised from 200 C to 400 C. fiALJ‘g T:' 673? 7‘67) W4 ((1) The solidification rate of the liquid metal in a sand mold is dependent on the thermal conductivity of the solifidified material. WE a yeti/Wit" (e) Slightly negative liquid phase gradient during solid-liquid solidification may lead to formation on dendrites. TKUE’ ...
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