Monday, May 14, 2007

HW #8, P7 - Entropy Generation and Lost Work in a Nozzle - 5 pts

Oxygen, O2, enters a nozzle operating at steady-state at 3.8 MPa, 387oC and 10 m/s. At the nozzle exit, the conditions are 150 kPa, 37oC and 790 m/s.

a.) For a system that encloses the nozzle only, determine the heat transfer (kJ/kg) and the change in specific entropy (kJ/kg-K), both per kg of oxygen flowing through the nozzle. What additional information would be required to evaluate the rate of entropy production in this process ?

b.) Using an enlarged system boundary that includes the nozzle and a portion of its immediate surroundings, evaluate the rate of entropy generation (kJ/kg-K) and the rate of lost work (kJ/kg), both per kg of oxygen flowing through the nozzle. Assume that heat exchange at the enlarged system boundary takes place at the ambient temperature, 20oC.




Treat O2 as an ideal gas with variable heat capacities. Verify that the ideal gas assumption is valid.

2 comments:

Anonymous said...

Hi Professor,
For a I use the 1st law equation with Wsh = 0, and got the equation
Qhat = delaH+(delta v^2/2)
I looked up the number for H for O2 from the NIST and plug in the equation, and I got -26.38 instead of -30.2. Please tell me what have I done wrong, Professor!
Also, can you give me some advice on how to do part b? Thank you very much!

Dr. B said...

Anon:
Don't use NIST. Use the ideal gas property table for O2 in the back of your book.

Use the equality form of the Clausius inequality that defines Sgen.

Best of luck.