AME 517, Fall 2009

Problem set #2

Assigned: 9/18/2009

Due: 9/29/2009, 4:30 pm

 

Homework may be submitted by email (ronney@usc.edu), by fax (213-740-8071) or put in my mailbox (down the hall from my office in the OHE 430J suites, in the small room with the copy machine).  DEN students should submit through the usual channels.

 

NOTE:  You can use the Excel spreadsheets to check your answers, but you need to show your work using the relevant equations.

 

5.1 – answers:  front 112 kW, rear 150 W, free convection 266 W.

 

5.7 – answer:

 

5.18

 

5.19 – answers:  T1 = 362K, T2 = 275K (note:  F1-2 isn’t particularly easy to find, but it turns out to be 1/3; use the inside sphere method)

 

5.20 – partial answers:  T1 =  364K for q = 0Ż; T1 = 306K for q = 60Ż

 

5.22 – answer:  -137.4 W/m of collector length

 

 

Problem not in text (from the midterm in a previous year):

 

We have discussed the “greenhouse effect” in class, which causes the interior of an enclosure to reach a temperature higher than that of the surrounding air when sunlight passes though a window of the enclosure.  Answer and explain briefly the following questions about this effect:

 

  1. Would running a fan inside the enclosure to increase the convective heat transfer coefficient between the interior air and the wall of the enclosure increase or decrease the temperature inside the enclosure?
  2. If the window glass absorbed more at short wavelengths than long wavelengths, would the interior of the enclosure be warmer, cooler or the same temperature as the outside air?
  3. To maximize the greenhouse effect, would you coat the interior of the enclosure with a low emissivity or high emissivity material, or does the emissivity not matter?  (Assume whatever coating you use, the emissivity is independent of wavelength.)
  4. At night, will interior of the enclosure be warmer, cooler or the same temperature as the outside air?
  5. If the window glass absorbed equally at all wavelengths, would the interior of the enclosure be warmer, cooler or the same temperature as the outside air?