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Unformatted text preview: ME 454 Spring 2017 Name h” «W06 Scott Exam #2
Total Points: 100 pts The points for each problem are listed. Be sure to include the symbolic version of the equation when you
are solving a problem. ifyou find yourself running short oftime, explain how you would finish the problem
if you had time (for partial credit). 1. 36 points. The top surface of the passenger car of a train moving at a velocity of 70 km/h is
2.8 m wide and 8 m long. The top surface is absorbing solar radiation at a rate of 200 W/mz, and
the temperature of the ambient air is 30 °C. Assuming the roof of the car to be perfectly
insulated and the radiation heat exchange from the surface to the surroundings is negligible,
determine the average temperature of the top surface of the car. You do not need to list your
assumptions. qusolar = 200 W/m2 be = 30 0C Air Properties:
k = 0.03 W/mK v = 1.6 x 10 ‘5 mZ/s
p = 0.6 kg/m3
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.‘wasols’naﬁ a. ME 454 Spring 2017 Name Frowkcié Sufﬁ 2. 38 points. Two long copper rods of diameter D = 10 mm are soldered together end to end, with
solder having a melting point of 650 “C. The rods are in air at 25 °C with an air velocity of l
0.4 m/s. Assume the ends of the rods are long enough that the tip is in equilibrium with the l surroundings. What is the minimumI steady state power input needed at the solder ioint to just
start melting the solder? List your assumptions Air Properties: k = 0.04 W/mK
' Fl :»=25°C;V=0. i
Air 0W T 4m/S v=5.0X10 >5 mZ/S i
llll ‘lllM’
Copper Properties:
D = 10 mm I Copper Rod Copper ROd k = 380 W/va
Ttip = Tm I 0 Trip = °°
SolderJoint
Tmelt = 650 0C q=? Assumptions: Slgmk/ SledQ) lob QavaxLQX‘K'OVK ‘iv\ (1045 (LE. ad like g'wx5\,
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09*: 51:ng => Nutt CR1» r u/ egg—~30 M‘OALDQ ME 454 Spring 2017 Name l’rowms Scﬁi 3. 8 points. What do the Biot and Fourier numbers represent physically? The Blot number is the ratio of conduction resistance of the solid to the convective resistance.
The Fourier number is a dimensionless time. 4. 6 points. What is the main difference between implicit and explicit numerical techniques? In solving the nodal temperatures, the implicit method uses the current temperature of the
surrounding nodes, while explicit uses the Qast temperatures of the surrounding nodes. 5. 4 points. Name one way you can verify your numerical solution and state whether it is an
internal or external check. There are a number of possible answers. For example:
Compare the solution to analytical or experimental date — external
Check that the solution is mesh independent — internal Check to make sure the solution is free of any nonphysical discontinuities — internal 6. 8 points. Using one of the empirical Nu correlations from the textbook, an engineer in your
facility does a quick calculation for the convection heat rejection for one of your process
comparing the current setup with a proposed new setup. She determines that by increasing
the velocity of the cooling fluid with the new set—up, you can increase the heat transfer by 5%
compared the current set up and slightly increase productivity. However, it requires down time
to replace the blower. Do you follow through on her recommendation? Why or Why not? Not without some additional analysis. The empirical Nusselt correlations can have a high degree
of uncertainty (1 10 — 25%). So the 5% bump is probably not statistically relevant and therefore
doesn’t warrant the investment. The additional analysis could include an uncertainty analysis
and a test for statistical relevance to try to show that it is a statistically relevant increase in heat
transfer. ...
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 Fall '08
 Goplen,S

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