Answers to MEC701 Final Exam W1999

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Answers to MEC701 Final Exam W1999 Multiple choice. ... 1(d), 2(e), 3(a), 4(e), 5(b), 6(b), 7(e) Here are the answers to the long questions, with some hints and intermediate results. Question 1 . (a) 2219 W/m^2 (get dT/dx=-3500 K/m at x=0 from the tabulated data. Multiply this gradient by k of the water, evaluted at the surface temperature (315K), k=.634 W/mK) (b) h = q"/(Ts-Tinf)=111 W/m^2K (use q" from part (a)) (c) dT/dx in the Aluminimn =13.05 K/m. (Heat balance at x=0: Equate -k_al dT/dx=-k_water dT/dx at x=0) Question 2. Length of Pipe L=105m (I used Hilpert for the external convection ho=42.7 W.m^2K and Dittus Boelter for the internal convection hi=1532 W/m^2K. LMTD=46.65K, U=41.5 W/m^2K. Be careful to get the fluid properties correct! See my notes for a dicussion of the usual pitfalls!) Question 3. (a) From Figure 13.9 Incropera & Dewitt Textbook (with abscissa value of 0.49) I got a fin efficiency of
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Unformatted text preview: about 0.82 (82%) (Some variability was permitted, since the chart cannot be read with high accuracy). (b) For one fin q = 5.6 W (Area of one fin 2.28E-3 m^2) (c) q_total= 1290 W (200 fins, base area 0.05667 m^2) Question 4. (This is a fairly tough question - I did more on radiation in W1999 than I usually get to. So, I may not have covered this material in detail this year.) (a) See the example for two parallel plates that I did in lecture (the sketch is identical). (b) q1=8757 W (Shape factors: F12=.367, F13=.633, F23=.79) (c) T2=382 K = 109 C (J_2=1211 W/m^2) (Note that since surface 2 is re-radiating (insulated), Eb2=J2=sigma T2^4. So, you have to find the radiosity of surface 2, J2)....
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